CA1268362A - Hollow waveguide - Google Patents

Abstract

HOLLOW WAVEGUIDE

Abstract A flexible hollow waveguide has an optically smooth, rectangular cross section internal channel, in which reflecting metal is overcoated with a ThF4 or ZnSe dielectric approximately one-half of the quarter wave thickness. CO2 laser propagation is promoted for guide curvatures down to about 5 cm radius of curvature.

Description

3~i~

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 ~y~

., ~

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~

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.

" -:. :.
: .~
~..... ~ . .

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;

,, - , , .:-; , :

. .

., . .

;3 ~ 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;

,..
~;~

. -.. . . .

33~-3~
jl 5b -(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;

: ' ,~ '" . ' ' :

.,., ' ,.

3~

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.

~;

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 ... .

~: ' . :' , 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 ~

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.

.. :

.3~

U~
oo o O ~D ~
....
r o ~ ~r ,~
P~
~ Lr o U~
u ~ ~ o O . ~ . .
;

~ U~
s ~ ~ I` C~
E~ 0 cc ~ ~ o ' U~ . - . .
~_ ~ O
o ~
Il Y 11 C U o U~
_, 0 U~ ~ ~ C
E~ ~ ....
U~ O
U~ ~
U~
U~

t~ ~ .-1 0 ;:~ O ....
s r~
.., L 3 --I ~

E~. M u~ ~1 ~ C

i~ ~ . . .
S

::L In a~ ~ r ~
l O O
~1 0 . . .
JJ

r~
Q In r~ ~ ~ o O
_I
,~
U~
o U~
n ~ a~ I
U~ CO O ~ O O
O ~ . . .
.

Il ~
E~ 0 ~ 1` ~
U~ ~ o r7 O ....

a)l ~ C~
C~ o:) oo ~

.. ' ` ,' ' .
, j2 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 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.

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 ,:

;3 ~!_ 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.

,~, . ~, :, .". :' . .

3~

- :L7a -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 ' ~ ~ .

33~

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.

..

.

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 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 :
' '~
..

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 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~

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.

:~ -, ..

,

Claims (23)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A narrow diameter, flexible, hollow wave-guide for high efficiency transmission of laser light by 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 in 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 reflectivity 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 90°.

2. The hollow waveguide according to Claim 1 wherein said metal coating is selected from the group consisting of silver, gold, and aluminum.

3. The hollow waveguide according to Claim 1 wherein said dielectric coating is selected from the group consisting of ThF4 and ZnSe.

4. The hollow waveguide according to Claim 1 wherein the internal surfaces of said housing are planar and the internal cross section of said housing is rectangular.

5. The hollow waveguide according to Claim 1 further comprising a coating applied intermediate said housing and said reflective coating to provide enhanced adherence of said reflective coating to said guide.

6. A narrow diameter, flexible hollow waveguide for high efficiency transmission of laser light by internal reflection, said waveguide comprising:
(a) a hollow flexible elongated housing;
(b) a metallic coating, highly reflective at normal incidence, on the internal surface of said housing;
(c) a plural layer dielectric coating applied to said metal coating, said coating composite having the reflectivities of the P and S polarization averaged together to be in excess of 98.5% for all angles of incidence ranging from 80° to 90°

7. A waveguide as described in Claim 6 and comprising three said layers of dielectric, first and third ones of which being a first select dielectric material, and a middle one of which being a second select dielectric material.

8. A waveguide as described in Claim 7 wherein said first select dielectric material has an index of refraction which is relatively low in comparison with the index of refraction of said second select dielectric material.

9. The hollow waveguide according to Claim 7 wherein the first and third layers are ThF4 and the second layer is selected from the group consisting of ZnSe and Ge.

10. The hollow waveguide according to Claim 7 wherein the first and third layers are ZnSe and the second coating is Ge.

11. The hollow waveguide according to Claim 7 wherein said first and third layers are Ge and the second layer is ThF4.

12. A waveguide as described in Claim 7 wherein said outermost of said dielectric layers has a thick-ness between approximately one-eighth and three-eights of the wavelength of light in said outermost layer.

13. A flexible hollow waveguide for high effi-ciency 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 use;
(c) a plural layer dielectric coating applied to said metal coating, said coating composite having the reflectivities of the P and S polarization averaged together to be in excess of 98.5% for all angles of incidence ranging from 80° to 90°.

14. The hollow waveguide according to Claim 13 wherein said guide is made of aluminum and said metal coating is silver.

15. The hollow metal waveguide according to Claim 13 wherein the interior cross section of the waveguide is square shaped.

16. The hollow metal waveguide according to claim 13 wherein the interior cross section of the waveguide is circular.

17. A narrow diameter, hollow flexible waveguide for high efficiency transmission of laser light by internal reflection, said waveguide comprising:

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;

c) a first thin film dielectric overcoat on a first opposing pair of internal surfaces of said waveguide adapted to engage a first polarization of said light;
and d) a second thin film dielectric overcoat, differ-ent from said first overcoat, on the second pair of internal surfaces of said waveguide, adapted to engage a second polarization of said light.

18. A waveguide as described in claim 17 wherein the composite coatings on said first pair of surfaces yields high reflectivity for one polarization at 80° to 90°
incidence, and the composite coatings on said second pair of surfaces yields high reflectivity for the other polarization at 80° to 90° incidence.

19. A waveguide as described in claim 18 wherein said first overcoat has a thickness equal to or less than one-half of the quarter wavelength of light in the material of said first overcoat.

20. A waveguide as described in Claim 19 wherein said first overcoat has a thickness equal to or less than one-tenth of the quarter wavelength of light in the material of said first overcoat.

21. A waveguide as described in Claim 17 wherein said second overcoat has a thickness approximately equal to the quarter wavelength of light in the material of said second overcoat.

22. A waveguide as described in Claim 17 wherein said first and second overcoats are of the same material, but of respectively different thicknesses.

23. A waveguide as described in Claim 18 wherein at least one of said pairs of surfaces has a multiple layer dielectric overcoat.