Note: Descriptions are shown in the official language in which they were submitted.
OS8'Lt~
1936-1~21D
This is a di.visional application of copending Canadian patent
application Serial No.37~,300 :Eiled March 31, 1981 and assigned to
Chevron Research Company.
Technical Field
The invention relates to the field of radiant energy modulation
in optical fibers.
Background of Prior AIt
The prior art for either phase modulation or frequency
modulation of, for example, light in an optical fiber utilized the
acousto-optic effect wherein the signal to be imposed onto the light
carried in the fiber is used to mechanically or acoustically excite
the fiber. This mechanical or acoustical excitation gives rise to
a variation in the optical index of the core of the fiber. The
result is a variation in the optical path length for the light
traveling in the fiber. This light is therefore modulated in phase
and frequency by the signal. For glass fibers the change in optical
index is quite small for a given energy of mechanical or acoustical
excitation. In order to obtain sufficient modulation, this necessitates
either high signal energy or long interaction lengths where the
interaction length is the length of fiber which must be acoustically
excited wherein modulation occurs. The sensitivity of optical
fibers to direct acoustical modulation is discussed by J. A. Bucaro
in Applied Optics; Vol. 18; No.6; March 15, 1979.
-- 1 --
5~3~ '',
--2--
Tlle invention constructs single mode fiber which has reduccd clad
thickness for use in sensors which allow signal energy to stretch single
mode fibers to cause phase modulation. The invention also constructs low
order mode optical fibers from large diameter optical fibers. The invention
accomplishes these two efforts by etching presently available optical fibers.
As discussed in "Acoustic Sensitivity of Single Mode Optical Power
Dividers", by S.K. Sheem and J.H. Cole in Optics L,etters, Vol. ~; No. 10;
October, 1979, the prior art etches-single mode fiber in order to decrease
its light guiding capabilities withou* any regard to its increased or de-
creased acoustic sensitivity or change in mode structure. Such an effect,
decreased light guiding capability, is considered detrimental to the inven-
tions purpose and the invention specifically provides means to minimize it.
This invention also uses length limited distributed wavelength
reflectors to reflect light within a single mode optical fiber. Such reflec-
tiOlI is discussed in our U.S. Patent No. 4,268,116, of May 19, 1981, entitled
METHOD AND APPARATUS FOR RADIANT ENERGY MODULATION IN OPTICAL FIBERS, and in
"Photosensitivity in Optical Fiber Waveguides: Application to Reflection
Filter Fabrication" by K.O. Hill et al in Applied Physics letters, 32(10),
15 May 1978.
The invention also uses reflectors which cause reflection within an
optical fiber in an arrangement which resemblas a Fabry-Perot interferometer.
Such an arrangement is discussed in "Fiber Optic llydrophone: Improved
Strain Configuration and Environmental Noise Protection" by P.G. Cielo in
Applied Optics, Vol. 18, No. 17; 1 September 1979. The invention provides a
novel detection system which uses this arrangemen* of reflectors as one of
its many components,
Brief Summary of Invention
The invention comprises a novel type o~ fiber optic energy sensor,
a method of manufacturing such type, and an optical demodulation system which
can be used to convert the output of this type of energy sensor as well as
other types of
lZ~5
-3-
energy sensors into a more easily handled electronic signal.
The invention uses etched single mode fiber for the
energy sensorn The energy sensor operates as follows:
The s-ignal energy to be sensed or detected is caused
S to stTetch an etched single mode fiber. An etched single mod3
fiber is a single mode glass clad fiber whose cladding thick-
ness has been reduced a specific amount so ~s to lower i~s
strength. The invention pro~ides that,~Yhen necessary to main-
tain the etched single mode fiber's light guiding properties,
the portion o the glass clad which was removed is replaced
with a plastic material whose optical index is lower ~han that
of the s-ingle mode fiberls core material and whose elastic
- modulus is lower than that of the glass clad which it replaces.
Such an etched singlls mode fiber is more sensitive to s*retch-
ing or compression because it is weaker. For a given amountof ~ignal energy~ a single mode fiber will stretch a greater
amount after it has been etched.
The pr;or art teaches that stretching a length of
single mode fiber causes ths optical path length for electro-
magnetic radiation traveling in its core to change. The artfurther teaches that this change in optical path length in-
creases as the amount which ~he single mode iber is stretched
increases. The prior art uses this change in optical path
length to modulate the elec*ro-magnetic radiatiorl traveling in
the core of the fiber. The prior art also teaches that the
amount of modulation increases as the optical path length change
increases in magnitude. Therefore, a fiber optic energy sensor
constructed with etohed single mode fiber and operating by
longitudinally stretc:hing or compressing the etched single
mode fiber will for a given amount of signal energy produce
greater modulation w}lich results in greater sensitivity.
The invent.ion also uses the etching process to produce
vptical fibers with low modal dispersion from op~ical fibers
having larger diameters.
The invention also provides a manufacturi~g prvcess
to construc~ devices using etched optical fiber~ This process
causes a form to be constructed of materials which will not
affect the etching process. These forms are used to maintain
the fiber to be etched in the same configuration as it is to be
. .
~ 58~
in the actual device. Various means ~re also detailed which
permit the form to be removed if it is to be absent in the
actual de~ice.
Finally, the invention provides an optical demodu-
lation system which renders energy sensors more useful byactually optically demodulat~s ~he output of the energy sensor,
thus substantially reclucing the previously enormous bandwidths
required of the electronic demodulation equipment. The optical
demodulation system presented also allows for multiplexing
several energy sensors on~o ~he same optical ~iber, thus sub-
stantially reducing cc,sts for a multi-sensor system such as
a hydrophone array.
The optical demodulation system locates each energy
se.nsor between the members of a pair.of length limited Bragg
reflectors formed inside of the optical iber. Each pair of
reflectors as arranged., forms a Fabry-Perot type inter~er-
ometer which only ont.ains resonances ~or those portions of
the electro-magnetic spectrum in which the Bragg reflectors
operate. Since each energy sensor is located between ~he re-
flectors of a pair, then as signal energy is.detected thesensor's resulting optical pa~h length change causes the
resonances of ~he Fabry-Perot intererometer to shit spectrally.
The system then partia.lly demodulates this spectral shift
using a second Fabry-Perot interferometer, called the analyzer
inter~erometer, whose resonances have a spectral separation
relative to that of the interferometer containing the energy
sensor so as to cause amplification of the spectral shiftO
The output o~ the combination of the energy sensor interfer- I
ometer and the analyzer interferometer:shifts spec~rally more
than the original spectral shift by an amplification factor
given-by equations presented-in the-detailed description of
the invention. The system also allows ~he use of more than
one analyzer interferometer, each causing different amplifi-
cation. The resulting amplifications can be arranged tG pro-
vide outputs each of which corresponds to a separate digit o~the number expressing the original s~ectral shift thus re-
ducing the bandwidth of the electronic detectors and time
demodulator.
~;~OS~9
~he optical demodulation system finally allows for
multiplexing several energy sensors onto the same fiber hy
causing each reflector ~air corres~onding to each sensor to
have different reflection bands from all other reflector pairs.
~he system uses a wavelength scanning laser, the
output of which scan~3 over the resonances of one reflector pair
at a time.
This divis:;onal application is particularly concerned
with apparatus and method for phase modulating electro-ma~netic
radiation travelling in the core of an optical fiber and appa-
ratus and method for guiding electro-magnetic radiation.
The latter apparatus comprises an etched optical
fiber having low modal dispersion, wherein the optical fiber
having low modal dispersion is coated with a material having
an optical inde~ lowelr than that of the core of the fiber.
The phase modulating apparatus comprises an etched
optical fiber; and means for longitudinally stretching or
compressing the etched optical fiber, wherein the etched opti-
cal fiber is coated with a material having an optical index
lower than that of the etched optical fiber's core and having
a modulus of elastlcity lower than that of the material which
was removed by etching.
The invention will now be described in greater detail
with reference to the accom anying drawings, in ~hich:
FIGURE 1 is a cross sectional view of a single mode
optical fiber greatly magnified;
FIGURE 2 is a greatly magnified, cross sectional
view of the single molle optical fiber of FIGURE 1, which has
been etched in accordance with the invention,
FIGURE 3 is a greatly magnified cross sectional
view of a large diame~ter fiber of core material;
5~9
FIGURE ~ is a grea-tly magnifled cross sectional
view of the large diameter fiber of FIGUP~E 3 after having
been e-tched and coated in accordance with -the invention;
FIGURE 5 is an end view of an acoustic ener~y sensor
shown in EIGURE 6;
FIGURE 6 is a cross sectional illustration on line
~-6 of the acoustic energy sensor of FIGURE 5;
FIGURE 7 is an enlarged partial illustration of the
acoustic energy sensor of FIGUR~S 5 and 6;
FIGURE 8 is an end view of another acoustic energy
sensor provided by the invention,
FIGU~E 9 is a eross section on line 9-9 of the
acoustic ener~y sensor of FIGURE 8;
FIGURE 10 is an illustration of a form and a single
mode optical fi~er as arranged by the invention for the purpose
of manufacturing fiber optie energy sensors;
FIGURE 11 is a eross seetion of only the form of
FIGURE 10 after having been coated with a guard material in
accordance with the invention;
FIGURE 12 is an illustration of the form and single
mode optical fiber of FIGURE 10 after etchiny and coating in
aeeordanee with the invention;
-5a-
PIG. 13 is an end view of a collapsible ~orm which
could be used in the etching process;
PIG. 1~ is a schematic diagram of an optical de-
modulation system provided by the invention;
FIG. 15 is an illustration of a typical transmission
of a reflector pair of the pairs 25 in FIG. 14;
FIG. 16 is a graph of a laser output suitable for
use in the optical demodulation system of FIG. 14;
PIG. 17 is a schematic diagTam of a multiple
analyzer interferometer demodulator which the invention p~ovides
as a substitute for the part of the optical demodulation system
which is enclosed by dashed lines W in FIG. 14; and
PIG. 18 is a schematic diagram o an example time
demodulator circuit illustrated in FIGS. 14 and 17.
Detailed Description of the Invention
__
The invention comprises a highly sensitive fiber
optic, energy sensor and an optical demodulation system which
can convert the output of the energy sensor to an electric
analog signal. First the energy sensor will be discussed and
second the demodula~ion system will be discussed.
The present art of fiber optic energy sensors teaches
*hat if a single mode optical iber is compressed radially, or
stretched, or compressed longitudinally~ then the optical path
length for electro-magnetic radiation tTaveling in the core of
the single mode optical fiber changes. The art further teaches
that as the amount which the single mode fiber is stretched or
compressed increases, then the change in the optical path
length also increases. The present art uses this change in
optical path length to cause phase modulation of the light
traveling in the core. The length of optical fiber in which
modulation occurs is called the interaction length.
The invention pro~ides etched single mode fiber for
use in fiber optic, energy sensors. Single mode fiber is a
fiber constructed so as to allo~y only the lowest order mode
to propagate. This lowes~ order mode fo~ some single mode
fiber constructions i5 two fold degenera~e. In ~hese cases,
the lowest order mode contains ~wo states of propagation which
are distinguished by the fact that their polarizations are
mutually perpendicular.
7~20s~t~
Etched single mode flber is defined herein as
single mode optical fiber ~hose clad thickness has been re-
duced by a chemical reaction ~e.g., etching in a bat~ of hy-,
drofluoric acid or a Datn of hydrofluoric as,id b,uffered with
S ammonium ~luoride), or ion millin~., ,
FI~. 1 is a magnified cross sectional ~iew of a fiber prior to
etching. FIG. 2 is a magnified cross sectional view of the
fiber after etching. In FIG. 1, the glass clad generally
designated,as 2-l, is shol~n to have a thic~ness designated K.
10' In FIG. 2, the clad 2-2 is shown to have a reduced thickness
designated as R. In both FIGS. 1 and 2~ the core designated
as 1 1 and 1-2 has a diameter V ~hich remains unchanged by
the nature of the etching process, which occurs only at the
exposed surface of the fiber.
The utility of such a fiber is e~plained first in
terms of sensitivi~y followed by an explanation of the ease
~f manufacture of de~ices employing etched single mode fibers.
For a given amount of signal energy, E, to be detected, the
, fiber of length L and total cross sectional area Sl, of FIG. 1',
will stretch an amount ~Ll as belo~:
~Ll ~ ~ EQ I
where YO is the modulus of elasticity of the fiber
material and for the explanation may be assumed constant and
equal to that of fused quartz. Using the same derivation above,
but substituti~g the reduced cladding-thickness into EQ l gi~es
the amount of stretch ~L29 which the etched fiber will undergo
for the same given amount of signal energy E,
~L2 ~ ~ EQ II
where S2 is the cross sectional area o~ the etched fiber
Since S~ is greater t:han S2 then from FQ 1 and II, ~L~ ~ ~Ll.
The present art of fiber optic sensing teaches therefore, that
for a given amount oi' signal energy the e~ched single mode
fiber will have a greater change in optical path length than
the normal single mode fiber, resulting in a greater amount of
2~ 3
phase modulation o light traveling in the core.
Further utility can be understood realizing that
fibers ha~ing very small overall diameters are difficult to
construct using present methods and even if constructed, are
difficult to handle. Through the teachings of this invention,
devices which might use fibers with reduced clad thickness can
be construc~ed with readily available larger diameter fibers.
When such devices are assembled to the point ~here the larger
fiber is in place, the fiber can ~hen be etched7 thereby elimi-
nating further handling o~ a thin fiber or a fiber with a Te-
duced clad. A more detailed explanation of this process follows
later.
Further utility can be recognized when the need to
construct fibers with small core diameters arises, and the
invention allows such fibers ~o be constructed from larger
diameter fibers. FIG. 3 shows the cross section of a large
diameter optical fiber, 3-3, which is a core material, (e.g.,
silica glass), of diameter F. The large diameter fiber is
etched thus producing the thin fiber whose cross section is
shown as 4-4 in PlG. 4, having a reduced diameter G. The in-
vention further provides that fiber having a diameter G can
then be coated with a material 5-4 having a lower index o~ re- ¦
fraction, such as RTV ~70 silicone rubber produced by ~eneral
Elec~ric Corp., than the~ fiber itself thus producing an optical
fiber wi*h a small core diameteru Such a small core diameter
fiber is useful for having a low number of guided optical modes.
As an example of ~he etching process, the fiber 3-3,
FIG. 3, may have a pre-etching diameter in the range o~ ~3JU~
to lOO~m and the etched core 4-4, PIG. 4~ may have a diameter
in the range of 50~m 1:o 5~m .
The invention provides the particular hydro-acoustic
energy sensor shown in l:IGS. 5 and 6, and in part, in FIG. 7.
FIG. 5 is an end view oi the sensor illustrating its cylindrical
shape~ FIG. 6 is a sec1ional view of the sensor. This sensor
consists o~ a rigid cyllndrical skeleton9 perhaps made of
aluminum, designated 6 in PIG. 6. The outside surface of this
cylindrical skeleton has a reduced diameter between planes H
and J. About this cylîndrical skeleton is a membrane of
2~
compliant material ~enerally designated 7, in which are
radial turns of sing~le mode optical fiber generally designated
as 8. Such a compliant material may be~ ~or example, silicone
rubber or PVC. This sleeve is cemented as at 13 and/or clamp-
ed as at 14, to the larger diameter ends 13' of the cylindrical
s~eleton, thus creating a space 9 between the compliant membrane
and the rigid cylindrical skeleton where the diameter of the
rigid form is reduced. In the wall of this rigid cylindrical
skeleton where the diameter is reduced are equalization holes
10 which extend from the inside wall of the cylindrical skeleton
to the space between the compliant membrane and -the rigid skele-
ton. On the inside walls of the cylindrical skeleton are pro-
trusions designated as 11 in FIG. 6. Also, lnside the cylindri-
cal skeleton is stretched a compliant bladder 12 which ser~es
as ballast supply and creates a reservoir 16, which communi-
cates with space 9 by holes 10. The spaces 16 and 9 are filled
with a second Viscou!; compliant material, such as air, He, or a
silicone oil. There is also provided end caps 17 which create
an additional space -:L6' sho~n in FI~. 6, and which are provided
with holes 15, which extend through the thickness of each of
the end caps. The hydrophone sho~n in FIGS. 5, 6; and 7
operates as follows:
The hydrophone is immersed in the fluid containing the
acoustical waves to be measured. At any particular clepth the
invention causes the stat;c pressure in the spaces 9 and 16
to be equalized with the static pressure in the fluid exterior
to the hydrophone by allowing some of this ~luid to enter the
hydrophone through holes 15 and then to stretch bladder 1~
around the protrusions 11, as illustrated by the dashed lines
labeled 12', thus compressing ~he second viscous compliant
material in spaces 16 and 9O l~hen the pressure in spaces 16
and 9, ~lus the additional pressure generated in stretching
the bladder 12 is equal to the exteT;or pressure, the ~luid
stops flowing through holes 15. The holes 15 and/or the equali-
zation holes 10 are sufficiently small so as to slo~ the rate o~equalization to time periods much longer than the time periods
bet~een the acoustic pressures to be measured.
The acoustic signals to be measured or sensed by the
hydrophone consist of alternating changes in the surrounding
~luid pressure. Because these chan~es are not equalized by
~~ -lO- ~ ~O S~ 9 ~ ~
he above bladder mechanism, they instead cause the compliant
membrane 7 to expand an(l contract radially, thus longitudinally
stretching and compressing the etched single mode fiber 8.
For ~hose applications which require the hydro-acoustic
5 sensor o~ FIGS. 5, 6 an~l 7 to be in motion while it is being
used to sense hydro-acoustic signals, the invention provides
strength strands, for ecample, fibers 8' in FIGS. 6 and 7 ,
attached parallel to the axis o~ the rigid cylinder. The fibers
8' are cemented to the outside and/or inside surface o ~he
lO `compliant membrane 7 and extend under each clamping ring 14
about the rigid cylinder 6. The clamping surfaces are the
portions 13' of the rigiid cylinder to which the compliant mem-
brane 7 is attached. Such strength fibers 8' may be of Kevlar,
a tire-cord fiber made by DuPont, OT glass. Such strength
15 fibers 8' are placed to increase the longitudinal strength of
the compliant membrane ;r. Therefore, if the hydro-acoustic
sensor of FIGS. 5, 6 ancl 7 should be accelerated in the direction
of the axis of the rigicl cylinder 6 then the resulting deform-
ation of the compliant membrane 7 will have been lessened by
20 the strength fibers ~' 3n FIGS. 6 and 7. Further, the strength
fibers 8' when placed pclrallel to the axis of the rigid cylin-
der 6 will not substant;ially increase the resistance of the
compIiant membrane to rcLdial contraction as caused by the acous-
tic signals to be sensecl. FuTther, the mass of the compliant
25 membrane may be varied as well as the density of the single
mode fiber turns 8 which wil~ ha~e the affect of shifting its
hydro-acoustic frequency responseO
The invention also provides the hydro-acoustic sensor
of FIGS. 8 and 9. FIG. ~, is an end view of the hydro-acoustic
30 sensor and FIG. 9 is a c:ross sectional view of the sensor of
FIG~ 8 wherein 7-9 designates a compliant membrane within which
is a helix of single mocle fiber 8-9. The assembly also in~ludes
an inner cylinder-202 oi resilient complian~ material such as
silicone rubber, which is in contact with the inside wall of
35 the compliant membrane 7-9. Strength strands, for example,
fibeTs 201 are located parallel to the axis of the compliant
membrane 7-9 and a~e in mechanical contact with the inner cy-
linder material 202 so as to increase ~he longitudinal strength
of the inner cylinder w;thout significantly modifying its
radial compliance. The strength fibers Z01 may be made of
Xevlar or glass an(l may also be continued in length beyond
the ends of the comp]iant material and then may be used to
anchor the sensor ;n place. The inner compliant cylinder
202 may also be continued in length to aid in positioning or
anchoring the sensor. The in~ention also provides tha~ the
streng~h fibers 20l be mechanically attached to the outside of
the compliant membrane 7-9 parallel to the axis of membrane
7-9 to increase it5 longitudinal strength. Increasing the
1~ longitudinal strength of the sensor not only increases sensor
durability but also reduces the amount of radial expansion and
contraction which results from longitudinal acceleration of the
sensor, without reducing i~s ~esponse, i.e., radial expansion
- and contraction, to acoustic signals.
The hydro-acoustic sensor of FIGS. 8 and 9 operates
as follows: The sensor is immersed in the solution contain-
ing the acousticsignals. The periodic alternations o pres-
sure present in an acoustic signal cause ~he compliant ~embrane
7-9 to expand and contract. As the membrane 7-9 expands and
contracts, it stretches or compresses the etched single mode
optical fiber 8-9, thus, as previously set forth~ modulating
the electro-magnetilc radiation traveling inside the core of
the fiber 8-9. Furth~r, since the inner cylinder is also
radially compliant it will ofer less resistance to the ex-
~5 pansion and contractill~n of the compliant membrane. The in-
ven~ion provides ~h~ ~he sensor of FIGS. 8 and 9 may use
etched single mode Eiber for the single mode fiber 8-9.
The etched single mode fiber of the invention has
utility in all eneTgy sensors, which use a signal energy to
longitudinally stretcll or compress a single mode fiber to
cause a change in the fibers optical path length~ Some energy
sensors may employ :Low order mode optical fiber when the modal
dispersion of such Eibers is low enough to maintain sufficient
optical coherence throughout the interaction length. For
these energy sensor; the invention supplies the thin fiber of
FIG. 4. It is to be noted ~hat any optical fiber can be etched
to inCTeaSe its sensi1:ivity to longitudinal stretching and com-
pression, such as m~ ;imode step index or graded index fibers.
-12~ S ~
In cases wherein the glass cladding of step index or
graded index ~iber is removed by etching so as to impair its
electro-magnetic racliat.on guiding abilities, the invention
provides that the resul1ing fiber may be coated with a material-
as at 2-2' PIG. 2 of the dral~ing, having an optical index lower
than the fiber core such as RTV 670 silicone rubber to restore
its ability to guide electro-magnetic radiation. Therefore,
the invention includes in its scope "etched optical ~iber" as
well as "etched single mode optical fiber" and throughout the
specification and claims these terms can be interchanged when-
ever etched fibeT has low enough modal dispersion to suffi-
ciently maintain optical coherence for proper operation of the
device and when the purpose of using etched optical fibers is
to increase sensitivity to longitudinal stretching OT compres-
sion, or when the p1l:rpose is to provide a low order mode-fiber,
i.e., one with low modal dispersion, or for both of these pur-
poses simultaneously.
Manufacturing -~
The inventioll also provides the following method
of manufacture of fiber optic energy sensors which may employ
etched single mode fiber. First, a orm is made which will
maintain the fiber 1:o be etched in the same configuration or
arrangement as it is to be used in the particular sensor being
constructed. In the case of the hydrophone of FIGS. 5~ 6 and
7, the fiber is config-lred as a helix. A suitable form
for this hydrophone is a cylinder 18 as shown in
FI~. 10, around whichL is cut a spiral groove 1~9 in ~Yhich is
wound an unetched optical fiber 20'. If it is desired that
the form be remo~ed af~er etching, the form material should
be a material which can be melted or dissolved into the liquid
state at temperatures or with solvents whichLwill not cause dam-
age to the fiber or the compliant membrane material. Such a
form material is beeswax. ~urther, because some form materials
may jeopaTdize even etching of the fiber ~wax may rub onto
the fiber in places, thus shielding it from the etchant~,
forms of such materials are first lightly coated by dipping
or spraying with solution of a guard material 21' in FIG. 11,
which upon hardening will not affect the etching process.
- 1 3 ~ )S~
Suitable guard materials are: ~ype 139 Low Andex Plastic
Cladding Solution produced by Optelecom, or Kynar, a viny-
lidene fluoride manufactLred by Pennwal~ Chemical Co In
cases where the fiber to be etched will not have a sufficient
s glass clad ~o guide lig~t, the invention provides ~hat the
guard material have.a lower optical index of refraction than
that of the fibeT core. Type 139 Low ~ndex Plastic Cladding
Solution or Kynar have op~ical indices lower than that of
silica glass.
; 10 ~hen necessary, the invention also provides that the
iber be cemented to the form at peThaps the ends of the
etching core as shown at 22' in FIG. 10. The guard materials
already mentioned will suffice as ceme~t.
. When it is necessary to protect portions of the fiber
from the etchant, these portions may also be coated with the
guard materials already mentioned as shown at 23 7 in FIG. 10.
If the fiber optic energy sensor is *o employ etched
singled mode fiber, the invention next causes the form 18'
with fiber 20' in place as shown in FIG. 1~ to be placed in
a bath of either hydrofluo~ic acid or hydrofluoric acid buffer-
ed with ammonium bifluoride 7 or any other chemical which can
dissolve or remove the glass clad of the ~iber. In general,
this etching bath is u.ltrasonically agitated, if necessary~ to
facilitate entry of the etchant around all portions of the fiber
which must be etched.
After the etching period, (which may be determined
empirically),has concluded the form wi*h the IIOW etched fiber
in place is removed from the bath, washed in water, dried and
then dipped in and Tem~oved from a bath of dissol~ed or molten
coating material, or sprayed OT othe~wise coated with a solution
o the material which when cured, dried, or cooled beeomes the
compliant membrane mat.erial. Ultrasonic agitation of the bath
is carried out when necessary to facilitate entry of the coat-
ing liquid around all portions of the fiber. The invention also
provides that the appl.ication of the coating material may take
place in a ~acuum to aid uniformity of coating and elimination
of air pocXets.
In cases using a fiber l~hich after etching does no~
have sufficient claddi~ng thickness to cause electro-magnetic
radiation to be guided in the core, the invention provides
that the coating material have an index of refraction which
is lower than the core material. Such coating baths may be
either of the guard materials already mentioned or silicone
rubber such as General Electric Company's RTV 670. The viscos-
ity of the coating bat}l may be varied as a means of control-
ling the thickness of the coating which remains on the form
upon its removal from 1:he bath. Lo~er coating bath viscosi-
ties will provide thimler coatings. The form having been r~-
moved from the coating bath is then rotated until the coating
has hardened in order to achieve an even coat in the presence
of gravity. ~IG. 12 depicts the form and fiber of FIG. lO
after ~he etching and dipping processes have been completed.
The etched fiber is den,oted as 20-E and the compliant membrane
material is denoted as 124.
After the cc~ating in FIG. l2 has solidified, holes
are drilled through''the coating and guard materials which ex-
tend into the form mat:erial. The loation of such holes mustbe chosen so as to allow the form material to be removed by
melting or dissolving but without damage to the fiber within
the coating. Such a hlole 125 is shown in ~IG. 12. The form
may be of a material such as teflon which may be cooled with
liquid nitrogen to caulse the ~orm to shrink away from the com-
pliant membrane and guard material 9 thus facilitating its re-
moval through a much larger opening 1~6 in FIG. 12 formed by
cu~ting away the membrane at the plane marked P in PIG. 12.
FuTther, it is contemp~lated the fo~m may be collapsed to
facili*ate its removal. For the hydrophone of FIGS. 5, 6 and
7, a suitable form whiic]i could be collapsed is shown in end
viewl PIG. 13.
FIG. 13 is an end view of a cylinder 257. A ~e-
movable section of th'is cylindeT called a key is shown at
256. The key 256 exten(ls parallel to the axis of the cylinder
and for the entire leng1:h of the cylinder. The arrow labeled
ZZ illustrates the mo1:ion of the key ~o facilitate its removal.
Upon the key's remova], the cylinder 257 ~-ill collapse radially
thus allowing its removcLl from the compliant membrane material
-15-
after etching and dipping.
If it is desired to produce a fiber optic energy sensor which uses
a compliant sleeve o~ envelope t:o contain unetched optical fibers, then the
invention also allows the etching ancl washing steps to be eliminated from
the above-detailed manu~acturing process.
Optical Demodulation System
The optical demodulation sys-tem included in the invention is shown
in FIG. 14. Referring to FIG. 14, 24 designates an optical fiber upon which
are mounted pairs 25 of length limited distributed Bragg reflectors. Length
limited distributed Bragg reflectors are, as used in the invention, devices
which cause particular wavelength bands of electro-magnetic radiation travel-
ing in the optical fiber to be in part reflected back to the source and in
part transmitted onward through the optical fiber and allow light which is
spectrally outside of these part:icular wavelength bands to be transmitted on-
ward through the optical fiber nearly unaffected. Such reflectors may be con-
structed by inducing spatially periodic perturbations of the optical index of
the clad surrounding the core of an optical fiber so that the spatial period
exists in a direction parallel to the axis of the core and the required
length of the spacial period does not exceed the length over which optical
coherence is maintained Eor the coptical fiber. Spatially periodic perturba-
tiOIls can be inducecl by partially removing the clad from a length of tho
fiber and then placing tho Eiber ugainst arl optical grating so that the teeth
of the grating aro perl~elldicular to the axis of the core. The ma~nitude of
tho reElectivity m.ly be increclse~l or decreasecl by removing more or less of
the clad thus placing the optical gratitlg nearer or further from the core as
disclose~l in above-lllentionod U.S. Patent No. ~,268,116. Such reflectors may
also be constructed using the metllod developed by ~lill et al and described in
"Photosensitivity in Optical ~iber Waveguides: Application to Reflection
~ilter labricatior~ pplied Physics Letters; #32~10), 15 May 1978, where it
is shown that a reflection wavelength band occurs at:
-16- ~ ~S ~
~ -~ 2nd ~ EQ III
where ACM i.s the ce~ter of the reflection
wa~elength b~nd for a particular
~21ue of 'I;
n is the effective optic index o~
- refraction ~or the optical fiber core,
d is the sp~tial period of ~h.e per-
turbations which create the Bragg
reflector;
and M is an inte~er which is ~eater than
zero ænd will be called the order o~
- the reflec~ion band.
The width, ~cM, is the full spectral width of a particular
reflection band meas~ired at half of the total reflected in-
tensity o which the particular Bragg reflector is capable.It is shown in the prior art.to be:
~cM ~ c~2 EQ IV
2nQ
- where ~ is t;he length of the length limited Bragg
reflector.
Referring again to FIG. 14,the pairs 25 of reflec-
tors are labeled A, B, C, .O.. Both reflectors in each pair
are made to partially reflect the same wavelength bands and to
have the same transmis;ion spectra by for instance~ adjust;ng
d and Q. However, each pair is made so that it reflects partic-
ular wavelength bands which are spectrally different than the
reflection wavelength ~ands of all other pairs, again by ad-
justing d and Q in accordance with EQ II1 and EQ IV, 50 that
: there is a wa~elength :interYal h.I. which contains at least one
of only these particular wavelength bands for each reflector
pair to be used.
~ )s~
-17-
Each pair 25 forms a Fabry-Perot type interferometer .
inside of the single mode fiber 24. This Fabry-Perot type
interferometer is sensitive only to electro-magnetic radiation
which is spectrally within the reflection ~Yavelength bands o~
the distributed Bragg reflectors ~hich form the particular pair.
FI`G. 15 is an illustration of the tTansmission of a particular
reflector paiT. Referring to FIG. 15, the ordinate represents
the tTansmission of electro-magnetic radiation through the par-
ticular reflector pair and the absicca represents the wave-
length of electro-magnetic radiation which is traveling inside
the fiber 24 and is incident on the reflec~or pair.
Electro-magnetic radiation which is spectrally outside of the
reflection wavelength bands of a particular pair is transmitted
pTactically unaffected. Such radiation is shown as regions
lS a in ~I~. 15.
The maximum amount of electro-magnetic radiation
traveling inside the fiber and spectrally within the reflector
bands of a particular reflector pair will be transmitted on-
ward through the reflector pair when the wavelength
.
~ _ 2( , ~ EQ V
where OPL is the opti~al path lQn~th between
the reflectors;
and N is a positive integer.
If ~ = 2(0PL) EQ VI
then a minimum ~mount of the elec~ro-magnetic
radi~tion will be tra~s~ltted onward
throu3h the T~e~lecto~ ~ir.
This results in a spectrally periodic tTansmission as is sho~Yn
in ~egion b of FIG. 15.
As is taught in the field of interferomet~y, the
spectral width of the transmission peaks which are designated
300 in FIG. 15 may be altered ~Yith respect to the spectral
: separation of the tra.nsmission pea~s ~ b~ changing the
~~ -18~ J~ ~ 3
magnitude oE the reflectivity of the length limited Bragg
re~lectors which for~l the pair of re~lectors responsible for
the transmission peaks. This can be achieved as previously
discussed.
The numbe* of peaks 300 in wavelength region b of
FIG~ 15 is given by:
.
X ~ Z EQ VII
Q
where Z is the geometric le~gth bet~reen the
ref:Lectors as mea~ured along the axis
of 1;he single mode ~iber;
~nd Q is 1;he length of the distributed .Bragg
re~lectors as measured along the fiber
axis .
As the optical path le~ngth between the two reflectors of a
pair changes, the transmission peaks shown in FIG. 15 within
wavelength region b will shift spectrally within this region b,
as is indicated by EQ V.
The invention causes some or all of the length of
the optical fiber 24, located between the two reflectors of
a pair, to be the interaction length of a fiber optic energy
sensor, e.g., the acoustic energy sensor of FIGS. 5, 6 and 7.
As previously explained, such sensors operate by allowing signal
energy being detected 1:o longitudinally stretch or compress a
length of optical fiber thus changing its optical path length.
Therefore, for a reflector pair B, for example, inside of which
is located an interaction length of a fiber optic energy sensor
which is detecting a signal the transmission peaks of region b
of FIG. 15 o~ this pair B will shift spectrally as caused by
; the signal energy being detected.
Referring again to FIG. I4, the invention uses a
wavelenght scanning laser 26 to supply electro-magnetic radia-
tion which is injected with suitable focusing lenses 27 into
the single mode fiber 24 upon which are located the reflector
pairs 25. The output of the laser 26 is scanned or chirped
over a particular wavelength range. FIG. 16 contains a graph
of a laser output suitable for the invention. The scan range
is ~L, and is so labeled in FIG. 16. The scan time interval
is ~T and is also labeled as such in FIG. 16. The scan rate is
L The invention chooses the scan range of the laser 26 to
~e the wavelength interval W.I. previously men~ioned so that a
reflector wavelength band region b of FIG. 15 of each pair
25 in FIG. 14 lies spectrally within the scan range.
.Once again referring to FIG. 14, the assembly includes
a beam spli~ter 127 to direct a portion of the laser output
beam to a Fabry-Perot interferometer 28 hereafter called
"ref-erence Fabry-Perot".interfe~ometex. When the output wave-
length ~L of the la.ser is such that
(Q-1-2) ~ D EQ VIII
where Q is a positi~o integer
and D is the optical path length between
the reflectors formi~g the Fabry-Perot
inter~erome~er 20,
then the reference ~abry-Perot interferometer 28 will transmit
some of this radiation to the photodetector 2g of FIG. 14, whic~
will then produce a:n electrical reference signal. Photodetector
29 is a commercial device, e.g. ~TIXL 45~ produced by Texas
Instruments Inc., the output of which is an electrical signal
: ~ 25 the amplitude of which is a known function of the amplikude of
the incident radiation. If the laser is scanning as in FIG. 16,
the transmitted output of the reference Pabry-Perot interfero-
meter 28 will be a ;eries of temporally separated peaks each
one corresponding to a resonance of the reference Fabry-Perot
interferometer 28.
The invention arranges the optical path length D of
the reference Fabry-Perot interferometer 28 and chooses the
spatial periods of the reflectors in each reflector pair 25
so that for the laser scan range ~L a transmission peak of
the reference Fabry-Perot interferometer 28 occurs at a wave-
length very near the reflector wavelength band of each reflect-
or pair 25.
Again referri.ng to FIG. 14, the output end of the
-20- ~ Z(~
single mode ~iber 24, i.e.~ the opposite end from that into
which the laser beam in injected, is associated with a suitable
focusing mechanism 3,2 focused into a Fabry-Perot inter~erometer
130 as shown in FIG. 14. The output o the interferometer 30
is directed onto a photodetector 31.
The prior art of interferometry and the preceding
explanatiOn of the spectral transmission of a reflector pair
; 25 shows that if the scanning laser is at some particular time
injecting a particular wavelength AL of electro-magnetic
radiation into the iber which falls wi~hin the wavelength
region b, shown in ~IG. lS, of a particular reflector pair A,
then-this electro-magnetic radiation will be transmitted
through the partioular reflector pair A~ through the remaining
fiber, *hrough all other reflector pairs (since the invention
causes all other reflection wavelength bands of all other
re~lector pairs, B, C, etc. to be different) 9 through the
Fabry-Perot interferometer 30 and onto the photodetector 31 at
maximum intensity whenever the injected radiation l~avelength
~L is spectrally centered on a par~icular ~ransmission peak
of PIG. lS of the particular reflector pair A and when this
particular transmission peak is also spectrally coincident with
a transmission peak of the Fabry-Perot interferometer 30~here- -
after called the analyzer Fabry-Perot interferometer.
The optical pal:h length TR ~(n)(Z) between the reflectors
~of, e.g., reflector pair B is arranged so that for a par~icular
wavelength region bet~reen ~lD and ~2D the reflector pair B will
produce SR trnasmission peaks. This will occur if:
.
TR ~ /1 - 1 ~ EQ IX
~A1D ~2D)
30 FOT the same wavelengl~h region between ~lD and ~2D 9 the
analyzer ~abry-Perot interferometer 30 will produce SA trans-
mission peaks if:
¦_ A _ ¦ EQ X
¦ (~lD . A2D~
where TA is the optical path length between the reflectors
o~ the analyzer Fabry-Perot inter~erometer 30.
-21~ S ~ ~-t~ ~
As previously explained, i~ a signal is detec~ed
by a fiber optic energy sensor which is located between the
length limited Bragg reflectors of, for example, pair B, then
the transmission pea};s of region b of FIG. 15 of pair B will
undergo a spectral shift, ~SR , within the region b. By
adjusting the relative values of SA and SR using EQ IX and X
the invention actually amplifies this spectral shift,a~sR by
causing the resulting spectral shift, ~SA of the transmission~
of pair B and the analyzer interferometer 30 combined as in
10 FIG. 14 to be:
a~SA ~z u~sR EQ XI
where U is the ampli~cation and is, for example:
U ~ fSR _ EQ XII
fsR _ SA
for sA = (f)(sR) ~ 1
and SA and SR ~ 2 and f i5 a positive integer
To better explain the demodulation system and to
demonst~ate some of .the ].ess evident constraints to be con-
sidered for its imple:mentation, an example of the system of
FIG. 14 with the addition of energy sensors placed within
the pairs .25 as previously detailed ~ill be detailed chrono-
logically through two laser scan intervals. The laser scan
begins at ~l which does not fall within the.reflection wave-
length bands of any of the pairs ~5. As the laser outputwa~elength srans in t.ime, it will eventually begin to scan
across the transmission peaks of a particular pair A. A~
this time the reference interferometer .28 transmits a pulse
_ of laser light to the photodetector 29 ~hich then delivers an
electrical pulse to t.he time demodulator 33. This electrical
Teference pulse is used in the time demodulator 33 to reset and
start an electrical c:Lock. The time demodula*or 33 ~lso counts
~he reference pulses in one scan interval and dependin~ on the
number of the pulse, directs the final output of the electric 1
clock to one of the electric outputs corresponding to the parti-
cular reflector pair ~-~hose transmission pea~s are being scanned
at that time. Such ell-ctronic circuitry can be readily a~quired
from present commercia.lly available products.
-22-
Returning to the e~ample of the system, the laser
ou~put is nol~ beginning to scan over the ~ransmission peaks
of pair A. When the laser output wavelength is within the
first peak of pair A, at ~2, the laser light travels through
pair A and all other pairs and eventually to the analyzer
interferometer 30O For the sake of the explanation, assume
.that the system of FIG. 14 is designed to provide an ampli-
fication U = 100 by ~laking use of EQ XII with SR =~10. Also
for simplicity, assume that the interval between ~lD and ~2D
of EQ X and XI for ea.ch reflector pair in ~he example system
is spectrally coincid.ent with region b of ~IG. 15 for each
reflector pair. Therefore, for U = 100, SR = 10, then SA = 99.
- Assume also that the analyzer in~erferomete~ 130
has a peak which is spectrally coincident with the first peak
of pair A. TherefoIe, the laser light is tTansmitted on~o the
photodetector 31 which produces an electrical output which when
delivered to the time demodulator 33 stops the electric clock the
final output of which is an electric signal corresponding to the
time on the clock and is delivered to the electrical lead or leads
labeled A. As the la.ser continues to scan eventually the wave-
length o its output nears the transmission peaks of pair B.
Again the reference i.nter~erometer 28 transmits a pulse of
light which causes photodetector 29 to produce a pulse which
resets and starts the clock and prepares electrical lead or
leads B for the final output of the clock.
As the sigr.Lal being detected by the energy sensor of
pair A changes, the t:ransmission peaks of pair A shift spectrally.
Assume that the signa!l has caused the peaks to shift ~9 t~A~
sometime prior to the second laser scan. When the second laser
scan begins, the output wavelength is again ~l . Soon after
the beginning of the scan, the laser output again nears the first
transmission peak of pair A, the output wavelength is approxi-
mately ~2 ~ .. This wavelength, however, is not coinci-
dent with a pea~ of 1:he analyzer interferometer 30 so no light is
transmitted to the photodetector 31 to stop the clock. However,
as the laser continues to scan its output will at a later time
be ~2 -~ gl9a~ + a~, which is the n~w spectral location of the
-23-
second transmission peak of pair A. By the previous equa~ions
for the amplification 1~ of this example system, ~2 ~ A
is also the spectral location of the analyzer inter~e-rometer
30 transmission peak ~hich is spectrally nex~ to that peak
5 located at ~2 . There~:Eore, transmission occurs through
analyzer interferometer 30 and the photodetector 31 produc~s a
signal which stops the clock. Even though the transmission
peaks of pair A shifte(l only ~ ) , the output of the com-
bination cf pair A and the interferometer 30 did not occur un-
lO til the laser outpu$ wavelength reached ~2 ~ result-
ing in a spectral amp]L:ification of lO0. The r9e~ainder of the
second scan interval p:roceeds as described in the first laser
scan interval.
The.implementa1::ion of the demodulation system ~equires
15 particular attention to the band~idth of the optical fiber 24.
The bandwidth must be sufficiently high so as to maintain the
narrowness of the retu-rning reflector pair transmission peaks.
Single mode fiber wil:L suffice in most cases. Note that the
. optical demodulation system can be used whenever it is desired
20 to determine the spectral motion of the fringes of a Fabry-
Perot interferometer 1~ith or without the use of optical fibe~
It is also allowed that the laser light may pass through the
analyzer Fabry-Perot :interferometer first and then to the Fabry-
Perot interferomete~ ~hose spacing one is attempting ~o me~sure.
25 Ho~ever, if optical f:iber is used to carry ~he laser light to
the Fabry-Perot inter:Eerometer being measured, and if the laser
light is to pass through the analyzer interferometer fïrst,
then it is necessary to select optical fiber ~ith low dispersion
for.carrying light from the analyzer interferometer to the Fabry-
30 Perot inte~ferometer ;ince this ligh~ will have an additionalamplitude temporal dependence as caused by the spectrally
periodic ~ansmission of the analyzer interferometer. Further,
the finesse of both t].l.e analy~er interferometer and the sensor
interferometer must be chosen so that if none of the transmission
35 peaks is exactly coincident spectrally there will still be only
su.fficient o~erlap to produce a meaningful combined output.
Finally, the example system will produce ambiguous outputs if
.
3~:3
the spectral motion of the reflector pair transmission peaks
is allowed to equal or exceed .1~ or is below gg~A~
Finally, the invention provides that the electrical
reference signal may be derived from the same signal which
s causes the laser to scan. The cTi~eria for a Teference signal
is only that it must have a known position in time with respect
to that position in time of any particular wavelength of the
laser scan. Further, the criteria for a suitable laser scan
is: first, the scan interval must occur su~ficiently often
in a period of time in order to detect the highest frequency
of the oscillation of the temporal position of the output of
the combination of a reflector pair and analyzer in~erferometer;
second, the output wavelength of the scanning laser must be a
known function of time.
To eliminate the said ambiguity for the excessive
motion of the peaks the invention provides the addition o a
second analyzer interFeroTneter 30B shown in FIG. 17. FIG. 17
is a schematic of a rlsplacement subsystem -for that subsystem
W enclosed by the das;~ed lines in FIG. 14. This additional
analyzer 30B is arranged by means of, e.g., EQ XII, to provide
a lower magnification when used with the same output of the
reflector pairs. From the previous explanation a lower ampli-
~ication combination can provide a higher threshold for trans-
mission peak motion at which ambiguity first occurs. If
EQ XII is used to establish the amplification U then the
threshold spectral shift is:
fA~ . EQ XIII
Such an arrangement using two analyzer interferometers ea~h
causing different amp:Lifications rould be implemented as
follows: the first interferometer 30 could have-as in the
previous example of the demodulation system SA = 99 transmission
peaks between ~lD and ~2D. The re~lector pair cculd have SR -- lO
-peaks between ~lD and ~2D and the additional interferometer
30B could have SA - 9 peaks between ~lD and ~2D If~ for
example, time demodulators 33 and 33B provided analog outputs
then for a particular shift ~SR corresponding to pair A
-25- ~ ~6)~ 8~3~D
~he electrical OUtpllt of time demodulator 33B corresponding
to pair A would be a voltage e such that
el = K~AsR Ul EQ XIV
whe.re K is a constant
Ul is the amplification which equals 10
for SR ~ 10 and SA = 9
and A~SR is the spectral shift of ~he peaks of
r~gio~ b of FIG. 15 corresponding to
~lect:rical lead A
The output e2 of demodulator 33 would be:
e2~ SR U2 EQ XV
= .LOOKA~SR
where 100 is the amplification, U2,
for SR = 10 and SA = 99
Such an arrangement of course can be expanded to
include many such analyzer interferometers with different
amplifications simp:l~ by adding more beamsplitters such as
127' to di~ide the output of *he reflector pairs among them.
Note that from the previous explanation analyzer interfero
meter 30B m-ay begin to produce ambiguity for spectral shifts
smaller than .laA. However, analyzer 30 will as described
produce meaningful ~utputs for spectral shifts below .l~.
One may therefore contemplate adding analyzer interfsrometers
which would cause e:ither lower or higher amplification than
those analyzer interferometers already pTesent in a systemO
The time ,lemodulator is an electrical device which
performs two unctions: first~ producing an e~ectrical signal
which by means o, :foT example, its amplitude, frequency of
oscillation or phase of oscillation contains or conveys the time
between the receipt of a reference pulse and the receipt of an
additional pulse ca:Lled ANAL pulse, which is the pulse from
the photodetec~or recei~ing electro-magnetic radiation from
the analyæer Fabry-:Perot interferometer, and; second~ directing
this electric signa:l to a particular output wire or to a par-
ticular group of output ~ires. There are many electrical
-26 ~Z~ 3 ~)
circuits which can accomplish this one of which is shown
achematically in FIG. 18. Re~erring to FIG~ 18, Ul and U2
are voltage comparators, for example, part #LM311 produced
by National Semiconductor Corp., U4 and U5 are counters, e.g.
part #74161 produced by Texas Instruments~ U3 is a clock
generator, e.g., part #74LS124 produced by Texas Instruments,
U6 is a demux, e.g. 9 part ~74155 produced by Texas Instruments
and U7, U8, and U9 are latches, e.g., part #74175, also pro-
duced by Texas Instruments.
The circuit operates as follows: Ul and U2,
voltage comparators, serve to convert the reference pulse and
the additional pulse into standard TTL logic voltage levels for
use in the demodulato~r. The regularly spaced reference pulses
serve to reset counter U4 which is continually counting at a
rate approximately 16 times as fas~ as the reference pulse
repetition rate, as driven by clock generator U3. The result-
ing output of counter U4 is a number which starts at 0 when
the reference p~lse ic; received and counts upward until again
reset to 0 by another reference pulse, starting its count anew.
Meanwhile, each time aL reference pulse is received, counter U5
is incremented. It is set to automatically return to 0 after
counting the appropriate number of channels (in this case, 3).
When an ANAL pulse comes, it is routed to the appropriate latch
(U7, U8, or U9) via demux U6. The counter output number is
latched into the appropriate channel latch and represents the
time bet~een the reference and ANAL pulses. The next ~AL
pulses causes the ~ext channel latch to store the number Tepre-
senting the time between those reference and ANAL pul$es, and
so on. Each time a new time count is latched~ the trailing
edge of the latch pulse notifies the user that new data is
available.
Statement of Industrial Application
,
An improved fîber optic energy sensor and method of
manufacturing the sens~r, and an improved optical demodulation
system is provided which is particularly sensitive to stretch-
ing or compressing by signal energy to be sensed or detected.
