CA1202507A - Polarization retaining single-mode optical fibers and methods of making - Google Patents

Abstract

Abstract of the Disclosure A single mode optical waveguide is constructed in a manner such that the core thereof is subjected to a stress-induced birefringence. In accordance with one embodiment the fiber comprises an oblong core surrounded by an oblong inner cladding layer. An outer layer of stress cladding glass, which has a circular outer surface, surrounds the inner cladding layer. The TCE of the stress cladding glass is different from that of the inner cladding glass. In another embodiment the stress-induced birefringence is accomplished by introducing into the cladding region of the fiber on opposite sides of the core longitudinally extending regions of glass having a thermal coefficient of expansion different from that of the remainder of the cladding. A
number of novel techniques are disclosed for forming such fibers.

Description

2~
,' , .'. ~

POLARIZATION RETAINING SINGLE-MODE OPTICAL FIBERS
AND METHODS OF MAKING

Back~round of the Invention .,, . . .
- In many applications of single mode optical wave guides, e.g. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external ~-~ 5 depolarizing perturbations. This requires the waveguide to have an azimuthal asymmetry of the refractive index , 1;, ....
profile.
` A slight improvement in the polarization perfor-mance of single mode optical waveguides is achieved by dis-` ` 10 torting the Eiber core symmetry as a means of decoupling j; .
` the differently polarized waves. Two such optical fiber ~`; ; waveguides are disclosed in U.S. Patent No. 4,184,859 and in the publication by V. Ramaswamy et al., "Influence of Noncircular Core on the Polarisation Performance of Single ` ~ 15 Mode Fibers'l, Electronics Letters, Vol. 14, No. 5, pp. 143-; 144, 1978. However, the Ramaswamy publication reports that .~ ., measurements on borosilicate fibers with noncircular cores ~` indicate that the noncircular geometry and the associated ` ; stress-induced birefringence alone are not sufficient to maintain polarization in single mode fibers.
The invention disclosed in U.K. Patent Application GB 2,012,983A is based upon the recognition that orthogonally polarized waves are more efficiently decoupled in a waveguide - ` ~2~

that is fabricated in such a manner as to deliberately enhance stress-induced, or strain birefringence. That patent teaches that such behavior is accomplished by intro-ducing a geometrical and material asymmetry in the preform 5 from which the optical fiber is drawn. The strain-induced birefring~nce is introduced by a-t least partially surround-ing the single mode waveguide by an outer jacket having a different thermal coefficient of expansion (TCE) than that ,:, .
of the waveguide and a thickness along one direction that is 10 different from its thickness along a direction orthogonal ~o the one direction. For example, the preform may be a three-layered structure comprising an inner core region surrounded ': by a cladding layer which is in turn surrounded by an outer jacket layer having a TCE different than that of the cladding 15 layer. Diametrically opposed portions of the outer layer .~:
-~ are ground ~way, and the resultant preform is drawn into a fiber approximating a slab configuration in which the thick-; . - :'-nesses of the outer jacket layer are different in two ortho-gonal directions. A similar result can be accomplished by ~ , 20 constructing the preform from an inner core region, a cladding region and two outer jacket layers oppositely disposed along . .;, ~
the longitudinal surface of the preform. Difficulty can be : ~ encountered in the manufacture of that type of preform since , stress is built up in the outer layer. When grinding the ~, .
25 outer layer or when cutting slots therein, the built-up stress has a tendency to cause the preform to break. Assum-ing that a fiber can be drawn from the preform, the stress-forming outer layer is far removed from the fiber core, and therefore, the effect of the stress on the core is minimal.

In one embodiment of GB 2,012,983A represented by Figures 10-15, a relatively thick substrate tube forms the :
; ~
; outer portion of the optical fiber. In order to impart to the fiber the desired characteristics, either the inner or outer surface of the substrate tube is non-circular~ Because ~, at least a portion of the substrate wall must be relatively 5 thick, the efficiency of deposition is adversely affected.
Also, since the substrate tube forms the outer, compressive ; layer of the fiber, commercially available tubes may not be usable in the process unless they fortuitously possess the ~ desired expansion and/or viscosity characteristics of the ; 10 resultant fiber outer layer.
In a fiber such as that illustrated in Figure 12 o :~ GB 2,012,983A, the outer layer 60 of cladding is referred to herein as the stress cladding. It has been found that the stress ô at the core of a circularly symmetric single mode ~, .
optical wa~eguide fiber is equal to the product of fxg where ~;" i f is a function of geometrical factors and g is a function of glass factors. The function f is given by the equation . .
``` ~` f = Asc (1) f ` where ASc is the cross~sectional area of the stress cladding and Af is the total cross-sectional area of the fiber. The ~;
~` ~ function f can therefore have a value such that o ~ f ~ 1.
`~ The function g is given by the equation ~;~ g = 2(1-v) (2) ~ 2 where E is the effective elastic modulus of the fiber, ~ is -~ ; the difference between the TCE of the stress cladding and ~` ; the TCE of the remainder of the fiber, ~T is the difference between the lowest set point of the glasses of which the fiber is comprised and room temperature and V is Poissons ratio. Since the aforementioned definition of stress generally applies also to non-symmetrical fibers such as those disclosed in GB 2,012,983A, it is necessary to maximize f to obtain the grsatest core stress and thus obtain the greatest stress birefringence. Values of f greater than ,. ~
0.9 should b~ achieved to provide maY~imum values of stress birefringence. The need to maximize function f is recognized ~-~ in GB 2,012,983A as evidenced by equations (7) and (8 thereof.
Another art-recognized design criteria for single mode optical waveguides is concerned with minimizing loss.
~ 10 A common method of forming single mode optical waveguide -; - preforms is illustrated in Figure 11 of GB 2,012,983A which ..
~: - -shows a plurality of vapor deposited layers on the inner .,., - .
~` surface of a substrate tube. The purity of the substrate ..... .
j tube is generally not as high as that of the vapor deposited ;; , 15 glass. Thereforej the vapor deposited core glass is isolated ~ from the substrate tube by a layer of vapor deposited optical :,.
~ cladding glass of sufficient thickness. For a single mode `, ~ fiber having a core cross-section which is circular or nearly circular, the radius rS of the optical cladding ~ 20 should be at least five times the radius ra of the core.

`~ This estimate is based on the findings reported in the : ~ .
; ~ publication: Electronics Letters r Vol. 13, No. 15, pp. 443-` 445 (1977). For fibers having cores of oblong cross-section, ... . . .
this relationship lacks meaningful significance. In such a fibex, the extent of the optical cladding is better described .~
; in terms of its thickness. Since the size of a single mode ` core is related to the transmission wavelength ~ the thickness ..
of the optical cladding can also be specified in terms of A.

The aforementioned cladding radius to core radius ratio implies that the thickness of the optical cladding be at least about 20~. When a single mode waveguide is designed 5~

in accordance with this cri~eria, loss associated with ~ladding ~hlckness is limited to an acceptably low -value.
~ he sollowing analysis of~ ~ 2,012,9~3 A is made by takin~ into consideration, inter alial the SpeCifiG embodi-ment described in conjunction with Figures lO-12 thereof.
The fiber of ~hat embodiment will satisfy the requirement that the ratio AsC/A~ exc2eds 0.9 except when the ~ubstrate tube i~ completely illed with i~ternal layers during the process o making the preform from which ~he fiber is drawn.
This aforementioned exception is, of course, an impossibility.
Since the substra~e tube cannot be completely filled during the internal layex deposition process, the total thickness of the internal layers is lLmited by the internal diameter of the substrate tube. It is well known that the core diameter of a step proflle single mode fiber is generally between 3 ~m and lO ~m. The outside diamet~r of the fi~er is typically about 125 ~m. If the preform described in G~
2,012,983 A is formed in accoxdance wit~ conventional practice so that the ratio Asc/Af exceeds 0.9, the thicknass of the optical cladding layer will be less than 20~ at conventional wavelengths. Thus, the excess fiber loss due to insufficient optical cladding thickness will not be sufficiently low for man~ applications.

Summary of_the Invention It is therefore an object of the present invention to provide an improved single polarization sin~le mode optical waveguide exhibiting stress-induced birefringence. Anotner object is to provide novel methods of making polarization maintaining single mode optical ibers. The method of this --S--"

~V~

. inven~ion ~s particularly advantageous in that it does no~
;;: include steps which weaken the fiber preform.
~. The invention relates to single mode, single polari-"
zation optical fibers ~omprising a ~ransparent core surrounded by a layer of transparent cladding material having a refracti~e index lo~er than tha of the core. The invention is broadly charac~eri~ed in that the cladding layer includes an assymetry that includes a birefringence in the core.
In accordance wlth one embodiment of the present invention an optical waveguide f iber comprises a txansparent glass core h~virlg an oblong cross sectional confi~uration. Disposed on the surface of the core is an eliptic~lly-shaped layer of optlcal cladding glass having a refractive index lecs than that o the core glass. Surrounding the eliptically-shaped layer of cladding glass there is disposed an outer layer of stress cladding glass having a temperature coeffi-cient of expansion beiny greater or less than that of the eliptically-s~aped cladding layer. The outer surface of the outer cladding layer is substantially circular in cross-section.
;' 20 The aforementioned optical fiber may be formed by providing a tubular intermediate product comprising an inner layer of core glass surrounded by a first cladding glass layer. Tha intermediate product is collapsed ~o form a fla~tened preform foreproduct wherein the core glass has .
been transformed into a unitary layer having an elongated cross-s~ction. This core layar is surrounded by an inner cladding layer which now has an oblong cross-sectional . -~
configuration. A layer of particulate glass, often referred :` to as soot, is deposited on the outer surface o,~ the inner cladding layer, the TCE of the particu.ate glass being greater or less than that of the inner cladding glass. The resultant article is heated to consolidate the particulate glass into an outer cladding glass layex, thereby forming a solid ylass draw blank which can be drawn into an optical waveguide f iber.
In aecordance wi~h one m~thod o~ forming the tubular intermediate product, a ~lurality of layexs are deposited by a chemical vapor deposition ~echni~ue on the inner sur~ace of a substrate tube which is ~ormed of a glass which may be of lower purity ~han the glass layers deposiked ~herein.
The inn~rmost layer forms the core and the next adjacent layer, which is thicker than the core layer, forms the optical cladding. This me~h~d of forming the fiber permits the thickness of the op~ical cladding ~o be greater than 0 at the operating wavelength. The core is thus ad~quately isolated from the ~mpure substrate tube.
In another em~odiment, the tubular intermediate product is formed by a flame oxidation technique. Reactant vapors are fed to a burner where they are oxidized in a flame to form layers of glass particulate material which is deposited on a cylindrical mandrel~ The first ap~lied layer forms the core material of the resultant iber. At least one additional layer is applied to the first layer to form the inner cladding.
After the mandrel is removed, the resultant hollow porous preform can be consolidated to form a hollow glass tube which is thereafter heated on opposite sides to cause it to collapse flat. Alt rnatively, a low pressure can be applied to the aperture of the soot preform to cause it to collapse flat during consolidation.
Both of these methods permit the formation ~ a very thick stress cladding layer so that the ratio AS~JAf i5 greater tha~ 0~9 ~2CI~

In accordance with one embQdiment of this invention there is pro~ided a polarization retaining single mode optical wave-guide fiber comprising a core of transpaxent glass, said core having an oblong cross-sectional configurationl an oblong inner cladding layer disposed on the sur~ace of said core, said inner cladding including an optical cladding layer of high purity glass surrounded by a layer of lower purity glass, the refractive index of said core glass being greater than that of said high purity and said lower puri~y cladding glasses~ and an outer layer of stress cladding glass suxrounding said inner cladding, said stxess cladding glass having a thermal coe~ficient of expansion greater or less than that of said inner cladding glass, the outer surface of said outer layer being substantially circular in cross-section.
In another embodiment the invention provides a method of fabricating an optical waveguide fiber comprising the steps of providing a preform foreproduct having an oblong, centrally dis-posed glass core surrounded by an inner layer of cladding glass, the refractive index of said core glass being greater than that of said cladding glass, deposi~ing a layer of flame hydrolysis-produced soot on the outer surface of said inner cladding layer,the thermal coefficient of expansion of said ylass soot being great-eror less ~han that of said inner cladding layer by at least 1 x 10 7~0C, heating the resultant article to consolidate said layer of glass soot, thereby forming a solid glass draw blank, and drawing said solid glass draw blank to form a polarization retain-ing single-mode optical waveguide fiber.

- 7~ -5~

In yet another embodiment of the invention a ~ingle mode, singl~ polarization op~ical waveguide fib~r comprises a core of transparen~ glass and a layer of cladding glass on the surface of the core, the refractive index of the core gla~s being yreat~r than ~hat of the cladding glass. ~wo diametrically opposed longitudinall~-ex~ending regions, whi h are locate in the cladding, are ormed of a glass having a TCE which is higher or lower than that of ~he cladding g}a~s. The fiber can further comprise a second 1~ pair of diametrically opposed regions which are orthogonally dispos2d with respect to the two diame~rically opposed regions. The ~CE of the ~wo regions is greater than that of the cladding glass, and the TCE of the second pair of regions is less than that of the cladding glass. ---The fiber of the last mentioned em~odiment can be formed by the following method. A first coating of particulate glass is deposited on a rotating mandrel. A second coating `~ o particulate glass having a refractive index lower than I that of the first coating is deposited over the first coating.
¦ 20 First and second longitudinally extending regions of particulate I glass having a TCE difexent from that of the second coating ¦ are depo~ited on diametrically opposed portions of the second coating. This can be accomplished by halting rotation of the mandrel, moving the deposition means longitud.inally .
along th~ mandrel, rotating the mandrel 180, and again moving the deposition means along the mandrel. Alternatively, the longitudinally extending re~ions can be formed by changing the composition of reactant materials supplied to the deposition means during the rotation of the mandrel so that the desired particulate glasses are formed and deposited during each increment of mandrel rotation. A coating o particulate i 2~

cladding glass is then deposited on the outer surface of the resultant body~ The ~CE o the cladding glass is similar to that of the second glass coating, and the refractive index of the cladding glass is equal to or lower than that of the second gla~s coating. The mandrel is remove~, and the resultant porous preform is formed into an op~ical wave-guide fib~r.
An alternative m~thod of manufacturing the fiber of the present invention comprises the steps of disposing centrally within a ~lass tube a first glass rod ha~ing an axially disposed core region surrounded by a layer of eladding glass. A firs~ pair of glass rods is diametrically situated with respect to the central rod within the tube, the first pair of rods being formed of a glass having a TCE different from that of the cladding glass. Rods of cladding glass are - situated in at least some of the interstices between the centrally disposed rod, the first pair of rods and the tube.
The resultant combination can be drawn into a iber. An increase in birefringence can be obtained by disposin~
~G within the tube on opposite sides of the first rod and orthogonally disposed with respect to the first pair of glass rods a second pair of glass rods having physical characteristics different from those o the first pair of glass rods.
~ he fiber of the present invention can also be manu-factllred in accordance with a process which includes the steps of passing through a first tube a gas which, when heated, forms glass paxticles, and moving a heat source along the outside of the first tube whereby at least a portion of the gas is converted ~o particulate material and at least a portion of the particulate material is deposited - _ g lZ~2~

on the inside of ~he first ~ube. Tha improvement of the present invention comprises moving a pair of tubes within the first tube while maintaining ~he ends of the pair of tubes, which are within the first ~ube, in spaced relation to the heat source and up~tream of the heat source, the pair o tubes being disposed symetrically on opposite sides of the center of ~he irst tube. The gas is passed hetween the first tube and the pair of ~ubes. ~hrough ~he pair of tubes is passed another gas which reacts in the hot zone to form 13 an oxide which çombines with the particulate material ~o form a region of glass having a coefficient of expansion diferent from that of the glass par~icles produced by the first gas alone. The pair of tubes may be at least partially retracted from the fi~st tube except when gas is flowed therethrough.

Brie~ Descri tion of the Drawin s _ _P

Fi~ure 1 is a cross-sectional view of an intermediate product which is employed in the formation o~ the preform rom which the fiber of the present invention is formed.
Figure 2 shows an apparatus for collapsing the int~r~
mediate product of Figure lo Figures 3 and 4 are schematic representation~ of an apparatus for forming a composite preform haviny an outer soot coating.
- Figure 5 is a cross-sectional view of a draw blank forme~ by consolidating the composite preform of ~igure 4.
Figure 6 is a cross sectional view of a single-mode single polarization fiber drawn ~rom the draw blan~ illus-trated in Pigure 5.
Figure 7 illustrates a flame hydrolysis procass for ~10-~2~D2S~7 forming a preform including a core portion and an inner cladding portion.
Figure 8 shows the ~oot preform of ~igure 7 after the mandrel has been removed.
Figure 9 shows the consolidated preform.
Fisure 10 is a schematic representation of a consoli-dation furnace which may be used to consolidate the preform of Figure 8~
Figure 11 is a cross-sectional view of an optical ;~ 10 waveguide fiber constructed in accordance with the present invention.
Figures 12 and 13 are cross-sectional views of further embodiments of the present invention.
Figure 14 shows an apparatus for forming one o the embodiments of the present in~ention.
Figure 15 is a cross sectional view of a finished soot preorm as ormed by the apparatus of Figure 14.
Figure 16 is a cross-sectional view of a fiber drawn from the preform of Figure 5.
Figure 17 is a cross-sectional view of a portion of a modified soot preform.
Figure 18 is a cross-sectional view of a rod-in-tuhe type draw blank.
Figure 19 illustrates an alternative apparatus which can be employed in the formation of a fiber in accordance with the present invention.
Figure 20 is a cross-sectional view of a preform formed by the apparatus of Fi~lre 19.
Figure 21 is a cross--sectional view o a iber which can be formed ~rom the pre~orm shown in Figure 17 or from that shown in Figure 20.

12~9~5 19~

Figures 1 through 10 pertain ~o an embodimen~ wherein a single mode iber comprises an oblong core surrounded by an oblo~g inner cladding layer and an outer layer of stress cladding glass having a circular ou~er surface. Referring more ~peciically to Figure 1, there is shown an intermediate product 10 formed by a well-known embodiment o~ the chemical vapor deposition ~echnique wh~r~by one or more layers of glass are formed on the inside sura~e o a substrate tube which latex fonms at lea t a portion of the cladding ma~erial.
The reactan~ vapor, together wi~h an oxidizing medium, 10ws through hollow, cylindrical substrate tube 12. ~he substrate and the con~ained ~apor mixture are heated by a source that moves relati~e to ~he substrate in a loni-udinal direction, whereby a moving hot zone is established within the substrate tube 12~ A suspension of par~iculate material, which is produced within the hot zone, travels downstrea~ where at least a ~ortion thereof comes to xest on the inner surface of tube 12 where it is fused to orm a continuous glassy deposit. Such process parameters as temperature, flow rates, reactants and the like are discussed i~ U.S. Patent No. 4,217,027 and in the publications: J. ~. MacChesney et al., Proceedings of the IEEE, 1280 (1974) and W. ~ ~rench et al., Applied Optics, 15 (1976) Reference is also made to the text Va~or Deposition edited by C. F. Powell et al., John Wiley ~ Sons, Ine. (1966).
A thin barrier layer 14 o~ pure silica or silica dope~
with an oxide such as B203 is sometimes initially deposited on the inner surface of tube 12 which is usually formed of silica or a high silica content glass, the purity of which is lower than thak of the vapor-deposit laya~s formed therein. The ~ar~iex layer prevents ~he migration of hydroxyl ions or other light absor}: ing impuri~ies from tube 12 into optical cladding layer 16. ~n order to reduce light transmission 105s caused by the Lmpurity of the substxate tu~e to an acceptably low level, the thickness of layer 15 is made s1lfficiently great tha~ the thic3cness of the optical cladding lay~x in the resultant fiber is greater than 20~o Since barrier layex 14 is opt70nal, it is ns:~t shown in Figures 3-6. The optical cladding layer is a r~latively thick layer of glass having a rela~itreïy low r~fractive index. It conventionally comprises pure silica or silica doped with a small amount of dopant oxide for the purpose of lowering processing t~mpo eratures. The addition of a small amount of P205 to the deposi~ed silica cladding layer is taught in ~he publication:
S. 5entsui et al., "Low Loss Mon~mode Fibers Wi~h P~Q5-SiO~
Cladding in the Wavelength Region 1.2-1.6~m", 5th European Conference on Optical Commun'cation, Ams~erda~/ September, 1979. ~he use of P~O5 along with either B2O3 or F in the deposited silica cladding layer is taught in the publication:
~0 B~ J. Ainslie et al., "Preparation of Long Length of Ultra Low-Loss Single-Mode Fiber", Electronics Letters, July 5, 197~, Vol. 15, No. 14, pp. 411-413. The use o such dopants has resulted in a deposition temperature of about 1500C, which is appxoximately 200C lower ~han the .

temperature required to deposit a pure fused silica cladding layer. Upon completion of optical cladding layer 16, a relatively thin layer 18 of core material is deposited on the inner surface thereof. Core layer 18 consists of a high - 5 purity glass having a refractive index greater than ~hat of cladding layer 16~ Layer 18 conventionally comprises silica ., ~ - doped with a low-loss oxide for the purpose of increasing . -:
the refractive index. Many dopants have been employed in .: .
` the fabrication o the cores of single mode optical waveguide fibers, GeO2 presently being preferred. Single mode wave-- guides having losses less than 1 dB/km in the infra red region ,, comprise cores formed of SiO2 doped with GeO2 as reported in the aforementioned Sentsui et a]. and Ainslie et al. publi-cationsO The resultant intermediate product 10 contains ~`~ 15 an aperture 20.
For operation at wavelengths in the range between 1.1 and 1.8 ,um a preferred intermediate product could be ` constructed in accordance with teachings of U.S. Patent No. 4,385,802 in the name P.E. Blaszyk et al. That appli-. ~ .
cation teaches the formation of a P2O~ doped SiO2 cladding layer on the inner surface of a borosilicate substrate tube followed by a thin layer of pure SiO2 to pxevent the P2O5 .....
from diffusing into the GeO2 doped SiO2 core which is deposited - on the inner surface of the pure SiO2 layer.
For purposes of the present invention it is merely ` required that intermediate product 10 comprise an inner ` layer of core glass surrounded by a layer of lower refrac-tive index optical cladding glass. Core 18 could be deposited directly on the inner surface of tube 12, for example, if tube 12 were formed of high purity glass. As used herein, ~ the term "inner cladding layer" means tube 12 and any other - layer or layers of glass surrounding core layer 18 in inter-mediate product 10.
It is an advantage of the present method that .. ~
- 5 commercially available glass tubes may be employed or sub-, ~ ~ strate ~ube 12. The cross-sectional area of cladding layer 16 -` can be made much greater, e.g. more than twice ~chat of sub-~ strate tube 12 so that the physical characteristics of .: , . .
deposited layer 16, rather than those of tube 12, predomi-nate in the determination of characteris~ics such as the ` thermal coefficient of expansion of the inner cladding.
.; , ~ In such a situation, the cross-sectional area of tha sub-:.-~ strate tube is so small relative to that of the entire ... . .... .
. . - . ~
~ resultant fiber that the physical characteristics thereof i~ 15 remain essentially insigniicant.
~ .. . .
Intermediate product 10 can be collapsed in the manner illustrated in Figure 2~ Burners 22 and 24 produce :
~;; flames 26 and 28, respec~ively which are directed onto oppo-i ~ site sides of intermediate product 10. During this process ;~ ~ 20 intermediate product 10 may be mounted in the glass lathe ;, ~ (not shown) in which it was mounted during the formation ;..
`~- of layers 14 and 16. During the collap~e process illustrated ~ ~ , - in Figure 2, lathe rotation is halted so that only opposite sides of product 10 are heated. The collapsing step is preferably done under a controlled internal pressure as .
taught in U.S. Patent No. 4,154,491. During this step, the -- heat source must cover a sufficiently broad axial region of intermediate product 10 to permit collapse thereof. Alter-natively, a single heat source may be employed in the manner described in U.S. Paten'c No. 4,184,859, whereby first one side and then the other is collapsed.

~20æ~s~7 Complete collapse of the intermediate product 10 resultsin preform foreproduct 30 in which opposlte sides of core layer 16 have been combined to form a core portion 32 which is elongated in cross-section. Very large core aspect ratios ` 5 can thus be achieved. The core issurrounded by inner cladding portion 34 and substrate portion 36, both of which have an oblong geometry.
~; Preform foreproduct 30 is then provided with a cladding portion, the outer surface of which is substantially circular in cross-section. The surface of foreproduct 30 is ~, .
prepared in a conventional manner prior to deposition of the ` outer cladding. The surface of preform foreproduct 30 is ..
, kept clean after the firepolishing step which resulted in the collapse of intermediate product lG by inserting for0product .; -: ., ~` ~ 15 30 into a clean sealed bag such as a polyethylene bag. If foreproduct 30 is handled or permitted to become dirty, - several cleaning steps are typically required. It is washed in deionized water and then washed in an isopropyl alcohol bath. It is then etched in HF to remove a few microns of ~; 20 glass or about 1% of the article weight. Then foreproduct . , 30 is rinsed in deionized water, degreased with isopropyl ~; alcohol and placed in a clean polyethylene bag. Soot of the desired glass composition is deposited on foreproduct 30 by a conventional flame hydrolysis process similar to that disclosed in U.S. Patents Nos. 3,737,292 and 4,165,223.
Referring to Figures 3 and 4, there is shown an apparatus -: ~ . . .
" which is now conventionally employed in the manufacture of low-loss optical waveguide Eibers. A flame 38 containing glass soot emanates from a flame hydrolysis burner 40 to which fuel, reactant gas and oxygen or air are supplied.
Burners such as those disclosed in U.S. Patents Nos.3,565,345;

3,565,346; 3,609,829 and 3,698,936 may be employed. Liquid constituents required to form the glass soot can be delivexed to the burner by any one o the many well known xeactant delivery systems known in the prior art. Reference is made in this regard to teachings of U.S. Patents Nos. 3,826,560;
. ~

4,148,621 and 4,173,305. Excess oxygen is supplied to the burner so that the reactant vapors are oxidized within flame .:
~; 38 to form the glass soot which is directed toward fore-, .
product 30.

In accordance with one technique for forming the ; outer cladding layer, longitudinal strips 44 and 46 are initially deposited on the flattened sidewalls of foreproduct ~i 30 to accelerate the formation of a circular outer cladding.

With the lathe halted, burner 40 makes a sufficient number ~ . ., ; 15 of longitudinal passes to form a soot layer 44. Foreproduct ~- 30 is rotated 180 and a second soot layer 46 is deposited opposite the first one as shown in Figure 4. Outer layer 48 . . .
of cladding soot is then deposited by rotating foreproduct ~ . . ,~ .
~" ` 30 while burner 40 traverses it longitudinally.
~; 20 The steps of depositing strips 44 and 46 o ~` cladding glass may be omitted without affecting to too great ``~ an extent the geometry of the res~1ltant fiber. If cladding `; layer 48 is deposited directly upon foreproduct 30, the soot ` stream from the burner will deposit a greater amount of soot when the flat side walls of foreproduct 30 are facing the ~; ` burner than when the rounded portions thereof are facing the burner since soot co]lection efficiency is a function of target size. This tends to decrease the noncircularity of the soot blank cross-section as layer 48 is built up.
Substantial circularity should be achieved when the outside diameter of layer 48 is sufficient, relative to the size of ~oæ~

:
the core~ to enable the resultant fiber to function as a single-mode fiber. The thickness of layer 48 must be sufficient to cause the ratio ASc/Af in the resultant fiber ~ to exceed 0.9.
,- 5 The flame hydrolysis-produced cladding layer is porous in form and must be heated to fuse or consolidate it into a glass layer free from particle boundaries. Consoli-dation is preferably accomplished by gradually inserting the ~; composite body 50 into a consolidation furnace in the manner taught in U.S. Patent No. 3,933,454. The resultant glass -~ draw blank 56 may not be circular if layers 44 and 46 are not applied or if they are applied in such a fashion that they do ~` not balance the initial non-circularity of preform fore-product 30. The amount that the outer surface of consolid-` 15 dated blank 56 deviates from circularity decreases with ~.: .
increasing amounts of outer cladding 48.
Draw blank 56 of Figure 5 is inserted into a draw furnace wherein at least one end thereof is heated to a . . .~ .
temperature that is sufficiently high to permit fiber 70 of Figure 6 to be drawn therefrom in accordance with conventional . .~ , ~ practice. During the drawing of fiber 70, surface tension ,; ..
tends to round the outer surface thereof.

~; Alternative processes for forming an intermediate ~:. .. .
product are illustrated in Figures 7 through 10. As shown in Figure 7, a first coating 84 of glass soot is applied to cylindrical mandrel 85 by a conventional flame hydrolysis process such as that referred to hereinabove. A flame 86 containing glass soot emanates from flame hydrolysis burner 87 and impinges upon mandrel 85. After a coating 84 of core glass is formed on mandrel 85, the composition of the reactant gas fed to burner 87 is changed and a second coating 88 of - ~æ~æ~
.. .
.
inner cladding ylass is app],ied to the outer surface of first coating 84, The refractive index of coating 84 is '' , greater than that of coating 88. The physical character-' ~- istics of coating 88, such as the TCE thereof, are selected ' 5 to impart the required amount of stress to the inner cladding .:
~ I of the resultant optical waveguide fiber~
, ! . . ~ ~
;'~''~''' After coating 88 has achieved the desired thick-:,.. :, ~' ','~ ness, the mandrel is removed as shown in Figure 8 to form a ;.
:'.`'.!:, '. porous preform 90 having an aperture 89. The resultant hollow soot preform can then be consolidated in the manner ' described hereinabove to form hollow intermediate product , 10' as shown in Figure 9. Intermediate product 1'0' can be ~, . .
;~-,''' ' collapsed in the manner illustrated in Figure 2 and further ~ '~' processed in the manner described in conjunction with , .. . .
4~ 15 Figures 3 through 6 to form ~ polarization retaining single-mode optical waveguide fiber.
~ ' , The porous preform 90 illustrated in Figure 8 can ,' , alternatively be consolidated in the manner illustrated in ......
;' ''~ Figure 10 to form a preform foreproduct having a high ' 20 aspect ratio core in a single processing step~ After man-, drel 85 has been removed from the soot preform, a tube 91 is inserted into one end of the preform. The preform is then ``-, suspended from a tubular support 92 by two platinum wires, ,'; of which only wire 93 is shown. The end of gas conducting . ~, . .
', 25 tube 91 protrudes from tubular support 92 and into the adjacent end of preform 90. The preform is consolidated by ` yradually inserting it into consolidation furnace 94 in the , direction of arrow 97. The preform should be subjected to gradient consolidation, whereby the bottom tip thereof begins to consolidate first, the consolidation continuing up the preform until it reaches that end thereof adjacent to tubular support 92. During the consolidation process a - flushing gas such as helium, oxygen, argon, neonj o.r the - like, or mixtures thereof flow through the consolidation . 5 urnace as indicated by arrows 95~ Prior to the time that ; preform 90 begins to consolidate, drying gases may be flowed into aperture 89 in the manner taught in U.S. Patent No.
4,125,388. During the time that the initial tip of the preform begins to consolidate, the pressure in aperture 89 ` lO is reduced relative to that outside the preform. This may ~; be accomplished by connecting a vacuum system to gas con-.~. ducting tube 9l by line 96. As preform 90 i5 inserted into . the consolidation furnace in the direction of arrow 97 the .-~ low pressure within aperture 89 causes aperture 89 to collapse .` 15 flat, beginning in the region of the initially consolidated .~:
tip portion of the preform. As the remainder of the preform . becomes consolidated, the remainder of the aperture continues , '- :' :`: to collapse flat. Thus, in a single consolidation step, . . porous soot preform 90 having aperture 89 therein can be ; ~
. : 20 consolidated and simultaneously have the aperture collapsed flat to form a preform foreproduct of the type illustrated by numeral 30 in Figure 3.
; Referring again to Figures 4-6, the composition of .., ,.,~, ,.
. soot layer 48 (and that of strips 44 and 46, if they are . 25 deposited) is such that the TCE o the resultant cladding . layer 74 is much greater than or much less than the TCE of the remainder of fiber 70. It is known that portion 72 (comprising core 80, substrate tube 82 and any layers forming inner cladding 78) will be caused to be in tension if the TCE

of the outer or "stress cladding" layer 74 is lower than the effective TCE of portion 7Z. Conversely, portion 72 will be : .

caused to be in compression if the effective TCE thereof is lower than that of stress cladding layer 74. See the publi-cation: S.T. Gulati and H.E. Hagy, American Ceramic Society A ~ 61 260 (1978). Moreover, a stress distribution will exist . .
~. .
- - 5 within the waveguide core 80 in which 6 ~ 6~ , where x and ,. .
~ ~y are the stresses in the core region parallel to and per-., . . -, ~; - pendicular to the long axis of ~he core cros.s-section.
` ~ Furth~rmore, this stress difference will increase as the ~. .. .
aspect ratio of the core region increases. This stress differential will produce the desired birefringence.
....
~ A stress of 20-40 kpsi in the core is needed to . .
~ provide the required birefringency. With the aspect ratios . . - .
; achievable by the processes described hereinabove, the TCE
difference between the inner cladding and the outer stress `~ 15 cladding should be greater than 1 x lU 7~oc. Following are . ~, . . .
two theoretical examples wherein the glass compositions of - the various parts of the fibers are chosen so that the fiber core is in compression and tension, respectively.
..;.~ . i ~ A fiber of the type shown in Figure 6 is formed . ~ :
```~` ~ 20 of the glass compositions given in Table 1. The TCE of each composition is also listed.
Table 1 .~ :
~ Composition (wt. ~) , . .;

`~ - 2 SiO2TCE (x10 / Cj .; .:
~ 25 Core 80 15 85 13 `; ~ Inner clad 78 100 5 Tube 82 100 5 Outer clad 78 30 70 23 The fiber defined by Table 1 has a core that is in compression and an outer cladding which is in tension.
~ 21 -~2~2!~

Although the core is adequately str~ssed, this fiber may be undesirable from a strength stanapoint. Such a fi~er could be streng~hened by adding to the outex surface thereof a further low expansion cladd.ing layer of SiO2, for example.
A i~er of the type illustrated in Figure 6 could be ~ormed of the materials specified in ~a~le 2 in order to put the core into a state of tension.

Table 2 Composition (wt.%) ,.
GeO2 P205 SiO2 ~i2 Core 80 15 1.5 83.5 15 Inner clad 78 1.59~.5 6 Outer clad 74 93 7 0 This type of fiber, in which the core is in tension, is preferred since the outer cladding will he in compression, a condition tending to strengthen the fiber.
Figureq ll through 21 are concerned with a further embodiment of the invention wherein the fi~er core is subjec~ed to a stress-induced bireringence by introducing into the cladding on opposite sides of the core longitudinally extending regions of glass having a TCE diffPrent from that of the remainder of the cladding. Figure 11 shows a cross-sectional view of a single polarization optical waveguide fiber comprising a core 110 surrounded by an inner cladding region lll. Diametrically opposed relative to core 110 are two narrow, longitudinally-extending regio~s 112 formed of a material having a TCE different from that of matPrial 111.
While regions 112 are illustrated as being of somewhat random cross-section in Figure ll, methods will be described -22~

~2~2~7 below which result in the formation of these regions in various specific shapes. ~hen such a fiber is drawn, the longitudinally-extending regions 112 and the cladding regions disposed orthogonally thereto will shrink diff~rent amounts whereby regions 112 will be put into a state o~ tension or compression depending upon the TCE thereof relative to that of the cladding. A strain induced birefringence, which is thus induced in the fiber, reduces ~oupling between the two or~hogo~ally polarized fundamental modes. Surrounding regions 112 is an outer cladding region 113, ~he refractive index o which is pr ~erably equal to or less than that of inner cladding region 111. Region 113 may consist, for example, of any of the materials specified ~bove for use as cladding region 111.
The outer surface of cladding region 113 may be cixcular as illustrated in Figure 11~ or it may have areas which are ~lattened in the manner disclosed in the afor~mentioned UO~. Patent Application GB 2,012,983A for the purpose of aligning a fiber with a polarized light source or with another fiber to which it is to be connected. If the outer surface of cladding 113 is substantially circular, means such as longitudinal depression 114 may be provided for alig~ment purposes. If it is preferred that the outer surface of the fiber be substantially circular, the input end o the fiber can be properly oriented during its install-ation into a sy~tem. The input end is connected ko a polarized light source, and an analyzer i~ connected to the output end of the fiber. The input end of the fiber is rotated relative to the source until a maximum or a minimum i5 detected in the light emanating ~r~m the output end. ~hen either the maximum or minimum light output i9 detected, the input end of the fiber is then fixedly mounted with re~pect to the ~23-polarized light source.
Regions 11~ sould be as close as possible to core 110 without inordinately affecting the light transmitting properties of the fiber. ~f regions 112 are formed of low . loss material having the same xefractive index as inner cladding region 111, then the minimum radius rm f regions 112 is about 1.5ra, wher~ ra is the radius o~ core 710. A
matching of the refrac~ive index of regions 112 ~o that of the cladding could be accomplished by employlng a cladding formed of Si.02 and forming the stress-inducing regi.ons 112 of/ for exampler SiO2 doped with one of the following combina-tions of dopant oxides GeO2 and B~03 or P205 and ~23 or GeO~, P205 and B203. An example of a suitable high TCE
composi~ion having a refractive index substantially the same as pure SiO2 is SiO2 doped with 12 wt.% B203 and 4 wt.%
P205. To ensure that the resultant fiber possesses low loss characteristics, a~ least the entire central region, i.e.
the core and inner cladding region, should be formed ~y a chemical vapor deposition (CVD~ process. I~ the refractive indices of these two regions are not matched and rm is too small, i.e. les~ than about l.Sra, regions 112 can cause light transmission loss due to scattering.
If regions 112 adversely affect the light transmission ! properties of the fiber, e.g. the regions 112 are formed of a material which inordinately absorbs light at the trans-mission waYelengths, the in~er radius rm of these regions should b~ at least three times and preferahly a minimum of ~ive times the radius of the core. ~his estimate .is based on the findings reported in the publication: Electronics ~etters, Vol. 13, ~o. l~, pp. 443-445 (1~77). Obviously, the adverse effect of light absorbing material increases as the distance from the material to the core decreasesO

, -2a-However, the magnitude of the bireringence at the core also decreases as the in~er radius rm of the stress-inducing longi~udinally extending regions decreases. .he optimal .inner radius of regions 112 depends upon the speciic type of single mode waveguide employe~, since the amount of light propagating ~eyond the core region of a single mode wave-guide depe~ds upon such parameters as core radius and refractive index.
A fiber may contain a second set of diametrically opposed longitudinally-extending regions havina ~hysical charac~eris~ics which are different fro~ those of ~he irst set of stress-inducing regions. The fiber illustrated in Figure 12 compxises core 116, inner cladding region 117, and outer cladding region 118. Two longitudinally-extending regions 119, which have a TCE differer.t from that of the cladding regions, are diametrically opposed relative to core 116. Or~hogonally disposed with respect to regions 119 is a second pair of longitudinally extending regions 120 which may comprise a light absorbing glass or a glass having a TCE
which deviates from that of region 117 in a direction different from the direction in which the TCE of region 119 deviates from that of region 117. For example, the T~E of regions 120 should be less than the TCE of region 117 if the TCE of region~ 119 is greater than the TCE of region 117.
If the cladding regions consisted of pure SiO2/ regi~s 119 could comprise SiO2 doped with B2O3 and P2O$ while regions 120 could comprise SiO2 doped with TiO2. Regions 119 will be in a state of tension while regions 120 will be in a state of compression. The ef~ect of the two tensive regions i~ additive with that o the two compressive regions, the r~sultant combination providing a greater magnitude of stress induced birefringence than that which would be 2~7 obtainahle with either regions 119 or regions 120 alone.
The TiO2 doped regions are lossy for two reasons. The TiO2-SiO~ glass tends to phase separa~e and form small inho~ogeneous scattering sites which increase ~cattering loss. Also, the TiO2 raises the refractive index of the region to a value grea~er than that of region 117 so that light from core 116 that reaches regions l20 will tend to refract into and through regions 1~0 and thus away from core 116. Stress regions can be made lossy by forming them from glass rods which have been m~lted in crucibles containing absorption impurities such as iron, nickel, cobalt, copper and the like.
The a~or~nentioned U~K. Patent Application GB 2,012,983A
states that the methods disclosed therein are capable of fabricating fibers with-a strain birefringence ~n as large as 40 x 10-5 and that the beat length L for such a value of ~n is 2.S mm at 1 ~m wavelength and 1.25 mm at 0.5 ~m.
Some applications, however, require even shorter bea~ lengths, thereby necessitating values of Qn around 10-3. ~he following theoretical example indicates that such values of ~n are easily achieved by the fiber construction of the present invention. Referring to Figure 13, there is illustrated a fiber having a core 12~, cladding 123 and two longitudinal regions 124 o~ circular cross-section. The diameter of core 122 is 5 ~m, that of stxess-producing regions 124 is 25 ~m and that of cladding 123 is 125 ~m. The centers of circular regions 124 are located at a radius of 25 ~m. The specific composi~ion of core 122 is immaterial, it merely being necessary that the refractive index thereof be greater than that o~ cladding 123 which consists of pure SiO~. The composition of regions 124 is 5 wt.% P205, 12 wt.% B203 and 83 wt.% SiO~. Birefringence calculations were based on the 2~7 publication: G. W, Scherer, "S~ress-Induced Index Profile Distortion in Optical Waveguides", Ap~_ied ~ , Vol. 19, No. 12, June, 1980, pp. 2000~2006. Using computer tech-niques, the bire~ringence in the region of the core and innex cladding due to one of ~he r2gions 124 was determined.
Then the birefringence in the central region due to the other of regions 124 was determined and added ~9 the first calculated value. The results are plotted in Figure 13.
Lines 125, 126, 127 and 128 are lines of eoual birefringence ; 10 of 0.4 x 10 3, 0.5 x 10 3, 0.6 x 10 3 and 0,7 x lC 3, P
respectively, the latter line passing through core 122.
One method for forming the fiber of the present inven~
tion employ~ a flame hydrolysis process similar to that disclo ed in U.S. Patents Nos. 3,737,292 and 4,165,223. Thç
; resultant fiber is shown in cross-section in Figure 15 wherein elements similar to those in Figure 11 are represented by primed reference numerals. The fiber of Figure 16 differs ~ro~ that of Figure 11 in that longitudinally extendin~
stress-inducing regions 112' are crescent-shaped.
Refarring to Figure 14, a layer 130 of glass soot is initially deposited on a cylindrical glass mandrel 131 ~rom flame 133 emanating from burner 132. After the first soot layer 130 reaches a predetermined thickness, the composition is changed and a second soot layer 134, which is to form the inner cladding layer 111', is deposited. During the deposition of layers 130 and 134, mandrel 131 i8 rotated and burner 132 is translated longitudinally with respect to the mandrel.
In order to form the soot wh ch is to be consolidated to form strips 112', the mandrel rotation stops and burner 132 makes a sufficient number of longitudinal passes to form a soot layer 12;. Mandre~ 13~ is rotated 1~0 and a second soot layer 135 is deposited opposite the irst one as shown in Figure 15. Layers 1~6 of cladding soot can be deposited on layer 134 be~ween s~rips 135 in the same manner.
layer 137 o cladding soot is then deposi~ed hy resuminy mandrel rotation. The soo~ preform, when completed, is porous in orm and must be heated to fuse or ~/collaPse~ it into a monolithic glass preform which can be drawn into an op~ical waveguide which is ~hown in Figure l6.
The steps of depositing strip~ 136 of cladding glass may ~e omi~ted without affecting to ~oo great an extent the geometry of the resultan~ fiber. I~ cladding layex 137 is d~posited dix~ctly upon that portion of the ~oot preforTn com~rising layer 134 and strips 135 as the outex surface thereof, the soot stream from the burner will deposit a greater amount of 500t when the surface of layer l34 is acing the burner than when the surface o strips 1~5 i s acing the burner since a greater surfacP area is presented to the soot stream when surface 13~ faces the burner. This tends to decrease the noncircularity of the soot blank cross-section as layer 137 is built up. During drawing o~
the f iber from a consolidated hlank surface tension tends to round the ou~er sur~ace o the fiber, thereby slightly affecting the circularity of ~he core. This is not, however, a detrimental feature for single mode waveguides of the type to which the present invention pertains.
A modified prefor~ produced by the flame hydrolysis proce3s is illustrated in Figure 17 wherein elements similar to those of Figure 14 are represented by primed reference numerals. After layers 130' and 13~' have been deposited in the manner described in conjunction with Figure 14, a layer comprisinq ~egments 139 ~nd l40 is deposited in the following manner. The soo~ deposi~ion apparatus should employ a reactant delivery system such as that disclosed in U.S.

, -2~-2~

Patent No. 4,314,837. That patent discloses a reactant delivery system whereby reac~ant va~ors are fed to the flame hydrolysis burner by way of flow controllers, ~he throughputs of which are cuntrolled by a system control circuit. A
shaft position indicator connected to mandrel 1~19 informs th 5y5te.m contro 1 circuit as to which part of ~he soot preform surfac~ presently faces burner 13~o ~ given reactant flow may be employed in ~he deposition of regions 139, and an additional dopant reactant which a~fec~s ~he expansion coeffici~nt of ~he deposited glass may ~e fed ~o th~ burner during ~he deposition of regions 140. Thus r as mandrel ~31' rotates at a cons~ant angular velocityr regions 14~ are formed by supplying the burner with "pulses" of dopant reactants. Due to the mixing o reactant v~pors, a transition region exis~s between regions 139 and 140. Outer layer ~.41 of cladding ma~erial can be deposited over the layer comprisinq regions 139 and 140 as discussed above. After mandrel 131' is removed, the preform of Figure 17 can be drawn in~o a fiber the cross-sectional configuration of which is similar to that of th~ fiber shown in Figure 21 which i5 ~0 be ~20 described hereinbelow.
Instead of depositing a soot-produced outer cladding layer by the flame hydrolysis techniou~, that layer can be partially or wholly eliminated, and the outer cladding may be provided by a glass tube. For example, after strips 135 and 136 of Figure 15 are formed, or after the layer comprising regions lll' and 112l of Figure 16 have been deposited, the mandrel is removed and the 500t blank is consolidated. The resultant dense glass blank is inserted into a tube, and the resultant combination is drawn into a fiber in accordance wi~h ~he teachings o~ ~.S~ P~tent No~ 3,932,1~2.

~ ZS07 Refexring to Figure 18 there is shown a rod-in tube type pxeform which may be ~mployed to form fibers of the type illu~trated in Figures 11-13. A plurality o~ rods of appropriate materi~l is inser~ed in~o tuhe 142 o a cladding material such as SiO2. Centrally disposed within tube 142 is a rod having a core ~43 of high purity glass and cladding layer 144 of high purity glass having a refractive index . lower tha~ that of core 143. Core 143 and cladding 1~4 are preferably formed by a CVD technique. Diametrically disposed wi~h re~pec~ to the central rod are two rods 145 o glass ha~ing a high TCE relative to tube 142. A second pair of rods 146 is orthogonally disposed with respect to rods 145.
orm a fib~r of the type shown in ~igures 11 and 13, rods 146 can be orm~d of the same material as ~ube 142. To form a fiber of the type illustrated in Figure 12, rods 146 can be fonmed of a ma~exial having a low TCE relative to tube 142 and/or a light-absorbing material. .Rods 147 of cladding material occupy some of the inters.ices bet~7een the afore-~ mentioned rods. If the resultant prefGrm contains lar~e ¦ 20 unoccupied areas, tha tensile regions of the resultant fiber I will appear non-cixcular in cross-section as illustrated in ! Figure 11 due to the distortion of the rods as they fill in !~ interstices adjacent ~hereto. ~he core of such a fiber will also tend to bP noncircular. If all of the illustrated interstices-are packed with small rods ~not shown) of cladding material, the core and the tensile regions will appear more circular in cross~section in the resultant fiber.
Figure 19 is a schematic representation of an otherwisP.
standard chem~cal v~por depositlon apparatus modified so as to be applicable to the practice of this invention. This system compri~es substrate or bait tube 150 which may have ~%n~

2~

an enlarged exhaust tube 152 ~ixed to th~ downstream end thereo~. Tubes 15~ and 152 are chucked in a conventional glass turning lathe (not shown), and the sombination may be rotated as indicated by the arrow. A hot zone 154 is caused to traverse tube 150 by moving heating means 156 as schematically ~epicted by ar~ows 158a and 158b. Heating means 156 can consi~t of any suitable source of heat such as a plurality of burners encircling tube 150. In one embodIment hereof, the heating means muct be capable of applying heat locally.
For example, a single burner or ~wo diame~rically opposed burners cDuld be emp3.oyed. Reactants are introduced into tube 150 via tube 160, which is connected to a plurality of sources of gases and vapors. Any of the aforementioned reactant delivery systems could be employed.
Burner 156 initially moves at a low rate of speed rela tive to tube 150 in the direction of arrow 15~b, the same direction as the reactant flow. The reactants react in hot zone 154 to produoe soot which is carried downstream ~y moving gas where at least a portion of the soot deposits on the inner surface of tube 150 in region 162. As burnex 156 conti~nues to move in the direction of arrow 158b, hot zone 154 moves down~txeam so that a part of the soot buildup extends into the hot zone and is consolidated thereby to form a unitary, homogeneous glassy layer on the inner surface o~ tube 150.
When burner 156 reaches the end of tube 150 adjacent to exhaust tube 152, the temperature of the flame is reduced and the burner returns in the direction of arrow 15~a to the inpu~ end of tube 150~ Thereafter, additional layers of glassy material are deposited within tube 150 in th~ manner descri bed above .
After suitable layers have been deposited to serve -31~

~7 as ~he core material and any other desired layers of ~he resultant optical waveguide, the temperarure of the glass is incxeased o c use tube 150 ~o collapse. ~his can be accom-pli hed by reducing the rate o txaverse of the hot zone.
Preferably, the interior of tube 150 is pressurized during collaps~ as taught in U~S. Patent NoO 4,154,591.
Th~ conventional apparatus heretofore described is suitable for the deposition o glass layexs of uniorm composition on the inner suxface o tube 150. In accordance with the present invention, the oonventional apparatus is modifi~d by th@ provision of means adjacent to and u~stream of the hot æone 154 for delivering ~o diametrically opposed regions of th~ hot zone reactant gases capable of forming soo~ having an expansion coeficient di~ferent from that of the cladding glass material. As shown in Figure 19 a portion of two gas conducting tubes 164 extend into that end of bait tube 150 into which the reactants are introduced.
Those portions of tubes 164 within tube 150 terminate just prior to hot zone 154. Tubes 164 are mechanically coupled by means represented by dashed line 155 to burner 156 to ensure that tubes 164 are maintained the proper distance upstream of the hot zone 154, Alternatively, the heat source and tubes 164 may be kep~ stationaxy, and tube l5n may be caused to move longitudinally. The input end of tube 150 is connected to tubes 164 by a collapsible member 16~, a rotating seal 170 being disposed between member 168 and tube 150. While not in use, tubes 164 can be completely withdrawn from tube 150, ox they can be partially retracted to such an extent that they do not disturb the f low o reactants from conduit 160 through tube 150. Referring to Figure 20, a layer 178 of cladding glass can be deposited on the inner surface oE tube 150 in a conventional manner. In order to ~æ~7 form diam~trically opposed longi~udinal expansion strips within the cladding, a second reactant material is flowed through tu~es 164 while the cladding reactan~ material continues to flow into conduit 163~ For exampl~, SiC14 and BC13 can be flowed into conduit 160 in order ~o deposit within tube 150 a layer 178 of cladding glass. ~fter layer 178 has become sufficiently thick, ~ubes 164 are posi~ioned adjacent ~o the hot zone and a reactant material such as GeC14 is flow~d therethrough while the SiCl~ and BC13 continue to flow into conduit 160. Oxygen to be ~mployed in the reaction is also flowed into the hot zone in a Manner k~own in th2 art. A layer 180 of borosilicate glass is deposlted on the inner surface o layer 178, the stippled portions 182 of layer 180 containing GeO2- ~hereafter~ an~
additional layer 184 of borosilica~e cladding material can be deposited on layer 180, and a layer 1~6 of core material, e.g. GeO2 doped SiO2, can be deposited on layer 1~4. The resultant preform is ~hen collapsed and drawn into a fiber in the manner described hereinabove. Regions 182 of layer 180 ~ontain a sufficient amount of dopant material, e.g.

GeO2, to form within the resultant fiber longi udinal stri~s of high expansion glass.
After forming the preform of Figure 20, it is collapsed to form a solid draw blan~ which is inserted in a draw fur~ace wh~re it is heated to a temperature suiciently high to pexmit a fiber to be drawn therefrom. The resultant fiber, which is illustrated in cross-section in Figure 21, comprises a core 190, an inner cladding region 192 and an outer cladding region 196. On opposite sides of core 190 and within region 196 are two longitudinally-extending regions 194 of high expansion glass. There ls a gradual change between regions 194 and the surrounding glass because ~,z~æ~

of the mixtlare o~ gases during deposition of the glass and because of the diffusion of dopants during various high tempera~ure s~eps to which the glass is sub; ected .
The cross-sectional conf iguration of ~1ber shown in - Figure 21 will also result from consolidating and dra~ing a soot preorm o~ the type shown in Figure 17.

--3~--

Claims (13)

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

1. An optical waveguide comprising a core of transparent glass and a layer of cladding glass on the surface of said core, the refractive index of said core being greater than that of said cladding glass, a pair of diametrically opposed longi-tudinally-extending glass regions located in said cladding, the thermal coefficient of expansion of said glass regions being greater or less than that of said cladding glass.

2. An optical waveguide in accordance with claim 1 wherein the thermal coefficient of expansion of said pair of diametrically opposed regions is greater than that of said cladding glass.

3. An optical waveguide is accordance with claim 1 wherein the thermal coefficient of expansion of said pair of diametrically opposed regions is lower than that of said cladding glass.

4. An optical waveguide in accordance with claim 1 wherein the thermal coefficient of expansion of said pair of diametrically opposed regions is greater than that of said cladding glass and said waveguide further comprises a second pair of longitudinally extending regions which is orthogonally disposed with respect to said two diametrically opposed regions, said second pair of regions having physical characteristics different from those of the pair of regions, mentioned in claim 1.

5. An optical waveguide in accordance with claim 4 wherein the thermal coefficient of expansion of said second pair of regions is less than that of cladding glass.

6. In the method of forming an optical waveguide comprising the steps of depositing on a rotating starting member a first coating of particular glass, depositing on the outer surface of said first coating another coating of particulate glass, removing said starting member, and forming an optical waveguide fiber from the resultant soot preform, the step of depositing another coating being characterized in that it comprises depositing a second coating of particulate glass on the outer surface of said first coating, the refractive index of said second coating being lower than that of said first coating, depositing on diametrically opposed portions of said second coating first and second longitudinally extending regions of glass soot having a thermal coefficient of expansion greater or less than that of the said second coating, and depositing on the outer surface of the resultant soot body a coating of cladding soot having a thermal coefficient of expansion similar to that of said second coating and having a refractive index equal to or lower than that of said second coating.

7. A method in accordance with claim 6 wherein the step of depositing said diametrically opposed longitudinally extending regions of glass soot comprises halting rotation of said starting member, moving said flame hydrolysis burner longitudinally with respect to said mandrel while depositing a first crescent-shaped longitudinally extending region of glass soot, rotating said mandrel 180° and thereafter again halting mandrel rotation, and moving said flame hydrolysis burner longitudinally with respect to said mandrel while depositing a second longitudinally extending region of glass soot.

8. The method of claim 7 wherein the step of depositing said diametrically opposed regions further comprises the steps of rotating said mandrel 90° and halting rotation thereof, depositing a layer of soot on the surface of said second coating which extends between said first and second longitudinally extending regions of soot, rotating said mandrel 180° and halting rotation thereof, and moving said burner longitudinally with respect to said mandrel while depositing a fourth longitudinally extending region of glass soot, the composition of said third and fourth longitudinally extending soot regions being different from that of said first and second longitudinally extending regions of glass soot.

9. A method in accordance with claim 6 wherein the step of depositing diametrically opposed longitudinally extending regions of glass soot comprises rotating said mandrel, providing longitudinal movement between said flame hydrolysis burner and said mandrel, providing to said burner reactant vapors for forming a base glass soot, and supplying said burner with a pulse of a second reactant vapor each 180° of rotation of said burner.

10. A method of forming an optical waveguide preform comprising the steps of providing a tube of cladding glass, disposing centrally within said tube a first glass rod having an axially disposed core region surrounded by a layer of cladding glass, disposing a first pair of glass rods diametrically with respect to the central rod within said tube, said first pair of recess being formed of a glass having a temperature coefficient of expansion greater or less than that of said cladding glass, and disposing a plurality of rods of cladding glass in at least some of the interstices between said centrally disposed rod, said first pair of rods and said tube.

11. The method of claim 10 further comprising the steps of disposing within said tube on opposite sides of said first rod and orthogonally disposed with respect to said first pair of glass rods a second pair of glass rods having physical characteristics different from those of said first pair of glass rods.

12. In a method of manufacturing an optical waveguide preform, said method being of the type that includes the steps of passing through a first tube a gas which, when heated, forms glass particles, and moving a heat source along the outside of said first tube whereby at least a portion of said gas is converted to particulate material and at least a portion of said particulate material is deposited on the inside of said first tube, the improvement which comprises moving a pair of tubes within said first tube while maintaining the end of said tubes, which are within the first tube, in spaced relation to said heat source and upstream of said heat source, said pair of tubes being disposed symetrically on opposite sides of the center of said first tube, passing said gas between said first tube and said pair of tubes, and passing through said pair of tubes another gas which reacts in said hot zone to form an oxide which combines with said particulate material to form a region of glass having a coefficient of expansion greater or less than that of the glass particles produced by the first gas alone.

13. The method of claim 12 wherein said pair of tubes is at least partially retracted from said first tube except when said another gas is flowed therethrough.