Note: Descriptions are shown in the official language in which they were submitted.
CA 02248912 1998-09-11 ' ' ~
,
~IETHO~} AND .~P~RATUS FOR ~IP~OVEr)
BE~ OPTIC LIGHl ~ GEMENT
STAlEhI~ REG.~D~ REL.~TED APPLIC~TIO~'S
This a~plica~ior. claims ~h~ benefit of U.S. Pro- ision;ll Aprlic~tion ~ios
6u,013,3~I en~itied Fiber Optic Lntert~~e with Manipul2ted D~ ery ~nd R~eption
S~n~itivities," ~;led ~ arch 1~, 19~6. 60,03~,~a4, entitle~ "Lrnproved Fiber Optic Probe
As~embly,'' filed January ~, i997, and 5C1038,395, entitl~ -'Improved Fiitering of
Optical ~ibers and Other R~l~ted Oevice~," filcd Februa~- 14, 1997.
TEC~lCAL FEL~)
Tnis invenlion rela~es oenerally lo optic~] fioers, and mc~re particula;iy ro
opt c~1 f~er ?.obes tha~ use mampu]ated d~livery a.~d rece~t~on r~vions to irnpro~e
se~sitiYity to spe~a~lc light-m~tter interacti~ns.
BACKGRO~,~D ~F 1~ ~E~'rIO~
~5 In rc~er.t years, thc use of optieal fibers has ~ercme increasir.~ly
widespread in a varie~ of applicati~.ns. Optical fiber probes have been found to be
esFecially useful for ~nalyzin~ mat ri~ls by employing ~arious :ypes o~ 1ight-scar~.ering
spectroscGpy .
Op~ical fibers offer numerous advantages ovet o~h~r types of
sourceJdetection equipment. In short, the ~iber provides a light conduit so ~hat the
~ource-generatin~ h~rdw~re and the recording ~pparatus are s~tioned indep~ndently of
the subj~ct under investigation and the po'nt ~f analysis. T~.us, analyses are conduct~
remolcly in o~herwi~se i~accessib}e loc~ions, Previously unattainable information is
acquired in ~itu, ctten in real ~ime. 1his c~p~bility i~ sought in numerous industrial,
~5 ~nvironmental ~nd biomedical applications. The l~boratory is moved on line in ~he
indu~trial re31m, to the feld in the envi~onrr,ent~ ctor, and in vivo in the biotec~nlc~l
AM~NCc3 S~
. . .
CA 0 2 2 4 8 912 19 9 8 - 0 9 - 1 1
~rena Addition3.1]Y, h~rd~are :~nd measure~ nts 2re more robust, quic~er, less
in~iive, ~ore ni~ged, less eo~tl~ and m3ny ot~ler adv~n~ges lre re~lized.
~C-HT SCATTERI~(~ SPE~T~SCOPY
~ 'hi!~ t~3njmission spectroscopv an~ly~es li_hs ~sinC thr~ugh a
subs~nce, lioht-ic~cct~rin~ spectroseon~ en~ils IJu:n-.~n~tion of ~ measu.;md ~nd
~nalyzlng light th~t is scatt~re~ at angles rel~tiYe to the inciden~ soclrce. The photo. -
matter in~er~ctions of the scattering events may be either elastic or inelast~c. Tn an
inela~tic ever~t, ~ photon'i ~nergy ~ av~]en~h) changes 3S a resul~ of th~ ]ight-m~t~er
10 interaction. In ~t ela3tic ~vent, a photon'~ energ~y (w3velenOt~) does not ch~nge.
Abso~pticn, the phenomena in which ~ fr:lction of photonc are ~n~irely ~bsor~ed, also
pl~ys a ro!e in light-sc~ttering s~ectroscopies. Raman, diffuse, ret'lectance, ar~d
.luorescence spectrGscop}es ~re of p~.~tic~lar interest as th~y reiate to vibr~ti~n~l and
;lOTlYibr~iO~ phCtOlllC reSpon.Se5 OI 3. mat~rial.
The R~m~n effec~ describes a subt!e light-macter 1nter~ction. ~inut.
fr~ctions of light illllrnin~ing a substance :~re ~rn2n-cc~cred in ra~dom airecuons.
Ra~n-~c~ctered light is color ~1~itted from Ihe incident beam (usually ~ laser). Th~ c 013r
(frequency) shifts are hiehiy specific ~s they relate tO mole~ul3r bond v1b~a~ons indueing
molecula~ pol.~ri~;~bility changes. ~aman spectro~copy is ~ po~erful technique for
~0 chemic31 anaiysis ~nd monitor ng. The resllltin~ low ilgh~ le~,els re4uire sophts~icated,
e~pensive instrumen-~ation ~n.d technical cornple~i~y. Suita~]~ technology and products
. for on-~ine dnalysis c, prccesses and environmeGt:ll conta~inal~ts ar just becomin~
available.
Speeular refl-ctance .elates to ~ surf~e's mirror-liXe ~spects. Dif~use
reflec~ce relates to ligh~ th~ is eias~ica!ly scatcesed *om a sur.~ace of materi~l at dimls~
angles relative to tne inci~ent ~e~m. For e~arnple, a projec~or ~cre-r! diffusely refl.ects
light while a gJossy, ne-~v waxed car has a high specul~r c~lr.pon~nt. Diff~lse ref~ectance
spectroccopy is important for chernica] analysis as well ~s me~suring visual percepti~n.
Arnong other things, it is based on particula~e-scatlenn~ and a~sorptio!l events.
~0 Fluorescenc~ relates to s~lbsta~ces ~hich absorb light at one waielength
then re-emit it at ~ longer wavelength as a result of clectronic transitions. As an c~mpl-,
a "hi~hlighte." fclt-tip rrark-r ap~e~rs to '~low" green as it absoros b]ue al~d ultra-~iolet
light the~ emits it ~s green. Fluorescenc~ provid~s a po~effu~ technique tcr chemic~l
monitcrin~.
~aman spectro~copy is a well-establij~led l~bor~tory techniq~e and is
~eneral~y reco~iz~d ~s having cnor~iolls pot~nti~l tor on-lin~ monitonng and sensin~.
With tne advent of slable la~ers, che;lp compuling power, efficient detectors, ;~r,d otker
new technologica~ ~dv~ncernents, Raman spe~roscopy is primed for widespread
A~!L~
..
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industrial monitorin~ deplovment. In addition to process control monitoring, it ~vili be
utilized in specialized monitorhlg and sensing device~ ranging from neuroimagino to
environmental monitoring~ to in vitro and h~ o medical testin~.
~aman spectroscopy involves ener~izing a sample with a high-power,
5 narrow-wavelength energy source, such as a iaser. The laser photons induce lowintensity light emissions as wavelengths shift from the laser's. The Raman effect is an
elastic scattering of photons The emitted Raman light is collec~ed and analyzed with a
specialized instmment.
The spectral positions (Colors! of the shifts provide fingerprints of the
10 chemicals in the ~ample. Th~ls. Raman spectroscopy provides a means t'or chemical
identification. The intensi~y of the shift (the spectl;ll peal~ height) correlatec ~o chemical
concentratiol) Thus. a properly calibrated instrument provides chemical contellt and
concentratioll. In pr.lcticality. Ralllan spectloscop! i~ t~chnically complex and require~
so~histic.lted. e.Ypen~ e instl ulllelltatioll.
Ram.lll spectroscopy is ~ell suited to aqueo~ls-based media ~vithout
sample preparation. From this standpoint. it is an ideal tool for process control medical
testing and environmental applications. Thus. Raman spectroscopy has great potential
for real-time monitoring and is being vigorously pursued.
The basic concept for a probe-based, on-line Raman instrument is simple.
Laser light is directed down an optical fiber to a remote probe. The laser liglh~ exits the
fiber and illuminates the sample medium. Another fiber picks up the Raman-emitted
light and returns it to the instrument for analysis.
In practicality, the engineering challenges for a robust physical probe
implementation are substantial. In addition to the optical performance expected by
labora~ory instruments, a probe must be hardened to withstand extreme physical and
chemical conditions. Optical characteristics must also remain constant as dynamic
conditions change.
Optical aspects of probe engineering require particular design finesse.
The Raman effect involves very weak signals. Raman emissions may be one trillionth as
intense as the exciting radiation. Subsequently, the probe must be incredibly efficient in
collecting and transmitting Raman-emitted light. And, the signal must not be corrupted
by extraneous influences. As an example of the sensitivity, Raman instruments typically
feature cosmic ray filters. The mechanisms identify and discard measurement datasamples influenced by passage of a single cosmic ray photon through the detector.
A phenomenon known as the silica-Raman effect has proven especially
troublesome for those engaged in remote Raman spectroscopy. As laser light is
transmitted o~er optical fibers~ a subtle light-matter interaction inherently occurs. The
laser light and the silica in the (glass fiber interact generating "silica-Raman" light. The
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extraneous silica-Raman light becomes wave~uided in the fiber and hopelessly mixed
with the laser light. The purity of the laser li'ght is corrupted. Fiber flJorescence cause
similar problems.
Remote Raman spectroscopy employs optical fiber between the base
instrument and the remote probe or process interf~ce. Optical fiber delivers laser light
from its source to the probe. Separate fiber returns sensed light from the probe to an
instrument for analysis. In both delivery and return, undesirable silica-Raman light
travels in the fibers concurrently with desirable l~ser and sensor light. A major obstacle
in fiber-optic-based Raman spectroscopy has been in separating the desirable light from
the undesirable silica-Raman li,ht.
FLAT FACE~ PARALLEL FIBER PROBES
Standard optical fibers deli~er.lnd recei\~e ligh~ within narro~ angul.
confine~. Consider a ' probe'' that is formed by mounting two s~andard. t'lat face fiber~
(i.e., a source fiber and a collection fiber) in parallel. The functionality, operation~ and
limitations of this probe will be analyzed to present relevant technical re~uirements. The
technical discussion addresses, among other things, issues of optical efficiency.
Efficiency is a critical parameter concerned with the ratio between the illumination energy
verses the energy of collected light.
Increased optical efficiencv has significant benefits. As efficiency is
increased, system performance is dramatically boosted. In sophisticated instrument
systems, enormous efforts, expense and other considerations are devoted to produce
small, marginal performance gains in the detector subsystem. With an optimized probe,
tremendous gains are readily realized. Gains in probe efficiency vastly dominatefractional electronic and detector improvements. With increased probe performance, the
overall system benefits with reduced noise, increased stability, faster response, and
better repeatability. Required illumination intensity is minimi7ed. This translates to
reducing intrusive aspects and ensuring the subject under analysis is not damaged or
altered. In addition, much less expensive opto-electronic components can be employed.
In the flat face, parallel fiber probe, the source fiber delivers illllmin~ting
light in the form of a diverging light beam. Th~ collection fiber has a receptivity zone
that is similar in shape to that of the illumination zone. However, the collection and
illumination zones are offset from one another, each originating from its respective fiber
face. As the zones expand outward from the fiber end faces, they begin to overlap.
Under normal circumstances, only in this overlapping region can the source fiber deliver
illumination and the collection fiber gather light from the target. The lack of overlap
between these regions produces numerous troublesome effects. A second, though not
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entirely distinct, set of problems is associated with the angular orientation of light rays
within the illumination and collection cones. These problems are described below.
In many common applications, the investigative medium is light
absorbing, the probe might be deployed in a chemical mixture that is slightly black but
5 not fully opaque. For example, various biological tissues are well known as light-
absorbing matrices. And, the sample need not be dark in the traditional sense. Even
visually transparent media often strongly absorb ultraviolet and/or infrared light. In a
light-absorbing medium, the illumination light must penetrate some distance into the
environment prior to reaching a position in which the detector fiber can actively collect
10 returning light. Since the source light is absorbed as it traverses this distance, its
intensity is ~imini~lled before it reaches an active target zone. Once the illurnination light
reaches an active target zone, it triggers release of potentially collectible sample light
from the target. Depending on the application, the sample light may be generated by any
of various photonic mechanisms. Assuming a passive target, the sample light is reduced
15 in strength from the illuminating source light. Depending on the phenomenon of
interest, the attenuation is severe. Before capture by the collecting fiber, the sample light
must traverse a path through the absorbing medium further reducing the signal strength
by attenuation.
Initially, this problem appears readily solved by increasing the
20 illumination intensity. While in certain cases this technique might be effective, in many
circum.~nces, it is not feasible. As the medium absorbs source light energy, it can be
irreparable damaged. Even without damage, minimllm light intensity translates tominiml-m intrusive attributes. And, in addition to damage, photochemical reactions are
inadvertently initiated in certain circumstances. Therefore, applications that wil! not
25 tolerate high intensity illumination may preclude the use of a flat-face, parallel fiber
probe. In addition, the goal of minimi7.ing illumination light intensity is desirable in
almost all uses that are ~;u~ tly being investig;~t.o(l
A second problem exists in environments that involve elastic particulate-
scattering media, such as slurries, mists, aerosols, paints, and various other media.
30 Biological tissues are well known for these types of light-scattering characteristics. Most
unpurified sarnples scatter light to a certain degree and often intensely. Although light
scattering occurs by various m~ch~ni.cm.c, Rayleigh and Mie-scattering is common and
produces strong influences. As with the previous example, the illumination energy must
- traverse a path of attenuation prior to reaching a target zone for which the collection fiber
35 is receptive. And, the target-generated light must likewise traverse a path through the
scattering agent prior to reaching the collection fiber. As with the example of the light-
absorbing sample, minimi7:ing delivered light intensity to prevent sample damage is a
factor.
SUBSTITUTE SHEET (RUI.~ 26
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For elastic lioht-scattering media~ additional detrimental effects are
observed. Assume a distinct tarae~ is stationed within the particulate-scattering medium
and is positioned within the region where the il]umination and light-gathering zones
overlap. Illumination is elastically scattered as it traverses a path to reach the target.
5 Although the direct pathway may lie outside of the collection fiber's receptivity zone, it is
incorrect to surmise that this scattered light cannot be captured by the collecting fiber.
The incorrect conclusion is based upon a sing]e scattering event which primarily redirects
a source ray to a new angular orientatiorl. The population of angular orientations for an
arbitrary single ray is statistically determined and is a function. among other things. of
10 the characteristics of the scattering agent. These characteristics include, but are not
limited to, pa.ticle size. shape. refractivc inde,~. and reflective ~ualities. Granted, for a
sin~le scattering event ~o oener;1te a ray to be received by the collection fiber, the event
mUSt~)CClll ~ithin the coilection fiber's recelttivily zone. Unfortunately, light scattering.
particulally Rayleigh and .~Iie-scattering~ often is a multiple event phenomenon.
15 Typically~ ;i source ray undergoes multiple scattering events and is redirected many
times. Thus, the ray path is complex as it interacts with various sample particles.
As an overly simple example, consider a ray exiting the source fiber
paral1el to the fiber axis at a zero-degree heading, and is scattered by an event
perpendicularly directin_ it to a 90-degree heading. At this heading, it enters the
20 collection fiber's zone of receptivity. 'Nhile in this zone, the ray undergoes a second
event directing it to a new heading for intersection with the collection fiber's end face.
The ray is then captured by the collection fiber.
Li~ht captured by the collection fiber prior to undergoing intended
interaction with the target is usually highly detrimental. The negative effect transcends
25 diminishing the intensity of source illumination delivered to the target. This light
becomes indiscriminately mixed with the desired light within the collection fiber. This
"stray light" severely corrupts the process of various analytical measurements.
Typically, the stray light becomes indistinguishable from the desired light. Stray light
levels may be dependent on various environmental factors. In the aforementioned
30 example, stray light is a function of the quantity of scattering agent present in the optical
path. Assuming this quantity is an uncontrolled application variable, the effect cannot be
readily eliminated by referencing of similar compensation.
The situation in which scattering medium separates an intended target
from the probe tip is quite common. For exarmple, for in vivo analysis of biological
35 samples, various light-scattering aqueous solutions separate the probe tip from the target.
For example, biological tissue is often surrounded by fluids containing scattering agents,
such as tissue particulales and blood.
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A distinct class of sensor measurements is concerned with analyzing
particulate-scattered light to ascertain particle characteristics. In this configuration,
returning light from the particles is analyzed to ascertain turbidity, particle concentration,
and related parameters. These measurements are highly sought in the biotechnology
field for both bioprocesses as well as in vivo and in vitro biomedical applications.
Industrial applications are likewise numerous. In this instance, it is desirable to collect
and analyze light that has undergone a minimal number of scattering interactions. It is
understood from the previous discussion that the greater the distance from the probe end
face to a zone of mutual illumination and collection, the more likely the collected light
will have undergone multiple interactions. Therefore, for this application, other related
criteria addressing the extent and spatial duration of the zone overlap, and various
illumination and collection angles can and should be optimized.
Consider an application in cie~r media. which exhibits neither light
~bsorption nor particul~te scattering. As the distance from the probe end face increases~
the zones of illumination and receptivity increasingly overlap and ~symptotically
approach full concurrence. However, it would be incorrect to assume the optimal target
location is at a position removed from the probe end face, where the illumination and
collection zones are basically in concurrence. An opposing factor must be considered.
As distance from the probe end face increases, the relative sizes of the fibers nonlinearly
decrease. At a point removed from the end face, the collection fiber possesses light-
gathering abilities within a solid angle. These two opposing factors can be modeled to
calculate an optimal target distance which maximizes the signal for a given set of
application criteria, including beam divergence, fiber size, and fiber separation. The
mechanism by which the target returns source light and the characteristics of this light are
also important. Nevertheless, the solid angle effect is dominant and the collection fiber's
light-gathering ability decreases dramatically as distance from the fiber end face
increases. From this perspective, it is highly advantageous to be able to position the
target as close to the probe end face as possible. As with the discussions of the probe's
other limiting factors, intensity is a major factor.
Consider an application in which a flat face, parallel fiber probe is used
for Raman analysis of a clear fluid. In this case, the medium through which the
detection and collection beams are projected and the target are one in the same. As the
collection and illumination zones extend from the probe tip, they overlap as previously
described. ~Jnfortunately, at a distance away f~om the probe tip at which significant
overlap occurs, the illumination beam has diverged, and its intensity has diminished.
For the collection fiber, a similar scenario exists. At a distance at which zone overlap
occurs, the relative size of the collectior, fiber is reduced. The solid angle within which
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the fiber has the ability to collect light is severely reduced over that close to the collection
fiber end face.
Along a similar line of reasoning, consider a probe investigating
fluorescence characteristics of a liquid in a flat-bottom beaker. If the liquid is sufficiently
5 transparent, a portion of the light penetrates the liquid to the beaker bottom and is
reflected back to the detector fiber. This reflection manifests itself as stray light and
corrupts the acquired data. If the probe had the ability to angularly control illllmin~tion
and collection, then the stray light problem would be avoided by directing the reflections
to miss the detector fiber.
The dependence of captured light intensity upon target dict~nce from a flat
face, parallel fiber probe tip is often utilized in the prior art to create a displacement
sensor for position measu,~ment. The dynamic range and characteristics of such sensors
are limited by available source and detector pattern geometries.
Another important factor related to the probe is power density of the
15 delivered illumination. Power density may be expressed in watts per unit area. Power
density in the medium is highest at the surface of the illumin~ting optical fiber and
decreases as the source beam diverges. Thus, fibers that do not rapidly diverge maintain
power density as the source beam is projected into the medium. Unfortunately, the
source beam must diverge in order to deliver illumination light into the collection fiber's
20 zone of receptivity. For a given quantity of light injected into the proximal end of a
source fiber, power density at the fiber's distal end face at the probe tip decreases as the
fiber core diameter increases. As previously described, the lower the power density, the
less intrusive the probe and the less potentially f~m~ging the source energy.
In addition to the described criteria, the angular orientation of rays within
25 the illumination and collection zones are of interest. Depending on the intended
application, this aspect is critically important. For a flat face, parallel fiber probe,
emitted illumin~tion rays are oriented within the divergence angle of the ill-lrnin~tion
pattern and centered about the fiber's axis. The fiber axis is, therefore, the average
angular orientation of the emitted light rays. A similar scenario exists for the30 receiving/collection fiber.
Consider gathering light from a theoretical point source positioned a short
distance from a collection fiber end face. The fiber's cone-shaped collection pattern
extends outward from its end face. If the point source is positioned outside thecollection pattern, no light is collected by the fiber. At this position, light rays incident
35 on the fiber end face are not ~ropelly angularly oriented for collection. Similarly, if the
point source is positioned within the collection pattern, a portion of the point source rays
are collected. For a given stand-off distance of the point source from the collection fiber
end face, the fraction of collected light varies across the collection pattern. With the
SIJBSlllUE SH~ F 26)
CA 02248912 1998-09-11
WO 97134175 PCT/US97/04365
point source at the center axis of the pattern, the fraction collected is maximum. Moving
at a right angle to the center axis of the fiber, the zone of maximum collection extends
across a portion of the collection pattern. Moving further towards the outer boundary of
the collection pattern, the fraction of collected light is reduced. This reduction in
5 collected light is due to the fact that near the edges of the collection pattern most of the
- point source rays strilcing the fiber end face are improperly angularly oriented for
collection. The described scenario is important in modeling and understanding the
effects of collection and illumination zone overlap in fiber optic probes. In the described
flat face, parallel fiber probe, the overlap occurs only in the outer fringes of the conical
10 illumination and collection zones. The center, more critical regions of the illumination
and collection patterns do not coincide with one another. Thus, efficiency is poor.
For many measurements, the angular orientation of illumin~tion and
collection light is crucial. As previously described, Rayleigh and Mie-scattered light is
often angularly biased and the bias orientation is analytically important. Similarly, for
15 measurements related to visual ,oercel~lion, the angular orientation is often crucial. Gloss
is measured at specific angles of il~umin~tion and collection. Various material parameters
such as paper brightness are likewise measured. For color measurements, illumination
and receptive angles are often specified according to the material under analysis and
various industry-specific standards. Often, diffuse illumin~tion is desired. Perfectly
20 diffuse illumination has no angular bias; the target is ilh-min~ted by light rays incident
from all directions. Perfectly diffuse illumination is never fully att~in~hle; nevertheless,
it can be approached.
In addition to visually oriented measurement such as color, texture,
smoothness, and gloss; diffuse reflect~nce mea~ul~llellLs are widely utilized in analytical
25 mea~ulcll,ents. In many of these mea~urenlel-ts, it is desirous to m;nimi7~ the specular
component of reflection. In so doing, collection of source light that has not undergone
the desired interaction with the target is minimi7.e~ This characteristic is desired for
diffuse reflectance measurements in the visible, ultraviolet, near-infrared, and infrared
regions. It is also often desired for general light-scattering measurements including
30 fluorescence and Raman spectroscopy. It is readily seen that a flat face, parallel fiber
prob~,is limited in its capability to deliver diffuse illumination. These measurements are
highly sought for a variety of industrial and biomP-lic~l applications.
In addition to attaining light-diffusion-related measurements based upon
illumination of a target with highly diffuse light, another technique is of interest. In this
35 technique, light is angularly directed at the target such that the specular light from the
target surface is reflected away from the light-collection device. By this means, the
collector is only receptive to light that the target scatters in a non-specular fashion and the
SLlBSll~Ul~S~lEE~ ~Ru~E 26)
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light-collection device is not receptive to specular light. It is readily seen that a flat face,
parallel fiber probe lacks the capability for angular light control to achieve this goal.
Consider a flat reflective surface placed in front of the flat face, parallel
fiber probe so that it is perpendicular to the fibers. If the surface is positioned within the
S region of overlapping receptivity and illumination, then the collection fiber receives and
transmits source light projected from the reflective surface. However, the received light
is a small fraction of that available. Because the angle of incidence equals the angle of
reflection, the majority of the light is directly reflected back and away from the collection
fiber. The axis of the reflected light remains concurrent with the source fiber's axis.
10 The fact that the optical axis of the illumination from the source fiber remains fixed
prevents manipulating the optical patterns to change the ~ercentage of surface-reflected
light from the collection fiber.
As previously described, light reaching the collection fiber is a function
of the distance from the flat reflective surface to the probe end face. This distance
15 dependence can be utilized for the purpose of displacement sensing. However, the lack
of ability to manipulate the optical axis of the illumination and collection cones limits the
controllability of the measurement dynamics. It further limits the overall ability to
achieve specific application goals, such as linearity, dynamic range, sensitivity and
related criteria. And as previously mentioned, the capability to manipulate the angle and
20 axis of ilhlmin~tion incidence facilitates the ability to maximize or ,ni"i..,;~., desirable or
undesirable surface reflections. This capability, which a flat face, parallel fiber probe
lacks, can be utilized to significant advantage.
In certain sensing applications, the parameter under investigation
responds inadequately to light of desirable wavelengths. For example, suppose an25 albill~y chemical has an infrared signature suitable for photonic sensor development,
but the a~plopliate infrared light does not readily transmit with conventional optical
fibers. In many situations such as these, visible light and standard optical fibers may be
succes~fully lltili7..ocl This may be accomplished by introducing an intlic~tor material that
undergoes a visible color change upon interaction with the chemical species of interest.
To successfully employ indicator-based fiber optic sensors, fiber must
ilhlmin~te the chemical indicator and collect light from it. Although this sensor
methodology encompasses many techniques, one method involves coating the fiber end
face with the indicator material. If a single fiber's end face is coated and the fiber is
utilized as a bi-directional light conduit, poor isolation between delivered and collected
photons can result.
Due to shortcomings associated with a flat face, parallel fiber probe and
its ability to control illumination and collection, complications arise in ill-lmin~ting and
collecting in~ tor light. A probe able to project illumination light onto a clearly defined
SI~STITUTE SHEET ffU~
'' CA 02248912 1998-09-11 "'~
i~dlc;ltor zone i~ hiohly pre~rred o~,-er a ~lat flce~ p~r~lJ~I fibcr probe. For m~ny
si~ ons, t~e i~eal prob~'s d~si~able fea~ures includ~ th~ capability to project
illurr.ln~lion lioht directly ~nto a coll~c~ion f:lber w'nose ~nd face coated ~ n the
,n~ or In t~.is su~erior conf~urduon, orl~ hL inter~cting with [he in~ic~tor re~.es
t~e d~tec~or--th~re~y ~ ting str~y light.
~ he prec~ding diàeuc~ion h3~- focu~ed on a pro~e -on~ ing of ~w~
p2rai~el-rnounted fi~ers (one source ~n~l o~ dctector ~ib~r) T~le pro~ression lr.
cGrrel~ion to bun~i~s of fibe~s in v~r ous configuration j is re~dily ar)preci~ed ~nd
~ollowed by thos~ âkilled in the ar~ ltholl~h blJndle3 pG~ntially overcorne ~om~ of the
~0 ~re-~escribe~ limit~tions, sioni~c~n~ ]i~itl~ions r~ in. An~, Lh' usag~ of b,lndles
in~roduces ad~linon~l prob]ems ~nd un~e~irable char~cte~istics.
~l~EMPTS TO l~ O-~E PROBE PERF~RMA~CE
~rom ~he precedirl~ discussion. it is app~rPnt th~t the ability ~o ~;ect ~r!d
I j ,nanipula~e illum~ ion and re~ep~ivlty zones of optical L-lbers is m,hly de~ired. SeYer~l
prior art techriques have b~en employed t~ manipula~e ~ probe's illuminatio;~ ~rd
recep~ivity char~cteris<tics :~nd to address the ir.pu~loutput con~traints ~ optical fibers.
For the reaaons se~ out below, .h~se methods ~re liJr~ited in terms of their e~fectiveneâs
for mar.y de~iled R~n~n inst~ nr~tion ~pplic~tions.
~0 One apprcach emplvys optic31 ~i~e~ with ~~ar~ng numerical apertur;s in
arder ~o gai.. b~rter con~ol o~er the entryJ~xit ch~racteris~ics. For ?xampl~ by~mplovin~ bers with higher n~me;,cal aper~ures, the lignt-gathering a~ility is inc e~sed.
This approach ii~clu~es severai dr~w~k~. First, ~he requir~d fibcr xaterials haYe
cha~cteristics not suitable f~. high-end instlumenla~ion appiication including
~5 ,~nvironmen~l sensitiYity, usage r~s~rictions, ~nd the ~,ener~tion of ex.rane~us
responses. S~cond, yhysic~l laws limi~ rhe ~tenl to which ~ fiSer's aceeptance
c~.aracteristics can be extend~d. Thir~, a fiber'~ deliYerJ p~tternffield-of-view can only
be bro~dened or Darrowed bu~ not sleered off axis or ~irectcd ~o ~/lew in ~ specific
reg;ion. Fourth, a wide 3cceptanc~ angle on the input end of ~he fiber translates to ~ wlde
0 àiv~rgence on the output end. While a high numerical ~perture fiber incre~ses lig.?lt
~a~hering Gn thc collec~ion end, it deliYers its ligh~ to a detector 3ystem in ~ ely
di~rel~,ing 3r;gle. rn many ca~cs, the delivery o~ ~idcly diverO~i~g ligh~ to the d~tector
syst. m is detri~ental to achieving accept~ble perf~ ncf~.
~ nother apDroach errploys ~panded b~am external elemen~s, such as
~5 lerlses ~nd milrors, to manipuJate the illumin~tion and rec~ptivlty characteristics. The~e
~l~rnents are bulky, e~pensivc, iometimes fr~ , of~en lossy, difficult to align, and
susceplible to ~nvironment~l Influences. .~ddition~lly, it i5 di~'fcult to engin~er a highly
CA 02248912 1998-09-11
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12
robust package. For example, larger, more robust components, have higher mass and
increase system susceptibility to mechanical shock.
If an individual lens is dedicated to each collection and illumination fiber,
then the ensuing device becomes large and bulky. Furthermore, the larger the assembly,
5 the further apart and less efficient the collection and illumination devices. If, on the other
hand, illumination and collection paths traverse the same optical element, a significant
portion of the source energy is inadvertently reflected into the collection fiber without
interacting with the sample. This stray light cont~rnin~tes the measurement and is
extremely detrimental. Additionally, the introduction of expanded-beam optical elements
10 complicates the assembly, causes manufacturing difficulties, yields additional variability,
and produces other undesirable results.
A special class of devices, termed confocal, involves the utilization of
multiple optical elements in conjunction with optical fibers. In these devices, focusing
optics create a converging illumination beam which is projected into the operating
15 medium under investigation. The focal point, or point of ray convergence, of the
illumination beam lies within the sample. The collection zone is also created with
focusing optics and is likewise formed, to the extent possible, concurrent with the
illumination zone.
The objective is to create matching focal points of illumination and
20 receptivity projected into the sample medium. The underlying theory is that stimul~ted
light originating at the focal point within the sample is collected from a large solid angle
defined by the angle of beam convergence. The intent is to optically re-image the source
fiber end face within the sample thereby creating a virtual fiber end face. In theory and
assuming 100 percent optical efficiency and no optical distortion, re-im~ging the end face
25 as described re-creates the illumination intensity of the actual fiber end face. Although
achieving the theory is physically impossible, in a perfectly transparent medium under
laboratory conditions, acceptable results is a reasonable goal.
Unfortunately, in the majority of applications in which fiber optic's
remote capabilities are highly sought, the materials under investigation are complex,
30 dark, and scattering. The situation is similar to that of the flat face, parallel fiber probe
pre~ ously analyzed; the converging illumination bearn is drastically attenuated and
distorted before it effectively reaches the optimum point of receptivity. And for similar
reasons, feeble stimul~tPd emissions from this point cannot return to the collector optics.
As a separate disadvantage to this technique, response is collected from a
35 potentially undesirably large sample area since measurement contributions areaccum~ te~l, to a certain degree, as the beam converges to its focal point. These devices
suffer additional drawbacks including complexity, environmental sensitivity, large size,
high expense, and failure in hostile environments. For example, unlike even the flat
_
CA 02248912 1998-09-11
face, parallel fiber probe previously analyzed, this type of device cannot be inserted into
a biomedical catheter.
As a separate consideration, suppose the focal point projecting apparatus
is utilized to investigate an undulating mass, such as a heart muscle. As the muscle
5 beats, the tissue moves in relation to the analytical zone. Thus, the measurement is
difficult or unsuccessful.
In another approach, which is described in U.S. Patent No. 4,573,761 to
McLachlan et al., a fiber can be bent at its tip in order to point in a direction of interest.
For example, one or more flat-faced optical fibers may be directed to view a common or
10 overlapping zone of receptivity. A second group of one or more flat-faced optical fibers
may be directed to illuminate the zone. By this method, receptivity and illumination
overlap is achieved. Unfortunately, this method suffers from several serious
~- drawbacks. The assembly is expensive and difficult to construct. The ensuing device
also lacks repeatability due to manufacturing constraints. If the fibers are gradually bent
15 from their converging orientations to parallel, then the assembly is very large. Even if
the fibers are rapidly bent near the assembly's distal end, the assembly is bulky and
relatively large in diameter. Such an assembly is much too large to be utilized in an
application such as in vivo medical. As the fibers are bent, they become inefficient and
lossy at the sharp bend resulting in light escaping from the fiber at this point. In
0 addition, bending the fibers to create a probe results in exceeding the minimum bend
limitations for most optical fibers. The fiber is subsequently prone to failure and suffers
increased sensitivity to environmental influences.
In another approach, the illumination and collection zones may be
manipulated by shaping the fibers' end faces to create a refractive surface. For example,
~5 a center fiber may be encircled by a ring of fibers with tapered end faces. This tapering
creates a refractive surface on the ring fibers to manipulate their field-of-view inward and
toward the center fiber's axis. A key aspect of this refractive-end-face approach is that
light manipulation occurs at the fiber end face boundary, and rays are bent as they enter
or exit the fiber and cross the boundary of the fiber core end face. Several problems are
30 associated with this approach and limit its effectiveness.
In manipulation of accepted and/or emitted light by the method of forming
shaped end faces into optical fibers, the refraction is due to the refractive index
differential between the fiber core and the medium surrounding the fiber end face. The
extent of refraction is a function of the difference between the two refractive indices and
35 the angular orientation of the light relative to the surface of the interface. Optical fiber
cores are typical!y glass or similar materials with relatively hi ,h refractive indices. In
order to achieve significant refraction at the fiber end face, it is usually desired to have a
c,aseous medium, such as air surrounding the fiber end face. This type of medium has a
~MEN~ED SHEE~
CA 02248912 1998-09-11
14
low refractive index thereby facilitating sufficient ray refraction. Most fluids have
relatively high refractive indices with values approaching those of common optical fiber
core materials. Therefore, media such as fluids, fluid-filled matrix, biological tissue,
and melts typically provide insufficient characteristics to achieve the requisite refractive
S index differential. In addition, shaped end faces typically protrude beyond the protective
housing in which the fiber is mounted. This delicate protrusion is susceptible to physical
or mechanical damage.
In order to address the dependence on refractive index, the fiber end face
must be surrounded by a medium with a known refractive index. The medium is
10 preferably air or a similar gaseous material. This may be accomplished by situating the
probe tip behind a window in a sealed chamber. However, use of a window causes
numerous problems.
- The assembly encompassing fiber, fiber mount, window, window
housing, and sealing mechanism is expensive and difficult to construct. The necessity of
15 the sealed chamber also forces substantial increases in the size of the assembly. Thermal
expansion and sealing problems also plague the windowed mechanism. The window's
optical material possesses low thermal expansion characteristics while the housing to
which the window is bound is typically of metal or other high thermal expansion
material. Bonding and sealing the window to the metal housing presents numerous
20 difficult engineering challenges.
As light enters or exits the sealed chamber it traverses the window. In
many instances, the window material has undesirable spectral characteristics. For
example, diamond windows produce strong spectral peaks of Raman scattered light as
laser light is transmitted. As a second example, sapphire windows often contain
25 impurities that fluoresce.
The window forces the fiber end face to be removed from the application
environment by, at least, the thickness of the window. Although the window may be
only a few millimeters thick, it remains large relative to the size of the fibers. On the
optical fiber scale, positioning the fiber tip even this distance from the physical target
30 often correlates to excessive light intensity losses.
For the proper optical performance, the fiber end face shouJd be
positioned as close as possible to, and preferably touching, the window. Accomplishing
this feat requires a complex means to adjust the distance and lock the assembly in place.
As previously described, the shaped end face is mechanically feeble and prone to35 physical damage. Thus, the assembly is prone to damage not only during positioning
but also as a result of thermal expansion, vibration, and general operations.
As the light is incident upon the inner and outer boundaries of the
window, it is refracted and reflected. The refracted aspect is either a boon or a hindrance
~EN~Er) SHEE~
- CA 02248912 1998-09-11
depending on application specifics. The reflection aspects are often highly
disadvantageous. For source fibers, window reflections not only weaken the emitted
light but also are directed back within the sealed chamber. Depending on configuration
and application specifics, these reflections are captured by the source fiber and thereby
5 redirected towards the source. For many applications, this back propagated light is
significantly detrimental. The window reflections also tend to interfere with elements
adjacent to the optical fiber. For example, a detector fiber positioned in proximity to the
source fiber captures a portion of the source light that is back reflected by the window
Light captured in this manner potentially mixes with and contaminates the desirable light.
10 Similar circumstances surround applications in which the shaped end face fiber's
principal role is to capture source light generated outside the confines of the fiber. The
housing to which the window is fixed, together with the window, forms a sealed
, chamber. Undesirable light tends to bounce around in this chamber amplifying and
exasperating the described stray light problem.
15A standard optical fiber, properly mounted in a typical fiber optic
connector, withstands tremendous hydrostatic pressure prior to failure. The fiber's
small surface area translates even high pressures into very small forces. Thus, extreme
pressures are required to generate sufficient force to cause the fiber to piston back into its
connector and fail. Conversely, a window is typically much larger in diameter than the
20 fiber positioned behind it. Hence, for a given environmental pressure, the window is
subjected to much higher forces than would be an exposed fiber. Additionally, the
window is thin and only supported around its outer rim. Therefore, it is susceptible to
breakage. Strong, thin windows can be produced from materials such as diamond,
sapphire, and similar materials. Unfortunately, these materials not only suffer from the
25 pre-described drawbacks but also have high refractive indices. A high-refractive-index
: ~window intensifies the pre-described reflection/refraction problem.
Another drawback to relying on refractive end faces results from the
nature of the refractive effect, which limits the extent to which light can be manipulated.
This is readily investigated and studied by applying Snell's Law through ray tracing.
30 Due to the nature of refraction, light cannot be aggressively steered off axis to achieve
optimal response.
Based on the foregoing discussion, it is highly desirable to redirect light
by means other than refraction at the fiber's end face. Specifically, it is desirable to
manipulate light within the confines of the fiber assembly's light path. Light
35 manipulation can be accomplished by creating light-shaping structures within the
confines of the optical fiber assembly. Thus, light that enters the fiber and would
normally be rejected can be redirected for propagation via total internal reflection
Similarly, light propagating via total internal reflection can be directed to otherwise
AM~ ET
CA 02248912 1998-09-11
- 16
unfeasible paths. By creating the light-shaping artifices within the confines of the fiber
assembly's internal structure, effects similar to those found in fibers with shaped end
faces are produced without the disadvantages or constraints associated with shaped end
faces.
One method of achieving light bending within the confines of the optical
fiber is to include a light-manipulating surface between two adjoining waveguidesections. This can be accomplished by inserting a light-altering component between two
sections of fiber. A highly advantageous method is to construct the light-shaping artifice
into or onto the fiber end face that adjoins another fiber segment or section. For
example, light-shaping contours are readily constructed into a fiber end face that is butted
to a second fiber. The second, adjoining fiber end face can be flat faced or also
encompass light-altering surfaces or characteristics. As an alternative to light-shaping by
~ ~ refraction, the light shaping can occur via diffraction, reflection, scattering, interference,
or other methodology. If light-shaping refractive surfaces are employed which are not
symmetrical about the fiber's central axis, the light tends to be steered or bent off axis.
Thereby, illumination and/or collection zones are directed off axis.
... .
A second method of achieving light manipulation and bending within the
confines of the optical fiber is based upon reflection. In this method, the fiber core's
exterior surface is modified to create a reflective surface other than the standard core-
cladding interface. For example, an optical fiber whose end face is formed into a sharply
inclined planar surface will exhibit these characteristics. Suppose the angular inclination
of the end face is sufficiently inclined to generate a totally internally reflective internal
surface. As light propagates within the fiber core towards this distal tip, it encounters
the special surface. The propagating light is re-directed by total internal reflection to exit
the fiber through its side or outer cylindrical surface. European patent document EPO-A-
~ 0210869 illustrates this approach in the context of a probe for measuring the absorption
of light through a fluid. Variations on this theme include creation of surface contours
which do not typically yield total internal reflection but to which internally reflective
coatings are applied. Additionally, various complex contours can be generated which
mix various optical effects.
Another method for fiber optic light manipulation entails forming a group
of flat-faced optical fibers consisting of illuminating source and collection fibers. A
typical orientation is a single source fiber surrounded by a ring of collection fibers. This
grouping of fibers is butted up to a single, large-core optical fiber. The single fiber's
large core has a diameter equal to or greater than the collective grouping of smaller
fibers. The large-core fiber is utilized in a bi-directional capacity. Its distal end both
delivers illuminating energy and captures target light. This method suffers from several
drawbacks. A certain degree of source light is reflected from the fiber's end face before
r' ~ ? ~ T
CA 02248912 1998-09-11
16A
the light exits the fiber. This light is prone to back propagation within the large core
fiber and returns to the collection fibers as stray light. Secondly, as the source li;,ht
traverses the large-core fiber segment, detrimental signals are often generated. For
AMENDED SHEET
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WO 97/3417~ PCT/US97/04365
example, Raman-scattered light is produced and radiates in all directions. Unfortunately,
the large-core fiber accllm~ tPs the Raman-scattered light and efficiently waveguides it
to the collection fibers where it is mixed with the desired target light. Fluorescence
generated within the large-core fiber is likewise delivered to the collection fibers and
5 corrupts the measurement process.
Therefore there is a need in the art for an improved fiber optic probe
assembly that allows effective and efficient manipulation of the light delivery and
reception regions. The light manipulation should take place within the fiber assembly's
light path and should allow significant off-axis steering of the fibers' viewing areas. The
10 probe assembly should be compact and easy to manufacture, and should not rely on
expanded optics and other complicated features found in the prior art.
SUMMARY OF THE INVENTION
The present invention satisfies the above-described need by providing an
15 improved method and apparatus for fiber optic light management . The invention
provides a number of novel fiber optic light manipulation and management techniques,
which are individually important for diverse fiber optic applications. For example, the
present invention provides an improved fiber optic probe assembly for low light
spectrographic analysis. The invention improves response to subtle light-matter
20 interactions of high analytical importance and reduces sensitivity to otherwise dominant
effects, thereby overcoming the technical difficulties associated with light-based
characterization in complex media. This is accomplished by adjusting the illumination
and collection fields of view in order to optimize the probe's sensitivity. Light
manipulation is applied internal to the fiber so that the probe's delivery pattern and field
25 of view do not require external manipulation and are not adversely affected by
investigated media. This allows the light delivery pattern or field of view or both to be
aggressively and reliably steered off-axis to achieve significant increased performance
levels. Aggressive beam steering is accomplished by employing internally reflective
surfaces in the fiber. A reflective metal coating or a low refractive index coating or
30 encapsulant can be used to ensure total internal reflection. The fibers also incorporate
filters, cross-talk inhibitors and other features that provide a high performance probe in a
robust package. Design variations provide side viewing, viewing through a common~el lure, viewing along a common axis, and other features.
Generally described, the present invention provides a probe having
35 selective sensitivity to specific photon-matter interactions. This selective sensitivity is
achieved by delivering light at one angle and collecting light at the appluyliate angle to
maximize the response. The delivery and collection paths are re-directed off-axis to
SUBSTml~E S~1ET p~ULE 26)
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18
intersect with one another at specific angles while the delivery and collection fibers
remain in close proximity to one another.
In another aspect, the present invention provides a means for segregating
inelastic and elastic photon-matter responses of a material by angularly manipulating the
delivery and collection patterns in relation to one another. The elastic response is
directionally biased such that its collection, in relation to the inelastic response, is
minimi7,ecl
In another aspect, the present invention provides a fiber with a tip having
a portion that is internally reflective and a portion that is internally non-reflective. The
reflective portion of the tip delivers light at angular orientations beyond the fiber's
norrnal propagation limits. Incoming light, incident on the reflective surface, is
angularly steered, so that light is received at angular orientations beyond the fiber's
normal propagation limits.
More particularly described, the internally reflective portion may be the
result of an internally reflective coating, or may be essentially total internal reflection.
Total internal reflection may be induced by placing a low refractive index material into
contact with the fiber. The low refractive index material may include a low index coat or
encapsulant, or the ambient medium. The internally reflective surface may include a
variety of shapes, which can be used to control the field of view with great precision.
In another aspect, the present invention provides a probe that incorporates
a reflective surface for steering the light path. The probe includes at least one delivery
fiber and at least one collection fiber. The delivery fiber or collection fiber include an
internally reflective surface for causing the light delivery path and light collection path to
converge.
In another aspect, the present invention provides a probe assembly that
includes filters applied directly to the interior end faces of distal fiber segments.
In another aspect, the present invention provides a method for mass
producing fibers with high performance filters.
In another aspect, the present invention provides a fiber optic probe that
collects light directly in front of, or at, the delivery apc.lule. The viewing angle is
directed in response to the extent of the elastic response, the strength of the inelastic
response, the desired depth of investigation, and the absorption of the medium.
In another aspect, the present invention provides a probe comprising a
plurality of fibers essentially parallel to each other and in close proximity to one another.
The coupling efficiency between the probe and the investigative medium is enh~nced by
fusing the bundle of collection and delivery fibers together. The fusing process entails
heating the fibers and compressing the fibers so that no gap exists.
CA 02248912 1998-09-11
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19
In another aspect, the present invention provides a means of optically
isolating two or more fibers that are in close proximity to each other, such that the
signals from each fiber do not mix. A light impenetrable barrier is stationed between the
fibers in the area prone to cross talk.
In another aspect, the present invention provides an optical fiber
enhanced at its tip to collect or deliver light beyond the fiber's normal limits of
propagation. The fiber adjoins another short fiber segment. The short fiber shuttles
light between the beam-steered fiber end face and the distal end face of the assembly.
The short fiber segment has the ability to carry angularly oriented light beyond that of the
unmodified, primary fiber.
In another aspect, the present invention provides a fiber optic probe
assembly that includes a central fiber and a plurality of fibers surrounding the central
fiber. The central fiber has a flat end face at its distal end. The plurality of fibers
surrounding the central fiber and have a shaped end face at their distal ends. The
plurality of fibers are parallel to the central fiber at their distal ends. The shaped end
faces provide an internally reflective surface for steering the fields of view associated
with the plurality of fibers toward the central fiber.
The present invention also provides a fiber optic probe assembly that
includes a first fiber and a second fiber. The first fiber includes an end face having a
first shape. The second fiber includes an end face having a second shape. The first and
second fibers are parallel to each other at their end faces. The second shape provides an
internally reflective surface for directing the field of view associated with the second
fiber toward the first fiber.
In another aspect, the present invention provides a fiber optic assembly
having a common axis for delivering and collecting light. The assembly includes a light
delivering fiber and a light collecting fiber. The light delivering fiber has a filter at its
end face. The light collecting fiber has a reflector at its end face and is mounted parallel
to the light delivery fiber. The first filter is operative to reflect delivered light through its
side wall and to allow collected light to pass through to the light collecting fiber. The
collecting fiber reflector directs light along the axis of the light collecting filter.
Alternatively, the present invention provides a fiber optic assembly
having a cornmon axis for delivering and collecting light. The assembly includes a light
delivery fiber and a light collecting fiber. The light delivering fiber has a filter at its end
- face. The light collecting fiber has a reflector at its end face and is mounted parallel to
35 the light delivery fiber. The filter is operative to pass delivered light and to reflect
collected light to the reflector on the collection filter. The reflector directs collected light
along the axis of the light collecting filter.
CA 02248912 1998-09-11
WO 97/34175 PCT/U~97/0436
In another aspect, the present invention provides a fiber optic probe
assembly using a common ape,tule for delivering and collecting light. This is achieved
by transmitting desirable light through a fiber's sidewalls. The assembly includes a
central fiber having a flat end face at its distal end and a plurality of fibers surrounding
5 the central fiber. The plurality of fibers have a shaped end face at their distal ends. The
plurality of fibers are parallel to the central fiber at their distal ends. The shaped end
faces provide an internally reflective surface for steering the fields of view associated
with the plurality of fibers through the side wall of the plurality of fibers and through the
end face of the central fiber.
In another aspectt the present invention provides a fiber optic probe
assembly for side delivery and collection of light. The assembly includes a first fiber
and a second fiber. The first fiber has a shaped first end face. The second fiber has a
shaped second end face and is parallel to the first fiber. The shaped first end face and the
shaped second end face direct light toward a common region.
In yet another aspect, the present invention provides a method for
fabricating a fiber optic probe assembly. The method includes forming a bundle of
fibers including a center fiber surrounded by a ring of fibers. The bundle of fibers is
bound together. A cross-talk inhibitor mech~nism is incorporated into the probe. The
bundle of fibers is shaped to form a pencil tip or cone. The cone is then flattened so that
20 the center fiber has a flat end face.
In another aspect, the present invention provides a fiber optic probe that
incorporates an integral reference material. The probe includes fibers for delivering light
to an investigative sight and fibers for collecting light from an investigative sight. In
addition to exciting a response from the medium under investigation, the delivered light
25 excites a response from the reference material. The light from the reference material may
be collected and used to calibrate the system, compensate for drift, establish accuracy,
and verify functionality.
In another aspect, the present invention provides a means for
manufacturing low cost, high performance probes for inclusion in a comprehensive30 analytical system. The probes are disposable following their utilization.
The various aspects of the present invention may be more clearly
understood and appreciated from a review of the following detailed description of the
disclosed embodiments and by reference to the appended drawings and claims.
35 BRIEF DESCRIPTION OF THE DRAWINGS
Fig. I is an isometric view of a flat face optical fiber.
Fig. 2 is an isometric view of a fiber having a planar, angled end face.
SlJ~lllUlt S11E~(RUI~ 2~
CA 02248912 1998-09-11
WO 97/34175 PCTIUS97/04365
Fig. 3, consisting of Figs. 3a and 3b, illustrates a fiber having a cone-
shaped end face, with the axis of the cone displaced from the fiber's center and the cone
point outside the fiber.
Fig. 4 is an isometric view of a fiber having a cone-shaped end face, with
S the axis of the cone displaced from the fiber's center and the cone point within the fiber.
- Fig. S is a cross sectional view of a fiber having a partial bevel applied to
its flat end face.
Fig. 6 is a isometric view of the partially beveled flat face fiber of Fig. 5.
Fig. 7, consisting of Pigs. 7a-c, illustrates a fiber having a complex
sectional contour.
Fig. 8 is an isometric view of a fiber having a complex sectional contour.
Fig. 9 is a cross-sectional view illustrating the illumination zone of a fiber
having a partially beveled flat end face.
Fig. 10 is a cross-sectional view illustrating the illllmination zone of a
fiber having a partially contoured flat end face.
Fig. 11 is a cross-sectional view of a fiber configured to direct light
through the fiber's side.
Fig. 12 is a cross-sectional view of a fiber configured to refract some
light at the end face and direct other light through the fiber's side .
Fig. 13 is a cross-sectional view of a fiber with a curved end face.
Fig. 14 is a cross-sectional view of a fiber assembly including two fiber
segments with adjoining end faces.
Fig. 15 is a cross-sectional view of a fiber assembly including two fiber
segments with shaped adjoining end faces.
Fig. 16 is a cross-sectional view of a fiber assembly in which an optical
element is inserted between two fiber segments.
Fig. 17 is a cross-sectional view of a fiber assembly in which a large
~i~rn~tf r segmlo.nt is coupled to a sma~ ter fiber segment.
Fig. 18 is a cross-sectional view of a fiber assembly in which a large
diameter segment is coupled to a small-tli~meter fiber segment.
Fig. 19 illustrates light manipulation via traditional mirrors.
Fig. 20 illustrates light manipulation through a short segment of a large-
core fiber.
- Fig. 21 illustrates light manipulation through an unconventional fiber.
Fig. 22 illustrates light manipulation through a hollow fiber.
Fig. 23 is a cross-sectional view of a fiber assembly that employs a
small-core primary delivery fiber and a large-core distal segment.
SUBSTITUIE SHEET (RULE 26)
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WO 97/34175 PCTAU~97/04365
Fig. 24 illustrates an index optical fiber positioned behind a short
segment of a gradient index fiber.
Fig. 25 illustrates the fabrication of a non-axially symmetric, gradient
index fiber segment by means of core drilling a common gradient index lens.
S Fig. 26 illustrates a off-axis beam steering gradient index fiber segment
joined to a primary optical fiber.
Fig. 27, consisting of Figs. 27a-c, illustrates a light-scattering probe
constructed in accordance with the present invention.
Fig. 28 is a graph illustrating the spectra acquired by three probes in a
blood sample containing a fluorescent aspect.
Fig. 29 is an enlarged portion of the graph of Fig. 28.
Fig. 30 is a graph illustrating the results of probe tests conducted on an
aqueous-based, red solution with minimllm particulate scattering characteristics.
Fig. 31 is a graph of the spectra collected in a red light scattering
lS medium.
Fig. 32 is a graph of the spectra collected in a red light scattering medium
with trace yellow-green fluorescent additive.
Fig. 33 is a top view of a probe assembly that employs a tightly packed,
fused bundle of fibers.
Fig. 34 illustrates bundle being fed into a single, large-core fiber.
Fig. 35 depicts a probe assembly that is adapted for filter application.
Fig. 36 illustrates a fiber assembly in which the center fiber extends
further into the test medium.
Fig. 37, consisting of Figs. 37a and 37b, illustrates a probe assembly in
which the collection fiber is larger then the delivery fiber.
Fig. 38, consisting of Figs. 38a-h, illustrate various arrangement for
projecting light in front of and parallel to collection fibers.
Fig. 39, con~isting of Figs. 39a and 39b, illustrates attachments that may
be attached to the end of a fiber bundle.
Fig. 40, consisting of Figs. 40a and 40b, illustrates a fiber assembly in
which the ring fibers are diametlically separated from the center fiber.
Fig. 41 illustrates a fiber assembly in which the ring fibers are shaped to
be fully internally reflective.
Fig. 42 is an isometric view of the fiber assembly of Fig. 41.
Fig. 43 illustrates a fiber assembly where the ring fibers have a longer
bevel.
Fig. 44 depicts a fiber configuration in which thick-wall capillary tubing
iS ~ltili7.ed
SWSmU~ RULE 26)
CA 02248912 1998-09-ll
W O97/3417~ PCTAUS97/04365
Fig. 45 depicts a configuration in which the center fiber is formed to
create an internally reflective surface.
Fig. 46 depicts a configuration Ihat is adapted for high sensitivity directly
at the probe tip.
S Fig. 47 depicts a convex element adjoining delivery and collection fibers
in which reflections from the outer surface of the element are directed back into the
source fiber.
Fig. 48 illustrates the light interaction associated with a scattering
powder.
Fig. 49~ consisting of Figs. 49a-d, depicts two configurations in which a
source fiber's illumination is directed over a shlgle collection fiber.
Fig. 50, consisting of Figs. SOa-(l. depicts various cut-away and
perspective views of an assemblv which incl~lde~ all optical end piece producing th~
requisite internal reflection.
Fig. 51, consisting of Figs. Sla and Slb, depicts a configuration in
which source light is projected through a gap between adjacent fibers.
Fig. 52, consisting of Figs. 52a and 52b, depicts a fiber optic probe
utilizing gradient index optics to bend illumination from source fibers to coincide with
the field of receptivity of collection fibers.
Fig. 53, consisting of Figs. 53a and 53b, depicts elements in which a
hole is drilled and the center fiber is inserted.
Fig. 54 depicts a cross section of a probe with a center fiber surrounded
by a ring of fibers.
Fig. 55, consisting of Figs. 55a-f, depicts various aspects of probes
employing light-manipulating artifices between adjoining fiber segments.
Fig. 56, consisting of Figs. 56a and 56b, illustrates cross sectional and
perspective views of a similar assembly which utilizes an end piece to create similar
performance results.
Fig. 57, consisting of Figs. 57a-f, depicts a probe configuration that
employs a variety of light manipulation techni~ues.
Fig. 58 depicts a method of enhancing overlap between the source fiber's
delivery beam and the collection fiber's zone of receptivity.
Fig. 59 illustrates a single fiber with improved performance
characteristics.
Fig. 60 provides a perspective view of a complete termination assembly.
Fig. 61 illustrates an exemplary probe for delivering and collecting light
along a common axis.
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WO 97/34175 PCT/US97/04365
24
Fig. 6~ illustrates a probe in which light enters and leaves the probe along
a common axis and essentially parallel with the fibers' axes.
Fig. 63 is a cross-sectional view of a probe assembly having a common
delivery and collection aperture.
Fig. 64 illustrates a two-fiber configuration in which tne delivery fibe
has a curved internally reflective surface.
Fig. 65 details a distal tip incorporating a large number of optical fibers
and in which the collection fibers are responsive to light at diverse angular orientations.
Fig. 66 presents an embodiment in which the outer end faces are
contouled to form refractive end faces and light purposely travels throu~h the fiber
sidewalls .
Fig. 67 depicts an alternate embodiment whose fibers have individual!y
concurrent optical and mechanic;ll aYes these axe~ collectively intersecting one another
and light purposely travels through the fiber sidewalls.
Fig. 68 depicts an embodiment which incorporates an internally reflective
optical element in conjunction with the fiber bundle resulting in a common delivery and
collection aperture.
Fig. 69 depicts an embodiment which utilizes gradient index optics for
beam steering and light purposely travels through the fiber sidewalls.
Fig. 70 depicts an embodiment in which the center fiber end face is
shaped for light manipulation and light purposely travels through the fiber sidewalls.
Fig. 71, consisting of Figs. 71 a and 71 b, depicts a side delivery/viewing
two-fiber probe that utilizes an off-axis, parabolic surface contour for internal reflection.
Fig. 72, consisting of Figs. 72a and 72b, depicts a side delivery/viewing
probe that projects the collection pattern through thc delivery fiber.
Fig. 73 depicts a side delivery/viewing probe that utilizes a bundle of
fused fibers for collection and a single, small fiber for light delivery.
Fig. 74 provides an expanded view of the internally reflective shaped
surface and light pattern as it relates to fabrication tooling and parameters.
Fig. 75 illustrates a probe manufacturing fixture in which fibers are fixed
to a mandrel.
Fig. 76 illustrates the relationship between tooling fabrication geometry
and resulting optical parameters.
Fig. 77 depicts a side delivery/vie~ ~ing embodiment that utilizes gradient
index optics for beam steering.
Fig. 78 depicts a side delivery/viewing embodiment that utilizes an
internally reflective end piece.
CA 022489i2 1998-09-11 " . ) ~ "
Fig. 79 illustra~es ;~ too]in~J ~ppa~ us suitable for ~pplyin,~ ~I]tcrs ~o fiber
~nd ~ces.
Fig. ~0 i]lustrates filters apDliea to lib~r~ havin8 cone-sh~ped end ~;~ces.
F~g. ~ illustraI~s a ~er ~e~ice that 5~p31~te~s light ~ccording 20
w~vel~r~
Fie. S~ G~piCt'; ~ .rl~d probe wi~h fibers ~h~se mech~ni~ai 3nd optic~
~xe~ intersect lt~ ~ist~nc~ bevon1 the dist31 tip ~r ~he probe.
F~ 3 depirts a filtered pro~ ~~ ith fi~rs whosc mech~nlc~l a~d ~ptic~l
axes inte~sect at a disunce ~eyond the distal tip of the probe.
Fig. S~ strates ~ w~ ~ec,~.lided cell f~r ~aIysis of ~-f~lid.
Fig. 85 illu~lr~es a cell ~or ~nalysis that does not rely on optic~ ers.
Fig. ~6 illus~rates a non-fiber-coupl~d wa~eguid~d c~ll for analysis of
inelasllc li~ht-m~tter inter~ctio~s.
Fig. S~ uitr~tes a w~vegui.~ed cell for low-concentr~tiGn ana!ysis of
15 chem~c~ls.
Fi~. ~8 depic~s ~ proce in ~hich only ~he in~lastic light resona~s wi~hin
ca~lty.
DET.~L~) D~SC~IGN
~O The prese~t invention i~ ~irected to an imFIolred fiber optic pr~b~
as~embly with manipulated deli~erv and reception ~en~it~ ies. In an exem?lary
embodimen~7 the invenLion is inc3rpor~ted into fiber optic p~obes that employ the
~GASER~ ht ma.~g~rr.ent ~ystem. 'rhis sys~em incorpo~es a number of no~el fiber
aptic li~ht rnaniFuiation and m~nagem,ent methods ~ ich are described herein. Each o~
'~ the~e methods is indiYidu211y imForta.nl ~0~ di~re~e rlber CptiC applica~ions spanning
from teleco~-~nic;lcions ~o high Fo~er laser de~ivery. N~ver~heless, they ~re described
in terms ~rom this per~pectt~,e, [he selcc.ion and usility of each method for ~his
applic~tion is taught. And, the s~rategies of combining methods for an in~gratedsolution is deYeloped. Such probes ~re m~nuf~ctured and sold by ~isionex, ~nc., o~
~0 W~rner ~obms, GeorEia. Br~etly lescribed, a ~iber-aptic probc in acco~d~nce with the
present invention provides selec~ive sensitivity for c~ptL~r~ng dis~roportionate responses
associa~ed with specific ~ight matter ir.teractions Light rn~nipu~a~on tecbniques ~e
applied internal to the fi~er in order to allow the illumination and colleotion zones to be
~ltered for specific li~ht m~u~r interactlons. Probe per~o~mance is enhanced by applyin~
~5 filten ~ fib~r se~ner.ts, isol~ting the fibers ~hat forrn rhe pr~be tip, and fu~ing ~he tlbers
toget~ler to make thcm ~s close as possible
Referrin~ ncw to the dr~win~s, in which like numeraLs represent li~ce
e!ements throughout the ~ever~ gures, zspects of the ~reseDt invention will be
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26
described. The relative sizes of some components, such as filters, cladding, coatings,
and the like, are exaggerated for illustrative purposes.
OPTICAL FIBERS IN GENERAL
The terrn "optical fiber" is used herein to refer generally to any optical
waveguide or structure having the ability to transmit the flow of radiant energy along a
path parallel to its axis and to contain the energy within or adjacent to its surface. "Step
index," "gradient index," and "single mode" fibers are subcategories within the optical
fiber designation. The term "multimode" optical fiber refers to an optical waveguide that
will allow more than one bound mode to propagate.
Step index fibers include a transparent cylindrical core of relatively high
refractive index light-conducting material. Typical core materials include silica, plastic,
and glass. The core is cylindrically surrounded by a medium having a lower refractive
index. Typically, this medium is a relatively thin cladding, which is an intim~t~ly bound
layer surrounding the core. The cladding may be a ~lirreient material than the core, or it
may be a similar material that has been doped in order to reduce its refractive index. The
core may also be unclad whereby the ambient medium, often air, is of lower refractive
index and acts in the capacity of the cladding. The cladding is usually surrounded by
one or more coatings, buffers, and/or jackets that primarily serve protective roles.
An a~ ldlily oriented ray within the core of a step index fiber travels
until it intersects the core boundary at the cladding and interacts in accordance with its
angle of incidence. Generally, rays angularly oriented close to parallel with the fibers
axis are efficiently reflected at the core boundary. Within certain angular limitations, the
ray is oriented to undergo total internal reflection at the core interface. These angular
lirnitations are a function of the refractive indices of the core and the c~ ing. The limits
determine the angular bounds within which the fiber can propagate light. Thus,
sustained propagation occurs via repeated total internal reflection within the fiber core. If
the arbitrary ray is oriented beyond the fiber's limits for total internal reflection, then
only a fraction of its intensity is internally reflected. The reduced intensity ray is further
attenuated as it undergoes subsequent core boundary interactions. The ratio of light
energy that is internally reflected to the energy that escapes varies according to the angle.
If the ray is oriented normal to the core boundary, then all of its intensity is lost. As the
angle of an hn~upelly oriented ray approaches the acceptance limits for total internal
reflection, the relative intensity of the reflected ray increases. Thus, for rays with angle
orientation close to, but outside of, the limits for total internal reflection, multiple
refJections can occur prior to significant power loss.
If the arbitrarily oriented ray within the fiber core has sufficient power
and orientation, then it sustains power and eventually reaches the fiber end face. It
CA 02248912 1998-09-11
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interacts with the end face boundary in accordance with the laws of reflection and
refraction. As the ray crosses the end face boundary between the fiber's core and the
- surrounding medium, it is refracted. The refractive effect is a f~nction of the refracti~e
index of the core, the refractive index of the surroundin;, medium, and the orientation of
5 the ray relative to the fiber end face surface. The factor of ray orientation is based upon
its angle relative to a surface normal taken at the point where the ray intersects with the
end face surface boundary. Angular orientation of rays outside the fiber end face and
propagating rays within the fiber core are distinctly correlated. Thereby, a correlation
exists between indi~idual and collectiYe external and internal rays.
10The previous discussion centered on rays internally propagating and
exiting the fiber. An analogo~ls situation e~ists for rays outside the optical fib~r entering
into the fiber core. l~he correlating development is readily drawn by those skilJed in the
ar[. For a fib~r utilized for single-dil-ection flow of li~ht~ light is typicall~ injected into
the fiber at one end and exits the fiber at the opposite end. However, fibers can also be
l 5 utilized in a bi-directional configuration. In this configuration, light purposely enters and
exits from a single end of the fiber.
As light propagates within the fiber core, it tends to become mixed or
randomly oriented over distance. Even highly directional sources, such as lasers,
become mixed or scrambled over distance following input into a long optical fiber. In
20 this mixing process, the fiber's modes are filled and all source characteristics, or so-
called launch conditions, are lost. The mixing process can be accomplished in shorter
fibers by tightly coiling the fiber, inducing micro-bends, or otherwise stressing the fiber.
Similarly, for very short fiber lengths, launch characteristics are retained. Also, for very
short lengths of fiber, light can be tran~mi~ted beyond the norrnal limits for propagation
25 dictated by the angular limits for total internal reflcction. This property is due to the
reduced number of reflections, which accumulate minim:ll attenuation. A fiber's ability
to sustain transmission beyond the normal limits for total internal reflection can be
enhanced by the application of internally reflective coatings applied to the fiber's outer
cylindrical surface. This coating can be applied to either the fiber's core or the cladding.
30 It should be noted that for long fibers, propagation cannot be totally reliant on reflective
coatings. In contrast to total internal reflection, even the best reflective coatings offer
less than l00 percent reflectivity. Losses associated with repeated reflections at less than
l00 percent efficiency quickly accumulate resulting in severe attenuation. Vast numbers
of reflections occur during propagation in even moderate fiber lengths.
AN IMPROVED PROBE ASSEMBLY
A probe constructed in accordance with the present invention allows light
emergence and collection patterns to be manipulated and controlled in highly
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~8
advantageous manners. The light manipulation occurs inlernal to the assembl~ andwithin the normal path of light propagation in the assembly. Thus. hlumination and
collection zones are manipulated and directed without utilizing refractive influences at the
point of light entering or exiting the assembly as the primary control mechanism.
S However, refractive influences typically remain as light enters or exits the assembly and
may he used as a supplemental means of light manipulation. Incoming light undergoes
the desirable manipulation following entering the fiber assembly and passing the point of
fiber boundary refraction. Likewise, light leaving the fiber assembly is manipulated
prior to passing the confines of the assembly and the final fiber interface, where it is
potentially refracted.
MET~ODS OF MANIPULATING LIGHT ~ITHIN A FIBER
Fig. I provides a view of a genelal flat faced optical fiber 100. The end
face 105. core 110, claddino 115, and coatin<J 120 are shown. Figs. ~ 3. and ~
l S depict optical fibers with various contoured end faces. The fiber 200 of Fig. 2 has a
planar~ angled end face 20~. The fiber 300 of Figs. 3a and 3b has an end face 305 that
is shaped like a cone with the axis displaced from the center of the fiber and with an
imaginary cone point outside the fiber. The fiber 400 of Fig. 4 has an end face 405
shaped like a cone with the axis displaced from the fiber's center, but with the cone point
within the fiber cross section. More complex end faces, such as aspheric shapes are also
possible. Examples of end face contours include, but are not limited to, convex
radiused, concave radiused, parabolic, hyperbolic, tapered, and cylindrical.
It is also possible to create fibers with end face forms that cannot be
readily characterized as a single shape. For example, a partial bevel may be applied to a
flat faced fiber. Thus, a fiber may be created whose face is flat on one side and planar
angled on the other. Figs. 5 and 6 are cross sectional and isometric views of such a
fiber.
Generally speaking, a fiber end face can be created in which a section of
the end face is characterized a.s one geometric form and another section of the end face is
characterized as a second geometric form. The piece wise contours may be employed to
approximate a more complex contour. For example, multiple angled are sections formed
to approximate a radiused, or spherical, shape. As a second, more important utilization,
each surface area may perform distinct optical functions. ~n this manner, advantageous
optical characteristics are created.
Figs. 7 and 8 depict fibers with sectional contours whose shapes are
more complex than planar. The end face geometry is created by first creating a complex
end face contour and then flattening the tip through grinding and polishing operations.
In the depicted shapes, the non-flat section has convex characteristics. In Fig. 8, the
CA 02248912 1998-09-11
WO 97/34l75 PCT/US97/04365
29
non-flat region is geometrically a section of a cone side. In Fig. 7, the non-flat region is
geometrically a section of a paraboloid of revolution.
- Fig. 9 depicts a representative ill~lmination pattern from a fiber 905 with
an internally reflective simple bevel, such as the fiber of Fig. 6. Two distinct5 illumination zones are created. A first illumination zone 910 is typical of optical fibers.
A second illumination zone 915 is created by internal reflection from the angled surface
920. In the second illumination zone 915, a prismatic surface is created at the fiber
core's outer boundary.
Fie. 10 depicts a representative illumination pattern from a fiber 1005
10 having an internally reflective complex contour 1010 in addition to the flat portion
101~. The properly created contour delivers light at precisely controlled ~rlgles. The
contour 1010 is created s-lch that light rays striking its in~ernal surface are directed to
exit the fiber at desirable aneles per application requirements. Thus, precision contro1ied
illumination pattern.s are created. Additionally, the light need not exit throu~h a planar
15 area of the fiber's end face; it can exit through the fiber's side at a desirable angle. Fig.
I l depicts a configuration that accomplishes this goal.
As previously described, standard optical fibers' flat end faces induce
refraction as light rays cross the boundary between the fiber core and surroLInding
medium. The refractive effect bends the light rays. Contoured end faces produce a
20 refractive effect on light rays passing through the end face. It is this refractive effect on
which the contoured end face typically operates and upon which its design is based.
However, an optical fiber end face also produces a reflective effect as a result of the
refractive index boundary. As an arbitrary ray intersects the contoured fiber's end face
boundary, a reflected ray is typically generated. The angle of ray reflection equals the
25 angle of ray incidence upon the fiber end face. For minor angles of incidence, the
reflected ray is weak relative to the strength of the incident ray. Depending on the
geometry, the reflected rays are back propagated by the fiber.
~ fiber end face may be formed to create a surface for total internal
reflection. For example, light propagating within the fiber and towards the fiber end
30 face, is directed out the fiber's side by an appropriately angled, planar end face. Fig. I l
presents a cross sectional view of such a fiber 1100. The extent and characteristics of
total internal reflection are functions of light ray angle relative to the surface encountered.
Optical fibers typically propagate light of various angular orientations; therefore, a
contour may transmit and refract certain angularly oriented propagating rays while
35 producing total internal reflection for others. For example, it is possible to angle a planar
end face such that a portion of the propagating rays are refracted while others are
reflected. Fig. 12 presents a cross sectional vlew of such an optical fiber.
CA 02248912 1998-09-11
WO 97/3417~ PCT/US97/04365
In addition to the factor of ray orientation, certain fiber face contours
create refraction and reflection according to rav placement on the fiber end face. For
example, a hyperbolic end face might be totallv internally reflective around the outer
radial portion of the core and refractive near the fiber center. Fig. 13, although not
hyperbolic, depicts such a fiber end face. A similar situation exists for fibers whose end
faces are sectionally contoured. The previously referenced example of a fiber whose end
face is partially beveled and partially flat exhibits these characteristics if the bevel is
sufficiently angled. Figs. 5 and 6 depict this type of fiber and the related light rays.
Referring to Figs. 7 and 8~ the convex aspects of the reflective section adds a focusing
l 0 effect to the projected light receptivity/delivery zone.
As stated earlier, total internal reflection is a fur.-tion of the ray's angle of
incidence upon the surface boundary. A second condition requires the external medium
to haYe a sufficiently lower refractive index than the inner medium. Thus~ if a properlv
beveled fiber end face is positioned in air, or similar media, total internal reflection is
lS produced as depicted in Fig. I l. Nevertheless, it is also possible to generate total
internal reflection without an air interface. By surrounding the total internally reflective
surface with an intimately bound medium such as low-index fluoropolymer, appropriate
optical conditions are generated. This technique produces mechanically robust
components capable of bearing physical abuse. With the described intimately bound
material, the total internal reflection occurs independent of the refractive index of the
application medium. Thus, the assembly may be utilized in high-index liquids and other
high-index media. The ensuing assembly technique has the additional benefit of
allowing for secondary grinding and polishing operations to create additional optical
surfaces.
Even without coatings and barriers, sufficient refractive index differences
are achievable between fiber and many liquid media. For example, with the properangle, total internal reflection is readily achieved for silica fibers in aqueous media.
Fibers constructed of higher index materials, such as sapphire, achieve total internal
reflection with less restrictive angle and in even higher index media.
In certain materials. the refractive index is affected by transmitted optical
energy. Optical elements constructed of these materials are sometimes referred to as
nonlinear optics. By utilizing these materials as the medium contacting the fiber's special
internally reflective surface. an optical switch o~ beam steering mechanism is created.
The medium's refractive index should be close to the threshold for total internal
mechanism. By subjecting the medium to controlled dosages of optical energy, total
internal reflection is manipulated as the medium's refractive index changes.
For internal reflection at angles beyond the total internal reflection limits,
reflective coatings can be applied to the ~'iber end faces. As with the previously
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WO 97/34175 PCT/US97/04365
described example invol~ing application of bonded low-index materials, secondarygrinding and polishing operations prove useful. These operations provide an entry/exi~
sector for light and facilitate the creation of various secondary contours. Additionally~ in
many instances, the reflective coating reduces wavelength sensitivity since for most
materials refractive index is a function of wavelength.
Following formation of the fiber's principal end face, the fiber is coated
with internally reflective material such as aluminum or silver. For applications in
chemically aggressive or high-temperature environments, platinum, rhodium, and gold
coatings are less sensitive to degradation. The fiber end face is then encapsulated in
material. such as epoxy, providing strength and mechanical integrity. Next, the
assemhly is gr~ und and polished to expose the fiber ~nd create a flat section at the fiber
center. Thus, iight of specific angular orientatiolls enter and leave the fiber through this
exposed section. The fiber can be utilized directlv hl liquid.s and other high-index media.
Light manipulation occurs independently of ally refraction occurring as light crosses the
l 5 boundary between the fiber and the surrounding medium. Depending on the refractive
index of the ambient medium, refraction at the final exit port may be inconsequential. It
is readily seen that the aforementioned techniques are easily utilized to create many
desirous effects. For example, contours are created in which light is readily directed off
the fiber's axis.
To generate a surface contour to accomplish a specific light manipulation
goal, ray tracing mathematical procedures are undertaken. Although many variations on
this theme are possible, the following example illustrates a procedure for a simple first
order approximation. This type of procedure is often referred to as a finite element
analysis and is readily undertaken via computer programming. The fiber is first
sectioned, end-on, into analytical regions. A desirable illumination pattern specifying
angle of illumination and point of illumination escape is established as a goal. Each
analytical region is separately analyzed to establish acceptable contour boundaries. Next,
additional constraints are added, which may include manufacturability, continuity
between neighboring regions, and other factors. The final illumination is the summation
of the contribution from each analytical region. Since a typical net illumination goal may
encompass various angles of illumination at various strengths, additional manipulation of
regional contours may be required to achieve the overall goal.
Fig. 14 represents a fiber assembly 1400 comprising two fiber segments
1405,1410 with adjoining end faces. One of the adjoining fiber end faces is shaped
into a cone. This allows the shaped end face's light manipulating properties to be
transferred to the flat distal end face. Filters and/or filter coatings are readily applied.
Al~hough the depicted embodiments utilize step index fiber segments, either or both of
the segments may be constructed of gradienl index fihers. An in-depth discussion of this
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technology and art is presented in U.S. Patent Application Serial ,~io. 08/561.484,
entitled "Optical Fiber with Enhanced Light Collection and Illumination and Having
Highly Controlled Emission and Acceptance Patterns," filed November 20,1995. which
is assigned to the assignee of the present invention, and which is incorporated by
reference.
Fig. 15 represents a similar fiber assembly 1500 comprising two fiber
segments 1505,1510 with adjoining end faces. In this assembly, contoured end faces
are formed into both adjoining end faces. Light manipulation is accomplished gradualiy
as light is refracted across both fiber end faces. Additionally, the end face surfaces are
not symmetric about the fiber's central axis. The aYially bias of the contours directs the
light rays off the fiber's axis. As with the previous example, the light maaipulation is
transferred down the short fiber to the distal tip. The alternate distal end face, depicted in
dashed lines. is created to enhance the optical characteristics. This end ~'ace is depicted
as angled planar and produces several desirable properties. Since the effective aperture
for light acceptance/admittance is increased, placement sensitivity of final fiber sidewall
reflection is reduced so that the length tolerance of the distal fiber segment is maximized.
Since light rays pass through the final fiber end face closer to perpendicular than for a
standard fiber end face, the refractive influences of the distal fiber interface is minimized.
The importance of this factor is dependent upon the refractive index of the application
medium and the desirable effects. And, the end face protrudes further into the
application medium than would a normal, flat end face. The fiber protrusion extends
farther into the application environment and thus presents the opportunity for precision
manipulation, delivery, and acceptance in scattering or absorbing medium. Although the
protrusion of the fiber end face might appear small and insignificant, closer analysis
reveals that on the optical fiber scale, this di~t~n~e is significant in many circum.~t~n~es.
Fig. 16 depicts a fiber assembly 1600 in which an optical element 1605
is inserted between two fiber segments 1610, 1615 producing the desirable light
manipulation. The optical element 1605 may be refractive, reflective, diffractive,
interference-based, or even based on holography, light-scattering agents, or other optical
mechanisms. As in the previous example, filter elements are readily applied to the
applol,riate surfaces or added as distinct elements. The light controlling element can also
be molded into the fiber such that it is an integral component of the optical fiber.
Figs. 17 and 18 depict two assemblies 1700, 1800 in which large-
diameter fiber segments 1705, 1805 are couFled to small-diameter fiber segments
1710, 1810. In the described manner, desirable effects are readily created. For
example, it is possible to transfer light from the larger fiber into the smaller fiber. Thus,
light intensity is increased. The previous statements related to the delivery and
transmission of the manipulated light are relevant in this scenario.
CA 02248912 1998-09-11
W O97/34175 PCTAUS97/04365
Fig. 19 depicts the transfer of light manipulation via traditional mirrors.
Although not shown, prisms are readily utili~ed in a similar fashion. In either case, the
flat reflective surfaces maintain delivery/receptivity pattern integrity as the light
manipulation is transferred. Fig. 20 depicts the transfer of light manipulation through a
very short segment of large-core fiber. Fig. 21 depicts similar transfer of light
manipulation through a non-conventional fiber. In this case, the fiber is an unclad
waveguide As the reflective surfaces of the waveguide approach flat planar geometries,
convolution of the manipulated light is reduced within the transfer path. ~lthough
depicted straight, the waveguide may be permanently bent. Fig. 22 depicts transfer of
light manipulation throu;,h the application of a hollow optical fiber.
Fig. '3 depicts an assembly 2300 configured with a number of
advantageous attributes. The primary deliverv fiber 230~ is a small-core, low numerical
~perture fiber. This fiber is butted to a distal je_ment of lar~e-core. step index fiber
2310. The distal segment 2310 utiiize.s a high numerical aperture fiber to better contain
the light. Additionally~ to enhance light containment, the fiber's exterior may be coated
with internally reflective material such as vapor deposited aluminum. The distal segment
2310 has a light-shaping contour 2315 formed into its end face adjoining the primary
delivery fiber. The light-shaping contour 2315 is formed to steer the light off axis. The
non-active, protruding portion of the contour is removed to minimize the distance
between end faces of the two fiber segments.
Although this embodiment features a light-shaping contour in only one of
the adjoining end faces, both end faces can be so formed. In doing so, the lightmanipulation is distributed over two surfaces thereby creating the opportunity for
additional manipulation and higher transfer efficiency.
The primary delivery fiber's ~iameter is increased at the fiber junction by
fixing a short sleeve over the fiber. Properly sized capillary tubing is acceptable as are
various fiber optic industry components. This sleeve increases the fiber's effective
physical diameter to approximately coincide with the large-core distal tip. Since a
precision alignment is not required, adequate tolerance is acceptable.
A second sleeve holds and aligns both fibers. The distal segment 2310
is fully encased in the sleeve while only a short section of the primary delivery fiber is
bound. The two fibers are epoxied into the assembly.
As depicted by the dashed line, the distal end face 2320 can be cut at an
angle so th~ a larger exit port is created fnr the escaping rays. This modification also
minimizes refraction as the rays leave the fiber.
Application of a band pass filter coating to either the primary delivery
fiber or the distal segment is effective in cleaning up laser light traveling to the distal end
facc. Fluorescence, silica Raman light, and other laser contamination which often
' CA 0 2 2 4 8 912 19 9 8 0 9 - 1 1
accumul~tes aS l~ser ~ight tr~nsmits over optical fil~er is r~jected prior lo fin~l dcLivery.
.~s -n ,~ 3ative lo a co ~tir.g, the band p3ss fil!er c03tint, can be ~pplied to a w~fer Y.~hich
i5 inse~ted ~t~ een th- fibe~ se~,~ents.
l~e primary d~livery fiber's sm~ll c~re ~nd 'ow numeric~l ~pett~lr~
cre~t~s several ~vantages. The !ow numerle~l ape~;~re rr~I~irnizes the lccu~u12iion ;~n~
wave3uiding o~ laser contanun1~ion ~urir~ conduit of laser li~ht to the dis~l end. Both
fac~crs minirl~ize be~m ,~re~d and s?~ti~l convolution as the be3~;1 tr~erses and e~p~ s
in ~h~ final fi~er segment.
Fi~ depicts an optical fiber 2~05 positi~ned bebind a shcrt segment
1~ of l.~g~ core gr~dien~ index fi~er 2410. The pr.m.ary fiber' s deliYer~/recep~vit~ pattern
is direct~d 3nd ~ided ot~f a~is by the g~adient index segment ~410. As li~ht pr~paga~es
within the gradienl inde~ s~gment. Iiobt controi is gentl~J app~ied over a rei~tiYely lon~
path 'ength ~f manipulation. Although the index of re~raction v~ries wich axi~l
symmetr~, tlle vari~ticn ste~rs the beam off-~vls. As is evident in th~ ~iagr~m, only ~
I ~ sectar of ~.hc gr2dient index se~nent ~ntcracts widl relevant lio~t rays. Thus, desirable
char~cteristics are readily aehie-~ed by or~ly ~ltilizing th~ relevant yort~on of the ~radient
inde~ fiber se~nt. By rem~vi~g the non-relevant por~ion of the gr~dier.t inde.Y fiber
se_menl~ ~ sm~ll dizm~ter specializ~d se~mcnt of gradient index fiber without axial
syrnme~ry is genc-3ted. F1~. 25 depicts a fabrication me~od by which the appropriate
_C se,ment 2510 ~s rernoved from the larger ~radient inde,~ cyLinder 2~ y me~r.s such
as core dri~ling th~ fib~r ~egment or off~enter cylindric311y grinding Fi6. '6 depicts the
resulting fib~r s~gment 2510 adjoined to the primary opcic~l fiber ~4~i. By thismethod, ~n optical fiber segment is created haYing a ;efr~ctiYe index ~r~dient th~t Is no~
~ialLy symme~ic and p.odllcing off-a~is delivery and acceptance eharacteristics. By
'5 uti~izing low nume~caJ aperture fiber ~s the pnmary fiber. the resulting 20ne of
rec~pt~ r or d~liYery is ti~ht rcla~ive to that resulting from hiaher nurner.c~l fibers.
Those skilled in d~e art reali~e ~bat ~anous methads ~re re~dily employed
to create t~e axialiy non-sym2T.e~ric~l gradient index ~iber. These techniques incorporate
methodclogies utilized in ~he mass pr~duction of s~rldard gradient index fibers and so
called~grin lenses."
~or example, a lar~e~ore, ~tep index fiber may h~e a region on one of
its sides with ~n 3rtificially increased refr~tive inde~. Thus as light tra~rels toward this
rcOion and interacts with it, ~hc li~ht is bent off a~is in accord~nce wilh the laws af iight
rer:r~c~ion.
By utilizing ma~erials whose rcfractive indices are a function of
~Jansmitted optical ene~gy, the light' s directional aspects m~y be s~red. By ir~troducina
a con~ollin~ m of oplical energy into ~he ma~erial, its refrac~ e ind~x ~s inten~iar ~lly
m3nipuk.ted. l~.us, as the primary ligh~ interacts with the re~ion whose refractivc inae~
AMEN~E~ c~ T
CA 02248912 1998-09-11
W O 97/34175 PCTGUS97/04365
is altered, the primary light beam is steered. The controlling light should be injected so
that it does not become entangled with the controlled light. For example, light
introduced perpendicular to the fiber axis is not waveguided
Additionally. the fiber may have refractive index gradients which are not
- 5 only axially non-symmetrical but also include axially symmetric aspects. By
supplementing the axially non-symmetrical gradient with an axialiy symmetric gradient
aspect, the delivery and receptivity characteristics of the primary fiber are more tightly
maintained as light is manipulated and directed off axis.
Gradient index optics are generally less environmentally stable than many
other optical components. The index gradient may be permanently changed upon contact
with various chemicals. Therefore~ their ~lsage m-lst be analyzed for a specificapplication prior to deployment.
IMPROVED PROBE ASSEMBLY UTILIZING I~'TERNALL~'
l 5 REFLECTIVE SURFACES
Figs. ~7a-c illustrate an exemplary embodiment of a light-scattering probe
2700 utilizing the principles of light manipulation according to the present invention. A
bundle of fibers is formed so that a center fiber 2710 is surrounded by a ring of fibers
2715. Fig. 27 depicts six surrounding fibers. However, in certain instances, seven
ring fibers prove to be preferable. Similar configurations with various fiber quantities
are preferable for specific usage goals. Depending on the application, the center fiber
may be dedicated to light delivery and the surrounding fibers to light collection or vice-
versa.
The bundle of fibers is bound together. To protect against cross talk, the
center fiber's outer cylindrical surface near the distal end is coated with a metallic, light
impenetrable film. Alternatively, a light blocking additive such as carbon black is loaded
in the bonding agent such as epoxy or inorganic cement which holds the fiber bundles
together. The bundle may be formed as a free-standing assembly by epoxying the fibers
together while the fibers are constrained by heat shrink tubing. Following epoxy cure,
the heat shrink tubing is removed. This technique minimi7es the diameter of the fiber
bundle. If minimum size is not a primary constraint, the fibers should be collectively
mounted in a tube or fiber optic connector. The internal dimension of this mounting
hardware should closely match the outer diameter of the bundle.
To maximize light collection, the fibers are tightly bound together in order
to rninimize the space between delivery and receiving fibers. Step index, silica core,
silica clad fibers with polyimide coatings are preferred. The polyimide coat should be
removed near the distal tip of the fibers. This further minimizes fiber separation. The
size of the fibers is dictated by the application requirements and overall system
. . . ~ CA 0 2 2 4 8 912 19 9 8 0 9 - 1 1 ~ ' J ~ ' J ~
36
pararneters. Fiber with a ~OO-micron core wor~cs well ~nd is large enou~h t~ f~cilita~
e~se of fabrication. Dep~ndin~, on th~ ~pplication, ~mall fibers are iusceptible to
de~rimental sensitivity ta dust. dirt. or o~hcr debris. Thin cladding ~.qlls 3re ~e~t,
because they n~inimize fiber core ~e~2raIion ~vert~ele5s, cl~dding thickness rr.us~ be
5 su~ficien~ so tl.a~ the ~i~htwav~ is fully cont~i~ed. i:3y [,e~ting ~s.d compre~sing th~ ~iber
blJndlc, the fit~ers can b~ d ~o~e~ r W~ t the need of epo~y ~nd furth~r
elimin~ing spacinp~ ~etween fibers.
Wh~t~er the fiber bund,~ is mo-lnted in a conn~ctor ~ssembly, n~,edl~
tubing, ~r i5 free-st~nding, the blmdle's dist31 end is shaped ~llowin~ its cre~tion. As
10 descri~cd e~rlier, various fiber shapes yield applic~tiorl-s~ccific ad~a~tagecus light
deli~es~ and lceep~ance p~t~erns.
A pencil-p~int ~ip is readily cre~ted with st~nd~rd fiber processino
equipment 3dapted for the faoric3ticn prccedure. The fi~er polishin" equipment
pr;t~rably is the vanety with rot~AtinV abra~i~,e disk platens. A holding nl~oe~nism~ such
l~ ~AS a coll~ct, ehuc~c, or simil3r deYic~, sup~rts and pcsitions the fiber for polishing. ~he
holding ATnech~nism mu~ aintain the ~iber's pri~Aary axis al the desire~ aAngle o~ polis
relative to th~ rotating disk. ~rheleas conventional, fl~AI f"Ac~d fiber po1ishinc is
accompl,shed by positior.ing the tlber's cen~al ~xis 3t ~A 90-de~ree an~le relative to the
s1~rface pl~Ae of the rct~tin~ disk, Ihe ~encil-point tip bun~le is formed by po~itioning the
~0 fiber at a lesser ~ng]~.
The holding rnechanism pre~er~bly includes a pTovi~iion ~o simultaneous1y
rotate tn~ fib~r about its A~aAjor a~is and s~eFp it bdcl~ and forth across the abr~siYe disk.
It is importa~t for the holdirA~ m~ch~lsm ~o possess sutIIcient precision ~o the a~i~ of
r~tatio~ is accurately m~inr~ d with respec~ to the fiber bundle's center longiludAnal
25 rr.~ch~ic~l and ()ptic~ 5.
To form ~ pencil-point tip on ~he bundle~ 2he b~n~le~ iS contin~ously
~ota~ed aS it is swept baAc~; an~ forth across the poJishin_ disk. ~rogressivcly a ~iner
~lishing A~Aedium is used to cre3te a highly polished sur~ac~.
For~nina the pe~cil-point ;ip ~ith an included an~e of approximateiy 40
3~ degrees resul~s in a probe with excellent per~ormanc~ ~r Ftaman ~pectroscopy, even in
hia~iy scattcnng and absor~ing media By de~r~asing this angle to 20 degre~s.
performance is rcduced iD dema~diDa media but ~creased in intermedia~e condihons. At
10 d~rees, p~rforrn3nce is oFtirnized ~or l~ss demanding me~iia
rn m e,~emplary ~mbodiment, the end i'ace is ?referably coaled wi~h
35 internally ~ellec~ e me~alIic film folloY~ing form~tion of th~ primary bundle shap~.
~rious vapor deposition techrliques a~e suitable.
Foll~wing fonnation of the probe tip, it is then flattened by erind{ng ~nd
polishin~ on the fi~er polishin~ equipm~n~. For the previously descnbed point
~Ea~ r
CA 02248912 1998-09-11
WO 97/34175 PCTIUS97/04365
parameters, the tip should be fl~ttene-~ so that the flat region extends beyond the center
fiber and into the ring fibers. The extent to which the ring fibers are flattened is
dependent upon the numerical apellule of the fibers and application specific parameters.
If the flat section extends approximately 50 percent of the center cross section of the ring
5 fiber, excellent performance is achieved for a point based on a 70-degree polish angle
(40 degree included angle). This ensuing tip is well suited for Raman analysis and
performs very well even in dark, scattering media. As mentioned earlier, by reducing
the included angle performance is readily tuned to various absorption and particulate
scattering conditions.
Fig. 27a depicts a fiber bundle created in accordance with the previous
techniques except that the application of internally reflected coatings is omitted.
Figs. 27b and 27c depict the probe contained within a protective housing.
Fig. 27b is a cross sectional view. Fig. 27c is an isometric view. Fig. 27b illustrates
the presence of an internally reflective coating 2720. Prior to the formation of the flat
l S surface 272S on the bundle, the bundle is inserted and fixed into a hollow tube. As per
application requirelllellts, various mounting connectors are also adequate. Fig. 25b
depicts a thin metal tubing 2730, which is often referred to in the industry as needle
tubing. Initially, the tube should extend slightly past the tip of the bundle point so that
the bundle is slightly recesseA The tube is next filled with epoxy or a similar material
20 encapsulating the tip within the tube. Following this process, the assembly is flattened
as previously described.
In many applications, protection against environm~nt~l abuse is desired.
By applying a high quality surface such as that achieved with .3-micron polishing film,
environm~nt~l abuse due to chemical attack is minimi7~ Chemical attack in optical
25 fibers is most severe at and may be initiated by surface imperfections. Nevertheless, in
many cases, additional environm~nt~l isolation is required.
The application of the probe behind a window should be avoided due to
the previously described negative aspects that windows impart. Environm~nt~l isolation
is achieved by the application of various protective co~tinE.c. Examples of these coatings
30 include diamond-like coatings or amorphous diamond coating, sapphire coatings, and
various oxide coatings. The application of these coatings is facilitated by fusing the
bundle into a solid mass as previously described. A thin, free-standing wafer can also
be bonded to the end tip. Certain windows and coatings such as those in the diamond
family exhibit strong Raman signatures. These ~ign~tllres can be utilized to significant
35 analytical advantage in certain application environments. As the source transmits
through this medium, a Raman band is ge~ d. When utilized in certain applications,
such as those containing back-reflecting, Rayleigh characteristics, a portion of this
S~ME~ RULE2~
CA 02248912 1998-09-11
WO 97/34175 PCT/US97/04365
38
Raman light returns from the primary measurand via the collection fiber(s). This signal
can be utilized as a reference to establish both wavelength and intensity.
As previously stated, the ~le~lic~ion of light delivery and collection fibers
is dependent upon application and system requirements. For Raman spectroscopy, the
5 outer fibers are usually assigned to collection. Conversely, for applications such as
diffuse reflectance measurement utilizing white illumination light, the ring fibers are
usually best utilized to delivering source light.
In isolated media applications, the measurand may be susceptible to
source energy. In these media, it may be advantageous to distribute the source energy
10 among the outside ring fibers. In doing so, the surface area under illumination increases
and, in turn, the power density to which the measurand is subjected decreases.
OPERATION OF AN EXEMPLARY PROBE ASSEMBLY
In addition to the earlier operational description of various components
lS and optical surfaces, a general operational overview of an exemplary probe is insightful.
Suppose the probe is configured for Raman spectroscopy with the center fiber utili7ec~
for laser delivery. This fiber delivers the laser light into the medium of interest. The
medium Raman scatters the light thus producing Raman bands. Typical media of interest
presents Rayleigh scattering, absorbing, and other characteristics that induce the often
20 undesirable effects previously described in the Background section.
As depicted in Fig. 9, each ring fiber 2715 (Fig. 27) has two distinct
zones of receptivity. The first zone 91~ is controlled so as to intersect the illumination
beam directly in front of the fiber end face. The internally reflective surfaces of the ring
fibers direct these fibers to be receptive to Raman-scattered light very close to the probe
25 tip. This first zone of receptivity extends between approximately 44 degrees and 32
degrees (where zero degrees is taken as coincidental with the fiber axis). Thus, the
precious light is collected and the detrimental scattering and absorbing characteristics of
the medium is circumvented. Tremendous performance increases are generated
co~ ared with those achieved utili7ing alternative mech~ni~m~. The second zone 910
30 of receptivity is responsive to light at a greater distance from the source fiber end face.
Witk, his configuration, the probe is responsive in highly diverse media.
The probe exhibits selective sensitivity to specific photonic mech~ni~m~.
The physics of this performance is based upon complex light-matter interactions as
follows. The Rayleigh and Mie-scattered light is angularly biased and is fre~uently a
35 multiple event phenomenon. The multiple event aspects produce complex paths of light
travel. Both the primary laser light and the silica Raman light emitted from the ~lber are
susceptible to Rayleigh scattering by the media. As such, their scatter is angularly
biased. Fluorescence and Raman-scattered light are more directionally random or less
su~sm~ St~ ULE 26)
CA 02248912 1998-09-11
WO 97/3417S PCT/US97/04365
39
angularly biased. Therefore, statistical bias exists between the directional aspects of
scattered photons according to the scattering phenomena. Thus by angular manipulation
of illumination and detection receptivity, the ratio of light collected from each photonic
mechanism is advantageously set. Thus, a new, previously undescribed filtering
S m-~ch~ni.~m is set forth for fiber optic measurement light-scattering phenomena.
The physics is believed to be somewhat akin to that of automobile
headlights in certain driving conditions, such as fog, snow and rain. In a snow- or fog-
ridden night, the driver sees better with headlights on "low beam" than on "high beam."
The key difference between the low and high beam settings is the angle of illumination.
10 Certainly, more light reaches the driver's retina driving on high beam, but it is the wrong
light--virtually all scattered by the fog.
Because the angular orientation of scattered light is complex and
dependent upon many factors, the probe is optimized by expe~ enlation for specific
application constraints. Empirical data indicates that orienting the collection zone
15 approximately as specified above maximizes Raman probe performance in heavy
particulate scattering media. In this configuration, the ratio of Rayleigh to Raman light
produces maximum perfollllance.
In a similar fashion, the probe achieves advantageous perforrnance in
analysis based on particulate scattering. It collects Rayleigh-scattered photons which
20 have undergone a Illinilllll,-- number of scattering events. Thus, the acquired data is not
convoluted by multiple scattering events.
PERFORMANOE OF AN EXEMPLARY PROBE ASSEMBLY
Figs. 28, 29, and 30 depict performance levels achieved with a probe
25 fabricated in accordance with the previous discussions. This probe was utilized in the
configuration depicted in Fig. 27. Internally reflective coatings were not required as the
test medium provided a sufficient refractive index dirr~l~,ntial to generate the required
total internal reflection.
Results from the analysis of three probes follow. The first probe is a
30 "Flat Paced/Parallel-Fiber" (FF/PF) Probe of the type described earlier. For the tests, it
was deployed without a window in order to maximize its performance. The FF/PF
Probe is heavily reported in liltldtu.e and is particularly noted in medical literature. The
second probe, denoted as the "Refractive End Face Probe" was manufactured in
accordance with U.S. Patent No. 5,402,508 to O'Rourke et al. under license from the
35 patent owner. The Refractive End Face Probe's configuration was optimized by ntill7.ing
a flat-center source fiber surrounded by a ring of collection fibers angled at 20 degrees.
The Refractive End Face Probe is positioned behind a .020-inch thick sapphire window,
which is required for proper operation. In order to make even-playing field comparisons
CA 02248912 1998-09-11
W O 97134175 PCT~US97/04365
with the FF/PF Probe, only two of the Refractive End Face Probe's fibers were utilized.
The third probe is Visionex's "Advanced Probe," which is fabricated in accordance with
an exemplary embodiment of this present invention. As with the Refractive End Face
Probe, only two of the Advanced Probe's fibers were utilized for the test.
For the test, all of the probe fibers were the same size and numerical
aperture, and all equipment and test conditions were as close to identical as possible.
For these tests, a broad-band, visible source was utilized with minim~l ultraviolet
energy. Performance from fluorescence and particulate scattering were the light-scattering mech~nicm~ under investigation.
Fig. 28 is the spectra acquired by the three probes in a blood sample
containing a fluorescent aspect. As would be expected, all three probes collected red
light from the blood. In the red spectral region, the Refractive End Pace Probe (line
2810) and the FF/PF Probe (line 2805) produce conlpa.dble results. The Refractive
End Face Probe's performance is slightly superior. However, the Advanced Probe (line
2815) collects the red, elastic-scattered light much more efficiently. Nevertheless, the
Advanced probe was not optimized for this response. In a shorter wavelength region
(slightly above 475 nm), further disparity is a~a~lll. A fluorescent agent present in the
blood sample generates a se~a~ale peak. The Advanced Probe picks up spectra from this
agent whereas collection is not evident from the other probes.
Fig. 29 is an enlarged view of the relevant spectral region. The FF/PF
Probe (line 2905) has no appalel t detectable collection of light from the fluorescent
agent. The Refractive End Face Probe (line 2910) produces a spectrum with a nebulous
structure in the region; however, it is not clear what portion of this peak is a result of
back reflection from the window and what is attributed to the trace agent. The Advanced
Probe (line 291~), with its selective sensitivity to specific photonic mech~nicmc,
efficiently gathers the required light and produces a clean spectrum. The ratios of elastic-
scattered light to fluorescence light from the various probes clearly demonstrates this
per~ormance.
Perhaps more clearly, Figs. 31 and 32 illustrate the selective sensitivity
characteristics and the achievable results. Fig. 31 is a graph of the spectra collected in a
red particulate scattering medium. Fig. 32 is a graph of the spectra collected in the sarne
sample but with trace yellow-green fluorescent additive. Note the dramatic increase in
ratio of collection of fluorescence light relative to inelastic light collection from the
Advanced Probe.
Fig. 30 illustrates the results of probe tests conducted on an aqueous-
based, red solution with minimllm Rayleigh ScatLe~ing characteristics. The test solution
appears to be a clear, vivid red to the human eye. It is sufficient]y red to quickly
attenuate non-red light. The solution contains a violet fluorescence additive, which is not
s~mwt S~T ff~E 2~)
CA 02248912 1998-09-11
WO 97134175 PCT/US97/04365
41
~pal~nt to the human eye. In addition to the violet response, the additive also induces a
yellow fluorescence response from the solution.
The FF/PF Probe (line 3005) generates the spectrum of lowest intensity
and fails to detect both the yellow (500 nm - 600 nm) and the violet (450 nm) fluorescent
5 aspects. Whereas the blood inherently exhibits particulate-scattering characteristics, this
test solution does not. In the blood, the illumination light returns to the collection fiber
via multiple bounces and interactions with the medium during which the light becomes
biased to the red. In the non-particulate-scattering test solution, miniml-m illlpurilies are
present to generate the pre-described effect. The FF/PF Probe's spectrum also exhibits a
10 structure above 600 nm which results from reflections returning from the bottom of the
sample beaker. (The solution is relatively transparent to these red and near infrared
wavelengths.) The probe collects no appreciable violet or yellow light. As light from
the source fiber excites the fluorescent agents, the fluorescent light is quickly absorbed
by the red solution. And, the red solution quickly absorbed the fluorescent-inducing
lS light. In fact, virtually all the excitation light is absorbed before it can reach a region to
which the collection fiber is receptive. Andt any photons which are generated in the
region of receptivity are immetli~tPly reabsorbed before completing the return trip to the
collection fiber.
The Refractive End Face Probe (line 3010) suffers essentially the same
20 fate as the F~/PF Probe. Although the Refractive End Face Probe is able to generate a
certain degree of light bending and light manipulation, it is insufficient to overcome the
solution's attenuation. Additionally, the Refractive End Face Probe suffers another
drawback. Reflection from the window is inadvertently capluled by the collectionfibers. The spectral structure captured by this probe is consistent with window reflection
25 tests (not shown). Under careful scrutiny, feeble peak structure can be "im~gined" in the
a~p,vpliate spectral regions; although, they are certainly not definitive.
The Advanced Probe (line 3015) excited and c~lules violet and yellow
light within the solution. Both peaks are captured clearly.
Under a battery of tests in various media, the Advanced Probe delivers
30 similarly impressive results. Dem~n~ling conditions are present for a vast majority of
applications where fiber optic instruments are considered beneficial and their usage is
highly sought. These applications span from in vivo biomedical to environmental to
industrial. Often, a species is believed to be a weak producer of the desired photonic
effect when in fact the photonic mech~ni~m is strong but the available in~umcntalion is
35 insufficient to adequately acquire the generated light energy.
UltSHEEl ~2~
CA 02248912 1998-09-11
W O 97/34175 PCT~US97/04365
42
COMPLEX SURFACES
As previously indicated, additional light manipulation, with advantageous
results is attained by forming alternate surfaces into an internally reflective portion of the
fiber's face. These surfaces are more complex than those described imm~ tely above,
5 yet they deliver superior performance for many applications.
By creating these surface contours in accordance with the previously
described design techniques, light entering or exiting the ring fibers is further
manipulated. The ray angles are directed as required for the specific application. Also,
the flat portion of the fiber, through which light travels, is made smaller and still
10 transmits all available light.
Whereas the kick-over zone is relatively thin in the pencil-point tip probe
version, the zone can be readily expanded. Figs. 7 and 10 illustrate representative
performance improvements achieved by the application of more complex surfaces.
However, these improvements complicate probe fabrication. The Fig. 7 embodiment is
15 particularly useful in its ability o maximize response from a specific "focal point" region
within the light delivery zone.
A fluorescence-spiked solution provides an excellent test bed for
evaluating various probe shapes and architecture. The distal end of an assembly is
immersed into the solution. Emitted light patterns are readily observed as the
20 illumination pattern glows. Illumination and collection patterns and their overlap is also
readily observed. To accomplish this, the test bath should contain a particulate sc~ttering
agent in addition to the fluor~scel ce additive. Titanium dioxide is ideal. By injecting red
laser light into the proximal end of the collection fiber, its receptivity is observed. The
red light does not induce intense fluorescence but scatters visibly from the suspended
25 particles. This general technique is useful as a design aid in configuring a fiber optic
interface for specific applications.
Complex fiber contours may be generated in several manners. CO1II~)UL~"
controlled laser ablation is feasible but requires expensive e~luip~l,ent. The shapes are
formed in a piece wise linear fashion in accordance with the previously described
30 techniques for creation of the preferred probe embodiment. The preferred method to
create the complex shapes is through a polishing process similar to that described above.
The machinery requires the additional capability of lifting and tilting, in unison, the
probe during the grinding and polishing process.
ADDmONAL ASPECTS OF THE PROBE ASSEMBLY: FILTERING
MECHANISMS
As previously stated, source light, particularly narrow-band light, such as
laser light, often becomes corrupt with extraneous signals as it travels within optical
. . . ~ . CA 0 2 2 4 8 912 19 9 8 - 0 9 - 1 1 1 ' ~ J ' )' )~ ' J~
tibers. This ~ffect is paFucula~ly tr~bie~ tor Raman spectroscopy. Th~, it ;S 03.
~dvan~ge~us to eli~r.inate ~his li~h~ by ;~pl~ing a b~nd pass ~ilte~ In ~he ~icinity of the
pro~e, ~o the l~ser iig~ conduit. Addi~on~lly, it is ~t~cn ~ ant~gecus tO simiI~ly ~Dply
a ~3nd itop ~lter lo ~ nal co~lection fibeI cond~ts,
s Filt~ring may be ~cc~m~iishcd in ~eve-~l rIl~nne~s. Inierf~renc~ filler
ce ~dn~s ~-e lp~ di~ to the fiber ~nd f~cc bunc.~. T'nic techni~ue requirei
difficul~ manutac~unng procedure~ also suf~ers because Ihe filter.~ d~ noc ~ncsion
weil ~,.her~ ligh~ i~ incident a~ diverse ~ng!es; ~d, th~ pro~e'i ring ~i~ers lr~ specifics~ly
desi~ned ~r wid~ accepunce angles ~f li~,hr.
The c~ter~ laser f3~cr c3n be filtered by applying ~ filter cvati;lg tc t~e
~are fiber~ T~e probe i~ ~en cGn~truct~d wit~, a lumen in ils center. .A~ ca~ i~y is cre~ted
~y conslr~cting the prob~ with ~ caFillary ~ube in placc of the cent~r fi~er. .~fter ~he
probe is constmcted in accordanc~ wi[~ ~he afor~rner;tioned procedurej, the center fi~er
is inse~ed into the lum,en cavlty ~nd ~t'fixed ir.to pi~ce
A~,other useful ~nd preferred .echniau~ is ~cr e~ch i~di~idual ~ber ~o be
separately fil~er~d in a cor.nector~unction a~ ~n appropria~e ~ist~nce fror~ the prooe tip.
Filter coatillg may b~ directly applied to th~ fb~rs or to tilter wafess that are inserted
between ~lber end f~es. This techniq~e may c~use the ~siembly tc incre~se in ~ize
~eycnd ~cceptablc lilILitr. if industr5~ standard ccnnectors ~re ~ e~. Thus. the prefe~ed
~~0 approach i~ to apply th~ filter coa~n~ directly to tne short ~iber se~m~nt er.d faces
~pproximatel~J one inch in length. The filtered end face i, but~ed to the primary fiber
. len~,th. Thi~ junction is ma~ie in a ~eed]e tubing with psecislon b~re. I~eally, th~ tubir.g
w~ll is .OOl- 003" lhick.
Allother filterin~ techniqlc is poten~i~ se~ul. In t~is ~echni~ue,
2~ un~ar.t~d w~Yelen~hs ~re mgularly ~isplace~ In doing so, lioht c~ undesirablewavelengths c~n be dirccled ~utsidq ~t~e ~iber' ~. inte~.al refl~c~i~n lirnits.
By ~ unding ~he .,enter fiber ~ e~,en smdller fibers instcad of ~ix of
the sa~e size, ~n adYant~g~ous gecme~ry is crea~ed, The s~/en ~ers are bundled
separately into a tight ,geome~ric p~ck The so-called pac~.in~, factor carl be fur~her
30 ~nhanced by h~ing ~he bundle '~ il soft ~nd squce2in~ i~ t-~ ~emove ~l voids. Fig. 33
depicts a ~und!~, s~ erea~ed. R~ardl~ss, thi3 conflou~a~lon pro~uccs ~l advunt~geous-
packing factor and thc bundle ~xhibits e.ccellent circul3r Oeometry. It ~s possible to ~ced
the b~nd3e into a ~ingl~. Iargc-core fiber ~s depic~cd in ~ig. '~. V~rious filtcring
meehanisms c~ be utilize~ in the junc~oa betweeD ~he bl~nd!e and large-core fiber
35 1 ~sing a large-core t'iber thal is 31icht~y l~r~cr than the bundle overco~nes the effect of
d~crgenc~ if the light sprc~d~ durin~ transmission through the ~llt~rinO mechanism.
A~thou~h light den~ity decreases lq a result uf the chang~ from m~ltipie fibers to ~ singl~
AM~ c~ c~''
CA 02248912 1998-09-11
WO 97/34175 PCT/US97/04365
44
fiber, total light energy is not significantly decreased. The effects of the slight decrease
in intensity are system dependent but are typically inconsequential.
The aforementioned configuration of connecting seven ring fibers to a
single, large-core extension fiber offers other advantageous considerations. Forexample, the ring fibers are often utilized in the illumination capacity to deliver broad
band light from a source lamp. In this in~n~e, the source lamp's spot size is typically
larger than the fibers. Therefore, no net light loss is realized as a result of the single-
fiber-to-bundle junction (as long as the single fiber is larger than the bundle and the
single fiber is smaller than the lamp's spot size). Additionally, any inconsistencies in the
lamp's illumin~tion pattern are mixed during transport to the junction so that illumination
energy is equally distributed between each ring fiber.
As an alternate to the large-core fiber for transport, the entire bundle can
be filtered. To accomplish this task, the individual fibers within the bundle are aligned
with one another. The size of the fibers also influences the effectiveness of the
technique. Larger fibers are obviously easier to align and typically exhibit less loss
during filtering. An appropliate bundle ~lignment technique is described in U.S. Patent
Application Serial No. 08/561,484, entitled "Optical Fiber with Enhanced Light
Collection and Illllmin~tion and Having Highly Controlled Emission and Acceptance
Patterns," filed November 20, 1995. Additional information is also described below.
The total length of the assembly is first established and a bundle of fibers
is created whose continuous length is the desired length of the overall assembly. The
bundle is tightly conslldilled and epoxied/bonded in the region in which the connection is
to be formed. Tubing is a suitable component to achieve this goal. Heat shrink tubing is
desired because of its ability to tightly constrain the fiber while allowing removal
following epoxy cure and assembly. Rigid tubing such as metal, glass, or ceramic may
also be used. Industry-standard connectors are also suitable. The fact that this type of
tubing is permanent may be a benefit or hindrance depending on the desired
characteristics of the assembly and overall space constraints. If the connection location
is far removed from the end, care must be taken to prevent damage to the fibers while
they are inserted into the tubing.
After the region is constrained and epoxied/bonded into a rigid section, a
m~ch~llical key or other identifying mark is placed along the section parallel to the axis of
the fiber. The section is then cut perpen~iC~ r to the fiber axis. The cut is best achieved
with a thin precision saw such as a fine grain, diamond impregnated wheel. Afterprocessing and polishing each side of the cut thereby forming the a~plopliate surfaces,
the individual fibers are realigned by mating the two bundles together. This mating
connection is accomplished by any of the methods typically utilized in the industry for
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mating single fibers. Rotational alignment is achieved by visually matching the
previously described identifying marks or with mPch~nir~l keys.
This junction can be produced directly into the rigid section of the probe's
distal tip. In doing so, care must be taken to isolate the delivery light from that received.
5 Although a blocking mechanism can be introduced, it is best that the center fiber's
junction be removed from the junction of the ring fibers. In accordance with the stated
methodology, the junction for mating the ring fibers may feature a lumen in which the
center fiber passes uninterrupted. The center fiber may be then broken for filtering, or
have filter coating applied directly at its distal end face.
Fig. 35 depicts a probe assembly 3500, similar to that depicted in Fig.
27, that is adapted for filter application. The embodiment of Fig. 35 employs the lumen
concept. It also utilizes the probe's outer metal tubing 3505 for a dual usage of bundle
alignmP-nt and physical protection. The filters 3510 are applied directly to the short fiber
segment end faces such that the filter is between fiber segments.
ADDITIONAL ASPECTS OF THE PROBE ASSEMBLY:
INSTRUMENT INTERFACE
For the configuration in which the ring fibers are employed to collect
light, the light from these fibers is transported to another instrument subsystem where
20 the light is analyzed. Various instrumentation systems require specific fiber input
configurations. Non-dispersive instruments typically accept the input in a circular
geometry. Therefore, these in~l~u"~ directly accept a fiber bundle or large core-fiber.
Another class of instruments is often referred to as dispersive. These
instruments typically perform best when the input is arranged in a narrow rectangle.
25 This configuration is often referred to as a slit; circular inputs are typically converted into
this geometry by positioning the input directly in front of a physical slit.
For ~ptilllUIII performance, it is often advantageous to configure the fiber
into a linear slit. This may be accomplished in several ways. For the bundle, the
individual fibers are typically positioned into a linear array in a connector designed for
30 this purpose. For the large-core fiber, the fiber is broken out into smaller fibers. In
effect, the connection described above and represented in Fig. 34 is reversed. Since the
desired slit width may be narrower than that presented by six fibers, a bundle with more
numerous fibers can be utilized. As with the previous discussions, the bundle can be
heated and compressed to minimi7e the non-active region of the bundle. Likewise, the
35 linear array of fibers is readily compressed to further enhance performance. Fused
bundles of fibers are often referred to as fused tapers and are often used for imaging
applications. By creating a similar structure, an instrument adapter is formed to
efficiently transfer the circular input into a linear, slit-style format.
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46
As a variation on this theme, the large-core fiber. or a similarly
constructed adapter is heated and compressed. In this compression, the total surface area
should remain constant. To accomplish this goal, a die is fabricated from material that
will withstand temperatures at which the fiber softens. The die should have an inner
5 rectangular cavity of the desired dimensions. Typically it needs a thin rectangular cross
section. It has a top and a bottom half which mate together. The die and fiber are heated
in unison, and the die is compressed to closure. As it is compressed, the fiber takes on
the shape of the desired rectangular cross section. Ideally, the transition from circular
cross section to rectanoular is accomplished gradually along the fiber's longitudinal axis.
ALTERNATIVE EMBODIMENTS FIBER GEC~.~ETRY
Various geometrical configurations increase the ring fiber s size relative
to thc center fiber. For example, a five-fiber ring placed around a sma!ler center fiber
provides potential results for increasing the extent of the intersection between the
l 5 collection and reception zones and providing increased receptivity.
Two fibers in a side-by-side configuration with both end surfaces
partial3y f]at (providing entry/exit locations) for light reflecting from the internally
reflective contour is another alternate configuration. It is well suited to instrumentation
systems accepting only one source and one detector fiber. Of course, cost and simplicity
20 advantages are also potentially realized. This set up also delivers illumination light into
the collection fibers field of receptivity very close to the fiber end face. This fact is
evident by noting that both source and detector fibers have components that are
manipulated into intersection.
The center fiber is shaped into various contours such as a cone. In
25 creating these contours, the center fiber protrudes farther in to the medium of interest.
This protrusion positions the center fiber's end face into closer proximity to the ring
fiber's zone of receptivity or illumination. Assuming the center fiber delivers energy, the
closer proximity minimizes the distance through which the center fiber's light must travel
in the medium until the light r~aches an area of ring fiber receptivity. l'his technique is
30 particularly effective in difficult to measure media; the refractive effects of the center
fiber's contours are minimized for media with refractive indices close to that of the fiber.
Fig. 36 is illustrative of the described effect of increased proximity for a multi-fiber
probe. Fig. 37a illustrates the potential benefits for a probe with the collection fiber
larger than the delivery fiber. Referring again to ~ig. 37a, the sharp point also facilitates
35 inserting the probe into materials such as biological tissue. In the Fig. 37a embodiment,
the optical components are depicted encased in needle tubing. Furthermore, response is
highly spatially specific within an investigative medium.
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47
If the angular orientation of the deliver and collection fibers are
aggressive}y shaped, the delivery light is forced to impinge on the collectior fiber's end
face. In doing so, indicator-type surface treatments can be applied to the fiber to produce
desirous measurements. Similarly, surface treatments can be applied to the deliverv
fiber, with the collection fiber directed to view this region. If the coating scatters the
delivery light appropriately, the scattered light is gathered by the collection fiber. This
technique is particularly valuable for the branch of Raman spectroscopy often referred to
as surface enhanced Raman spectroscopy. Fig. 37b provides an illustrative
representation .
As stated, Fig. 37 depicts a single source fiber directing light in front of.
and .somewhat parallel to, a collection fiber's end face. In the figure~ the collec.ion fiber
is shown to be larger than the de~ivery fiber. Depending on the medium and application.
this may or may not provide enhallced performance. It is evident that the outer side
portions of the collection fiber are somewhat remo\ed from the source beam~ which
skims over the fiber's center. Therefore, it may be surmised that an oval fiber would be
preferred. By slightly mashing the collection fiber results into an oval cross section, and
the overlap between the delivery and receptivity zones increases. Similarly, an oblong,
rectangular, or linear array of collection fibers can be utilized. This array provides
additional useful light scattering information from the sample medium. By analyzing the
light received from the individual fibers in relation to one another, valuable information
can be obtained related to the medium's properties. Figs 38a - h are relevant, genera
illustrations.
SEPARATE ELEMENTS
In addition to utilizing the fiber's outer surfaces for internal reflection to
achieve light manipulation, control is achieved with intimately attached elements. For
example, an internally reflective fustrum of a cone may be ~tt~rhed to the end of the flber
bundle as in Fig. 39a. As previous discussed, the internal reflection is a result of total
internal reflection or reflection from internally reflective coatings. The internally
reflective end piece functions in accordance with the previous discussions. Preferably,
the center of the end piece is drilled to facilitate insertion of the center fiber through the
hole. This adaptation is important to prevent source energy from prematurely entering
into the coll~ction conduits. The end piece is fabricated from a materials in accordance
with application requirements. By utilizing high-ref ~c~ive index materials, total internal
reflection can be realized for even liquids of relatively high refractive index. And if the
high-refractive index material is a material such as sapphire, increased environmental
isolation from chemicals and physical abuse is also realized.
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48
Fig. 39b depicts another end piece embodiment. In this case the end
piece sides are created with convex sides thereby generating effects previously
discussed. In this embodiment, the contour is a frustum of a paraboloid of revolution
with the geometric focus of the parabola positioned in front of the center fiber in the
relative position of desired maximum response.
Figs. 40a and 40b depict a probe in which the ring fibers are diametrically
separated from the center fiber. The end piece, although in many ways similar to that
shown in Figs. 39a and 39b, exhibits an additional prupelly. The end piece's side walls
create internally reflective surfaces for internal reflection thereby generating a
waveguiding effect. In this manner, light is not only angularly oriented but is also
directed into spatial proximity with the center fiber.
RELATED EMBODIMENT ALSO UTILIZING INTERNALLY
~EFLECTIVE SURFACES
The previous embodiments utilized internally reflective surfaces in
conjunction with an adjoining section formed to create an inlet/outlet for light. However,
internally reflective surfaces are also utilized to advantage without forming a special
inlet/outlet contour. Fig. 41 depicts a probe constructed in accordance with this
principle. In this probe, the ring fiber's end faces are fully internally reflective. Fig. 42
provides an isometric perspective view of the fiber assembly of Fig. 41. The fiber's
coatings are removed so that light readily passes through the side walls. Additionally,
the center fiber is recessed into a clear capillary tubing. Thus, light passes unintellupled
between the area in front of the center fiber and the internally reflective surfaces of the
ring fibers. The agent holding the assembly together, typically epoxy, is optically
transparent, and its refractive index is carefully chosen in accordance with desirable
operational effects. For this assembly, it is particularly important that the center fiber is
metalized or similarly blocked from inadvertent cross talk with the ring fibers.Among other things, this probe embodiment allows extremely minute
samples to be analyzed by inserting the sample into the recessed cavity. For applications
in which it is desired for the sample to inherently migrate into and out of the sample
cavity, a portion of the raised side can be removed. By this action, fluid readily flows
within the probe's sensing zone.
As with the previous probe embodiments, the internally reflective
surfaces can be formed with complex contours so that additional light manipulation is
realized. For example, a hyperbolic cross sectional shape is very efficient in light
collection for energy em~n~ting near its focus.
Creating a sharper bevel and subsequent longer taper on the ring fibers
generates additional effects, as depicted in Fig. 43. A deeper sample cavity is produced.
~ CA 02248912 1998-09-11 I~ IO:I,'J')- ~'J '~ I'J'J-I 1~
~9
Addition~lly. the fiber's 20nes of c~lle_tion~recepti~ity is pr~jected ou~side of th~
confines o~ the rec~ss.
F ~ d~piets a simil~r eollfi~ur~tion in which thic~-w311 capill~rY
tub~ng ~s lltili7ed so thilt the principle ~i.Tht tr~nsmission occurs tl.rough the ~
5 tubin,~'s en~ tac~. Si nil;lrly, light c~n transmi~ ~hr~ gh ~'oe ce~te~ er'~ cl~.ddina or
bu~I~r if it is suf,fici~n'~v thick~ tr~nsnlr~eo ~nd ~onst~ .~ s-d of a materi~ ith ~ ~uitabl~
index of refi~cuon.
Fig. ~5 d~picts a configur;ltil~n in ~hich the c~nter fibe,r ~ forrneci to
cre:~te ~n intern~lly retlecti~e surf~ce. Assuming the center fiber is utili2ed t~r
10 i~lu~Linaltion. light interacticn with this ~urface results in illumina~on exiting the fiber's
f~t 3ection at di~erse ~ngles. Th~ voi~ cre~ted by tapering the cen~er f.ber is fi]led with
clcar ma~erial such ~s optic~ transparent epoxy. Pet Gt~r lig~t readilr passes through
this transparent mat~ri;~l.
Flg. 40 dcpicts a cotûiguration ~hct is ~dapted for h1gh scnsiti~ity directlv
a~ ~he probe ~ip. l his e m bodiment collects surfac~ lieht ~Yithout requirin~ passa~e of
li~htth~cugh th~ mediurr. underin~esti~ati~n. Thus. surlace m e~ure m ~nts ~re re ~ ily
laken on ~ar.c~ n~ate~a~. lhe zones ofill~ min~tion and re~ptivit; 3re in coincidenc~
a~s t~ey int;lseCt the probe's b~unaary and proJect nutward.
Assu m e che c~nter fib~ri~ utilized torl'ght deli~ery. The center f~r-s
light r}ansrr.its rllrcugh a section of tr~n~p~rent m a~erial cuc~ as clear eFoxy, vlass~
sapphire, etc. Since many epo~ies are pron~ ~o interfcrin,, ~luorescen~o,th~ rnateri~l
~eleftlon for ~h~ ap~lic~lion param eterisim portant. The ~uter iUtlace of ~his section is
fcrme~ into a convex shape such th~t ipecularre~lecuon fro m ~he outer opuc31 surf~c~ is
directcd bPck into the cen.er fiber. Ho w ~er,lar.d~ mly ~e~ttercd li~ht, such as FCarn~n,
collec~ed by tb~ ou~e~ ring fibers. Thi~ embodimenl pro~ ides th~ added a~Yantage of
faci]itating co~l ~ ti~n of a refcrence jpectral ~ignal from ~he el~3l ~ncapsuJant "wir.do w."
If used to ~his end, the windo w macerial's ~atural sign~l c~n be aug m ented bv the
?ddi~ion cf a d~paDt Ihat prcdllces thc desired response. Ideally, the reference sp~tral
peak should be suffici~ntly remo~ed from the an~Ytical ~a~elength ta mi~imize
30 interfcren~e. But, it should be close enouF7h to yield sirr~ldr respons~ to exlraneous
influer.ces. Yarious oth~r tcchniques c~n be employed to p~eclude ~he suAa~e reflec~ion
~rom entenng7 th~ collec~on fibe~s.
This control of a sec~r,d-~urface window, or transparent encapsulant.
retlections is unique and ao~el. In ~,eneral, suppose the ~dius of c~rvatur~ of the
35 i'c~eond" ~u~er surf~ce is such that ~e s3urce f~ber is positioned aS the geometric c~nter
of curvature. In ~his collrl"ur~uon, sec~nd-surface rcflec~ions 3lising from light ~m~tted
by the s~urcc fibcr 3se direct~d baci; il-~o ~he fib~r. In practice7 significant de~iation of
the radius of c~rv:~lurc from ~!~e stated cri~er~on is ~cceptable. ThI~ fact is ba~ed on
A~lEN~ r'
. CA 0 2 2 4 8 912 19 9 8 - 0 9 - 1 1 ~ ' 3 -~
S~
dir~ing the reflzcted li~ht ;o ar~as s~ch aj source ~iber cladding. This g~ ner~ ic is
design~d to preclude r~flection enerC~y trom ~nte.in~ ~he detec.or fiber. To ~ ~c~ically
~ffectiYe~ r~ c~ed light c~ be directed 10 a l~tion o~lle~ than b;~ into the source
~i~er. ~cr e~Illpl2, it ea.~ merely be directed a-vay ~rom ~he collection f~bers. Ho~even
5 lirec~ing ~he ~~flections back into the sourcc Fj~ r's core 3nd.1~dcl~g is pr~f~ ~n,~ since
it ~rovid~s !he iQ ~ii 'light trlp.''
Sup~rior ~rform~n~e-is ~chieved in rnan~ medl ~ typ~ by c~nfigur.ng a
~ro~ a m~n~er that provides illumination ~nd receptio~ zones ~ ich ~r~ coincident
~overl~pping) on the second ( :listal) surf~ce of a transp~r~nt cnclGcure
10 (windowienc;~psul~lt) which interfaces the ~ dla As ~ cnlci~l co!~ition tO ~c~.ieve the
~uperiar pc.fo~n~nce, second-surfac~ rc~leclions (and first-surface) must ~ c~n~ro~le~
so they do not inadvertentl~ project into ~hl collection fi~er and under~ waveeuidin~,
If tbe~, an~er the ~ollec~ion ~!ber. they should be ~mgularl~ orien~- ~ vutside th~ li.nils for
ve~,uiding.
iC7~ ~7 provi~es an illustr2~ c r;?resent~tion ~ccomplishinc tt e ss~ted
~orOls. This embodimen~ ii cle~rl~ (iistinct from tha~ of Fig. 46, yet is operationally
si~ilar The dei~e3~ ber ~710 proj~,tC light thr~ugh tr e ~u~er conve~ vpac~l elem~t
~71i nd onlo, i~s cutcr surface. R~flec.ions fIom this surfac~ are principally dlrected
~ck into ~u'le fiber. Th~ colle~tion fiber ~7t~ h~, a rnat~ipu~t~d ,ield of view such ~at
~O i~ is ~eptive tc inter~ior.s 3t and beyor.d ~he optic~l element's outer ~urface.
I~n1ike the pro~es illus~1ted in F33~. 41 and ~ ~5, the embodi~nent oi
Fig. 16 effec~iYely caph~res si~nal from t.~.e Foin~ o~ ~irst light ccn~act with t~.e ~ample,
and beyon~. It is ll~)t only eff~ctiv~ lor d~.~ solids su~h as n~bber, but also for fin~
cryst~lline powd~rs. The*:rob~ is eff~cu~e on cryst~ ne powders, becAuse thc cryst~l's
planar surt~ces tend to orient ~hemselves along the surface. Thus, s~ecui~r reflection
from che cr-~st~!line powder's ~un~ac~s is direct~d in~o che centcr fiber'~ core ~nd bac~,
from wh~r2 it orig.inated. Fi~,. a,8 provides ~ ~ener~i understanding of how ~his
crys;~l1;ne powder phenomenon beha~s.
Fi" ~9 depicts two conf~ rations in which ~ source fi~er's illumirl3ticn
30 is direct~d o~cr ~ single collection fib~r. ~n on~ ca~e the ~ollection rlber's face is llat. ~n
tll~ o~her c~e, ~he c~llectio;2 fi~er is angled to produce a clos~r proximity o~ the
iilumin~tion ~ to ~he collection ~iber'i fa~e.
hg ~O d~picts Yarious ~ut-away aDd perspecliY~ views of an as~emblY
~ hich includes ~n optical end piece produci~g in~ernal retlection. This ~nd piec~
3~ performs slr.~arly to th~ end pi~ces yrev~ously describ~
Fig. ~ l ~ epicts lnothe~ riation in which s~urce iight is prolcc,ed lntO 3.
paLh ori~!lted ~or direct for c~llec~lon fiber rec~ipt. In this configuration. ~arious
co~ungs, such lS those used for sur~3ced e~ nced Ram~n spectroscopy, are reldily
A~ E~'J E' -
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51
applied to the optical surfaces through which the light passes or appf~,pl;ate materials are
introduced into the gap between the fibers. Although depicted as bare fibers, each fiber
can be encased or encapsulated in appl~fiate structures. For example, the structure may
provide environmental isolation, additional light manipulation elements, or materials
S which provide enh~nced photonic response to specific sensing conditions.
GRADENT REFRACIIVE lNDEX EMBODIMENT
Figs. 52a and 52b depict a fiber optic probe utili7ing gradient index optics
to bend illumination from source fibers to coincide with the field of receptivity of
10 collection fibers. In keeping with previously described principles, the center of the
gradient index waveguide is drilled to allow for passage of the center fiber. If an
alternate configuration is chosen in which the center fiber does not pass through a cavity
in the gradient index segment, increased stray light is collected. Optical matching
materials applied to the interface surfaces minimi7e this effect. Additionally, the distal
15 end face of the gradient index segment should be formed into a shape such that outgoing
surface reflections are directed for exclusion by the collection fibers.
In keeping with the principles previously developed in this document,
individual gradient index segments can be deflic~ted to and applied on to individual
fibers. And, as previously described, introducing a high-index region directly into the
20 fiber core directs the region of receptivity or illumin~tion as required for a specific
application. The distal and faces of the adjoining fibers may be directly filtered.
REFRACI IVE END PEOES APPLED TO THE PROBE TlP
Figs. 53 and 54 depict the optical components of probes in which light is
25 manipulated by refraction that occurs from light interaction with refractive elements.
Figs. 53a and 53b depict elements in which a hole is drilled and the center fiber is
inserted.
Fig. 54 depicts a cross section of a probe with a center fiber surrounded
by a ring of fibers. An element is fixed to the probe tip; in this case, it is a semi-sphere
30 whose radius equals half the bundle diameter. In addition to refractive effects at the
distal side of the element, this configuration yields another advantageous effect. With
the source fiber positioned in the center, unwanted, internally reflected light from the
radiused surface of the semi-sphere is directed back into the center fiber. Likewise, if an
outer, ring fiber is utilized in a source capacity, internally reflected light from the outer
35 surface of the element is directed away from the center fiber and towards the diametrical
opposing fiber. In a similar sense, the zones of receptivity are also advantageously
manipulated. For this probe to achieve refractive, beam-steering effects, the refractive
index of the semi-sphere must be sufficiently dir~lG~ te~l from the application medium.
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Similarly, if the refractive indices are sufficiently removed from one another, total
internal reflection occurs at the outer, side portions of the spherical element.
PROBES EMPLOYING LIGHT MANIPULATION BETWEEN
S ADJOlNING FIBER SEGMENTS
Figs. 55a-f depict various aspects of probes employing light-
manipulating artifices between adjoining fiber segments. The depicted probes areconfigured with a ring of fibers surrounding a center fiber. The elements between the
fiber segments modify the light entry/acceptance characteristics to create application-
specific performance advantages.
An in-depth discussion of the operation and theory is presented within
this document. Additional descriptions and specifications are presented in U.S. Patent
Application Serial No. 08/561,484, entitled '~Optical Fiber with Enhanced Light
Collection and Illumination and Having Highly Controlled Emission and AcceptancePatterns," filed November 20, 1995. In light of this background, the operation and
construction of these probes is clear.
By utilizing the principles and methodology presented, embodiments are
readily op~ill~ized for specific applic~lions and desirable effects.
Fig. 56 illustrates cross sectional and perspective views of a similar
assembly which utilizes an end piece to create similar performance results.
Fig. 57 depicts a probe configuration which is in keeping with principles
developed within this document and utilizes the light-manipulation methodology
developed within this section. This type of configuration is particularly effective for
probe deployment in applications such as biological tissue and other complex matrices.
SEPARATE METHODOLOGY
Fig. 58 depicts a method of enhancing overlap between the source fiber~s
delivery beam and the collection fiber's zone of receptivity. In this depiction, a channel
is formed into the collection fiber. The source fiber is bent and placed into the channel.
Thus, the source fiber's energy is not only directed into a zone of receptivity, but also
the fiber is almost fully encased by this zone. By utilizing small delivery fibers, light
loss at the fiber bend is minimi7~.rl Variations on this theme include a hollow collection
fiber with the source fiber formed into the center cavity. This configuration offers the
disadvantage of manufacturing difficulty and susceptibility of damage to the fibers.
A S~GLE ~ ER EMBODIMENl
Fig. 59 illustrates a single fiber adapted according to the presented
methodology and achieving previously lln~tt:~in~hle performance characteristics. In this
5UgsmulESltEET(RuLE 26t
CA 02248912 1998-09-11
WO 97/34175 PCT/US97/04365
embodiment~ the fiber's illumination/collection characteristics are manipulated while
maintaining symmetry about the fiber's ]ongitudinal axis. A fiber with a numerical
aperture of .~ is adapted to achieve an effective numerical aperture of .63.
Silica core/silica clad optical fiber typically has a numerical apelture (NA)
of .22. This is a common fiber type employed in diverse applications and is the variety
depicted in the ill~stration. NA = .22 corresponds to illumination/receptivity field
diverging in air with an included angle of 25 degrees. In a medium whose refractive
index matches that of the fiber's silica core (approximately 1.46). the divergence angle is
l 7 degrees. Equivalently, the light is oriented within a range of +/- ~.5 degrees from tne
fiber's longitudinal axis. And, this is the orientation of the light propagating within the
fiber under fu~y filled conditions. The patterns and angles are depicted in the
illustration.
Fig. 59 al.so illu~trates the pat[ern~ created as a re~ult of the adaptation
The resulting illuminatioll/collec~ion patterns corresponding to the new numerical
lS aperture (NA = .63) are also shown. The included angle is 78 degrees in air and 51
degrees in medium with refractive index matching that of the fiber core.
To achieve the desired result, a cone is shaped into the fiber tip. The
included angle of the cone is 17 degrees. This angle purposely matches the propagation
limits for light within the fiber. In this case, those limits are +/- 8.5 degrees. By this
geometry, light striking the sidewall of the cone is reflected to headings between 8.5 and
25.5 degrees and directed inward towards the fiber's center axis. The internally reflected
light combined with the light that has not interacted with the cone sidewalls creates a
diverse angular population of light rays spanning +/- 25.5 degrees (51 total degrees).
The light enters/exits through the flat, planar portion of the fiber end face
that is typically created by grinding and polishing the cone tip flat. In the illustration, the
tip is flattened only to the extent that the rays reflected from the fiber sidewalls fully fill
the inlet/outlet flat section. Simple ray tracing or algebraic equations are employed to
fulfill this geometric condition. By tracing the rays contacting the cone section at the
point of transition from cylinder to cone, the condition is easily met.
In simple procedural terms, the fiber is first drawn to scale. Next, the
cone is drawn as specified. Three rays are drawn impacting the cone at the point of the
fiber's transition from cylinder to cone. These three rays represent the fiber's normal
angular limits for propagation and the average (+8.5, 0, and -8.5 degrees in theexample). Next, the corresponding reflected rays are drawn. The cone is graphically
flattened until the desired effect is achieved. In this case, the reflected rays fully fill the
entrance/exit aperture.
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54
Since actual performance deviates from the theory due to influences
including the fiber's fill factor, experimental tests must be conducted to finalize the
design specifications for a specific application.
If the fiber tip is created with a smaller inlet/outlet section (equivalently a
longer cone section) the "sidewall" light strikes the opposing wall before exiting. In this
manner, the illumination/collection pattern open wider. Similarly, if the flat section is
larger, the percentage of illumination diverging at wide angles decreases.
As described in previous text, the angle of the conical portion of the fiber
may be manipulated to create various effects. By increasing its included angle, a halo
pattern is created by the "sidewall" light and two distinct illumination patterns are
created. Similarly, by creating compound tapers (segmented cone angles) the NA can be
further increased. And, this principle is taken another step by creating a complex
contGured surface such as a paraboloid of revolution t'or the internal re~lection.
The fiber tip is packaged and encapsulated in accordance with previously
described methodology and/or application specific dictates. Fig. 60 provides a
perspective view of the a complete termination assembiy.
In keeping with the multifiber probe embodiments. several adaptations
are readily incorporated. The light-manipulating surfaces are readily formed into an
element which adjoins a standard, flat-faced fiber. This adaptation offers fabrication
advantages, in terms of facilitating rnass quantity manufacturing of reproducible
components.
The element is formed as a flat-ended cone (a frustum of a cone). This
element is fixed to the end of a flat-faced fiber. By recessing the flat-faced fiber into a
fiber optic connector, or similar tube, the element is readily attached and aligned.
Similarly, the special end face is formed into a short segment of fiber
(typically a few mil~imstçrs). One end of the fiber segment is flat faced; the other end is
shaped into the modified cone. This segment is adjoined to a standard, flat-faced fiber.
The best method of attachment i~ to place the fiber into a needle, or capillary, tubing with
the flat portion of the fiber's syecial end face parallel with the end of the needle tubing.
If desired, the fiber can be inserted and fixed into the tubing while it is in the full cone
condition. Then, the tubing and fiber are ground and polished to create the planar
section. The flat end of the fiber must be free from contamination and recessed into the
tubing so that an internal cavity is created. The male fiber is then inserted into the female
end cap. It is attached with epoxy or similar bonding agent. With an optically clear
bonding agent, migration of the agent between the fiber end faces does not result in an
inefficient junction. In fact, the properly chosen agent minimizes transmission losses by
acting as an optical "matching" material. Thermal melting adhesives are particularly
CA 0 2 2 4 8 91 i 19 9 8 - 0 9 - 1 1 ~ ir
u~e~l if ch~en to wi.hst~r~d the ~pplicltion's environm~nt conditions. For ~hi~,rluoropolymets are well suited.
DELI~ER~ AND COLT ECTI:~G LI~H~ .~LO~G .~ CO~ O?~J
~YIS
T~.~, techu~:c. ~.e~erio~ ~hove m~y be usc~ ~o Ccnstruct ~ ~rabe
3ssem~1y ~ui~able fGr :ieliYering monoc~rolnatic light ~nd col~e~ùng wav~ngth-snirte~
li~ht ~long a co~nmon a~is~ ~ ~robe of chij tYpe is advanta~e~ls because it pro~ es
approximate concurrence be~ween the optic~l axis of the delivery lig~t pattern 3nd th~
10 optic~ of ~he collection fieid-of-view. Tbe probe .~iniInizes the number and size of
~he ~lements ~'optic~l and ;nech~nical) req~ired for the asscmbly and elim~nates ~h~ need
for exP~Ddcd be~r,q opti~al ~le~ent~ in the primary light-delive~y scheme. The probe
aiso mj~imi7eS th~ e~tent of r~f.active inde~ intert~c~s wilhin ~he op~ical ~vstem~ ~hich
can cause reflecuon within th~ ~se~bly. Puno~her ~dv ntage ot *.e probe i~ it c~n be
1~ u~e~ ~o deli~/er li~ tc .~.nd collect light ~rom :~n investig-a~ive medium withollt nec~ of
~cc~sin~ ele~nents ~nd, ~hen advantageotis, c~n be dir~ctly inserted into the in~cs~igati~e
med~um. The need ror these attribu~es is stron~ in applic~ion~ ~uch as biomedic~!.
process control~ down hole oil ~ , ccmF~site cunng, poiymcri~aùon re~cticns.
~cientific rei~arctl~ an~ ny cimil3r ~FFIic~ions. A device, me~ting the d~fined
~O objectives is p~icul~rly ne~d to on~ble ph~tonic applic3tions in biomedical usages. T~is
probe configura~io~ offers ~he adv3nt~ges ~f the prior art conf~c~l devices wi~hout the
.nhernt dr~wbacka.
Fi~3 ~1 an exemplary probe 6100 ~or deli~ering and collectinc light
~30ng a common ~is. The wa~elen~ 7 intensi~ie~ represented in the figur~ a~e mer~y
'S ili~cE;a~ e ~f the ~e~eral ~u~cticnalitt~; Eh~ ~cnual inrensi~ie~, relauve to the l~ser (or o~her
monachroma~c SGUX5) l~aveleng~h are ~y~ic31iy extremeiy ~,veak.
In oFer~tion, highly mcnochrom;~tic laser li~ht i~ launched into tlle
proximal end of the deli~ery fiber 6103~ ~s the light 6101 is uided tow~rd t~.e
1p~aratus tip, its wavelerJg~h purity degrades due to iigh~-ma~ter in~eractior!s with the
30 ~ r'S materials which produce w~vel~oth~shifted light 6lO2. This "silica-R~man"
interference light is due to R~rnan scat~ring an~ er flu~resconce; it is nct sFecific t~
silica materiais. The ~aveguid~ng n~lure qî the fiber accumulates t~i~ e,~t~neous light,
whlch can interfere with ma~eriai ~nalysis techni~ues 3uch ~s fiber-cptic-based l~ser-
R~man spec~roscopy. ~nterfering light c~n ;llso ~rise frcm o~her sourc~s, such ~s laser
35 in.stabi]ily (mcdG hopping) and am~ient light sources entering the pathway The l~ser
li~hr centcred ~t wa~leng~h ~ and the ~vavelenglh-shifted interfere~ce ligh~ 61~2 is
trm;m~tt~d to the delivery ~ibcr filt~, 6lO5. T~,~ bar d-p~ss fil~er 6105 passes the lascr
CA 02248912 1998-09-ll
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56
light 6115 with only minimal transmission of the interference light 6116. The off-
wavelength, interference light 6110 is reflected back off the filters 6105.
Instead of orienting the filters 6105, 6180 perpendicular to the fibers
6103, 6108 axes. orienting the filter 6105,6180 at an angle offers advantages for
certain applications. Referring to the delivery fiber filter 6105 for simplicity of
explanation, it can be angularly oriented greater than the fiber's angular propagation
limits (approximately 8.5~ for silica core/silica clad fiber with .22 numerical aperture).
As a result, the back reflected light cannot be back propagated towards the source. This
is useful for several reasons. First, on very long fiber runs. a single, distal filter may
not be sufficient. In this case~ it can be advantageous to provide filters at interval
distances along the fiber run so that a fiber segment within the run has filter~ on both the
distal and proximal ends of the fiber segment. If the rejected interference light at the
distal end of a fiber inter~al i.s allowed to bac~ propagate within the fiber. then it v~ill
encounter another filter at the proximal end of the fiber interval and be reflected back
] 5 again. Thus, the interference light can be trapped between filters. The filtering scheme
loses its effectiveness as the trapped light passes through the filters; nonlinear effects
also contribute to problems and corrupt the system performance. By angling the filters.
the back reflected light is rejected outside of the fiber's angular propagation limitations.
Second, back-reflected laser light, due to normal filter inefficiencies, increases the laser
power intensity within the fiber. The additional laser intensity generates additional fiber
interference without benefit of increased laser power delivered to the sample. Although
this reflected laser is traveling away from the sample, it generates multidirectional
interference light within the fiber core, which, in part, travels towards the sample.
Therefore, the back reflected laser light should not be allowed to propagate. Third,
lower-efficiency, less expensive filters, which inadvertently back reflect a larger
percentage of desired light can be used. Fourth, back reflected light can interfere with
the laser's stability if it is back propagated into the laser. However, for short fibers and
stabilized, isolated lasers, this angled-filtering technique is typically not required,
especially if high-efficiency filters are utilized.
As the light 6120 travels down the distal fiber segment 6162, a small,
but increasing, amount of interference 6121 is present. The light 6120 is incident on
the angled filter 6125. This filter is a notch (band stop) filter that reflects the laser light
6130 outward along with a small portion of unwanted interference light 6131.
The majority of the interference light that is incident on this filter 6125
passes through the filter 6125 along with a small portion of laser light 6135, 6136.
This unwanted light 6135 must be eliminated from the signal path. Several methods are
useful depending on the application environment. This light can be allowed to simply
exit the assembly if the surrounding materials do not tend to re-introduce it into the return
h~ \ " i ~ 1.: ' ... I ~ ". . . .
CA 02248912 1998-09-11 --
~i"ht p~th. l~e pTef~rre1 nlethod is t~ fill in the c~vi~y s~rroundin~ the filter 61~ with
optically trar.sFlter.t m~teri~l, such as 1 sili~a plug or cle~r epoxy 6140. .~ IiCht-
absorbing ,~tion 61~5 ~f~r e,~rnpIe ]amp-bl~k-loaded epoxy) ~n the dist~l sid~ ~f ~r.e
tran~p~rent section 61~ tr~ps ~nd attenu~les the unwanted lighl 613j. Prefer3bi~., the
5 a~sorbing sectic~l 61~15 h3s s.mil~r refr..~ e index to that of th~ transpa~es~t sec;,or.
61-tù so that iU~T'-e re~1ecti~n is ~ i;nized. Other li~,ht-trapping confi~urations c~
utili~ed. By sh~ping the lin31 surf3ce ~t an an~le, or pointing it iike a conc, interfererlc~
frcm surf~ce reflections is minimi~ed ~nd the assembly's inse~tion ability throueh
bi~logic31 materi~s i~ also ~mFro~
The filter 61~5 directs th~ Fure l~s~r light 6130 tl~rough the fi~er
sidewall ~d into the invesllgative rr~edium. The l~er inter~c~ ith the m~ n throuPh
elas~ic and inelastic processes 6150. ~hus. Iigh~ Nhose sp~ctral composition
~56 inciL:des the lase r wavelen~th 6110 ~nd shif~ed waYeIellgths radiates bac~cthrough the t~ibcr side~ll 3nd is incident cn ~he tllter 61~;. Thc filter 61~ ~asses th~
nwanted ;ight 6160 spectr~lly comprised 6161 of laser li~ht and ~ smalI poltion of th~
desired, ~aveleng~h-shiftea lieht. Th~ fi]ter 612~ pas,es the desired light 6165 which
is illeident on reflec~ive surface 617S m~ is directed for prcpagasion 6170 to the
d~tec~or. The ~i"h~ 6170 is f~ltered aga~n wlth ~ nocch ~ban~-stop) 618~ to elirnina
re.sidual, unw~nted light 5163 att~e laser ~veleng.h G110. Thc residual light may be
~0 due to suc~. factors as cross talk ~1~0 and ;mperfect t;itenn~, In keeping with the
d~scribed re~soning, ulting the filter 61~0 c~n imorove performance. ~he residual l~ser
ii~ht 61~3,~16~ i~ filtered out here to prev~nt it r~rcm ~ener~tin~ interference light in
the main run 6108 of the col3ection fiber and ~o rn~nimize filter.ng requiremlcrlts at the
de~ctor. Pure. w~Yeleng~h-shifted 6172 ligh~ 6171 is guide~ to the detect~r over rhe
'~5 main deli~ery fiber 6108.
For systems ar.alyzing ~ç inves;ig~ti~e medium by S~okes-shlIt liPht
processors, a 1O~4-p~lss filtcr can be utiii7ed in place of the band-~ass filter 6I05 of the
delivery fi~er. Likewise, a hi~h-p~ss filt2r can b~ used on the collection fiber in plac~ of
~he notch filter ~180 And, a hlgh-pass filter c~n l~e us~d ~s th~ tip filter 6125.
The filter fiber segmen~s 6162,6173 are best ~ehed to the main fiber
segmerlt 6103,6108 ~ holl~w sl~eve (noL depicted in drawing). For ~naximum
perIormance, the sIce~e must be ~ized ~or precise ~ Jnmént. I his may be ac~ie~ed by
~ppin~ the usldersized ne-dle mbing to the preci~e ba~e. C)r, the tubin~ c3n be fabricated
~y electroforrnin~ me~al over a pr~perly sized fiber ~hen remo~in~ the fiber. A split
s]eeve, slight]y undersized and spring tem~ered so th~t it firm)y hold~ th~ fibers and
achie~e~ extreme alignment between f1ber segTnents, is an ;~l~emative The junction c3n
be ~cund toge~he~ wit~ optic311y transp~rent e~c:cy which pro~/ides mechanicaL integrity
and aids coupLirlg effieleney. C3re must be t31cen ~o ensure that the epoxy does not
1~3~!S
CA 02248912 1998-09-11
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58
fluoresce. For certain wavelengths~ the fluorescence is unavoidable. In this case~
matching gel may be used to ensure the efficiency of the junction. In most instances, the
index matching compound is not required.
The area of the assembly through which light is delivered to and collected
S from the investigative medium can have a reference material applied to it.
The cross talk 6190 can be minimized by metalizing the fiber's outer
sidewalls in areas not requiring through-the-wall light transmission. Similarly, an
opaque foil can be utilized to block cross talk.
A low numerical aperture fiber, such as sin~le mode. can be utilized for
the primary delivery fiber 6103 run so that the percentage of the gener~~ed silica-
Raman/fiber fluorescence light which is accumulated and wave~uided minimized.
This also allows for a shal-per filter pertormance since it delivers light to the filter with
reduced anrular deviation.
The area of the assembly throu~h wilich light en~ers alld e~its th~
in~estigative medium can be treated with an anti-reflective film~ such as magnesium
fluoride, so that reflection is minimized.
The collection fiber 6108,6173 can be larger than the delivery fiber
- 6103,6162 or comprised of a fused or unfused bundle of fibers so that collection
efficiency is bolstered.
The main delivery fiber 6162 can have a smaller core than the delivery
fiber tip segment 6162 such that mechanical alignment sensitivity is improved and the
light transmits through the center of the filter 6105 so that filter performance is
maximized. The main delivery fiber 6162 can also have lower numerical aperture so
that filter performance is increased and less interference light is generated within and
waveguided by the fiber 6162 .
The reflective surface 6175 of the collection fiber 6173 may be
internally reflective by a metallic coating or dielectric (stack) reflector or even be a filter
itself.
The length of the distal fiber segments can be very short so that they do
not provide significant waveguiding performance. In this manner, their interference
contribution is rninimi7eci
Fiber materials are preferably silica core/silica clad with a coat such as
polyimide, which strips cladding modes.
As an alternative to directly applying the filters to the fiber end faces, they
may be applied to a thin wafer which is placed up to the fibers' end faces or permanently
attached.
The main delivery fiber lO0 can b~ filtered with a Brag filter, which is
preferably applied to a single mode fiber.
CA 02248912 1998-09-11
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59
One or both of the filters 610~, 6180 on the delivery and collection
fibers can be elimin~t~ d if reduced pelroll,lance is acceptable.
Projection optics or light pipes can be utilized to project the investigative
site further--for example, through a window.
The cylindrical surface through which light enters and exits the
investigative medium can be reshaped to minimi7ç distortion by flattening with heat
softening, grinding flat, or building up the surface with optically transparent n-~tçri~l.
The assembly can be utilized to monitor parameters within the human
body.
One, or both of the end fiber segments 6162, 6173 can be formed of
high purity sapphire or similar material such that hardening against physical and chemical
hostilities is bolstered. They may also be hollow, waveguides such as tubes internally
coated with reflective metallic coating.
The optics can be housed in various bodies to accommodate application
15 parameters.
The assembly can be utilized for in~ nta~ion analyzing inelastic light-
matter inte~ ons such as Raman and fluo,escen~e.
Fig. 62 illustrates a fiber assembly 6200 in which light enters and leaves
the assembly essentially parallel with the optical fibers' axes.
The depicted assembly operates similar to the assembly in Fig. 61 (as
indicated by common element numbers) with several important differences. The source
light 6220 in the delivery fiber tip segment 6211, which is incident on the distal filter
6225, functions differently. The distal filter 6225 is a band pass (or low pass for
Stokes-shift analysis only). This filter 6225 allows the laser line light 6230 to pass
through, unimpeded, into the investigative medium. The interference light 6235 is
rejected by the filter 6225 and directed outward through the fiber sidewall. (By treating
this area with absorbing material. the inte.l~re.lce light can be trapped). The laser light
6230 that is transmitted through the filter 6225 passes through the transparent region
6240.
This region 6240 may be comprised of transparent conformal material
such ~s epoxy, loaded with a reference standard, filled with investigative media, or
plugged with a solid glass, sapphh~, or similar piece. This area may even be joined with
a short waveguided segment, although interference accum~ çc rapidly in even a short
segment of optical fiber (more than a few inches).
Regardless of the material, if the outer surface of the region 6240 is
opelly shaped (beveled, coned, or similar), back reflected light from the outer surface
will be less prone to inadvertent propagation to the detector. Regardless of the outer
surface shape, the surface should be finely polished and is preferably treated with an
CA 02248912 1998-09-11
WO 97134175 PCT/US97/04365
antireflective coating. Ideally, the refractive index is m~tchecl to that of the investigative
medium.
After passing through this region 6240, the laser light 6230 interacts
with the investigative medium. Elastic and inelastic light 6255 returns from theinvestigative medium and is incident on the filter 6225. The majority of the elastic (no
wavelength shift) light 6260 passes back through the filter 6225 and is reflected again
by the internally reflective surface 6275 for propagation to the detector.
This configuration is well suited to operation in conjunction with
projection optics which facilitate capturing information at a stand off or through a
window. However, superior light-coupling performance is achieved by direct insertion
of the device into or onto the investigative medium. Similarly, the delivery/collection
field can be directed to the side with a mirror, prism, or similar optical component which
may be directly attached to the distal end.
The assembly can be adapted for and the methodology is applicable to
15 producing a similar image-acquiring device.
As with the side-viewing embodiment, this assembly can be produced
with micro-scale optical elements attached to fiber end faces. A defining aspect is that
the delivery and collection pathways are not significantly interrupted to allow for
exp~n-ied bearn optical elements.
The filters may be formed directly on the fiber end faces or first applied to
a wafer intimately associated with the fiber. Direct application is preferred. For
maximum performance, the filters should be high quality. However, depending on the
application, lower performance filters are possible since one objective of this assembly is
to minimi~e the need for elaborate filters. Nevertheless, better filters correlate to
25 deployment of light-based characterization in previously un~tt~in~hle applications. The
highest performance filters are produced by processes which create high-density thin
films. These processes include ion beam spull~ling (single and dual beam), ion plating,
magnetron sputtering and to a lesser degree non-assisted deposition. Not only is high
efficiency achieved, but also environmental stability is bolstered. Since filtering losses
30 equate, in part, to increases in back reflected light, and the det~ lents of back reflected
light have been explored, efficiency is an hllpol ~nl factor.
By butting a capillary tube cont~ining the investigative medium up to the
optical inlet/outlet, signal strength can be enh~nce-l The capillary tube should have a
lower refractive index than that of the liquid sample. (Or be internally reflective).
35 Biological fluids such as blood plasma, urine, amniotic fluids, and cerebrospinal fluids
are well suited to this method. Dupont's Teflon F~P fluoropolymer is a suitable material
for some fluids due to its, relatively low refractive index. Many of the common
fluoropolymers have low refractive indices. However, the commonly known ones are
CA 02248912 1998-09-11
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61
higher than water and many aqueous solution. Water's refractive index is approximately
1.33. Hence, for aqueous solutions, a novel methodology is preferred. The capillary
can be formed of Dupont's Teflon AF amorphous fluoropolymer. This material is also
- the preferred overcoat film or encapsulant to induce total internal reflection a the surface
S of the optical fiber contours which are described throughout this document.
- This fluoropolymer may also be coated on the inside of a glass (silica)
capillary tubing with internally diameter approximately equal to that of the delivery fiber
end segment. The internally coated glass is preferred due to its advantages for reduced
fluoropolymer cost and increased rigidity. This amorphous fluoropolymer is best
10 applied to the capillary tubing in the solvent-dissolved state which is available from the
manufacturer. The polymer-laden solvent can be swapped repeatedly through the
capillary until a uniform film is built up. The film should be at least five microns thick
for operation below 1000 nanometers and ten micron for near infrared usage (a thickness
of approximately five percent of the tubing's internal diameter is best). The preferred
15 method of polymer application is to add the mixture to the end of the capillary tubing
while spinning the capillary lengthwise so that the mixture is forced down the capillary
and a consistent coat is achieved. The solvent is driven off in accordance with the
suppliers standard usage guidelines while directing air flow through the inside of the
tubing. For volume manufacturing application and long capillary lengths, the polymer is
20 applied as an internal film during the glass capillary manufacturing process. Added
performance will be achieved with increased length of the capillary up to the distance
through which light is attenuated. The maximum beneficial length is dependent upon the
absorption of the investigative m~ium at the analytical wavelengths.
DELIVE~RING AND COLLECT~G LIGHT THROUGH A COMMON
APERTURE
The techniques described above may also be used to construct a probe
assembly suitable for delivering monochrolllatic light and collecting wavelength-shifted
light through a common aperture. The utility and adaptations for broadband usageapplications, as opposed to monochromatic, are readily seen and are based on theteaching presented herein. A probe of this type is advantageous because it is capable of
inducing and ca~Lu~hlg light-matter response interactions at the surface interface (contact
plane) between assembly and investigative medium. This is particularly useful in media
that exhibit optical propagation difficulties, such as absorption. By varying parameters
within the described configurations, the investigative depth beyond this contact interface
is readily selected in accordance with application re4uile."t;n~.
This characteristic is in contrast to devices that are only responsive to
light-matter interactions occurring at a depth within the investigative medium. As a result
SU~I~UIE SHEET ff~ULE 2*
-
CA 02248912 1998-09-11
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62
of this characteristic, the exemplary probes are capable of generating and capturing light-
matter interactions in fully opaque materials. Similarly, information from thin layers and
films can be acquired. For example, a film on, or layer of, any material (opaque or
transparent) can be monitored with direct contact or minim~l standoff. Furthermore, an
5 indicator layer can be applied to the assembly surface through which light is transmitted
to the subject and responses in this layer can be captured. As another special case, an
jm~ging fiber optic probe assembly can acquire im~ging information from a surface
while in direct contact with the surface; this characteristic is particularly valuable for
medical endoscopes.
This arrangement is also advantageous in the sense that specular
reflections, which arise as delivered light passes the refractive index interface of the
boundary between the fiber optic assembly and the investigative medium, are controlled
to advantage. In applications in which specular reflection would interfere with the
collection of desired light-matter response, its collection is minimi7.ed. Specular
reflections are directed outside of the collection fiber's angular receptivity limits,
waveguided for back propagation within the source fiber, and/or projected away from
the collection fiber. As a special case, the micro-surfaces of crystal powders align
themselves with the surface such that unwanted collection of specular light from these
surfaces is ",i~li",i~e~ In applications in which the specular interactions are of interest,
the assembly's configuration is readily tuned to optimize specular collection and
subsequent analysis.
In addition to the foregoing advantages, the number and size of the
optical elements are minimi7ed, and the optical elements within the delivery andcollection paths are configured to minimi7e and control surface reflections from these
elements. As a result, optical efficiency is maximi7~A, and con~min~ion of collection
light with stray light interference from delivery light is minimi7f~d Reliance on image
trains of expanded beam optical elem~.ntc is also elimin~ted.
As in the devices discussed above, by selecting the delivery and
collection angles of light, the devices are readily configured to exhibit selective
sensitivity to specific photonic mech~ni.cm~ that are angularly biased. For example, the
perc~ntage of captured inelastic light-matter interactions, such as Raman and
fluorescence scattering, can be increased relative to that from elastic processes such as
Rayleigh and Mie-sc~Llt;fillg and specular reflection.
Fig. 63 is a cross-sectional view of a probe in which a center fiber 6375
is surrounded by a ring of fibers 6380. The center fiber 6375 is utilized to deliver light
and the ring fibers 6380 are utilized to collect light. This is the O~)tilllUIII configuration
for laser-based analysis since the quantity of laser light injected into the delivery fiber
6375 at its proximal end (not shown) is not typically limited by fiber size. Other light
CA 02248912 1998-09-11
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63
sources, such as broadband lamps, are less effectively coupled into single fibers and are
often better coupled into fiber bundles. Therefore, in many non-laser applications, the
ring fibers 6380 are utilized to deliver light. This configuration offers the additional
characteristic of angularly rich (diffuse) illulllilla~ion of the investigative medium 6320.
S In operation, light 6301 travels from the source down the optical fiber
towards the distal end face of the center fiber 6375. In the illustration, the assembly is
configured for laser-Raman spectroscopy. As such, the source light is highly
monochromatic laser light. Through interactions with the fiber core 6310, the chromatic
purity of the light 6301 is degraded. The band pass (or optionally high pass filter)
6355 rejects the unwanted, off-wavelength light so that highly monochromatic light
6301 is introduced to the measurand 6320. Specular reflections, arising as light rays
6301 cross the refractive index interface between the center fiber 6375 and the media
6320, are angularly oriented within the fiber's limits for angular acceptance. Likewise,
in the depicted Raman configuration, Mie-scattering is predominantly directed backwards
within the center fiber's waveguiding capabilities. Raman-scattering events are in~lucec~
at the surface of the investigative medium 6320 and beyond (depending on the opacity
of the medium). These events produce light rays that enter the center fiber 6375 end
face, travel through center fiber core 6310, pass through 6340 center fiber cladding
6330 and travel into the collection fiber 6380 and intersect with internally reflective
20 surface 6345 and are re-oriented for propagation within the collection fiber 6380 for
tran.cmi.csion to the detection system. The collection fiber filter 6360 rejects the laser
light via band stop (notch) or, alternatively, high pass.
The ring fibers 6380 are polished at an angle 0 to create an internally
reflective surface 6345. The ring fibers 6380 may be individually faceted such that the
internally reflective surface 6345 is planar (flat) or preferably, and as illustrated, they
are contoured so that they collectively form a frustum of a cone. The fibers are stripped
of their protective coatings/buffers 6335 near the distal tip such that tr~n~mi.csion of
applo~liately angled, desired light 6340 between the fibers 6375, 6380 is not
encumbered. The bundle is held together by optically transparent bonding agent or are
fused together. When bonding agent, such as epoxy, or inorganic cement (binder) is
used, its refractive index should be close to that of the fiber cladding 6325, 6330. By
approximately m~tching the refractive indices, the influence of refraction on desirable
rays 6340 passing between delivery 6375 and collection 6380 fibers is minimi7f.~1
In some applications involving transparent liquids, the fiber bundle can
be bound with only a thin heat shrink (preferably Teflon). The fluid medium can creep
into the voids between the fibers and serves to transmit light rays 6340. Regardless, a
certain degree of refractive index deviation is acceptable as the negative influences of the
distortion is minim~l. Although the refractive effects can be modeled and the
SUBST ~UlE SHEET (RUlE 26)
CA 02248912 1998-09-11
WO 97/34175 PCT/US97/04365
64
configuration optimized to compensate, a trial-and-error approach is sufficient for a
given set of materials (fibers and epoxy). The contour angle 0 is readily changed until
desired results are achieved. A visual method is valuable in optimizing the
configuration. By sending light down the ring fibers 6380 towards the distal tip, it is
bent at the internally reflective surface 6345 and redirected towards the center fiber
6375 end face. By placing the assembly into a bath, the e~ ging light can be viewed
under magnification. For white light, the bath can be composed of water with a trace of
fluorescence indicator. Alternatively, the batch can be composed of water with a small
quantity of scattering agent, such as titanium dioxide. The water's low refractive index
10 (approximately 1.33) yields total internal reflection at the internally reflective surface
6345, and thus, the need for coating 6365 is elimin~ted for visual testing. The light
patterns are readily inspected by conducting these tests in a clear container with flat
sides. A common aquarium is suitable; a cell culture flask (available from most
laboratory supply houses) is ideal. This method is adequate as long as the angle 0 is
15 within the limits for total internal reflection given the other optical p~alllcLt~
Internal reflection at surface 6345 may be generated by several means.
Total internal reflection will result if the medium contacting this surface 6345 has
sufficiently low refractive index for the other relevant parameters (angle 0~ refractive
index of fiber core 6315, and angular propagation limits for waveguided light 6305).
20 Typically, angle 0 will be between 45~ and 90~. In certain applications, such as
monitoring of solid surfaces, the surface 6345 need only be exposed to air to create the
conditions for total internal reflection. However, this open-air approach is less robust
than is often required as the tip is mechanically delicate and losses can be created by
cont~rnin~tion of the optical surface 6345. Still, the open-air approach is acceptable,
25 and even preferred, when the device is utilized in pristine environments such as clean
rooms - especially when precision~ tom~te(~ equipment is used to delicately position the
device against the m~asu,dl d.
Similarly, if the angle 0 is high (typically 75~ - 85~ for all silica fibers)
then total internal reflection is in~ ced in an aqueous medium when the surface 6345 is
30 bare. If this approach is taken, the aqueous media should not contain absorbers, which
can "frustrate" the internal reflection.
The surface 6345 can also be coated with a low-refraction-index film
6370. Although magnesium fluoride can be applied through various thin film
deposition techniques, it is difficult to provide a sufficiently thick coat to ensure the field
35 of the collected light rays 6305 do not extend beyond the coat 6370 and become
frustrated by adjoining materials. Several fluoropolymers are capable of forrning this
film. These include those known by the trade names FEP Teflon, PFA Teflon, TFE
Teflon, Teflon AF and Tefzel - all manufactured by Dupont. Some of these polymers
CA 02248912 1998-09-11
WO 97/3417S PCT/US97/04365
are available from other manufacturers under various trade names. Of these polymers,
Teflon AF Amorphous Fluoropolymer is superior and FEP Teflon is next best. Teflon
AF, s~metimes referred to as amorphous Teflon, has the lowest refractive index, adheres
well to the surface 6345, and is optically transparent for most wavelengths of interest.
5 It has proved to provide excellent results on angles 0 as small as 70~. Furthermore, it
exhibits excellent p~o~e.lies for chemical inertness. The procedure for applying the
Teflon AF follows.
Although the Teflon AF can be used as a melt extruded solid to
encapsulate the assembly, applying the polymer in a dissolved solution is more
10 economical and is better for short fabrication quantities. The assembly tip is dipped in a
solution of Teflon AF (6% perfluorinated solvent (CS- 18) 4,5-difluoro-2,2-
bis(trifluoromethyl)-1,3-diolole (PDD), polymer with tetrafluoroethylene) ~lesign~ted by
Dupont as Teflon AF 1600. Dupont's Teflon AF 2400 is also acceptable. Other
percentage solutions are also acceptable. The assembly is allowed to air dry and then
15 dipped again to build up a coat. The assembly is then allowed to air dry thoroughly
(about 10 minutes). Next, the residual solvent is driven off by baking the assembly at
approximately 112~ C for 5-10 minutes. The temperature is raised to 165~ C for five
minutes. The te,l,pelatulc is raised to 265~ to 270~ C for 15 minutes. At approximately
240~, the Teflon melts, uniformly coats, and adheres to the fiber surface 6345. The tip
20 is in~pected under m~gnific~tion for anomalies; if present, the procedure is repeated until
an acceptable coat is established.
Following the application of the polymer, the delivery fiber 6375 end
face should be re-polished to remove the polymer from this surface. To increase the
hardness of the assembly, prior to this step, the bundle may be inserted into a tube filled
25 with epoxy so that the entire assembly is encapsulated. The end of the assembly is then
polished to expose the bare fiber core 6310 of the center fiber 6375.
As another alternative, a thin, internally reflective film of mçt~1i7~ion can
also be utili7.~.d It has the advantage over the Teflon AF coat in that it is not limited by
the angle 0. And, all metallic reflectors are less efficient than total internal reflection.
30 Therefore, the metals should be reserved for conditions (such as small angle 0) in which
the Teflon AF approach will not produce adequate performance. For reflection in the
near infrared, gold is an excellent reflector, and it also resists chemical attack. Since
gold films do not adhere well to silica, a thin, essentially transparent layer of another
metal must be applied as an undercoat. Chromium is suitable for this purpose it adheres
35 to the silica, and the gold adheres to it. Although chromium has reduced reflectivity in
the near infrared, this interme~ tç layer is very thin and does not significantly degrade
reflection efficiency. For ultraviolet-visible light, alulllinulll works well. Silver is also
suitable for visible and near infrared. However, neither silver nor aluminum exhibit
CA 02248912 1998-09-11
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66
good resistance to chemical attack and should be avoided in harsh environments. For
harsh environments, rhodium or platinum is preferred in areas of the spectrum where
their reflective pr~elLies are acceptable.
As yet another alternative, a dielectric mirror, created by the application of
5 multiple thin film layers can also be utilized. By carefully designing the dielectric mirror,
it can also pass selective, unwanted wavelengths of light (such as laser) so they are
precluded from propagating within the collection fiber 6380.
If the distal segment of fiber is made of high index material, such as
sapphire, the conditions for total internal reflection are more easily maintained.
10 Therefore, this approach is an excellent alternative when the application requirements
dictate the premium cost of the sapphire fiber.
Referring again to Fig. 63, when configured for laser-Raman
spectroscopy, a filtering scheme can be utilized to enhance performance. As the delivery
light 6301 travels towards the exit aperture, it is filtered by an interference filter 6355
l5 such that only the laser light passes and the extraneous wavelengths are rejected. As
depicted, the filter 6355 is applied to the distal fiber segment so that filtering is very
close to the exit ~e~tulG. The filtered segment is joined with the main fiber segment in a
small capillary (needle) tubing 6350. For mPflical applications, minimi7.ing the di~TnPter
of the bundle is important, and the capillary 63S0 can formed of very thin wall
20 platinum-alloy. As a result of the alloy's strength, the wall thickness can be minimi7Pd
while m~int~ining structural integrity. The precision of the internal diameter of this
tubing 6350, comp~Gd to the fiber's outer diameter governs coupling efficiency and the
effectiveness of the filter's blocking. A split sleeve may be utilized in place of the tubing
6350. The split sleeve's internal diameter should be slightly less than the outside
25 diameter of the fiber. Since the sleeve can be expanded slightly, it accommodates the
insertion of both fiber segments and m~int~in~ extremely accurate alignment. (During
final assembly, the unit of filtered fiber segments if fixed into permanent position with
epoxy or similar binding agent.)
For Stokes-shift Raman analysis, the delivery fiber should be filtered
30 with a low pass or band pass filter. The filters should be of extremely high quality and
should be sharp cut on/cut off. The purpose of the filter on the delivery fiber is to block
light cont~min~ted with intclrclGllce, such as silica-Raman and fiber fluorescence. Thus,
light delivered to the medium is clean in the wavelength regions of analytical interest.
The collection fibers 6380 should also be filtered; although, this is often not a
35 requirement. The purpose of the filters 6360 on the collection fiber is to block returning
laser light and to permit Stokes-shift Raman light to pass. (Laser entry would generate
silica-Raman interference on the return.) Therefore, these filters 6360 should be band
stop (notch) or high pass and should likewise be of high quality. Index m~ching gel can
~murE~Er(~ 26
CA 02248912 1998-09-11
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67
be added to the mating surfaces so that coupling efficiency in maximi7e~1 By angling the
mating surfaces of the adjoining fibers, another ~ropelly is created. This technique
prevents back propagation of reflected light within the fiber if the angle is properly
chosen. The filter angle should be greater than the limits for light propagation within the
fiber (8.5~ for silica core/silica clad fiber with numerical ape.lul~, equal .22). This
prevents light from bouncing back and forth in a fiber segment multiple times in a
fashion similar to a resonate cavity.
As previously described, collection light 6340 enters and delivered light
6301 exits the assembly through the same aperture (the distal end face of the delivery
fiber 6375) but at different angles. This property presents the opportunity to create
advantage by applying a coating 6365 to the delivery fiber 6375 end face. By utilizing
an anti-reflective coating (~uarter-wave m~gn~ium fluoride or similar) optical efficiency
is enhanced and stray light performance is improved. By applying a coating whichproduces a known signal, a wavelength and relative intensity reference can be
15 established for analytical comparison. For example, if the film has a Raman signature,
the peaks of this film can be utilized by the system to verify proper operation, calibrate
wavelength, and check relative intensity. Diamond coatings exhibit excellent Raman
signatures. An in-lic~tor can also be applied which is responsive to specific measurand
6320 analytes. For example, fluorescent indicators which respond to specific biological
20 chemicals can be ~nached to the end face. Various indicators and techniques for their
attachment to matrices are available in the art. Similarly, films which undergo color
change in response to specific physical and chemical conditions can be applied. In this
instance, the film should have a scattering agent, such as titanium dioxide, to direct a
portion of the light between the delivery and collection optical paths. Coatings which
25 enhance a material's native response, such as those utilized for surface enhanced Raman
spectroscopy can also be applied.
This approach of applying coa~ing~ (and even filters) to a fiber end face
also lends itself to mass production. The prerell~d method is to apply the coating to the
short fiber segment (approximately one-inch long). The end segment is then joined to
30 the main fiber in a fashion similar to that described for the filter. However, another very
valuable option is available when the junction does not contain filters or sirnilar artifices.
The segments can be fusion spliced together by any of the means commonly available.
To improve stray light performance, all the optical end faces should be polished to a high
finish (.3 micron or better).
The illustrated configuration also lends itself to another referencing
scheme. By injecting reference light into one of the ring fibers 6380 and choosing the
applo~fiate angle 0, a portion of injected light can be captured by a collection fiber on
the opposite side of the center fiber (following reflective bounce off of the center fiber
SUBSIIIUIE SHE~RUl E 21~
CA 02248912 1998-09-11
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68
6375 end face). The internal reflection will onlv be significant when the end face of the
center fiber 6375 is not in contact with the measurand 20 (prior to taliing a
measurement).
The application of the described device to a multitude of photonic
applications should be obvious to those skilled in the art. Neverthe]ess, several specific
application parameters are particularly noteworthy.
If the center fiber 637S is chosen to be an imaging fiber or bundle, the
assembly wil! capture spatial image data directly from a surface. Thus. a standoff
between end face and imaged surface is not required for the introduction of light. This
l 0 characteristic is particul~rly valuable in medical imaging of the human body.
Often. it is desirable io collect information from a layer slightlv within a
medium ~nd minimize the inflLlence of the initial boundary. For example. it may be
desir~ble to investieate tissue layel-s below the sl;in s o~lter surf3ce or under a finger nail.
This cap~bility can be enhanced by ;Ipplying a matching gel or cream to the center fiber
6375 cont~ct point. In biomedial spectroscopy, this method is valuable in numerous
skin-to-window applications.
The ring fibers 6380 can be recessed below the assembly surface: angle
0 need only be set appropriately. Similarly, these fibers may be moved outwardly and
downward from the center fiber such that a separation is maintained between center fiber
6375 and ring fibers 6380.
The region adjoining the internally reflective surface 6345 can be
encapsulated in a solid mass. The entire assembly can be potted into a rigid body.
By incorporating a lens assembly in front of the assembly tip, a
projection system can be created to extend the measurement point to a short standoff
location.
Fig. 64 illustrates an alternative, two-fiber embodiment. The delivery
fiber 6475 has a curved internally reflective surface 6445. As a result, the delivered
light is more angularly rich than would be the case if the surface were as described in
Fig. 63. This assembly can be formed by the rotational polishing procedure described
earlier. Note that the form of internally reflective surface 6445 is rotationally symmetric
about the mechanical axis of collection fiber 6380. This is accomplished by forming the
optical surfaces with dummy fibers positioned opposite the collection fiber (to maintain
symmetry during fabrication). The surface form 6445 is readily designed utilizing ray
tracing (manual or computer aided) to meel application dictates. The contour is then
generated into the surface by controlling the tip interaction with the polishing platen
Either computer control or cam-based raising and tilting the assembly while it is rotating
is acceptable. By forrning the surface a.s a paraboloid of revolution with the geometric
foci approximately at collection fiber 6480 end face, li~ght delivery to this area is
CA 0 2 2 4 8 91 i 19 9 8 - 0 9 - 1 1 ~ ' J' ~ ' J I ~ 1; ~ j, ' ~,
C9
~axirnized. The ill-lstr~tion ~I.o depic~s ~ t ~rea on th~ de!iY~ ber nd fa~ .hrcugh
~vhich a p~rti~n of Ihe d~liv~red'~ght 6~1 passe~ ~l~interrupted. 'fhis ~e~ture enhanc~s
ne;lsuremenl in cle~r In~d ~ ~ince it e~nd~ the me;~uremen~ r~ng~ signific3n~ly b~,yor.d
~he ~iistal ~nd race. ~3y ~his me~hod, the ii~ht deliYery p~.te~n fully engulfs the collec~iotl
5 ~ield-of- Jie-Y ~he corre!~tin~, ~dvanta ,c is T~roduced ~hen Ihe deliv~r~ ~nd colle~on
r?~e,s of the t~b~r., ~ -e .e J~l je.,~ Thls p~ lar ~onfigura~ion is w~ ted to bio;nedical
appl~c~tior.s in which th~ t~ber assembly is mo~lnted in a r.e~ i~e ~r is use~ in ~ sirrularly
m~ll configura~ion
deri~ti~e, the colleot~on fiber 6~80 car. have ~ sh~ped cnd face,
la ~-lch ~ Flanar angl~d. This confi~u~tion i5 especially .,seful to ~r~en~ b~
prop~gaticn of out~,oing light wh~n th~ fiber ~ is utili2ed in ~ ht deli~er~ C'lp3.Ci[y.
Fii,. 65 il]1ls~rates ~n cm~odir~ent 6~00 in which ~he ass~mbly
incorporates a quantity or ~l'oe~s. E;l~h ot ~e coll~cuon fibers 6~0 is ~esponsi~ to
light at specific angular orientatiotls. l~le dr~win_3 ~ icts intcmally reflecti~re ~urrac~
I ~ 6~4~ pr~filed ~s ~ c~mr~und (two) angle. The number ~nd ~ngular orient2tion cf lille~r
s~gment~ is re~dilv ir.creased to ~rly dcsired quan~ity to rne~t ~pplicr~licn requircments
~nd 1s reai~ily det~rn;ined -~hr~ugh ray rracing. Like~is~, the c~nt~ur rn~y be 5m~0th,
such as a para~ id. The dl~wing a]so ~ccentuates the path of th~ sllrf3ce reflection
~0 and the ~.~nse~ by .~rhich this reflec~on ~ contained ~y the cenccr fi~e~ 657~.
~O For best performatsce, the ~sse~nbly end se~ nt is ~used toge~her. Tne
indivldu~l f~b~rs can chen be coup]ed to a lar~o,e, single-co~e fiber. P~efer~bly, this
junction is accomplished b~ f~ing thc ir,di~ddllal fi'~els together so thal the junction k.as
max~m~m efficiency ~here are no gaps }~et~Yeen fibers). And, the appropriat~ filters can
be directly applied to this ~ssed end fa~e. At the spectrograFh, ~or detectors in ~hict
~5 line~ (sli~ r.put is desir~ble, the large, single-core fiber can ~e readily braken cut into
~he ~ppropriate configuIation. For this, a fused bundle of f.bers is best. r,ne bundle
should ~e fused ~n~o ~ ci~cle on onc esld a~ld ~ rectangle on the other. The fiber c~n be
loose in bet~en the r~ en~s or con~n~u.cly fused. C~re must ~ t~en lo ~ in~in ;he
arne ~ppro~malc surface area on cac~ end sc th~t the numerical ap~r~ure of the "~.used,
~0 round-to-slit ~dapt~r' is no~ alte~ed inad~ertcntly. Or likewise, a desired m~gnific~tion
can be cre3~ed.
~s described e~rlier, tl;e c~nter fiber 6~7~ can be m~ l;7ed vn i~s ~uter
jurface ~o ~hat cr~ss ta3k betwee~ d~iivery and c~llectioc flbers is rninJmized. This
metali~a~ion ~echnique i~ ~ery effec~iYe in fu~ed bundles which ar~ particul~rly p~onç to
i~ cros~ ta~k. W~en an assembly is u~ilized for Ra~nan spectroscopy, the crass-~alk
problem c~ be especiaJly trouble~ome The problern is troublesom~ because the
interactlon of r~e ]~er liCh~ 65~1 ~ith the deliYery tiber core 6510 produces unwanted
ber tl~,loresceslce ar~d silic~-R~man ligh~. This li~ht r~diates in all directions ~nd is
AM~F~ )Fn.~ T
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WO 97/34175 PCTrUS97/04365
prone to inadvertent collection and propagation by the collection fibers. The delivery
fiber 6575 may be thought of as "glowing" with unwanted, potentia]lv interfering light.
Highly efficient collection mechanisms~ as those described in this document. caninadvertently acquire this detrimental light. However. the metalization technique can be
5 readily used to advantage.
Since the nature of the assemblies (fused and unfused) described in this
document employ light transmission through fiber side walls to advantage, the
metalization technique must be carefully applied. For best results, the center fiber 6575
is the only one metalized. The metalization 6~96 should not extend fully to the distal
10 end face; a section of the fiber end should be left uncoated. This bare segment facilitates
the transmission 6540 of light rays between fibers only where this property ij desired.
The length to ieave bare is obvious through simple ray tracing and is readily
accomplished by masking durinr the metalization process. Altern~tively~ the metal is
applied fuliy and then removed chemically in the desired area.
Fig. 66 illustrates an alternative embodiment 6600 in which refraction,
created by contouring the outer fiber end faces 6635 into a refractive surface, is
employed to manipulate the fiber's delivery/collection pattern 6660. The refractive
effect as the light traverses 6640 the fiber sidewall is ignored for illustrative purposes.
However, this additional effect increases the coincidence of the pattern 6660 with the
20 center fiber 6675 end face.
Fig. 67 illustrates an alternative embodiment 6700 that is particularly
useful for monitoring surfaces in open air. It offers the advantage of facilitating the
direct application of filters 6760 on the ring fiber 6780 end face. Ring fiber 6780 end
face can be shaped such that the collection of light em~n~ting from the center fiber 6775
25 distal end face is enhanced. For example, by forming the ring fiber's end face into a
cone the fiber 6780 better collects this light.
Fig. 68 illustrates an embodiment 6800 that utilizes a solid, internally
reflective end piece 6850. Th.s configuration offers several advantages, particularly for
harsh environments. The end piece 6850 can be formed of sapphire, which has a high
30 refractive index ( I .77) and therefore is conducive to total internal reflection. The filters
6860 can be applied directly to this end piece. It can be manufactured in mass quantity
through techniques common to the jewel bearing industry and its internally reflective
surface 6855 can be shaped into complex contour. As an alternative to butting the main
segment of the center fiber 6875 to a short segment of identical construction, the main
35 segment can be adjoined to a short segment of sapphire rod. Filter 6855 can be applied
directly to the sapphire rod. (The mechanical alignment component is not depicted in the
illustration so that the function of optical elements can be best communicated.) By
coating the rod with low-refractive-index polymer, preferably fluoropolymers such as
CA 02248912 1998-09-11
WO 97134175 PCT/US97/04365
Teflon FEP or Teflon AF, a short waveguide is formed. And unlike sapphire optical
fibers, this segment does not need to be flexible. By encapsulating the entire tip in low-
refractive-index fluoropolymer such that no voids exist between end piece 6850 and
center fiber 6875 end segment, light rays 6801 transmit through the end piece sidewalls
5 6840 and are redirected for propagation. Other techniques include the addition of
optical matching gel, clear epoxy, or other methods common to the optics industry.
Since the wetted surfaces of the tip can be formed of sapphire and Teflon, this assembly
is particularly well suited to harsh environment applications.
Fig. 69 illustrates an alternative embodiment 6900 that utilizes gradient
index optics 6955 to steer the light 6901 so that it can pass 6940 through the center
fiber 6975 side walls. As an enhancement for certain applications, the outer surface
6985 of the gradient index element 6955 can be coated with an opaque substance, such
as metal, so that response to light em~n~ting from locations other than the distal end face
of the center fiber 6975 is minimi7ed Also, the distal surface of the gradient index
15 element 6955 can be contoured to create an additional refractive or internally reflective
effect.
Fig. 70 illustrates an embodiment 7000 in which the center fiber 7075
end face is shaped for light manipulation. In the illustration, the end face is formed into
an internally reflective, frustum of a cone. As a result of this adaptation, the sensitivity
20 to a small area at the distal tip of the center fiber 707~ is enh;lnced. This technique is
particularly useful in instances in which a large~ meter fiber improves source-to-fiber
light coupling. For example, when a lamp is utilized for source light as opposed to a
laser.
~Lt3ER OPTIC LIGHT MANIPULATION APPARATUS YIELDING
SIDE VIEWING AND SIDE DELIVERY
The techniques described above may also be used to construct a probe
assembly suitable for extracting information to the side. Such probes are particularly
useful for biomedical applications in which light is utilized to characterize biological
tissues and processes. Examples of the usage includes monitoring vessels and artery
sidewalls, probing various body canals and channels, and insertion into small needles.
Based on the intended use, the probe assembly needs to be configured to view various
off-axis angles ranging from 90~-off-axis, to forward-looking-off-axis, to back-directed-
off-axis. While the foregoing sections provide great detail to off-axis light delivery and
collection field-of-view, this section teaches more detail in these regards. Specifically,
the methodology through which the relevant variables are manipulated to optimality are
described herein.
SUeSlTME'S~E2~
CA 0 2 2 4 8 9 1 2 1 9 9 8 - O 9 - I I
Th~ OptiCa described hereiI~ deli-~er lioht to and collect li~,ht ~rom th~ sid~
of ~ ~robe assembl~. ln Lhi~ m~r.a~r, infomL~tioII reg~rdine the chemic3~ ~nd E~hysic31
p~r3meterà of 1 m~.~orial ~r pracecs is acq~ ed u~ inO opticql ~ers. Intem~ Qe~tIon
ia u-ilized ~o ~[e~r the lig~, to th~ desired loc~tion. I~he con~ollr of ~h~ mtelnally refl~c~ive
~urface i.s shaped ~c rrouuce v~no~lj results. I~he~ results in~lude the posi~ion ~nd si-?
of lh_ ins~ct~d rcOiorl within th~ ig~tive Ini;diwm- ~r ~lso inc~ud~s ~he an~,ul~r
- ~ri~nt~u,~n o' ihe coll~c~.ion field-of-v;ew optic~l 3xis reIatiYe to th~t or the lIght d~ llv~ry
p~ttern
S~p~rate fib~rs ~e uuli~ed to deliver and collec~ light. The iight m~y be
1~ monochrom~tic (like ~ ier~ or bro~dba~ld (whitej. Th~ collected light 31~ e shifted in
wa~el~g~h rel~tiYe to tb~ deli~ered li~ or it m3y !~ot
.4s an a~ternacive to int~ ! reJ~ec~ n, gradient inde:~ optics are utilized
for ~~r~ng light.
FlJ. 71 Illustrates a probe 71~0 that inclu~s on~ delivery t-lb~ 7175 to
1~ project a lighl delivery pa~tern 71~1 in~c e~e investigative re~ion. A s.cond7 coll~ction
fi~er 71~0 has a field-of-vi~w '71~2 th~t is lik~ise direc~ed into ~he inves~ig~tiYe
reaion. The op~ical Lxis 71~ of the ccil~ction fiber 7180 intersects with rhe optical
a~i~ 719~ of thc deliver.~ ~ber 717~ at a poin~ 7185 tO the ~ide of the fib~r pair.
~n ~he illusts~tion, to ~romc;e cc;lceptU~ cl~ity, the diverging ~spects of
'O the li~ht p~tterns 7101, 710~ are n~c represented Wh~re tne drawing assumes par~llel
rays ~ithin the fiber cores 7110, 711~, ~e rays are actually r~ndomized within angular
i~mitati~ns of the fi~ec~' ~ropa~ation c~pa~ilities (~i-8.5 for 3iltc3, cor~/silic~ clad fi~er
With n~lmeric31 ~p~rture .2~). Tr.is ~roduc~s light pal;erns 7101, 7102 that ar~ less
sharply focu~d than those depicted. FGr the s~me reasons, ~e r~,fractiYe in~luer~ces of
25 ll~ht crossing the fib~r bo~lndaIies ~re nol illustrated.
The in~e~nally r~tlcctive su~ac~ 71~5 of ~he fibers are conroured inta ~he
tcrm or ~ pa~aboIoid oF revolutloll 1175. Thc ~xis of revolution 717; intèrsects wtth
the geom~tric foc~s 7185 of the parabolic form. By choosing the a~plopriate parabolic
~orm, the peak respons~ regicn can be s~leeted in a out-vard position - a desired distance
30 from the ~iber pair. Sinularl~!, the re~i~n. Cal't be f~rw~d of the distal ~nd such that ~h~
angle B between the optic~ es 71gO, 71Yi and an ~is parallel 7170 t~ the ~ibers
znd dispiace~ through the in~ersection of these ~xes ~190, 7195 is greater than 90 .
Similarly, the response region can be ~ckward f~om the distal end sllch tha~ thed~scribed ar~gie n is ~ss than ~0'. In shart, t~e pr~be is re~dily confilgured, ~y selecting
he appropriate parabolic form, to gen~rate pe3~ response a~ various dijtanccs and
an~,ular dispositions relaîive to the probe distal end face.
Clcarly, rhe apparatus c~n be configllred such ~hat the intcrsection point
7185 of the ~ptical aY~s 1190, 7195 is a laIg~ dist~nc~ from the di~tal end. Howe~fer,
AM'I~J"~3 Srlc i
_
_ _ _ _ _
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at a larger distance, the region of maximum sensitivity may not surround this
intersection; it will be closer to the apparatus tip. The larger the distance between
intersection point 7185 and apparatus end, the greater the influence by medium
transmission effects (such as absorption), multiple scattering events, and light5 divergence. These factors produce disprol,ollionate response close to the end face for
' larger axes 7190, 7195 intersection 7185 distances d.
For configurations in which the angle between the optical axes 7190,
7195 and the fibers' 7175, 7180 longitudinal axis is approximately 90~, light will
transmit between fibers' core 7110, 7115, fibers' cladding 7130, 7135, and the
external medium with least interference from the refractive index differentials at the
material boundaries. However, this configuration requires a large refractive index
differential between fiber core 7110, 7115 and contacting material at the internally
reflective shaped surface 7145. Therefore, this configuration favors metallic coating
applied over the contoured surface 7145 to generate internal reflection. Nevertheless,
lS total internal reflection is achievable with very low-refractive-index material, such as a
gas, contacting internally reflective surface 7145 and/or high refractive index material,
such as sapphire, used as fiber core material 7105, 7110.
Less aggressive light bending is required of the internally reflective
shaped surface 7145 when the intersection of optical axes 7190, 7195 of delivery7101 and collection 7102 patterns is more forward. In this case, the elsewhere-
described fluoropolymer overcoat, generating total internal reflection, is the preferred
method.
As light transitions the side boundaries of the fiber, the cylindrical
contour distorts the light. The influence of this effect are lessened for liquid investigative
media with refractive indices approximately ln~tching that of the fiber materials. The
cylindrical surface may also be re-contoured by such methods as filling-in with optically
clear material such as epoxy (or full tip encapsulation with the described
fluoropolymers). This material can also provide a measurement reference. A similar
result can be created by polishing or heat softening and compression. The fibers may
also be fused together.
The fibers' coating/buffer should be removed in the area through which
light passes. This also enh~nces performance by promoting closeness of the fibers. The
met~li7~tion techniques may be employed to ,llhli",;,~ cross talk between delivery 7175
and collection 7180 fibers. The preferred method utilizes a thin foil fixed between the
fibers during the fabrication process. The assembly also incorporates the ability to
provide selective sensitivity to specific photonic mech~nicm.c via angular bias of light-
scattering processes.
a BSmUE~T~ 2~
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74
As an adaptation, a portion of the end face can be polished flat (such as
perpendicular with the fibers' longitudinal axis). By properly choosing the shape of the
internally reflective contour 7145, the light patterns 7101, 7102 are directed forward
and to the side through the flat section.
The internally reflective contour 7145 is not limited to the shape of a
revolved parabola 7175. It may also be the forrn of a right triangle revolved about the
axis, flat (each fiber faceted), elliptical, or any derived form, preferably revolved about
an axis.
As a general rule, media in which light propagation in encumbered (such
as absorbers) favor a configuration in which the optical axes 7190, 7195 of the
delivery and collection patterns 7101, 7102 intersect close to the apparatus tip. This
m;nimi7.eS the flict~nce within the medium through which light must propagate. This
configuration can also be utilized to create an angular orientation between delivery and
collection optical axes 7190, 7195 in which sensitivity to specific photonic
mech~ni.cm.c is enh~n~ed
As another important variation on the theme, the two fibers 7175, 7180
depicted in the illustration can both be utilized as light collectors. A smaller delivery
fiber can be positioned directly adjacent to and between the two fibers (in the location
~esign~tt~.d 7160 in the illustration). By creating a similar, internally reflective contour
on this fiber, the optical axis of its delivery pattern can be manipulated to intersect the
other fibers' 7175, 7180 optical axes 7190, 719S intersection 7170. The optical
axis of the delivery fiber is not necessarily in the same plane defined by the collection
fibers' optical axes 7190, 7195 (it can merely intersect this plane). Thus, light
collection is ma~imi7~1 By forming the delivery fiber end face into a planar surface, a
filter coating can be applied to the surface.
Fig. 72 illustrates an embodiment 7200 that projects the collection
pattern 7202 through the delivery fiber 7280. This configuration is readily adapted to
meet various application re~luilcme.ll~. For example, the collection pattern 7202 can be
set so that the field-of-view encomr~ses the region of the source fiber 7280 through
which the source light passes and emerges into the investigative medium. In other
wor~ the light delivery and light collecting aperture can overlap on the assembly's
outer surface. The individual shapes of the internally reflective contoured surfaces 7245
of the source fiber 7280 and the collection fiber 7275 are readily designed with ray
tracing and/or geometric equations to generate this effect. Similar advantages to those
offered by the previously described through-fiber-sidewall, straight-viewing assemblies,
are available in this configuration. These advantages include the ability to conduct light-
based characterizations in extremely opaque media. Configuration variations include
fusing the fibers, utilizing multiple fibers, sizing either the source or delivery fiber(s)
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small in relation to one another, and forcing the delivery and collection optical axes
essentially concurrent with one another.
Fig.73 illustrates an embodiment that utilizes a large bundle (preferably
fused) of collection fibers 7380 and a single delivery fiber 7375. The optical axes of
the delivery pattern and the collection pattern are depicted as essentially concurrent.
By creating an angled, flat planar surface on the end face of the delivery
fiber, a filter can be applied to this surface. If the filter is a notch (band stop) filter and
the source light is highly monochromatic, the filter will reflect only the desired,
monochromatic light into the investigative medium. The undesirable light (off-
wavelength) will pass through the filter. Thus, by coating the filter with a transparent
material, such as the previously described fluoropolymers, and over coating this with a
stronger absorber (such as lamp black), then the undesirable light is trapped and
excluded from co~ g the measu,~ ent.
Another important aspect of this, and similar configurations, is that each
of the collection fibers 7380 are re~onsive to light traveling at various angular
orientations and spatial originations. Thus, light in each collection fiber 7380 has
undergone a form of spatial/angular filtering. And, by colllpaling the relative strengths
of the light from each collection fiber 7380, additional information can be gleaned from
the investigative ~
Fig. 74 is an expanded view of the internally reflective shaped surface
7445 and light pattern 7450 (with the simplicity of parallel rays within the fiber) of a
fiber assembly 7400. The fiber's internally reflective surface 7445 is contoured into a
form defined by a right triangle's hypotenuse's path as the triangle is revolved around its
upright leg with the upright leg positioned outside the fiber and the triangle's base
perpendicular with the fiber's longit~ in~l axis. The internally reflective shaped surface
is the surface region of a cone intersected by a vertical cylinder whose axis is parallel
with that of the cone. In contrast to the parabolic profile detailed earlier, this profile is
linear (not planar). In the cross sectional view, light is less focused than in the described
non-linear profiles. Viewed from overhead, a strong focusing aspect is observed.This surface lends itself well to manufacturing operations and is easily
generated with high repeatability. Referring to Fig.75, the set of delivery and collection
fibers 7575, 7580 are bound together. The cross-talk-inhibiting mechanism is
preferably integral at this stage. The fibers 7575, 7580 are fixed to the side of a
mandrel 7585. The mandrel is ground and finely polished into a point by spinning the
mandrel about its center axis while contacting a rotating abrasive disk with the mandrel
oriented to the plane of the abrasive disk at the desired polish angle. As the fibers
7575, 7580 are polished, their centers sweep out a circular path 7590. From an
overhead perspective, the lines defined by e~h fiber' s center and the center of revolution
8UiSTllU~ SHEE~
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76
7510 are approximately the optical axes 7515, 7520 for each fiber. The optical axes
7515, 7520 intersect one another at the center of revolution. The fibers are removed
from the mandrel 285 and moved to later stage fabrication procec.cing.
The fabrication assembly may be fixtured as follows. A mold release
agent is applied to the mandrel 7585. The mandrel 7585 is encircled with fibers 7575,
7580, which will make a ~uantity of assemblies. The fibers are held in place on the
mandrel with a heat shrink (preferably TFE Teflon) so that the ends are exposed. Each
fiber set (two or more fibers per set) 7575, 7580 is isolated from the adjacent fibers
with a piece of thin TFE Teflon "plumbers" tape. Each fiber set will comprise a fiber
optic a~ Lus. Metal shims, which prevent cross talk are placed between delivery and
collection fibers. Optically transparent epoxy is applied to the fabrication assembly. The
epoxy is allowed to harden. The assembly is polished, as described, to form the
internally reflective shaped surface. The fabrication assembly is split apart so that each
fiber set can be processed into a completed probe and housed accoldi,lgly.
A benefit of the described technique, for some applications, is the surface
form which is generated by the epoxy's contact with the mandrel 7585. The epoxy fills
in the cylindrical surfaces of the fibers. For large mandrel diameters, distortion
~C~soci~te~ with light enteringlexiting the fiber through the fiber's cylindrical sidewalls is
controlled.
Variations on this general theme are readily accomplished to generate
various desirable results. For example, the mandrel 7585 can be made of wax and then
melted away for each fabrication batch. By varying the angle of contact with theabrasive disk and concurrently raising and lowering the assembly (in staged steps,
continuously under computer control, or mechanically with a cam assembly) complex
internally reflective surface contours are readily created.
Fig. 76 illustrates another variation on this fabrication process, which
produces devices with close-in focused optics. In this variation, a fiber bundle is
polished without a mandrel. In the illustration, four fibers are bound together with heat
shrink. A shim 7610 is used to segment the bundle into probes (in this case, two fiber
pairs). After processing, the assembly is split apart. If both fibers 7675, 7680 in the
pair are to be utilized as collection fibers, a side-delivery source fiber is readily fixed in
the groove between the fibers.
Fig. 77 illustrates an alternative embodiment 7700 that utilizes gradient
optics as an alternative to reflective optics. In this embodiment, a gradient optical
element 7730 is attached to the delivery and collection fibers 7775, 7780. The
element 7730 can be formed by core drilling a grin lens (the core is taken offset from the
center axis of the lens). Alternatively, the grin lens is cylindrically ground {and
polished) while spinning it about an axis of rotation offset from its normal optical axis.
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If a non-cylindrical form is desired, the element can be ground as desired utili7ing a grin
lens as stock. The refractive index gradient of the lens steers both the collection pattern
7702 and the delivery light pattern 7701 off axis. The element is held in place with a
sleeve 7725 in conjunction with binding agent. The sleeve may encompass the entire
S assembly and, if metal, include a cutout for the passage of light 7701, 7702. The end
surface 7740 of the element 7730 can be beveled, as shown, to facilitate insertion in
biological tissues. Filters 7755, 7760 can be applied to the fibers' 7775, 7780 end
faces which adjoin the element.
Fig. 78 illustrates an embodiment 7800 whose m~ch~nical configuration
is similar to that described in Fig. 77; however, the light is bent with internal reflection.
The preferred method of fabrication includes metalization of fibers 7875, 7880 to
inhibit cross talk. The fiber bundle can be fused together to minimi7e spacing when
distal filtering 7855, 7860 is not required. The end piece 7670 is preferably
fabricated from high refractive index material such as sapphire with the internally
IS reflective surface 7845 coated with low index fluoropolymer to generate total internal
reflection. The area of desired light exit/entrance can be coated with an anti-reflective
film, such as magnesium fluoride. The end piece can be formed from a optical fiber
whose diameter is the desired size. For highest efficiency, the internally reflective
surface 7845 is shaped to facilitate overlap between collection 7802 and delivery 7801
light patterns within the investigative me(lium. For highly absorbing media, the contour
shape 7845 is chosen, as previously described, into a form such as a parabola such that
overlap is generated close to, or directly at, the medium/probe boundary. For clearer
media, the contour shape 7845 is chosen to project the overlap deeper within the media.
This end piece component is also useful for mounting on the end of a single fiber utilized
for laser delivery in cutting and treating biological tissue.
Another adaptation is also useful. A clear (silica) capillary tubing is
sub~ uted for the sleeve. The tubing extends over the distal end, is sealed, and encloses
a first-surface mirror. The mirror, oriented towards the fibers end faces, re-directs the
light collection and delivery patterns to the side. The reflector is preferably aspheric
concave (such as paraboloid) such that optical axes of delivery and collection light
intersect. The reflector can be formed by shaping a fiber's end face. The fiber should
have a outer di~m~.ter approximately equal to the inner diameter of the capillary tubing.
The fiber end face is coated with an a reflective film such as metal or dielectric mirror.
Alternatively to making the mirror on the end of the fiber, the mirror may be ground and
35 polished directly into a short metallic rod of the proper rli~meter.
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78
IMPROVED FILTERING TECHNIQUES FOR OPIICAL FIBERS
Many of the phenomena associated with light propagation in optical fibers
are dependent on travel distance in the fiber. The accumulation of wavelength-shifted
light with fiber length is an important example. Wavelength-shifted light is generated
5 due to inelastic light-matter interactions between propagation light and fiber materials.
These inelastic interactions include fluorescence and the Raman effect. Wavelength-
shifted light arising from these interactions radiates in essentially all directions from an
arbitrary region in the fiber core. Conceptually, it is useful to visualize an arbitrary
region of the fiber glowing. The portion of r~ ted, inelastic light which is angularly
10 oriented within the fiber's propagation limits is captured by the waveguiding l,ropellies
of the fiber. The fiber is overfilled with this wavelength-shifted light. This captured
light travels both forward and backward with respect to the primary, exciting light in the
fiber. The ra~ te(l, inelastic light accllmlll~tes with fiber length such that it is more
intense in longer fibers. This wavelength-shifted light manifests itself as interference in
15 many fiber optic applications. Applications in which the signals of interest are weak -
similar in strength to that of the interference - are particularly susceptible to detrimental
influence. These applications include low-light spectroscopy such as Raman, somefluorescence analyses, and ilumin~sce.n-~e.
Filtering techniques can be utilized to address this interference problem.
20 Filtering capabilities are also hllpo.~ll and needed for numerous fiber optic applications,
such as wavelength division multiplexing in telecommllnications. By directly applying a
high-quality filter to a fiber end face, the need for expanded-beam filtering techniques
can be elimin~t~.d
Previously, thin-film filters have been applied to wafers which were
25 placed between fiber end faces mated in standard fiber optic connectors. This technique
suffers from multiple drawbacks. 1 ) The assembly/fabrication process is difficult and
expensive at best. 2) It is not conducive to the fabrication of micro-sized assemblies,
such as are needed for biomedical applications as well as many other usages. 3) Light
diverges as it passes through the wafer's thickness; this leads to filtering and coupling
30 inefficiencies. 4) The performance demands of low-light applications, such as Raman
spectroscopy, necessitates high-performance filtering, which are not compatible with this
design architecture.
Filter pelro~ ance requirements for demanding applications, such as
Raman spectroscopy include: a) high throughput in tr~n~mi~ion wavelength region; b)
35 high-attenuation (dense) blocking in rejection wavelength regions; c) steep transition
between wavelength regions of rejection and tr~n~mi~ion; d) environmental stability; e)
low ripple in passage regions, f) minim~l sensitivity to temperature variation; g) no
performance fluctuation with ambient humidity or chemicals; h) the ability to withstand
CA 0 2 2 4 8 9 1 2 1 9 9 8 0 9 - 1 1 " , . I, ' . 1' !-- ~ I ' J ' .: I _; S: !~ ' J
79
higr" ~nd r~pidly cAanging, temper~ures present in steriliz~on prccess~s ~nd industrial
process~s; i) physic~ ou hness; ~nd j) tenaci~)us ad~esi~n be~ween filter md sub~trdte.
Th~se desiI~ble l'llter perform~nce prope.ties ~re ~chieved in thin-film
fiiRrs hlYiDg ~ rg~ umber ~f ~It~ in~, hlgh~lo~v refr~ct!v~ indice~, stac~e~ layers
J deposited on ~su~st! ~. Bet~en ~ and 1~0 lay~rs are usu~iiy requi~e~depcnding on
such f~CL~ .e p~rfo~nce requir~ e ~n~l u~e; ~) the r~o,rr;~ctive ir.de~
~ifferen~al betw~es~ m~t~r.~li in ldjdcellt filter llyers; ~) the eonsist~ncy and purity of the
fi~e. layer; and 4) the scphistic~tion of ch.e ~llter d~3ign process. An~ the Is.yers must bc
~ree from defects ;md voids such th;~t th~ matel~al characteristics of the llyer dpproac~es
!~ th;lt of a bul~c solid and th~ pachng f~c~or of the layer ~pproach~s 100%. Achievinc,
high-1ensity packing requires ~he molecules depositing onto th~ s~str~te ~o be hi~hl~
enercetic. Durirg the l~yer deposition proces3, th1s ener~y prevents the forming layer
frorrl onenting itseiJ int~ c~lumnar or sim~lar sauc~ures which are riddled with ~cids
While t~e depositlng laycrs ar~ ~redispcsed to forming the mperfecl structures, the hi~h
1~ energy forces p~c~; the moiec~les (~r ~m,) into a~y Yoids or pinhole~ which mlv exist~
This high ener~y tends to i-npar~ ~esidu~l mechanical s~ress to t~e substrat~ n~se
stre~se~ c~use curlin~ ~nd other ~roblems in ~hin ~ubstrates. Thu~ is ~iffic-~lt lo
produce hi~7,h-quality ~ rs. with the dPs~ribed attributes ~nd witho"t e~anded-beam
op~cs, on thin ~ rs whic~ wculd be suit~bi; for ~nscmon between fi~r end r~ces.
'0 U S. P~tent 5,037.18~ to Stone de~cribe~ th~ applic~ti~n ~f a thin-fiïlm
f~lter ro 3 fil~er mount~d in ~ industry-~andar~ ~erlule; :he ferIule-fiber uni~ is proces~d
(pGlished a!~d ~he ~ilter applied) a~ a unic. Stone attri~u.es d~vi~tions in fi!te,
per.ormc,nce to temper3turc dift~rences between terns in t~e filter chamber and tO
re.racti~e index errors in t~e fi3~r l~y~r rnaterials. ~he filter de~osition prGcesscs
~5 described typic~.liy produce il~efr~cient fi~tcrs withoul the ateri~ut~s described aboYe.
Stone describes rwo techni~ue which can ~ u~ e~ to overcome the nee~ti~e aspects of
filter inef~lc1encies. The first techniriue is the us~e of hi,,~ rcfrdctl~e index material ~or
the pertincnt ~;lter layers. B~ ~lsa~e of silicone (ref~cti~e inde~ apprc;~lmateiy 3.2~
tllt~r ~erforrna~cc is bolstered ~ompared with such materi~ls ~s ~it~llliUm dio~ide.
30 Unfortunately, silicone does ~ot r~nsn~il ~ell in elther the visible re~ion or ih~ ne~
infrared region be]~w~ microns. Ther~forc, its ~Isage is ~recluded f~r man~ impo tant
applicalions. Simil;~rly, the ~,hoices of dielectric matcri.lls is also 1imited by toxicity
Iss~es' enYlronmental stability, an~ other factors. The ~cond d~scri~ed technisue is to
bevel ~he filt~rl~Lber ~nd face such ~hat th~ ~llter' s rejected li~hc is reflected at an angle
. which cannot be propagatcd by the fiber. In this nlanner, the de~mental effects of i'ilter
ine~ficiencies ~ minimi7~d fcr rnan~ app1ic~tions.
It ~ill be seen that th~ techr~ques descr~bed below proYide an ~xtremely
~ttracti~, novel means of tilteri~g optica! fibcr~. They are well s~lit~d ~o lo~cos~
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fabrication and are useful for instrumentation applications, such as Raman, fluorescence,
and other spectroscopic analyses. They are also devised for wavelength division
multiplexing, telecommllniç~tions~ general fiber optic sensor usage, photonic co~ ulillg,
photonic amplifiers, pump blocking, fiber-integral active devices such as fiber-coupled
S (pigtailed) lasers and lasers utili7:ing the fiber as the lasing cavity.
In accordance with the present invention, a thin-film illl~lrerence filter is
applied to a fiber end face. The filter has a packing density of at least 95%, but
~,e~lably greater than 99%.
A fiber with an integral filter is utilized for analytical
10 instrl-ment~tion/sensing applications generally and spectroscopy more specifically with
significant benefit over the prior art for analysis involving low-light, inelastic light-matter
processes, such as the Raman effect.
In an exemplary embodiment, a short fiber segment, preferably less than
24" but optimally 1.5" or less, has an integral filter applied to its end face. The segment
15 can be joined to a longer fiber. With the filter on the distal end, the splice between the
two fibers can be formed with a fusion process. With the filter on the proximal end
(between the segments) the junction can be made with a sleeve. The sleeve is precision
mated with the two fibers. The sleeve is best formed through nonconventional
metalworking processes: electroforming or electrolysis plating over a precision mandrel.
20 The mandrel can be a section of the optical fiber. The coupling can also be made with a
split sleeve whose relaxed internal ~ m~t~r is slightly less than the outside diameter of
the fiber.
In one embodiment, short fibers are bound together in a bundle and filter
coated as a group. Preferably, they are polyimide buffered fibers and are held in a P l ~k
25 fluoropolymer (common trade name Teflon), heat shrink tubing. The bundle, so
constructed, may be large enough that it is simply held by the filter coating chamber's
fixtures. Alternatively, it is held in a plate; the plate is preferably silica so that it can be
used as a witness to control the coating process and also used after the batch process to
grade the filters. The fibers can be held at an angle in the plate such that an angled fiber
30 end face is flush with the planar surface of the plate; in this manner, an angled filter is
applied to the fiber.
The filter can be applied at an angle of approximately 45~ such that the
reflected and transmitted light can be transmitted to locations in an optical assembly for
subsequent processing. The filter can be oriented at an angle greater than the maximum
35 angle of light propagation within the fiber so that reflected light from the filter cannot
back propagate during low-light spectroscopy application, such as Raman.
Variability can be introduced into the thin-film application process so that
filters of various wavelengths can be produced within a batch. The variability can be
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81
provided by masks, off centering, and raising and lowering the substrate. The slightly
different filters can be graded and sorted.
Several short, filtered fiber segments can be aligned end-to-end with one
another. One end of each fiber segment is angled and has a filter applied to its surface.
5 The opposite, unfiltered ends of the fiber segments may be flat or formed with mating
bevels. The filters are slightly of~set in wavelength from one another. A clear, capillary
tubing, preferably with one side polished flat, can be used to hold and align the
segments. The assembly can used to tap off signals according to wavelength, or input
wavelength-separated signals.
In one embof~im~nt a waveguided Raman cell is produced by introducing
a fluid sample into a tube. Light is trapped and collected in the tube such that the signal
is amplified. The tube's inner surface is preferably formed of the material known to the
industry as amorphous Teflon.
THE FILTER APPLICATION PROCESS
As described above, the filter application processes that create filters with
desirable performance attributes often produce residual stresses in the substrate. These
residual stresses cause difficulties in applying the filter coatings to thin wafers of the
types which, in the prior art, has been placed between adjoining fiber end faces in
20 standard flber optic mating connector junctions. However, by applying a filter coating
directly to the fiber end face with a highly enelg~tic filter process, previously unrealized
filtering of optical fibers is achieved.
The prcf~.~d thin-film deposition processes imparts sufficient energy to
the depositing molecules so that the forming structure is essentially fully packed (100%
25 comprised of the desired molecules, essentially nonporous, and free of voids and
pinholes). For best performance, the structure should approach or equal 100% (greater
than 99%) p~c~ing density, but at least 95%. Due to this and other factors, adherence to
the fiber substrate is tenacious. The effects of the residual mechanical stresses are
negligible since the fiber is very strong in relation to its fljam.otçr. Several thin-film
30 processes are particularly well suited to produce this high-density, hard-coated filter.
These processes include m~netron s~uLtc,illg, single- and dual-beam ion sputtering, ion
plating, and ion-assisted deposition (typically slightly less performance and lower
packing densities). Reactive and non.ca.;live versions of these processes are available.
The reactive processes are typically faster in terms of the time required to produce a thin-
35 film coating. These and similar processes contrast with conventional processes, such as
- evapo-~Live films, which achieve packing densities of approximately 80%. Ion-~c~i~ted
deposition produces films with densities typically in the 95% range and for this reason
are less preferable. In short, a filter with high packing density - greater than 99%,
~BSmU E~E26)
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82
preferably approaching or equaling 100%, but at least 95% - is applied directly to the
fiber end face utilizing highly energetic, non-conventional thin-film depositionprocesses.
Variability can be introduced into the filter coating process so that fibers
5 with filters of various wavelengths can be produced in a single batch. Several methods
are useful in achieving the controlled variation. Selective fibers can be raised and
lowered relative to the source which releases the film coating material. Selective fibers
can be offset from the coating center. A mask can be applied to select fibers such that the
amount of filter material deposited is varied according to position. These techniques are
10 especially valuable in filtering optical fibers since a large quantity of fibers can be coated
simultaneously - given the tooling methodologies described herein. Furthermore, fiber
optic applications benefit from the availability of filtered fibers with slightly varied
wavelengths. These applications include: 1) wavelength division multiplexing (input and
output); 2) tapping off spectroscopic wavelengths for detection; and 3) m~tç~ ing filters to
15 lasers with varying but closely grouped wavelengths.
TOOLING
In prior art methods of coating optical fibers, the fibers have been
individually mounted in termination connectors. This and similar methods result in
20 nll-nelous problems. First, it does not make efficient utilization of the available coating
surface area in the ch~lbe.. Relative to the size of the fiber end face surface area, a large
space is required for the termination-connector assembly and also for assembly-
disassembly working room. Coating chambers which produce the best coating control
are small. Each batch run is expensive for a very high-quality filter. Thus, yield is a
25 critical economic factor. Second, the materials in a finished fiber assembly include
plastics and epoxy which can out-gas and cause problems with the coating process.
Third, the material a~ re~t to the fiber end face is the termination ferrule. This m~t~ri~l
difference can lead to incon~i~tencies in the filter coating, particularly at the boundary
region between the two materials. Fourth, the various materials in the chamber have
30 dirr~lent thermal conductivities. The bulk witness's (test plate's) heat transfer is vastly
different than the fiber-termination assembly. Therefore, tclll~ urc consistency of the
various materials is difficult to m~int~in This is a problem since substrate telllpe,alLllc is
an hllpollallt coating variable. Fifth, the filter must be situated on the fiber end face.
Here, it is susceptible to physical damage and envi-un..l~nt~l influence if left unprotected
35 (depending on the coating type). There is no clear means to position the filter slightly
behind the fiber end face. Sixth, the long fibers present space problems in the chamber.
For these, and other reasons, an improved methodology for applying thin films, in
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83
general, and filters specifically, to optical fibers and similar cylindrical components is
needed.
In contrast to the prior art, the methodology of the present invention
produces high-quality, consistent filters with significantly reduced fabrication expense.
5 The new methodology can be readily adapted in accordance with various needs.
Nevertheless, the following description, referring to Fig. 79, is illustrative of the
preferred techniques for a given application.
For Raman spectroscopy probes, step-index, silica core/silica
clad/polyimide buffer fiber is the preferred fiber for these filtering operations. Gradient-
10 index fibers and single-mode fibers are also compatible with the filtering processes and
are preferable for some Raman probe applications. Low OH fiber is p~f~ cd unless the
wavelengths of interest are outside its tr:~n.~mi.ccion capabilities. Fiber ~ nleters of 300-
and 400-micron core diameter are preferred for ease of workability (when compatible
with application req~hcl"ellt~,). The fibers are scribed and parted into short segments
15 7901; one-inch lengths are ideal for ease of handling. The fiber segments are grouped
into bundles 7902. For 400-micron fibers, 33 fibers per bundle 7902 are inserted in a
PTFE (DuPont trade name Teflon) heat shrink tubing 7905 of size 14 gauge standard
wall thickness. The tubing is approximately two inches long. The fibers are aligned in
one end of the tubing. A heat gun is used to shrink at least the first one-half inch of the
20 heat shrink/fiber bundle assembly 7912.
During the heat shrink process, the polyimide buffer on the fibers bonds
slightly together so that the fibers are tell~pG~a~ily fused together inside the heat shrink
tubing 7905. The bundle is ground and polished on the end 7906. Although not
required, the polishing process can be aided by pumping, or pulsing, cle~n~ing distilled
25 water through the bundle during the grinding and polishing operation. This can be
readily accomplished by coupling the tag end of the bundle's heat shrink tubing to the
output tubing of a suitable pump. A pulsating pump is especially effective in ~ ;llt;~
cle~nlin~~s during the polishing procedure. The flow minimi7~.s the extent to which
debris is trapped in the cavities between the fibers. It also improves the fiber end face
30 surface finish by removing ground particles from the polishing surface. The bundle is
polished to a .3-micron finish.
After acceptable finish is achieved, the bundle is cleaned in a low-power,
ultrasonic cleaner. No traces of debris should be visible under microscope inspection.
- The bundle is cleansed again with isopropyl alcohol and rinsed with acetone. The heat
35 shrink tubing's loose end is shortened so that about 1/8 inch remains extended pass the
fiber bundle.
Fused silica tooling plates 7910 are prepared to hold the fiber bundles.
Square plates (1" x 1" x 1/4") are suitable. The size and shapes of the plates are not
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critical--they are selected for compatibility with the coating chamber's tooling. The
thickness of the plate is more important: the plate's stated l/4-inch thickness functions
well with the given bundle size. The plates are drilled with four holes 7911 in a cross
pattern. If desired, a large number of holes can be drilled to increase filter yield. The
S internal diameter of the hole 7911 is .145"-. l 50" which is slightly smaller than the fiber
bundle/heat shrink tubing 7912 outer diameter. The fiber bundle assemblies 7912 are
inserted into the plate's holes 7911 with a light interference fit. Since there is a certain
degree of variability in the outer diameter, the fiber bundle assemblies can be matched to
various sized holes and/or trimmed on the outer surface for a good fit. After inserting
l(! the fiber bundle assemblies 7912 into the plates 7910. the entire assembly is cleaned
and inspected again. First, it is cleaned with standard-grade acetone and finally with
high-purity acetone. Ideally, suction is ~laced on the tubing back side so that any
residual contaminatioll is sucl~ed away from the critical end faces. The assembly can b~
he~ted at lS0~ - 175~ F for several hours to drive off any moisture from the fibers'
po~yimide buffer. The fiber bundle assemblies should be stored in a desiccated container
until the thin-film coating is applied. These moisture reduction steps are not
requirements but are recommended.
The orientation of the bundles 7912 in the plate tooling 7910 provides
that two o f the bundles will be aligned along the coating chamber's line of maximum
consistency. And, the other bundles will be situated on each side of this axis. In this
manner, the displaced bundles are slightly shifted in wavelength. The plates arepositioned and held in the filter coating chamber's tooling for thin-film deposition
according to the preferred processes described herein.
The fused silica tooling plate also provides a means to actively monitor
the thin-film deposition process. And, since the material of the tooling plate is the same
as that of the fiber, the thermal characteristics are similar. This factor minimi7es the
deviation between the thin film on the tooling plate and than on the fiber end face. The
tooling plate is a witness and record for the filter coating batch. By sc~nning across the
plate with a optical test jig, a contour representation of spatial filter deviation can be
rapidly ascertained. In this manner, the optical characteristics of the filters on the fibers
can be estimated without individual fiber testing. This proves especially valuable in
filtering chambers/processes which exhibit regional variability. It is also useful in
grading filters in runs in which variability has been deliberately introduced into the
filtering process. The testing jig is composed of a fiber-coupled spectrometer with broad
band light source. Collimating optics are attached to the source fiber. The collimated
output is passed through the plate; the spectrometer's collection fiber is th~ receiver on
the opposite side of the plate.
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To separate the fiber bundle assemblies. the heat shrink tubing is first slit
approximately 1/16" on unfiltered end so that a lengthwise halving is started. This is
best done with a razor blade, being careful not to scratch the fibers. By simultaneously
grabbing and pulling each side of this split with pliers or forceps, the split travels up the
5 tubing and the bundle pops out unscathed and unscratched. It is akin to peeling a
banana. The bundle can be readily separated into individual fibers by gently pulling with
fingers.
Each fiber may be tested individually if high reliability is required for the
final application. To minimize the complexity of this operation, the light can be coupled
lO into the fiber from a diffuse source. Ideally, this source is a large planar surface. This
type of source minimizes testing complexity as it minimizes positional sensitivity of the
fiber relati~e to the source plane. For most wavelengths, a suitable source can be readily
constmcted by positioning a common, househoid flood light behind a frosled glass plate.
For infrared filters, a heat lamp is preferred.
As a variation, the fused silica plate can be made thicker such that
increased handling protection is provided to the fibers. Although the fused silica material
characteristics provide the described advantages, other materials are acceptable. The
alternate materials may be preferred for volume manufacturing operations in which the
quantity warrants modeling and stabilizing all the variables associated with the thin-film
20 process. Aluminum and PTFE plates have both been successfully employed in testing.
The polymer manufactured by DuPont under the trade name Vespel reportedly has low
out-gassing characteristics and is probably suitable or even preferred; however, its usage
is cost prohibitive.
The described tooling configuration also ~u~olls the application of filters
25 to angular fiber end faces. The fiber bundle, prepared in keeping with the described
methodology, is ground and polished at the desired angle. The tooling plate is drilled at
an angle relative to its planar surface. The bundle is inserted in the angled hole such that
the end faces of the fiber in the bundle are co-planar with the surface of the tooling plate.
For high-yield filtering runs, an adaptation is well suited. In this
30 adar!ation, a larger quantity of fibers is bundled. The bundle can be formed and held in
a variety of fixtures; however TFE fluoropolymer heat shrink tubing is preferred. This
polymer has very low out-gassing characteristics, yet is gentle on the fibers. The outer
diameter of the finished fiber bundle assembly is l/2" to l-l/2".
The bundles of fibers, packaged in heat shrink tubing, are readily
35 produced. Fibers are first cut to a manageable length. This length may be anything from
a few inches tc many meters depending on the required volume. The desired number of
fibers are collectively inserted into the heal shrink tubing. Individual fi~er insertion
should be avoided since it may result in the fibers scratching one another due to the sharp
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end faces. Following filling the tubing with fibers, the fiber bundle acsembly is heated
so that the tubing shrinks. During this operation, the fibers temporarily bind together (if
the fibers are of the type coated with polyimide buff~r). Next, the fiber bundle assembly
is sliced into segments of the desired length. The best method is by using a fine-grit,
sintered-diamond, high speed saw. Each fiber bundle assembly is ground and polished
to a fine finish, preferably on both ends.
This large bundle is held in tooling within the filtering chamber. If both
ends of the fiber bundle assembly are ground and polished prior to the thin-film filter
deposition, then the filtered fibers can be graded or verified while the bundle is intact.
For the grading Ihe described test jig's configuration is slightly altered. The finished
filtered fiber bundle assembly is passed between the spectrometer's source and collection
fibers. No collimated optics are required; the light is fiber-coupled directly into and Ollt
of the fibers in the fiber bundle ~ssembly. The fiber bundle assembly is moved with an
x-y positioner so that each fiber can be separately inspected. The spectrometer-coupled
fibers should be similar, or smaller, in diameter than the bundle fibers so that individual
fibers can be readily isolated for testing.
As another tooling option, a thin PTFE plate (approximately l/8" thick) is
drilled with holes slightly smaller than the fibers' outer diameter. The fibers, with at
least one end face polished, are individually inserted through these holes so that an
20 interference fit holds them in place. The plate is mounted over a small aluminum box so
that the rear, unpolished, protruding ends are protected. The polished fiber end face is
mounted such that its surface is flush with the plate's outer filter coating plane. If
desired for extra protection, a thin aluminum plate with holes positionally matched to
those of the Pr~ plate can be fixed over top of the PTFE plate. The aluminum plate' s
25 holes are slightly larger than the fiber diameter. The fibers are firmly held by tight holes
in the TFE plate and protrude upward through the larger holes in the alllminum plate so
that the fiber end faces are flush with the outer surface of the alllminum plate.
The described tooling options also support thin-film coating of complex
contoured fiber end faces. Referring to Fig. 80, an important subset of these end faces
30 are fibers 8045 with cone-shaped end faces 8054. For this variation, the fibers are
individually ground and polished to create the cone contour 8054. Following thisoperation, a bundle is forrned with the fiber ends aligned with one another along a plane.
The filter 8050 is applied over the cone surface 8054. This cone filter method is
particularly effective in making filtered fibers X045 that will not back propagate light
35 reflected 8060, 8062 from the filter (both in the filter's pass and reject spectral
regions). This filter configuration offers the advantage over a planar-angled filtered end
face in that the fiber's optical axis remains essentially concurrent with the fiber's
mechanical axis. In single-mode fibers, the core is typically about four microns. Thus,
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the change in height of the active core area, across the conical contour. in relation to the
coating plane, is small. Hence, the influence on filter spectral characteristics due to the
distance deviation between the cone shape substrate and the deposition source ismanageable.
To control reflections, the angle 13 between the cone's base (perpendicular
to the fiber's mechanical axis) and cone's outer surface (hypotenuse), is between 0~ and
20~. To significantly reduce back-propagated reflection in multi-mode fibers, the angle
should be greater than the fiber's angular limits for sustained propagation. ~y increasing
the cone angle beyond this value, propagation is further reduced. The optimal angle
depends on sev~ral factors including the population angles of propagation (the fiber may
be over filled or under filled). propagation modes in the cla~ing, and the cladding's
tendenc~ to propagate back-re~lected light. Experimentation based on usage parameters
is ~he pr~t'erred method of optimization; nevertheles~ times the stated value is il
good starting point for experimentation. Other factors influencing the design include the
desired light pattern on the light-exit side of the filter and the spectral shift associated
with an_le of light incidence with respect to the filter surface. For sin~le-mode fibers, 4~
significantly reduces the back propagation of filter reflections. For 8C, the reduction is
greater still.
This filtered, cone-shaped end face 80~4 can be positioned against
another fiber end face 8064 such that filtered light transmits between the fibers. Bv
filling the void areas between the fiber end faces with index-matching gel or index-
controlled material. such as epoxy 8052, the refractive influences of the shaped surface
is controlled. In these manners, the back propagation of filter-reflected light is
controlled, yet the refractive effects of the cone surface is overcome, or controlled.
Similarly, the filtered cone can be encapsulated; the outer surface of the encapsulant is
shaped into a flat, beveled, or other surface.
The filtered, cone-shaped end face 8054 controls back propagation of
reflected light regardless of the direction of incident light. Back -reflected light 8060
due to light 8070 incident on the inner surface of the filtered cone is directed outside the
fiber's angular propagation capabilities. I,ikewise, back-reflected light 8062 due to light
8072 incident on the outer surface of the filtered cone is directed outside the fiber's
angular propagation capabilities.
FLTER DESIGN
The filter's physical implementation (number of film layers, film layer
thickness, etc.) is readily generated with thin-film design optimization software.
Dependence on the angle of incoming/outgoing light is a well-understood parameter.
The primary variables in the design process ls the wavelength blocking and transmission
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characteristics. The filter design process for fiber-optic-based laser-Raman spectroscopy
warrants discussion.
For the laser-delivery fiber, the objective of the filter is to deliver light tothe investigative medium with minimal extraneous, fiber-generated light in the
5 wavelengths of analytical importance. Wavelength shifts from laser line is typically
measured in wavenumbers (cm- l ). The wave number shift of analytical importance is
application dependent. The Stokes (red) shift is more often utilized due to its strength
relative to the anti-Stokes (blue) shift. The fingerprint region (400 cm- l to I 800 cm- l )
is widely important: however, the region beiow 400 cm-l is critical for certain
l0 applications. The required blocking is dependent on conditions of the l-~vestigative
medium, the probe configuration, the relative strength of the desired signal (the strength
of Raman-scattering of the analyte given it.s concentration) and the fiber materials of
construction and length. Generally speaking, the hi,~her blocking in near the wavelenath
of interest, the better. The closer the blocl~ing to the laser line~ the better. The higher the
15 transmission of the laser line, the better. For Stoke-shift analysis, a low-pass filter is
acceptable. A band-pass filter facilitates Stokes and anti-Stokes analysis. The fiber-
generated interference overfills the fiber and therefore is incident on the filter at more
diverse and less perpendicular angles than the primary laser light. Lower numerical
aperture and single-mode fibers waveguide less interference and also deliver the laser
20 light to the filter more perpendicular.
In keeping with these principles, the collecting fibers can be filtered with
a notch filter so that the returning laser light is blocked. The higher the blocking, the
better the performance. The incidence angle of the laser light returning into the notch
filter from the investigative medium, due to elastic scattering processes, is typically less
25 perpendicular than the incidence angle of the outgoing laser light on the band pass filter.
This factor leads to incoming laser light passing through the filter if the filter is designed
for angular incidences based on the fiber's normal angular propagation limits. However,
this off-angle light mostly escapes through the fiber side walls before it can generate a
significant level of fiber interference.
FIBER COUPL~NG
The following novel fiber optic coupling techniques can be used to join a
filtered fiber segment to another fiber (or cylir der-shaped optical element) segment.
However, these techniques are also applicable to general coupling of optical fibers. The
35 basic problems of joining optical fibers with precision alignment, inexpensive
components, and minimal size is addressed.
The fiber segments can be i~ined by a split sleeve with an internal
diameter slightly smaller than the fibers' ou(er diameters. The fiber segments can be
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joined by a thin capillary tubing into which the fiber segment ends are inserted and butted
against each other. The challenge associated with this approach is economically
fabricating the tubing with sufficiently tight tolerance to provide precision fiber
:~lignm.ont. The challenge is increased by adding the additional constraint that the tubing
wall thickness should be extremely thin and/or light weight. For numerous applications
such as m~ l, military, and avionics instrllm~nt~tion, these are critical parameters.
Non-traditional metal working processes can be employed to meet the
described constraints. The coupler is produced by depositing metal on a precision
mandrel. The mandrel is removed. If required, the coupler is chemically machined to
modify dimensions.
A fiber can be utilized as the metal-deposition mandrel. The fiber is
coated with silver or similar conductive material. This coating is applied with
electrolysis coating
For the silver application, the following procedure is suitable. Silvering
chemicals are available from Lilly Tn~hlctries, Woodbridge, Connecticut. Two solutions
are required: MS-400 and MA-300. The MS-400 is, according to the manufacturer's
literature, a solution of 26.6% silver ~ mmine complex, 13% ammonium hydroxide,
and 60.4 % water. According to the manufacturer's lileldtu~c, the MA-300 is a solution
of 15% sodium hydroxide, 2%-10% ammonium hydroxide, 75% water, and a trade
secret chemical. The MS-400 solution is diluted in a ratio of one part to 30 parts
deionized water. The MA-300 solution is likewise diluted. The solutions are mixed
together with the fiber in the bath. After the silver film has formed to a desired
thickness, the fiber is removed from the bath and rinsed. Similar chemicals are available
in the electrolysis plating industries from a number of suppliers. Numerous similar
application methods can be u~ili7P.~I The silver solution and the reducer are combined in
a spray over the work piece.
Metal is deposited over the fiber's silver coat through electroplating
(electrolysis). Nickel is the preferred deposition metal for this electroforming process.
When the nickel reaches suitable thickness (approximately .001" wall thickness for the
aforementioned 400-micron fibers), the piece is removed from the deposition process.
The assembly is heated until the fiber buffer degrades, and the fiber can be removed.
Alternatively, it can be removed by chemical attack. The sleeve is cleaned with acid
which also removes the silver. By flowing acid through the sleeve after silver removal,
its internal diameter dimensionally increases through a chemical machining process .
The removal of the silver increases the internal diameter of the sleeve. This factor can be
utilized to create sufficient gap to facilitate insertion of fibers into the sleeve during the
fiber coupling procedure.
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If desired, the sleeve can be produced entirely with an electrolysis plating
procedure. If platinum-based metal or other high-strength metals are utilized,
extremely thin sleeves can be fabricated. If using platinum then bare, un-buffered fibers
can be employed as the mandrel. Since platinum withstands high tenlpeldlules, the fiber
5 coated with platinum can be heated to sufficient t~ Lur~ that the glass melts from the
sleeve.
Using optical fiber as the electroforming mandrel offers the advantage of
short-run fabrication. To reduce fiber expense, low-grade glass can be substituted in the
fiber for high-purity materials. In volume production, a copper or brass wire can be
10 lltili7.eA; however, specialized equipment must be set up to precisely control the diameter
of the wire. Of course, electroplating can be accomplished directly on the conductive
wire. And, the wire is readily eroded with chemicals to which nickel is essentially
mpervlous.
To better facilitate fiber insertion, the ends of the sleeve may be flanged
15 outward so that the ends taper to the correct diameter. This may be accomplished by
electroforming over a mandrel having the desired shape. Alternately, a straight sleeve
may be swaged outward at each end. Similarly, a sleeve can be chemically machined at
each end or opened with electro discharge ~ g (EDM).
As a general rule, the longer the sleeve, the lower the precision of the fit
20 which is required for low-loss coupling.
In a related procedure, ceramic materials are utilized to form the sleeve.
The ceramics are packed around the optical fiber mandrel. After curing the ceramic
material, the fiber is removed through heat and/or chemical attack.
When inserting the fiber segments into the coupling sleeve, optical
25 m~tching gel or optically transparent epoxy/cement can be used to increase coupling
efficiency. For low-light spe~ oscopic applications, care and testing must be taken with
these materials. Some materials fluoresce or have strong Raman signals which caninterfere with the desired mea~u.~ment.
As an alternative to simply bonding the fibers to the sleeve to form the
30 coupling, the sleeve can be swaged around the fibers. This can be utilized in connection
with or without the epoxy. With precision swaging, fiber alignment can be increased;
thus, the coupling effciency is bolstered.
As another method of joining the filtered fiber segment to another fiber,
the segment can be joined with a fusion splice. This method typically subjects the
35 joining surfaces to extreme temperatures. Therefore, fusion splicing methods can be
effective when the filter is utilized on the distal end face of a fiber optic assembly.
For example, the short fiber segment is coated with a high-performance
filter. The unfiltered end of the segment is joined to a longer fiber segment. The
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assembly is utilized for fiber optic sensing in medical applications and is disposed after
each medical usage.
SPECIAL CASES
As stated, the described methods of coupling optical fibers is applicable
to general optical fibers. Nevertheless, they are particularly useful in solving problems
associated with filtering optical fibers. These filtering applications are neither limited to
thin-film depositions on the optical fiber end face nor to filtered fibers in Raman
spectroscopic applications.
A bundle of fused optical fibers can be directly coated with a filter. This
is particularly valuable for im~ging applications involving inelastic light processes, such
as the Raman effect and fluorescen~e. The subject is ilhlmin~tecl via optical fiber or other
source. The filter rejects the source light from collection by the bundle and passes light
at the analytical wavelengths of interest.
A fraction of the filter layers can be applied to each of the mating fiber
optic end faces within the coupler. The filtered segment of fiber may be filtered with a
Bragg filter.
In contrast to traditional thin-film interference filters, which include
alternating layers of high/low refractive index materials, rugate filters may be applied
directly to the fiber. Since methods described herein produce a high filter yield per
batch, they are particularly useful for rugate filters since the cost of a rugate filter batch
run is very high. And, variability is readily introduced such that numerous, closely
wavelength-spaced filter can be produced in a single filter run.
Although less plefellcd than applying the filter directly to the end face,
the coupling method supports u~ili7.ing filtered wafers. A filter may be applied onto a
thin wafer which is situated within the coupling, between the two end faces. The wafer
may be attached to one of the fiber end faces prior to the assembly; or, it may be simply
inserted as a separate unit.
The end segment of optical fiber can be formed of sapphire so that the
assembly is hardened for harsh environments. The sapphire rod is coated with
amorphous Teflon (DuPont trade name Teflon AF) so that the amorphous Teflon creates
a cladding for the sapphire. This novel method of fabricating sapphire fibers isparticularly well suited to making short sapphire fiber segments. Not only is the fiber
suited for extreme environments, but also it has a very high numerical aperture and
transmission capabilities at longer wavelengths than silica fibers. And, by minimi7.ing
- the length of the sapphire segment, the impact of sapphire's negative characteristics
(birefringence, high Raman signature, expensive, poor flexibility, and others) is
minimi7P.tl The negative impact of the fluoropolymer (differing thermal expansions,
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near infrared attenuation, and Raman signature) is likewise minimize~. AmorphousTeflon is available in the refractive index forrnu~ations of 1.29 and l .3 l . Sapphire has a
refractive index of 1.77. This leads to a numerical aperture (ca~culated as the square root
of the difference between the squares of the refractive indices of the core and cladding)
5 greater than one for both forrnulations. For short lengths, the sapphire rod (unclad
sapphire fiber) is dipped in a bath of solvent into which the polymer has been dissolved.
Amorphous Teflon is available from the manufacturer in this form. Alternative todipping, the rod is spun in a circle like the hands of a clock. It is spun at high
revolution. The solvent/polymer mixture is applied to the end of the fiber whlch is at the
l 0 center of revolution. The revolution forces the fluid down the fiber segment such that a
uniform coat is appiied. Regardless of the application method. the fiber is dried and the
solvent is driven off in accordance with the manufacturer's gerleral processing
instructions. For volume applic~tions. the molten polvn,er may be eYtruded over the
sapphire fiber.
Fig. 81 illustrates a fiber device 8100 that separates signals according to
wavelength and which can be readily fabricated utilizing filters fabricated in accordance
with the current teaching. This device is equally useful for combining wavelength-
separated signals into one fiber. Short, filtered fibers segments 8186 are stacked in a
clear, glass capillary tube 8184. The fibers' end faces are angled - 45~ is preferred.
20 One end face on each short fiber segment 8186 is filtered with a high-pass, low-pass or
notch filter 8188. Each filter is separated slightly in wavelength according to the
wavelength separation between signals. The wavelength separation can be generated by
the methods of introducing variability to a filtering process which are taught herein. As
illustrated the fibers are stacked end-to-end with one another such that the end face
25 surfac~s mate with one another. A fraction of the filter layers can be applied to each of
the mating end face surfaces.
Alternatively to the mating end face having equal mating angles, one end
of the fiber segment is angled and the other end does not mate (it may be flat or have a
lesser angle). The gap between fiber segments is filled with transparent material such as
30 optically transparent epoxy or index-matching gel.
~ or added perfor~nance, the filtered end face may be shaped (for example
into an off-axis paraboloid) so that the reflected light 8192 is focused. The assembly is
joined to a primary transmission optical fiber 8180. The fiber can be smaller in diameter
than the short fiber segments 8186 and the annular spacing filled with a sleeve. This
35 adaptation, not depicted in the illustration, facilitates better performance when the end
faces are shaped for focusing. Light 8190 emitted from the primary optical fiber 8180
core 8192 is incident on the first filter 8188. This filter is a high-pass, low-pass, or
band-stop which rejects the desired wavelength light 8192. This light is directed out
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through the capillary tubing 8184. The remaining light is transmitted through the filter
and is incident on the next filter which similarly rejects the next desired wavelength and
- so on with the remaining filters. The side wall of the capillary tubing 84 can be polished
flat or encapsulated in a clear material so that the refractive effects of light transmission
- 5 through the capillary tubing's cylindrical side walls is minimized. Alternatively, the light
can be coupled into a device, or assembly of optics which preferentially accepts light of
the pattern shaped by cylindrical optics. The device can readily be constructed as either a
combiner of wavelength-separated light or, as in the depicted configuration, as a
separator of such light. This device is useful for sensing applications which require the
comparison of wavelength-separated signals. It is also useful for wavelengt!1 division
multiplexing and related data transmission applications.
Figs. 82 and 83 depict probes whose optical fiber~ ha-e mechanical and
optical axes which intersect at a distance beyond the distal tip of the probe. Fi~ 8~ is a
cross sectional view of a probe 8200 with a center fiber 820~ surrounded bv a ring of
fibers 8202, 8203. For laser spectroscopy, the center fiber 8205 is best utilized to
deliver light since a laser's full energy output is readily coupled into the single fiber. For
spectroscopy utilizing a less focused lamp, better overall performance is typically
achieved by coupling the ring fibers 102,103 to the source. Filters have been applied to
the distal end faces of the fibers so that interference from fiber-generated light is
minimize(l. For laser spectroscopy, the center fiber filter 8204 blocks interference in, at
least, the spectral regions of analytical importance and passes the laser light. The ring
fiber filters 8206, 8208 block laser light and pass light in the spectral regions of
analytical importance.
Fig. 83 is a cross sectional view of a similar probe 8300 using two
fibers 8320, 8322. This figure illustrates important advancements in the art as
compared to that taught by McLachlan et al. in U.S. Patent 4,573,761. First, the fibers
8320, 8322 are filtered on their distal end faces as described above. ~econd, the
fiber's end faces have been flattened by removing a portion 8328,8330 of the fibers
8320, 8322 so that the probe tip is finished in a planar surface. This advancement
serves several important functions. In application media with refractive index lower than
that of the fiber cores, a beneficial refractive effect is created on the delivery and
collection optical axes of the fibers 8320, 8322. Whereas the optical axes 8332,8334 would otherwise intersect some distance away from the probe, they are bent due
to refraction to more converging positions 8336, 8338. Another benefit of this
advancement is that the fibers are more robust and less likely to be damaged; thus, the
need for a co nplex window assembly is reduced to only environments requiring
additional protection. (As described earlier, fiber end pieces segments ca I be formed
from sapphire fibers which are coupled to the primary fibers so that extreme robustness
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is achieved.~ Another benefit of this advancement is that back reflection of source light
which is incident of the fiber's distal end face surface cannot be back propagated. Still
another benefit is the angled filtering, prevents back propagation of filter-reflected light.
In Fig. 83, the axis 832~ is perpendicular to the plane defined by the
S probe's end face. The fibers 8320, 8322 are angularly offset ,01,~2 from the
perpendicular axis 8325. The two angles 01, 02 do not necessarily need to be equal to
one another. When the prove is utilized to either monitor, a flat surface or project
through a window, these angles can be manipulated so that specular reflections are
precluded. Similarly, the angels 01, 02 may be equal, and the entire probe tilted with
respect to the analytical surface/window such that the axis 832; is not pe pendicular to
this surface.
Although not emphasized in the drawing, the filters can be removed from
the end faces of probes configured similar to the illustrations of Figs. 87 and 83. In
keeping with the teaching described herein and in related patent applications~ the filters
can be moved inside a coupling, near the distal tip.
One class of probes utilizes an optical fiber with a fla~, end face (without
intentionally induced refractive effects) surrounded by a ring of fibers which are
essentially parallel to one another and to the center fiber. The end faces of the ring fibers
are contoured such that the optical axes of all the fibers converge. A filter may be
applied directly to the end face of the center fiber so that optical performance is
enh~nced.
Two or more fibers may be positioned as a group with filters directly
applied to the fiber end faces. One or more of the fibers is utilized to deliver light and is
filtered accordingly. One or more of the fibers is utilized to collect light and is filtered
accordingly. When high performance is not required, either the delivery or the collection
fiber filtering may be eliminated. The fiber bundle is positioned against an optical
element which re-directs the optical axes of the delivery and collection fibers so that they
overlap more than they would have had the element not been utilized. This element may
be any element such as: l) a gradient index component, 2) a lens, ball lens, a sphere, or
other refractive component, 3) a concave mirror, 4) an internally reflective paraboloid, 5)
a prism with contour on its internally reflective surface, 6) a diffractive optical element,
7) a waveguide, 8) a light pipe, 9) a partially or fully waveguiding hollow tube into
which a sample is placed; the tube is preferably made of low refractive index material
such as many fluoropolymers (such as the DuPont's Teflon family with Teflon AF the
best); airy solids, so-called frozen smoke, can also be utilized to benefit, 10) a multi-pass
cell such as a White cell or a Harriot cell, l l ) a complex element which combines the
refractive and reflective effects, 12) a holographic beam shaper, 13) an off-axis
paraboloid, or 14) a non-imaging optical element. Similarly, the element may simply re-
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direct the optical axes of the fibers without substantially redirecting them into
convergence. Examples of these elements include mirrors, prisms, and certain gradient
index optical elements.
IMPROVED COLLECTION AND FILTERING OPTICS FOR
CONFOCAL PROBES
For confocal probes, of the type described by Carrabba et al., U.S.
Patent 5,112,127, and Owen et al., U.S. Patent 5,377,004, the image of a source fiber
is projected onto or into an investigative medium. The light beam exiting this fiber is
expanded and then re-focused into the medium. A collection fiber is similarly re-imaged
along the same optical axis, so that its field-of-view is re-focused essentially to the same
focal point as that of the source fiber. Although the devices in the prior art incorporate
imaging optics, they may utilize non-im~ing optics; thus, the terms image, re-image,
focus, project and concentrate are used loosely herein.
One shortcoming associated with the general configuration of these and
similar devices is the inability to intensify the response arising from the focal point.
Although larger fibers and bundles of fibers can be uti1ized as the collection fibers, this
approach is not always effective. The lack of effectiveness is due to the manner in which
the focal point of the larger fiber/fiber bundle is re-imaged around the focal point of the
source beam. In short, most of the images miss one another at the critical focal plane.
Nevertheless, the sensitivity increase which is achieved as a result of increasing the size
of the fiber/fiber bundle is related, in part, to the medium's light transmission
characteristics (particulate scaLI~ g and other factors).
Increased performance can be achieved by shaping the end face of the
collection fiber/fiber bundle. Preferably, the center portion best remains flat so that the
image of this region is projected to concurrent focus with the source beam focus. The
surrounding areas of the collection fiber/fiber bundle is best adapted for lightmanipulation. This manipulation may be produced by either refraction or internalreflection. Internal reflection is readily accompli~he~ with the tÇ~C~ g described herein.
Refraction is readily produced by shaping the end faces of the fibers to create refractive
surfaces. Based on the specific optical configuration of the probe, the medium
characteristics, and application parameters, the desired effect can be optimized with ray
tracing and/or optical design software. For a fiber bundle, consisting of a ring of fibers
surrounding a single central fiber, the ring fibers can be beveled at a refractive angle.
The refractive angle is typically between 5 and 30 (measured between the base of the
bevel angle or cone and the hypotenuse). Likewise, a large single fiber can be formed
into a refractive frustum of a cone. Internal reflection can be utilized by applying a
frustum of a cone to a large single fiber (the side walls are internally reflective). The flat
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region of the frustum should be approximately the same diameter as used for the non-
enh~ ce~l probe. Similarly, the bundle approach can be utilized by forcing the ring fiber
to look through the center fiber's sidewall and on through the center fiber's end face.
The choice of methods which are employed are related to the focusing
S abilities of the probe to which the enhancement is applied. For example, if the enh~ncecl
area of the fiber/fiber bundle is too aggressively redirected, its field-of-view can miss the
focusing optics, and simply look at the optical housing - this is ineffective.
Another method which is valuable is to utilize a collection bundle in
which the fibers are positioned so that their optical and mechanical axes are not co-linear
10 but are converging. This may be accomplished with two or more fibers. In the
preferred approach, a central fiber is surrounded by a ring of one, two, or more fibers.
The ring fibers are tilted slightly inward so that their (its) mechanical/optical axes
converge and intersect with the axis of the center fiber.
IMPROVEDFILTERING
Fibers, filtered in accordance with the teaching described herein offer
significant perform~nce, mini;~ Lion, cost, and robustness improvement for confocal
probes. They can be utilized to filter the delivery flber and/or the collection fiber. They
can also be utilized for the bi-directional, angled filter which combines the optical axes of
collection and delivery.
AMPLlt~D RESPONSE
The earlier section regarding delivering and collecting light along a
common axis describes a method for fabricating a waveguided cell that substantially
increases analytical sensitivity to light-matter interactions. This cell is particularly well
suited to fiber optic interfaces which are filtered in accordance with the current te~ching.
However, its utility is certainly not lirnited to these interfaces.
Fig. 84 depicts a waveguided cell 8400 configured for transmission
(absorption) analysis of a fluid. Preferably, the cell is formed such that the intemal
surface of the tube 8464 is made of the material which is sold by DuPont under the trade
name Teflon AF, or more commonly known as amorphous Teflon. For fluids with
sufficiently high refractive index, other materials are acceptable; these include the
fluoropolymer sold by DuPont under the trade narnes Teflon FEP, Teflon PFA, and less
preferably Teflon 1~ and Tefzel. At one end of the tube 8464 is a source fiber 8450.
At the other end of the tube 8464 is a collection fiber 8452. An inlet port 8458, at one
end of the tube, delivers fluid 8454 into the tube 8464. Fluid 8456 exits the tube
8464 through an exit port 8460, at the other end of the tube 8464. As light 8462 is
waveguided down the tube 8464, it interacts with the fluid in the tube. By spectrally
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comparing the light received to the light delivered, the tr~n.cmi.~sion characteristics of the
fluid is readily ascertained. One method of making this co~ alison is to fill the cell with
a reference material before or after taking the primary mea~uie,llellt. Another method is
to divert a fraction of the source light to a detector.
The cell can also be used for analysis based on the Raman effect and
other weak inelastic responses. By delivering laser light through the source fiber 8450,
inelastic light-matter interactions (such as Raman scattering) are waveguided along with
the primary laser light. By applying a filter, in keeping with the teaching described
herein, to the end face of the source fiber 8450, purity of laser light is enhanced.
Furthermore, inelastic light radiating from the sample, captured by the waveguiding
capabilities of the tube 8464, and back propagating towards the source fiber 8450 is
reflected by the filter, towards the collection fiber 8452. By applying a filter in keeping
with the teaching described herein, to the end face of the collection fiber 8452, the laser
light is reflected back and the inelastic light is allowed to pass into the collection fiber
l S 8452 for propagation to the detector.
By choosing the collection fiber's filter's spectral characteristics are
chosen so that a small fraction of the inelastic light is transmitted and the remainder is
back reflected, then a resonate cavity is created for the inel~ctic light.
The utilization of a Bragg-filtered, single-mode fiber as the source fiber
8450 can offer increased pe,rollllance. By using an optical isolator on the delivery
optical path, light re-entering the laser can be ~.~i"i"~ e(l
The laser light may also be precluded from multiple transverse reflections
and from back propagation into the laser by positioning an angled filter at the collection
end of the cell (before or after the primary collection fiber filter). To accomplish this, the
angled filter passes inelastic light and reflects laser light at an angle so that the laser light
is diverted outside of the cell.
For simple operation in the analysis of weak inelastic light-matter
interactions, a bundle of one or more, pLefel~bly filtered source fibers and one or more,
preferably filtered collection fibers can be butted up to the waveguided tube which is
filled with the sample.
An amorphous Teflon tubing is well suited for producing the waveguided
cell since it exhibits a favorable refractive index in relation to water-based media.
However, another technique is also useful. The liquid is spiked with substance which
increases it refractive index. For example, the addition of sodium chloride to an
aqueous-based solution raises the refractive index of the solution as the salt dissolves.
In essence, the analytical medium is doped with a refractive-index-altering additive -
similar to doping the glass which is used in optical fibers. If an additive is chosen which
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exhibits wea~ inelastic light-matter interactions, then the additive's response can be used
as an analytical reference for comparison wi~h inelastic responses from the analyte.
Fig. 85 illustrates an embodiment that is configured for usage without
optical fibers. This configuration offers an advantage, compared to fiber-coupled
devices in that there is no interference from signals generated by light interactions with
the fiber materials. Since there are no fibers, the filtering requirements are simplified.
The waveguiding tube 8574 is plugged with a filtered end piece 8580. The filter 8578
allows transmission of the laser light 8570 so that this light can enter the tube 8574 and
interact with the investigative medium 8576. The filter 8578 rejects inelastic lioht
l O 8572 and directs it for subsequent processing and analysis.
The opposite end of the waveguiding tube 8574 can ha~e a light trap
which reflects inelastic light and absorbs laser light. A suitable light trap is created by
applyillg a filter 8584. which passes the laser ligh~ and reflects the wa~elength shifted
light, into a transparent plug 8586. The outer side of the plug 8586 is coated with a
light-absorbing material 8582 such as a carbon film. Alternatively, the plug 8586
contains light-absorbing material which is preferably loaded to the outer end (so that the
desired reflection is not inadvertently impeded).
The non-fiber coupled configuration is well suited to laboratory analysis
in general and analysis of plasma blood chemistries in particular.
Laser beams are readily narrowed to small diameters. This attribute can
be exploited for benefit in a non-fiber-coupled configuration of a waveguided cell for
analysis of inelastic light-matter interactions. Fig. 86 depicts a cell which utilizes this
attribute. The laser beam 8600 enters the expanded-beam, light acceptance/delivery
pattern 8612 of the cell 8610. It is reflected by a mirror X602 into the cell 8610. The
light emerging from the cell 8610 (laser line and inelastic) is collim~ted with expanded
beam optics 8606 and filtered. Preferably, the filter 8604 passes a portion of the
wavelength-shifted light and reflects the laser light back into the cell 8610. A light trap
for the laser line 8614 controls the extent to which the laser light resonates within the
cell.
In a similar manner, the cell can be configured such that it has a high
numerical aperture As such, it is capable of accepting light beyond the angular limits of
the expanded beam optics. Thus, the laser beam can be introduced into the cell along
side of the expanded beam optics and at a slightl~ larger angle.
Fig. 87 depicts a schematic representation of a probe assembly 8700 that
incorporates a waveguided cell for low-concentration analysis of chemicals in remote
locations. The probe is particularly well suited to in situ analysis of environmental
conditions such as in ground waters. The probe housing 8718 is streamlined to
promo~e deployment in minimal space conditions and is readily hardened for cone
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penetrometer usage. The source and collection optical fibers 8710 interface 8712 with
the waveguided cell 8714. The length of the cell tubing 8714 is chosen to facilitate
response from the analytes in the sample. The tubing is coiled around a mandrel within
the housing 8718. When positioned to the desired measurement location, the
surrounding medium (such as ground water) is drawn through a particulate filter 8716
and into the cell X714 for analysis.
The waveguided cells described herein, when energized with sufficiently
intense laser light, can intensify the Raman response in a nonlinear fashion. This
intensification is based on the stim~ tPd Raman effect. As the intensity increases, the
ratio of stim~ t~d Raman light to spontaneous Raman light increases.
Related patent applications have described methods which yield side
viewing and/or side delivery optical fibers and incorporation of the fibers in probes. The
fibers are also well suited to producing probes which amplify the Raman response by
surface enhancement ("SERS"). The surface-e~h~nced coating/treatment may be applied
lS directly to one, or more, of the probe fiber side walls. Thus, the side wall of the probe's
delivery fiber and/or the collection fiber is treated directly. In either case, the laser light
in incident on this sensitized area and the desired response is collected by the collection
fiber. As an alternative to treating fiber side walls, a film, plate, or similar material, is
introduced into the probe. By placing the film between the fibers, the light is incident on
one side of the film and collected on the other side. The film may also be positioned to
the side of the delivery and collection fibers (as opposed to between them) such that the
source fiber projects light onto and the collection fiber receives light off of a common
region of the film. The configurations described above are suitable for indicators and
fluorescence enh~ncers as well as surface enh~nned Raman spectroscopy.
The side delivery/side collection fibers are also suited to creating a
resonate cavity/semi-~esondle cavity micro-probe. The cavity is created between parallel
fibers such that Raman-scall~,led light and/or laser light bounces back and forth multiple
times within the cavity. To form the cavity, the side walls of the fibers are flattened in
the area between the fibers. Reflective filters are applied to the flat regions. ~aser light
may be introduced into the cavity through side delivery from one of the fibers. Inelastic
light may be collected from the cavity through side collection by the other fiber. Fig.88
depicts a probe in which only the inelastic light resonates within a cavity. Laser light
8836 is introduced into the cavity by a filtered fiber 882~, which excites the sample
molecules within the cavity. Inelastic light resulting from the excitation bounces back
and forth within the cavity between the mirror 8832 on the side wall of the "dead" fiber
8828 and the filter 8830 on the side wall of the collection fiber 8826. A fraction of the
inelastic light passes through the filter 8830 and into the collection fiber 8826. The
shaped surface 8834 of the collection fiber 8826 redirects the inelastic light for
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propagation to the detector. The filter 8830 serves the additional role of preventing the
introduction of laser light into the collection fiber.
By employing methods described within the microscale filtering and
manipulating fiber optic light micro-sized resonate cavities are readily created in
numerous configurations in accordance with application requirements.
S~MMARY OF THE DETAILED DESCRIPIION
Prom the foregoing description, it will be appreciated that the present
invention provides a method and a~aratus for improved fiber optic light management.
By applying a various light m~n~gemP.nt and manipulation techniques, one may construct
a fiber optic probe assembly that is ideal for low light spectrographic analysis. In an
exemplary system, the probe improves response to subtle light-matter interactions of
high analytical illl~JVI ~nce and reduces sensitivity to otherwise dominant effects. This is
accomplished by adjusting the illumin~tion and collection fields of view in order to
I5 optimize the probe's sensitivity. Light manipulation is applied internal to the fiber so that
the probe's delivery pattern and field of view do not require external manipulation and
are not adversely affected by the investigated media. This allows the light delivery
pattern or field of view or both to be aggressively steered off-axis to achieve significant
increased performance levels. Aggressive beam steering is accomplished by employing
internally reflective surfaces in the fiber. A reflective metal coating or low refractive
index coatings or encapsulants can be used to ensure total internal reflection. The fibers
also incorporate filters, cross-talk inhibitors and other features that provide a high
performance probe in a robust package. Design variations provide side viewing,
viewing through a common apellul~, viewing along a common axis, and other features.
Various embodiments have been described herein. However, the
illustrations and text are intentle-l to teach various aspects of light manipulation that can
be readily applied to various fiber optic applications. ~ellllulalions, derivatives, and
combinations of these methodologies can be readily formed to solve numerous
application-specific problems that have previously plagued both the fiber optic industry
in general and photonic hlsl~ .rlt~tion practitioners. In order to present this t~rlling as
effe~ ,'vely as possible, an exhaustive list of applications and variations is not presented.
Additional variations and applications should be within the level of skill of those who are
knowledgeable in this general subject area.
The present invention has been described in relation to particular
embodi~,lellls which are intended in all respects to be illustrative rather than restrictive.
Alternative embodiments will become appa,cnt to those skilled in the art to which the
present invention pertains without departing from its spirit and scope. Accordingly, the
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scope of the present invention is defined by the appended claims rather than Iheforegoing description.
