Note: Descriptions are shown in the official language in which they were submitted.
3~i~
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HOLLOW WAVEGUIDE
Technical Field
_
This invention relates generaLly to flexible, narrow
diameter, hollow waveguides and, in particular, to those
capable of high efficiency transmission of CO? laser
energy suitable fvr medical applications.
10 ack~round of the Invention
For almost as lon~ as CO2 lasers have been viable tools
for medical applications, the search has been on for
improved modes of guiding the laser beam to the desired
operating area. For the most part, lasers have been
coupled with multi-section articulated arms having any
number of large bore tubular sections hinged together with
a reflective surface at each hinge to permit the laser
light to traverse the length of the arm and to be aimed
toward the desired site.
While such articulated arm laser systems have experienced
wide spread acceptance lately in a variety of medical
specialities, they are generally somewhat clumsy to use
since the arm typically offers some "resistance" to move-
ment by the surgeon. Such arms are inherently limited in
the scope of their medical applications, because of their
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size and limited flexibility. Present C~2 surgica] appli
cations are essentially limited to those in which there is
direct access to the area to be treated. CO2 endoscope
procedures are still rare, as the present technology
requires a relatively wide, but short and straight
endoscopic channel to "shoot" the CO2 beam down. In
addition, most articulated arms experience problems with
beam alignment particularly if the surgical application
calls for a small spot size. These arms also tend to ~e
expensive, especially if precision optical alignment is
required.
It is an ob~ect of ~he present invention to provide a
small diameter, flexihle fiber for carrying C~2 laser
emissions, which can be threaded down a longer, narrow or
flexible endoscope, or alternatively be used as a second
puncture probe.
A variety of optical Eibers have been proposed as the
2n transmission medium for laser energy, but to date, not a
single one has become commercially accepted for the
ln.6 micron wavelength which is characteristic of CO2
lasers. Optical fibers or light pipes for the transmis-
sion of infrared light at l~.~ microns have however been
proposed: in one instance a polycrystalline fiber, such
as the ~RS-5 fiber developed by Horiba, Ltd. of Japan; and
in another, a flexible, hollow waveguide, various versions
of which have been suggested ~y among others ~. ~,armire
and M~ Miyagi~ See, for instance, M~ Miyagi, et al.,
3~ "Transmission Characteristics of nielectric-coated
rletallic Waveguide or Infrared Transmission: Slab
Waveguide Model", IEE~ Journal of Quantum Electronics,
Volume Q~-l9, No. 2, February 1983, and reerences cited
therein. Recently, Miyagi, et al. suggested fabricating a
dielectric~coated metallic hollow, flexible waveguide for
IR transmission using a circular nickel waveguide with an
~ ~i8~3~
--3--
inner ger~anium layer applied by rf-sputtering, plating
and etching techniques. Miyagi, et al. predict extremely
small trans~ission losses for a straight guide, but in
fact, actual transmission deqrades substantially with but
nominal bending radii (20 cm). To understand this, the
mechanism of transmission must be considered.
Transmission of laser light through a flexible, narrow
diameter hollow waveguide is subject to losses largely ~ue
to successive reflections of the beam along the interior
sur~ace of the curved guide. For the size and curvatures
contemplated for a medical fiber, rays will lnter.sect the
wall at angles of incidence ranging from~ typically, 80
to 90. nending a hollow fiber increases the loss as it
1~ tends to increase the number of internal reflections and
decrease the angle of incidence. In general, as the angle
of incidence decreases from 90 to 80, the loss per
reflection bounce increases. It is an object of the
present invention, therefore, to provide a coating which
has high reflectivity over angles of incidence ranging
froTn 80 to ~0.
A difficulty of curving metal walls is that at these
angles of incidence, metals tend to exhibit high reflecti-
25 vity for onl~ the S polarization hut low reflectivity(<~6~) Eor the P polarization. The losses for a 1 meter
curved guide are of the order 10 d~. Garmire et al.
atte~pted to avoid this problem by using a metal/di-
electric guide in which the guide was oriented relative to
3~ the incoming beam such that the metal walls "saw" only the
P polarization. This approach is flawed, however/ because
the dielectric walls show high reflectivity for only very,
very high angles of incidence, typically in excess of 89- ;
-requiring, in essence, that the guide must be straight
along the direction of the dielectric.
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:. :.
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~..... ~ . .
Some have sug~esteA reme~ying this situation by overcoat~ing a reflecting surface with a quarter-wave dielectric
coating~ Such a coating will yield high reflectivity for
the P polarization, but low for the S polarization.
r1iyagi et al. attempt to strike a compromise by choosing a
coating of thickness somewhere between those favoring the
P and and those favoring the ~ polarization. He chose a
germaniu~ coating of approxiamately 0.4 to 5 micrometers
in thickness. This coating yielded relatively good
results (>90%/meter transmission) for straight guides, but
rather poor for bent guides.
This disparity appears to result from two factors: l) The
transmission with the ~ell mode in a straight guiAe corre-
lates poorly with the transmission of very high multiorder mo~es in a bent guide; and 2) The imaginary part of
the refractive index of the dielectric coating is
extremely crucial in the transmission of a bent guide.
2n It is an object of the present invention to provide di-
electric overcoated waveguides which are tuned to perfor~
well althouqh bent in compound curvature.
Summary of the Invention
~e have invented a flexi~le, narrow outer diameter, metal
coated ~ielectric-overcoated hollow waveguide capable of
transmitting in excess of 68% of the entering C~2 laser
energy over a one meter section even when subjected to
compound curvatures. The waveguide is sufficiently thin
to be passed down the esophagus of an adult patient and is
safe for endoscopic applications.
The principles of the present invention are premised on
dealing with refractivity as a complex (i~e., real plùs
imaginary) ~uantity, taking into account both P and ~S
3~
~l_ 5 _
polari.zations over a clesignated. range of angl.es of
presenta-tion.
In accordance with a particular embodiment there is
provided a narrow diameter, flexible, hollow wave-
guide for high efficiency transmission of laser lightby internal reflection, said waveguide comprising:
(a) a hollow flexible elongated housing;
(b) a highly reflective coating on the internal
surface of said housing; and
(c) a thin film dielectric coating overlying
said reflective coating, said thin film having an
index of refractivity n of about 2.6 or less and a
thickness i.n the range of .075 to .175 of the wave-
length of the laser light in the medium of the
dielectric, whereby the average of the refl.ectivity
of the P polarization of the laser light and the
reflectivity of the S polarization is greater than
99.0% for any incident angle in the range of 80 to
9oo
In acccordance with a further embodiment of the
invention there is provided a narrow diameter,
~lexible hollow waveguide for high efficiency trans-
mission of laser light by internal reflection, said
waveguide comprisi.ng:
(a) a hollow flexible elongated housing;
(b) a metallic coating, highly reflective a-t
normal incidence, on the internal surface of said
housing;
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.,
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~ 5a
(c) a pJura]. l.ayer die].ect:ric coati.ng applied
-to said metal coating, said coating composir.e havinc3
-the reflectivi-ties oE the P and S polarization
averaged together to be in excess of 98.5% for all
angles of inci~ence ranging from 80 to 90.
In accordance with a still further embodiment of the
invention there is provided a flexible hollow wave-
guide for high efficiency -transmission of CO2 laser
light which comprises:
(a) a guide having an internal surface and an
external cross section sufficiently small to allow
for endoscopic application;
- (b) a metal coating applied to the internal
surface of said guide, said coating characterized by
a high degree of reflectivity of light at normal
incidence for the wavelength of usei
(c) a plural layer dielectric coating applied
to said metal coating, said coa-ting composite having
. the reflectivities of the P and S polari~ation
~o averaged -together to be in excess of 98.5% for all
angles of i.ncidence ranging from 80 to 90.
In accordance with a still further embodiment of the
invention there is provided a narrow diameter, hollow
flexible waveguide for high efficiency transmission
of laser light by internal reflection, sald waveguide
comprisng:
(a) a hollow flexible elongated housing having
a generally rectangular internal cross-section;
(b) a metallic coating having high reflectivity
at normal incidence on the internal surface of said
guide;
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(c) a firs-t thin Eilm diel.ectric overcoat on a
Eirs-t opposing pair of i~-ternal. surfaces of said
waveguide adapted to engage a first polarizati.on of
said light; and
(d) a second thin film dielectric overcoat,
different from said first overcoat, on the second
pair of internal surfaces of said waveguide, adapted
to engage a second polarizat.ion of said light.
In accordance with the principles of the present
invention, a flexible, narrow diameter, hollow wave-
guide has an outer reflective structure coated on its
inner walls with suitable dielectric material of
thickness equal to about one eighth the wavelength of
the light to be transmitted by the waveguide. Such a
dielectric construction will, on average, have
relatively minimal adverse effect (i.e., loss) for
both P and S po].arizations, because the extinction
coefficient of the complex index of refraction will
have been reduced substantially over the quarter wave
thickness shown in the prior art. For example,
thorium fluoride (ThF~) and zinc selenide (ZnSe) are
suitable dielectric materials useful in preferred
embodiments for transmission of CO2 laser emissions.
In such preferred embodiments) silver is an
; 25 appropriate reflecting outer layer, and the
dielectric -thickness may be within about i20% of an
eighth wavelength in thickness, and the refractive
index n is less than about 2.6.
The materials of the waveguide walls are chosen for
1) ability to obtain/retain the requisite inner wall
flatness, smoothness, and dimensional control;
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2) Elexibilityi 3) utili2ation of low loss
dielectric overcoatings; and 4) coating adhesive-
ness. The inner wall is coa-ted with a high
reflectivity metal and then overcoated with a
dielectric overcoa-t.
..
,
A fur-ther embodiment of the presen-t invention features plural
dielectric overcoatings of select materials and thickness
to pro~ote transmission of laser energy through a flexible
(i.e., hent) waveguide~ Preferably, three thin film
coatings of two different dielectric materials will have
the first and thir~ coatings of the same, relatively low
index of refraction (e.g., ThF4 or Zn~e), with the inter-
mediate layer having a relatively high index of refraction
(e.g., ~,e). For example, this arrangement may include the
layer first contacting the beam having a thickness o~
either ahout one an~ one half times (+0.~ the thickness
of a quarter wave layer, or one-half t+n.2) of a quarter
wave thickness for the laser energy in the medium of the
low index dielectric. In such instance, the two inner
layers have quarter wave thickness. It is also feasible
to have germanium constitute the first and third layers,
and ThF4 constitute the intermediate layer.
In accordance with a still further embodiment of
the present invention there is provided
utilization of a square tor rectangular) cross-section,
with each pair of opposing inner walls having a select,
different dielectric overcoat, respectively designed
separately to optimize P or S polarization. In a
preferred embodiment, the inner wall has a reflective
te.g., silver) coating, and two opposing walls have a
dielectric overcoat less ~han half of the quarter wave
thickness (and preferably less than 20~ of the quarter
wave thickn~ss) while the other two opposing walls have a
thickness which is an integral multiple (preferably
between 1 and 5) of the quarter wave thickness. ~hF4,
ZnSe, and Ge, and combinations thereof, are suitable
~ielectric materials.
~ à~
In a preferred embodiment, a light pipe cornprises two
generally V-shaped aluminum rods ~oined to form a guide
havinq a square interior cross-section, and interior
surfaces coated first with chromium for adhesiveness, then
with silver, and then with a thin film dielectric coating
of either thorium fluoride, ThF4, or zinc selenide, ZnSe,
of thickness of about 0.5 + 0.2 of 1/4~m of the laser
light in the dielectric. In such an arrange~ent, the
reflectivities of both the P and the S polarizations of
the laser light averaged together are greater than 99.0%
for any incident angle in the range of ~0 to ~0.
~rief ~escription of the ~rawinqs
.
Fig. 1 is a diagrammatic representation of a section of a
curved light pipe illustrating schematically the multiple
reflections to which a coherent lightwave is subjected
while travelling through the light pipe;
Fig. ? is a straight section of a portion of a hollow
netal waveguide according to the present invention;
Fig. 3 is a section taken along line 3-3 of Fig~ 2;
Fig. 4 is an enlarged section taken along line 4-4 ~f
Fig. 3;
Fig. 5 is a view similar to Fig. 3 representing an alter-
native embodiment of the waveguide; and
Fig. 6 is a view similar to Fig. 3 showing another alter-
native embodiment of the waveguide.
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3~
netailed nescription of the Preferred Embodi~ents
Includin~ the ~est Mode for Carrying ~ut the Invention
__
In general, for a guide to be useful for meflical including
en~oscopic applicationsp the average reflectivity of P and
S polarizations combined must be greater than 97% and
preferably greater than 99% for both P and S polarizations
for all angles of incidence from about 80 to ahout 90.
The reason for requiring a high reflectivity condition
over such a broad range o~ angles is that a curved guide
in effect introduces lower angles of incidence as the beam
is propagated through the guide. The extreme angle o~
incidence ~ that neeAs to be considered in a curved
guide of inner cross section d anA radius of curvature R
is given by the relationship:
= cos~i ~ 2d/~
~ence, for a guifle with d = 1 mm and ~ = ln cm, the
~n extreme incident angle is 82. A waveguide in actual
medical use will have, of course, a non-uniform radius of
curvature introducing in effect even smaller incident
angles. ~owever, for a waveguide with an inner cross
section ~iameter on the order of 1 mm the angles of
incidence will normally be in the ~0 to 9~ range.
In practice, a portion of the waveguide will have compound
curvatures such as shown in the diagrammatic illustration
of Fig. 1 wherein the laser beam, modeled in Figs. 1 and 2
as a one dimensional ray, enters the waveguide in a
direction normal to a plane orthogonally intersecting the
waveguide at one end of the guide. The beam is then
reflected off the interior surface of the waveguifle at
intervals determined by the curvature of the guide. ~or
the tvpes of guifles under considerationr i.e., those
having an inner diameter of about 1 mm and curvatures o
... .
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3n cm or less , a typical ray will hit the interior ~all
about every 1 to 2 cm. ~ence, for a one met~r length of
the gui~e there will be about 7~ reflections or bounces.
Assuming an average energy loss of 0.5~ per bounce, a one
meter guide will transmit hR% of the light entering the
guide. 11ith just a half percent increase in loss per
bounce to 1%, the overall transmission falls to about
47%.
For purposes of this application, "transmission rating"
shall describe the percentage of cn~ laser ~nergy
transmitted by a one meter section of a curved guide.
Thus, a 6~% transmission rating represents a one meter
section of a guide that transmits at least 68% of the
energy of a propagating cn~ laser beam entering the gui~e
after the beam is sub~ected to up to 75 internal reflec-
tion.s.
~ith reference to Fig. 2, ~, and 4 there is shown a
straight line section of a flexible hollow waveguide
referred to generally a.s 10. The waveguide includes tube
~n of a material, preferably stainless ~eel or aluminum,
chosen on the basis of mechanical performance including
ductility and strength hygroscopicity. An additional
requirement of the material forming the guide (i.e.,
halves 25 and ~6 in ~ig. 3) is that it be easily coatable,
for example, in a vacuum chamber by an adhesive material,
to yield a low loss surface. Plastics, from a mechanical
viewpoint, may be preferable material for tube 2~; how-
3n ever, it is more difficult to obtain low loss coatingsbecause OL the limited temperature to which plastics may
be heated~
Enclosed within the tube 20 are opposing halves 25 and 26 of a
base material adapted to receive reflective and dielectric
coatings pursuant to the present invention. Preferably,
3 ~ 3 ~
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the halves 25 and 26 are each formed from a me-tallic wire
and machined with a triangular (i.e., half square) groove
-therein, as shown. When the halves are mated within the
tube 20, -the square hollow waveguide is formed. The inner
S walls 100, 101, 200, 201 of the halves 25 and 26 must also
be optically smooth, relative to grazing incidence at 10.6
~m. Onto the interior surface of tube hal~es 25 and 26, a
metal coating 30 is applied. Coating 30 must be a high
normal incidence reflector of light at a wavelength of 10.6
microns, such as silver. Other suitable metal coatings
include gold and aluminum. The thickness of the silver
coating 30 is not critical and is preferably in the range
of something less than approximately 100 angstroms. To
improve the bonding between silver coating 30 and the
halves 25 and 26, a high adhesion coating 40, preferably
of chromium, is applied onto the tube prior to the appli-
cation o:E the silver coating. With the silver coating 30
bonded to the metal tube 20, a thin film dielectric coat~
ing 50 is applied.
In accordance with a Eurther embodiment, the thin film di-
electric coating comprises a multiple layer 50a, 50b and 50c.
The nature of the layers will be discussed below.
As above stated in general terms, the present invention
features a dielectric overcoat of a thickness of ahout
one-eighth a wavelenqth, that is, about one half of the
quarter wave standard. In greater specifics, the
dielectric overcoat of the present invention is to be
0-5~0 2 ~m/4~ where ~m is the wavelength of the light
in the medium at the 80 angle of incidence. More
precisely,
~m = ~ ~
n / 1 - sin~ ~0
~ n~
where ~ is the wavelength of the same light in a vacuum
and n is the index of refraction.
In the embodiment illustrated in Fig. 4, the thin film
dielectric coating is a single layer 50 such as ThF4 or
ZnSe. ~he coating thickness of the thin film dielectric
is critical to performance. With single layer coatings,
best results are realized with a thickness of about 50~ of
the l/4A thickness. Thus, for a silver guide coated with
ZnSe, the optimum thickness of the coating is about 0.7
and for the silver guide coated with ThF4 9 the optimum
thickness is about 1 4~.
n
The losses obtained for a variety of dielectric coatings
are influenced by the thickness of the coating as well as
by ~, the complex index of refraction of the material. N
is given by n + ik, where the extinction coefficient k is
the imaginary part, related to the absorption properties
of the material. The real part, n, commonly referred to
simply as the index of refraction, is the ratio of the
speed (or wavelength) of light in a vacuum to the speed
(or wavelength) of light in the material. While certain
materials exhihit unacceptably high losses regardless of
thickness, the optimum thickness of acceptable materials,
s~ch as Zn~e and ThF4, is consistently in the range of
ahout 0.5 of the quarter wave thickness~ r.osses are lower
as the imaginary component, k, of the refractive index is
minimized. It is crucial to the performance (i.e.
transmission) of the waveguide to keep the value of k to
some low numberO ~ven though k is related to the
properties of the material, to a significant degree the
magnitude of k is ~uality controllable through proper
vacuum deposition techniques.
As the value of k decreases, the greater is the tolerance
allowed on the coating thickness. For example, with a
ThF4 layer w~th a k = n, thickness in the range from about
.6~ to ahout ~.3~ (or alternatively .2 to .R ~m/2~ will
yield an average reflectivity of P and S combined greater
than 99~ from 8n to 90. ~n the other hand, with k = 2 x
10-3, the thickness may only be from about O4 to .6 ~m/2
to still yield the same minimum limit on reflectivity.
Tables 1 through 3 illustrate this comparison in detail.
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,~
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u ~ ~ o
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;
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s ~ ~ I` C~
E~ 0 cc ~ ~ o
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o ~
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Y 11 C U o U~
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E~ ~ ....
U~ O
U~ ~
U~
U~
0 0 N
t~ ~ .-1 0
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s r~
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1 0
E~. M u~ ~1 ~ C
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S
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l O O
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a)l
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TA~LE 2
Loss of Silver coated with ThF4 of varying thickness,
with k = 2 x ln-3.
T = o6~ T = 1.2~ T ~ 1.8~ T = 2~4
Angle Loss P Loss,S J~ss P ~ss S L~ss P L~ss S Loss P Loss
__ _ _
812.79 ~13 1.55 .~4 1.12 .72 l.nl ~.86
1083 2.64 .10 1.30 .lg .91 .56 .80 3.?.8
852.~5 .07 .99 .1~ .67 ,41 .58 2.52
~71.5fi .04 .62 .n8 .41 .25 .~5 1.60
89.56 .01 .~1 .03 .14 .n8 .12 .55
TABLE 3
Loss of .Silver coated with ThF4 of varying thickness,
20with k = ln-3.
T = .6~ T = 1.2~ T - 1.8~ T = 2.4~
Angle Loss P Loss S Loss P Loss S Loss P Loss S Loss P Loss S
~12.~6 .131026 .21.86 .53.73 2.57
832.32 .101.05 .16.fi9 .49.58 2.1R
851.98 .0~ .80 .12.51 .30.42 1.6~
871.37 .n4 .. ~0 .07 .31.1~ .26 1.06
89 .49 .~1 .17 .~2.10 .~6.09 .3
: 30
Similar dependence of allowahle~coating thickness on k
value can be found with %n.Se as illustrated in Tables 4
and 5.
TABLE 4
4n Reflection loss of ~ coated with ZnSe of varying
thickness in which k = ln-3O
T = .1~ T = .4~ T - .7~ T = 1.0~
Angle Loss P Loss S Lo_s P Loss S Loss P Loss S L~ss P Loss S
813~99 ~111.98 .161.16 .34.~2 2.08
834.68 .091.76 .12.93 .26.72 1.66
855.43 .061041 .09.69 .19.S2 1.21
875.62 .0~ .92 .05.42 .11,31 .74
892~9~ ~(11o32 ~f)2~12 ~06~10 ~25
, ~
3~
1~ -
TA~LE 5
Reflection 105s of ~ coated with ZnSe of varying
thickness, in which k = 10 3.
T ~ T = .4~ T - .7~ T = 1.0~l
Anqle ~oss P Loss S Loss P loss ~ Loss P Loss S Loss P Loss S
81 3O97 oll 1.93 .15 1.0~ .32 .~2 1.8~
85 5.40 .n6 1.37 .ng .65 .1~ .46 1.06
~ 2.97 .01 ..~ 2 .13 .~4 .09 .22
Even with k = 0, for high refractive indices, unacceptably
high reflectivity losses occur. ~,ermanium, for example,
even with k = 0 never yields a low loss reflectivity
condition as can be seen in Table 6.
TA~LE 6
Reflection loss of Ag coated with ~e and varying
thicknesses in which k = 0.0
T = .1ll T = .3~ T - ~5~ T = .7~
An~le Loss P Loss S Loss P Loss S Loss P Loss S Loss P Loss S
30 ~1 ~.0~ .12 2.75.20 2.31 .35 l.9Q 11.07
83 ~.65 .09 2.46.16 1.63.6fi 1.4~ 9.42
5u23 .07 1.~9.11 1.19.47 1.07 7.26
87 5.11 .04 1.31.07 .73~28 .~4 4.h3
89 2.~2 .01 .~6.02 .24.09 .21 1.60
,:
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~!_ 17 -
As shown in Figs. 3 and 5, the square cross-section
in accordance with the principles oE the present
invention defines respective opposing pairs of
in-terior walls 100 and 101, and 200 and 201. In
accordance with the prnciples of the present
invention, walls 100 and 101 will be provided with
a first dielectric layer, having select material
composition and thickness, and the other pair such
as 200 and 201 will have a second select choice of
material and thickness. In this fashion, one pair
of walls may be designed to promote optimally one type
of polarization, such as P polarization, whereas the
other may be configured for optimal transmission of
the other polarization, such as S polarization.
Clearly, the pair 100 and 101, and/or the pair 200 and
201 may be single layer dielectrics, or as desired may
be multiple layer dielectrics as set forth in U.S.
Patent 4,688,893.
In accordance with the principles of the present
invention, the dielectric overcoat on opposing walls
100 and 101 will have a thickness less than one-half
the quarter wavelength of the light to be tr~nsmitted
in the medium, and preferably less than two-tenths
such quarter wavelength thickness. If so, the other
pair 200 and 201 will be an integer multiple of the
quarter wavelength thickness of light in that
material, advantageously one plus or minus 0O5 quarter
wavelength thicknesses. Ideally, the latter coatin~
will be as close to the quarter wave thickness as
possible.
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Materials in accordance with -the principles of the
presen-t inven-tion wi.ll preferab].y be those commonly
described in accordance with the concurrently fi.led
copending appl.ications, that is, Ge, ThF4, and/or
ZnSe. It will be apparent that the different thick-
nesses on either walls will require separate process-
ing for each, and therefore
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33~
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that the same or diferent materials may be use~, as
desired, for the respective opposing pairs of surfaces.
It will be noted that in accordance with the principles of
the present invention, a square or nearly square cross
section will be featured, but that rectangular or nearly
rectangular designs may also he utilized~
The coating in accordance with the present invention is
ln advantageous over the others, primarily when it is found
hard to control the k values of the dielectric to an
acceptable small limit. ~ven with high k values, it is
possihle to obtain very low loss coatings in accordance
with the present invention. On the other hand, t~is
approach has to be halanced by greater dif~iculty of
manufacturing due to the non-homogeneous coating on walls
(i~e. different coatings on adjacent walls), and the need
to control parallelness and perpendicularity to about 1~
or less. This second requirement is raised because lack
~0 of parallelness and perpendicularity causes polarization
preservation to be less as an optical beam propogates down
a guide. Twists also can lead to this result, hut one or
two 90 twists over a meter long fiber should have only a
negligible effect.
..
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.
Regar~lless of choice of dielectric coating materials, the
preferred geometry of the metal guide 10 is square shapecl
as shown in ~ig. ~ and an alternativ~ approach is shown in
Fig. 5. In Fig. 3 respective halves have V-shaped grooves
formed therein, and the square guide results when the
opposing portions are joined. The square shape is
particularly advantageous for ease of fabrication. In
Fig. 5, two essentially ~I-shaped portions 20a and 20b may
be separately coated and subsequently combined to form the
10 guide 20. When ~I-shaped sections 20a and 20b are placed
in a vacuum chamber, resting on their respective pointe~
ends ~1 with their interior surface that is to be coated
facing a source of thin film dielectric, a relatively
equal thickness coating by the well-known vacuum deposi-
1~ tion technique is achievable. A generally circular cross
; s~ctional guide such as shown in ~ig. 6 may also be fabri-
cate~ by joining semicircular sections 20'a and 20 ' bo
~owever, the circular interior surface of sections 20'a
an~ 20' b require that they be oscillated during the vacuu~
deposition step in order to ensure a relatively uniform
thickness of the ~ielectric coatings.
After fabrication, the waveguide ?.0 is preferably inserted
into a plastic or metal sleeve 6n for safety consideration
should the guide ever crack during use. To enhance the
ease with which metal guide 20 is encased in a plastic
sleeve 60, the metal guide 20, as shown in Fig. 5, has a
; planar interior surface but has a circular exterior.
When the k values can be controlled to be less than about
-3
2 x 10 n, it becomes advantageous from a performance view
to consider multiple layer dielectrics pursuant to the
present invention. There are a wide range of coating
designs which, if the k value can be kept small, will
yield quite acceptable results. From computer modelling,
3;3~,~
- 20 ~
we have foun~ a wide range of possihle coatin~s ranging
from 2 layers to 3 layers and coating thicknesses of
substantial variations. Indeed, we found no way we can
analytically give a convenient formula for stating the
~ood coatinq ~esign--short of actual computer modellinq.
In the preferred embodiment illustrated i~ Fig. 4, there
is shown three layers of thin film dielectric coatings
with the inner and outer layers 50a and 50c being, for
example, ThF~ with the middle layer 50b of ~.e. As sho~n in
Table 7, Case ~, superior results are obtained for the
range o~ incident angles ~1 to 89 by reducing the
coating thickness of the final (outer) ThF4 layer 50c to
about 4n% from the 1/4~ thickness to about 1.67~ and using
1/4~ thicknesses for the other two layers SOb and 50a.
TA~LE 7
Percent loss of silver coate~ with ThFLl/r~e/ThF4 stack of
varying thicknesses, k=ln-3 for all three layers.
Percent_Loss
~ Case 1 _ Case 2 Case 3 _ _ Case 4
; 25 Angle Loss P Loss S Loss P Loss S Loss P Loss S oss P Loss S
81 .52 6.07 .52 .90 .61 .20 .99 .07
83 .42 7.14 .42 .73 .50 .15 .76 .05
.30 8.44 .31 .54 .38 .11 .59 .04
87 .18 8.74 .lg .33 .23 .07 .38 .02
89 On8 ~.76 .06 .11 .n8 .02 .13 .01
Case 1: 2.79 ~m, .67 ~m, 2.79 ~m (all three-quarter wave)
Case ?.: 2.32 ~m, .fi7 ~m, 2.79 ~m (all two-quarter wave)
35 Case 3: 1.67 ~m, .67 ~m, 2.79 ~m (all two-quarter wave)
Case 4: 1.16 ~m, .fi7 ~m, 2.79 ~m (all two-quarter wave)
~s Table 7, Case 3, above suggests the maximum loss is a
modest .61 percent at ~1 and the mean lo~s is substan-
tially less than 0.5 percent per reflection. Good results
:
' '~
..
- 21 -
are also obtained using the same sequence of coatings as
in the above example, but with the outermost ThF4 layer SOc
ahout 4~ percent thicker than a 1/4~ design. As the
results indicate in Table 8, the maximum loss was .9
percent, again at ~1.
TA~LE 8
Percent loss of silver coate~ with ThF4/~1e/ThF4 stack of
varying thicknesses~
Percent Loss
Case 1 Case 2 _ Case 3
Angle Loss P ~.oss S Loss P Loss S Loss P Loss S
~1 .62 2.~ .86 .9~1.49 .57
8~ .~9 ~.46 .~8 .821.~1 .47
.35 2.~ .4~ .63.87 .3~
2n 87 .21 1.43 .30 .39.~3 .~1
R9 .n7 n.~l .10 ~13.1~ .07
Case 1: 3.34 ~m, .67 ~m, 2.79 ~m
Case ~: 3.90 ~m, .~7 ~m, 2.7~ ~m
~S Case 3: 4.46 ~m, .67 ~m, 2O7g ~m
k~e = ~ x ln~3 k h = 1n~3
As in a single layer coating, we find extreme sensitivi~y
to k value. Indeed, if k = 0, three Am/4 layers of
ThF4/Ge/ThF4 will yiel~ extraordinarily lo~ losses. As is
shown in Table 9~ there is a substantial increase in loss
with this coating by having a finite k. If any generali-
zation can be made, it is that at least one non-quarter
wave coating is required when k values are even in the
ln-3 range. So long as k/n is smaller than about 1.~ x
10-3, we found that the best of the single la~er coatings
were inferior to the best of the multiple layer coatings.
~2$~
TA~LF 9
Percent loss of Silver coated with quarter wave
ThF4/~,e/ThF4 stack for k = n and inite k.
Percent Loss
Case 1 Case 7.
-- p S
Angle Loss Loss Loss Loss
~1 .ln .21 ~2 ~.07
83 .0~ .2~ .42 7.1~
.0~ .2~ .3~ 8.33
~7 . n4 . 29 .18 8.7~
~9 .01 .lfi .n6 4.75
~ther three layer stacks o~ dielectric coatings 50 were
investigate~ ~esi~es the combination o~ ~e and ~hF4;
however, these produce the best results because of the
~0 large ratios of refractive index (4~n versus 1.35).
.
~ne combination investiqated was a ThF4/Znse/ThF4 layer
coated onto Ag. ~ere, i~ was found that ~here was quite a
range of coating de~igns which yielded acceptable coating
reflectivities, although none Yielded as high perfor~ance
as the optimized Ge/ThF4/Ge stack. In tests conducted,
it was shown that one o the best performances
occurs when the first layer's thi~kness
.
.
.
~ 3
-- 23 --
is :I.lh (.4 ~/4), the secon~ layer's 1.79 (.~5 A/4), and
the third layer's 2.5 (.9 A/4). This particular design
yiel~s a loss of .85% and .45% for the P and ~ polariza-
tion, respectively. A k value of 1 x 10-3 was assumed for
each layer~
Another design considered was a ~nSe/Ge/Zn.Se stack coated
onto a thin layer of silver. As with the other multiple
layer coatings IThF4/~el/ThF4 and ThF4/Zn~e/ThF"), there
1~ are a wide range of coating designs (various layer thick-
nesses) which yield acceptable performances. One of the
optimum coating ~esigns with this stack are where the
coatings are of the following thicknesses:
1~ Layer 1 = .71~ (.6 Am/4)
Layer 2 = .69~ m/4)
Layer ~ = 1.19~ (Am/4)
2n
~lith a k value equal to ln-3 for each of the three layers,
the reElection loss for the P and ~ polarization of 81 is
computed to be .65% and .17%, respectively. Another good
design is:
Layer 1 = .95~ (.8 Am/4)
Layer 2 = .43~ (.63 Am/4)
I.ayer 3 = 1.19~ (~m/4)
~ere, the computed loss is .87~ and .17~ ~ at ~1.
3~ 3~
- 24 -
Concerning -the three layer embodiment,
during fabrication, care should be taken to insure good
layer-to-layer adhesion with minimal mechanical stresses
in and among the layers. Excessive mechanical stresses
will at the least degrade performance, and might even
cause the coatings physically to fracture and flake.
Stress conditions will be a function of material selec-
tion, coating thickness, and the coating process. ~or
example, Th~4 and Ge both are characterized by tensile
stress, and thus in combination are more apt to have poor
layer-to-layer adhesion. ~n the other hand, ZnSe is
characterized by lower tensile stressl which favors its
com~ination with either Th~4 or Ge.
:~ -, ..
,
