CA1100001A - Method of making optical waveguides - Google Patents

Abstract

Powers 2 METHOD OF MAKING OPTICAL WAVEGUIDES

Abstract of the Disclosure A method of making low loss glass optical waveguides, wherein at least one coating of glass soot is deposited by the flame hydrolysis process on a starting member. The starting member is removed from the coating material or glass soot preform leaving an aperture therein, and a tube is secured to one end of the preform. While the preform is heated to its consolidation temperature, an atmosphere including a drying agent flows from the tube into the aperture and through the porous preform, thereby removing water from the preform while the soot is consolidated to form a dense glass article. The tube can be removed, and the resultant dense glass article can be drawn into an optical waveguide fiber.

Description

Backgro~md of the Invention Field of the Invention This invention relates to a me~hod of form;ng, by the flame hydrolysis technique, high optical purity blanks fr~m which high quality optical waveguides, lenses, prisms and the like can be made. This invention is particularly appli-cable to optical waveguides whi.ch m~st be formed fro~
extremely pure materials.
Optical waveguides, which are the most promising medium for transmission of signals around 1~15 ~z, normally consist of an optical fiber having a transparent core surrounded by transparent cladding material having a refractive index lower than that of the core.
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1100~01 The stringent optical requirements placed on the trans-mission medium to be employed in optical communication systems has negated the use of conventional glass fiber optics, since attenuation therein due to both scattering and impurity absorption is much too high. Thus, unique methods had to be developed for preparing very high purity glasses in fiber optic form. Glass preparation techniques which have shown much promise are based on the flame hydrolysis process which employs vapor phase reaction of high purity vapors. This approach to the formation of low loss optical waveguides is based on methods described in U.S. Patents Nos. 2,272,342 and 2,326,0~9 issued to J. F. Hyde and M. E.
Nordberg, respectively. The flame hydrolysis technique has been employed to prepare single mode waveguides and multi-mode waveguides of both the step-index and graded-index type. Various methods employing the flame hydrolysis technique for forming glass optical waveguide fibers are taught in U.S. Patents Nos. 3,737,292; 3,823,995 and 3,884,550.
The usefulness of glass optical waveguides in optical transmission systems depends upon the attainment of very low loss transmission over the entire wavelength range of about 700-110~ nm. This can be achieved by reducing attenuation due to optical scattering and absorption to a level which approaches the minimum theoretically attainable level.
Waveguides in which at least 80% of the scattering loss can be accounted for by intrinsic glass scattering have been made by the aforementioned flame hydrolysis technique.
However, due to the presence of residual water produced by this technique, absorption losses betweèn 700 nm and 1100 nm have been excessively large. By residual water in glass is meant that the glass contains a high level of OH, H2 and 11~ QO~ 1 H2O. One explanation of residual water may be found in U.S.
Patent No. 3,531,271 to W. H. Dumbaugh, Jr. The maximum attenuation in the aforementioned wavelength range that is attributable to residual water occurs at about 950 nm. The remaining portion of the attenuation at 950 nm, which is due to factors such as intrinsic material scattering, amounts to about 3 dB/km. For example, a glass optical waveguide having an attenuation less than 6 dB/km at 800 nm may have an attenuation greater than 100 dB/km at 950 nm due to the presence of water therein. To be useful in optical communi-cation systems, optical waveguide attenuation is preferably less than 10 dB/km at the wavelength of light being propa-gated therein. In order to achieve such low attenuation over the entire range between 700 nm and 1100 nm, a glass waveguide fiber must be rendered substantially water-free, i.e., the amount of residual water within the fiber must be reduced to a level of less than 10 ppm.

Description of the Prior Art Since it is impossible to reduce the water content to acceptable levels after flame hydrolysis-produced soot has been consolidated to form a solid glass coating, the water must be removed before or during the consolidation process.
Heretofore, various methods were employed to reduce the water content in optical waveguides produced by flame hydrolysis. Such disadvantages as long processing times, equipment problems and incomplete water removal were encountered.
One prior ~rt method that has been very effective in reducing the water contenc in fused silica produced by the flame hydrolysis process is disclosed in U.S. Patent No.

3,933,454. In accordance with that patent a soot preform produced by the flame hydrolysis process is consolidated by inserting it into a consolidation furnace wherein the soot preform is heated to a temperature within the consolidation temperature range for a time sufficient to cause the soot particles to fuse and form a dense glass layer. The soot preform is simultaneously subjected to a stream of a sub-stantially dry chlorine containing gas which flows through the furnace. The chlorine permeates the interstices of the soot preform during the consolidation thereof and replaces hydroxyl ions by chlorine ions, thereby resulting in a glass article that is substantially water-free. However, prior to making contact with the soot preform, the chlorine containing gas can react with the walls of the consolidation furnace to produce volatile compounds such as iron chlorides which can then contaminate the preform. Thus, while the resultant glass article exhibits very little excess attenuation at 950 nm due to water absorption, the overall attenuation thereof across the entire visible spectrum is increased due to impurities transported by the drying gas.

SummarY of the Invention It is therefore an object of the present invention to provide an effective and economical method of removing residual water from a flame hydrolysis-deposited glass soot preform during the consolidation process. A further object is to provide a method of forming optical waveguides having extremely low concentrations of water and contaminants.
Briefly, the present in~ention relates to an improved method of forming a glass article by the flame hydrolysis process. This process conventionally comprises the steps of 11~1Q001 depositing on a starting member a coating of flame hydrolysis-produced glass soot to form a soot preform and consolidating the soot preform to form a dense glass layer. The consolida-tion step conventionally comprises subjecting the soot preform to a temperature in the consolidation temperature range, for a time sufficient to permit the soot particles to fuse and consolidate, thereby forming a dense glass layer which is free from particle boundaries. It is also conven-tional to subject the preform to an atmosphere such as helium, oxygen, argon, neon or mixtures of these gases, or even to a reduced pressure for the purpose of removing gases from the preform interstices during consolidation to thereby reduce the number of seeds in the resultant glass article.
In connection with the fusing of glass soot particles formed by flame hydrolysis, this process is some~imes referred to as sintering even though no particle boundaries remain.
In accordance with the present invention, the step of consolidating is characterized in that it comprises dis-posing the preform in a furnace wherein it is sub~ected to a temperature within the consolidation temperature range for a time fiufficient to cause the soot particles to fuse and form a dense glass layer. Simu~taneously, a stream of an stmosphere containing a drying agent is flowed through the interstices of the porous soot preform, that portion of the stream which contacts the furnace being prevented from thereafter contacting the preform.
In accordance with a preferred embodiment of this invention the starting member is removed prior to the con-solidation step. During the consolidation process, the stream which contains the drying agent is flowed into the aperture formed by removal of the starting member. The _5_ stream thereafter flows outwardly from the center of the preform through the interstices therein to the outer surface thereof.

Brief Description of the Drawings Figures 1 and 2 illustrate the application of first and second coatings of glass soot to a starting member.
Figure 3 is a graph which shows the attenuation curves of a plurality of optical waveguide fibers.
Figure 4 is a schematic representation of a consolida-tion furnace and consolidation atmosphere system.

Detailed Description of the Invention It is to be noted that the drawings are illustrativeand symbolic of the present invention and there is no inten-tion to indicate the scale or relative proportions of the elements shown therein. For the purposes of simplicity, the present invention will be substantially described in connec-tion with the ~ormation of a low loss optical waveguide although this invention is not intended to be limited thereto.
Referring to Figure 1, a coating 10 of glass soot is applied to a substantially cylindrical starting member such as a tube or rod 12 by means of flame hydrolysis burner 14.
Fuel gas and oxygen or air are supplied to burner 14 from a source not shown. This mixture is burned to produce flame 16 which is emitted from the burner. The vapor of a hydro-lyzable compound is introduced into flame 16, and the gas-vapor mixture i~ hydrolyzed within the flame to form a glass soot that lea~es flame 16 in a stream 18 which is directed toward starting member 12. The flame hydrolysis method of forming a coating of glass soot is described in greater ~1~ 000 1 detail in the aforementioned U.S. Patents Nos. 3,737,292;
3,823,995 and 3,884,550. Starting member 12 is supported by means of support portion 20 and is rotated and translated as indicated by the arrows adjacent thereto in Figure 1 for uniform deposition of the soot. It is to be understood that an elongated ribbon burner, not shown, that provides a long stream of soot could be used in place of the substantially concentric burner illustrated in Fi~ure 1 whereby starting member 12 would only have to be rotated. Further, a plurality of burners 14 could be employed in a row to similarly require only rotation.
To form a step-index optical wa~eguide, a second coat-ing 26 of glass soot may be applied over the outside peri-pheral surface of first coating 10 as shown in Figure 2. To form a gradient index fiber, a plurality of layers of glass soot are appiied to the starting member, each layer having a prog~essively lower index of refraction as taught in U.S.
Patent No. 3,823,995.
In accordance with well known practice the refractive index of coating 26 is made lower than that of coating 10 by changing the composition of the soot being produced in flame 16. This can be accomplished by changing the concen-tration or type of dopant material being introduced into the flame, or by omitting the dopant material. Support member 20 is again rotated and translated to provide a uniform deposi~ion of coating 26S the composite structure inc,uding first coating lO and second coating 26 constituting an optical waveguide preform 30.
Since glass starting member 12 is ultimately removed, the material of member 12 need only be such as to have a composition and coefficient of expansion compatible with 1 10 QOO ~

the material of layer 10. A suitable material may be a normally produced glass ha~ing a composition similar to that of the layer 10 material although it does not need the high purity thereof. It may be normally produced glass having ordinary or even an excessive level of impurity or entrapped gas that would otherwise render it unsuitable for effective light propagation. The starting member may also be formed of graphite, low expansion glass or the like.
In the manufacture of optical waveguides, the materials of the core and cladding of the waveguide should be produced from a glass having minimum light attenuation characteristics, and although any optical quality glass may be used, fused silica is a particularly suitable glass. For structural and other practical considerations, it is desirable for the core and cladding glasses to have similar physical characteristics.
Since the core glass must have a higher index of refraction than the cladding for proper operation, the core glass may desirably be formed of the same type o glass used for the cladding and doped with a small amount of some other material to slightly increase the refractive index thereof. For example, if pure fused silica is used as the cladding glass, the core glass can consist of fused silica doped with a material to increase its refractive index.
There are many suitable materials that can satisfac-torily be used as a dopant alone or in combination with each other. These include, but are not limited to, titanium oxide, tantalum oxide, tin oxide, niobium oxide, zirconium oxide, aluminum oxide, lanthanum oxide, pho~phorus oxide and germanium oxide. Optical waveguides can also be made by forming the core from one or more of the aforementioned dopant oxides, the cladding being made from one or more materials having a lower refractive index. For example, a core made of pure germanium oxide may be surrounded by a cladding layer of ~used silica and germanium oxide~
The flame hydroIysis technique results in the formation of glasses having extremely low losses due to scattering and impurity adsorption. Optical waveguides made by this technique have exhibited total losses as low as 1.1 dB/km at 1,060 nm. However, even when optical waveguides are formed -~ of glasses having such high op~ical quality, ligh~ attenua-tion at certain regions of the wavelength spectrum may be so great as to preclude the use of such waveguides for the propagation of light'in those regions. For example, an optical waveguide having a core of 81 w~./O SiO2, 16 wt.%
GeO2 and 3 wt.% B2O3 and a cladding of 86 wt.% SiO2 and L4 wt.% B2O3 was made by the ~lame hydrolysis process, no attempt being made to remove water therefrom. A~tenuatior.
curve 36 for this fiber is illustrated in Figuré 3. Water was responsible for such an excessive attenuation in the 700-1100 nm region that the waveguide was useless for the propagation of optical signals at most wavelengths within that region. At 950 nm the attenuation was about 80 d~/km.
Various oxides from which such glass optical waveguides are formed, especially SiO2, have a great affinity for water.
However, after such glass waveguides are completely ~ormed, the inner, light propagating portion thereof is inaccessible to water~ The tendency of these glasses to absorb water is not detrimental to water-~ree glass optical waveguides ater they are formed since most o the light energy is propagated in and around the core, and the presence o water on the outer surface has a negligible ~ffect on the propagation o~
- ~uch energy. However, in the ormation of optical waveguides _g_ 1100~0~

by flame hydrolysis, residual water, which is produced by the flame, appears throughout those portions of the wave-guide that have been produced by flame hydrolysis. Also, water is readily adsorbed by the soot during handling in air prior to the consolidation process because of the extremely high porosity thereof.
The starting member is removed from the soot preform so that a gas conducting tube can be affixed to an end of the preform. This can be accomplished by merely securing the preform while the handle is pulled therefrom. Preform 30 is then suspended from tubular support 50 as shown in Figure 4.
Two platinum wires, of which only wire 52 is shown, protrude through preform 30 on opposite sides of aperture 54 and are affixed to support 50 just above flange 56. The end of gas conducting tube 58 protrudes from tubular support 50 and into the adjacent end of preform 30. The preform is con-solidated by gradually inserting it into consolidation furnace 60. It is preferred that the preform be subjected to gradient consolidation, a technique taught in t-ne afore-mentioned U.S.-Patent No. 3,933,454 wh~reby the bottom tip of the preform begins to consolidate firs~, the consolidation continuing up the preform until it reaches that end thereof adjacent to tubular support 50.
In accordance with the present iDvention an optical waveguide preform is dried by subjecting the preform to a high purity drying agent during the c~nsolidation process.
The purity of the drying agent is m~intained by preventing it from contacting the refractory walls of the consolidation furnace prior to coming into contact ~ith the preform. The preferred method of maintaining the p~ity of the drying ~gent involves flowing a stream of an atmosphere containing - ' - 10: ' -~ lO ~Q~lthe drying agent into the center of the preform and through the porous preform walls to the outside surface thereof.
The resultant gases are flushed a~-ay from the blank by a gas such as helium, oxygen, argon, neon or mixtures thereof.
Thus, the drying agent is unable to transport impurities from the furnace muffle to the preform. This method there-fore results in an optical waveguide fiber that is essentially water free, thereby exhibiting low loss at 950 nm, and which exhibits low loss at other wavelengths as well. Examples of drying agents which may be employed are C12, SiC14, GeC14, BC13, HCl, POC13, PC13, TiC14 and AlC13. Compounds of the other halogens such as bromine and iodine sho~-ld also be effective. The particular drying agent to be employed is im~aterial insofar as the drying process is concerned.
However, such preform properties as refractive index, thermal coefficient of expansion and the like should be considered in the selection of the drying agent. In one embodiment of the type illustrated in Figure 4 the drying agent was delivered to the center of the soot preform by a system comprising only glass, Teflon~and a minimum 2mount of stainless steel, and thus, the drying ag~t was maintained substantially free from contgmination prior to contacting the preform.
The consolidation temperature depends upon the com-position of the glass soot and is in the range of 1250-1700C. for high silica content soot. It is also time dependent, consolidation at 1250C. requiring a very long time. The preferred consolidation temperature for high - silica content soot is between 1350C. and 1450C. Other glasses can be consolidated at lower temperatures, pure germania, for example, consoLidating at about 900C.

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11~00~1 Referring again to Fi~ure 4 the vertical sidewalls of furnace 60 are broken to illustrate that the relative depth thereof is greater than that shown. In this figure flow regulators are schematically represented by the letter "R"
within a circle and flowmeters by the letter "F" within a rectangle. Sources ~2 and 64 of oxygen and helium, respec-tively, are connected to orifices 66 in the bottom of fur-nace 60. Undulated arrows 68 represent the flow of the flushing gas from the orifices. Sources 72 and 74 of helium and oxygen, respectively, are connected to containers of SiC14 and GeC14, respectively, so that helium, oxygen, SiC14 and ~eC14 are present in line 76. Additional helium is coupled to line 76 by line 78.
The consolidation atmosphere system of Fi~ure 4 is - merely representative of a number of systems which may be employed to provide the consolidation furnace and preCorm with appropriate gas and vapor mixtures. The flushing gas could be caused to flow from top to bottom of furnace 60.
The system illustrated, whereby flushing gas flows into the 2Q bottom of furnace 60, is preferred since gas naturally tends to flow upwardly through the furnzce. Also, many different arrangements may be employed to provide the desired drying gas mixture, and the present invention is not limited to the arrangement illustrated in Figure 4. It is only necessary to provide tube 58, and ultimately preform 30, with the desired drying gas mixture, the particular means employed to achie~e this mixture being immaterial.
As indicated by arrow 80, preform 30 is inserted do~-wardly into furnace 34. The rate of insertion is preferably 3~ low enough to permit the tip of the preform to consolidate first, the consolidation process then continuing up the ll~QO~

preform until it reaches that end of the preform adjacent to tubular support 50. The maximum furnace temperature, which is preferably between 1350C and 1450C for high silica content soot, must be adequate to fuse the particles of glass soot and thereby consolidate the soot preform into a dense glass body in which no particle boundaries exist.
As soot preform 30 enters furnace 60 the drying gas passes through tube 58 into preform aperture 54 from which it passes into and through the interstices of the preform as indicated by arrows 82. Successful drying of the soot preform has been achieved by employing well known drying agents such as chlorine gas and SiC14. However, since the inner portion of a soot preform that is to be formed into an optical waveguide fiber contains a dopant to increase the refractive index thereof, the application of such conventional-drying agents to the preform aperture where the dopant concentration is greatest causes leaching of the dopant from the preform. This results in a decrease in the refractive index of the glass at the center of the resultant optical waveguide. Although the resultant fî~er functions as an optical waveguide, certain properties thereof may be adversely affected, especially in the case of ~aded index fibers.
Therefore, the drying gas preferably contains a component that will, upon reaction in the pref~m, produce that dopant oxide, the concentration of which te~s to be reduced by the aforementioned leaching action. The Fequired amount of the compensating component depends upon ~2rious factors including the concentration of the dopant oxid~ at the center of the soot preform. It has also been found that excessive amounts o the compensating component in th~ d~ying gas can cause the formation of a thin layer of a g~ass rich in the dopant '13-11~0001 oxide at the inner surface of the hollow preform. This cancause breakage due to unbalanced stresses in the resulting consolidated blank. Moreover, even if a fiber can be drawn from such a blan~, the refractive index at the center thereof may be excessively high due to the high concentration of dopant oxide at the fiber center. Thus, for each different waveguide composition the amount of the compensating c~mponent in the drying gas will have to be determined, determination by empirical means having been found to be satisfactory.
The excess attenuation at 950 nm for fibers drawn from untreated fibers is typically 50-100 dB/km or more. Curve 38 of Figure 3 illusèrates the attenuation vs. wavelength curve for one of the best fibers produced by this method.
It has been observed that treatment in accordance with the method of the present invention also decreases the attenuation of cor.solidated soot blanks by nearly ~ dB/km at each of the standard measurement wavelengths of 630, 800, 820 and 1060 nm. The improvement over the prior art can be seen by comparing curve 38 of Figure 3 with curve 34, which illus-trates the loss characteristics of a waveguide fiber formed from a soot preform dried in accordance with the teachings of ~.S. Patent ~o. 3,933,454. The drying process of the present invention appears to either decrease the light scattering property of the glass or remove metallic impuri-ties by forming volatile compounds th~reof which are then flushed from the soot preform. Thus, in addition to pro-viding substantial drying of a soot p~eform, the method of the present invention also decreases ~ttenuation at wave-lengths which are unaffected by water.
Excessive amounts of SiC14 and ~æC14 tend to increase the overall attenuation of the fiber, probably either by llO~Q~l contaminating the preform or by changing the oxidation state of impurities always in a blank. Excessive amounts of oxygen (insufficient helium) cause the blank to be seedy.
Insufficient oxygen causes the attenuation of the fiber to increase because of oxidation state changes of the impurities.
The method of the present invention broadly involves flowing a gaseous drying agent through the interstices of a soot glass preform while simultaneously preventing that drying agent from contacting the walls of the furnace in which the preform is being consolidated. In the embodiment of Fi~ure 4 this is accomplished by flowing the drying agent into the top of the preform aperture and flushing it away with a counter-current flow of flushing gas supplied to tke bottom of the furnace. The flushing gas could be supplied to the top of the consolidation furnace and thus flow in the same direction as the drying agent, means being provided at the bottom of the furnace for removing both drying and flushing gases. In another variation of the method of this in~ention, a laminar flow of drying agent is provided at the bottom of the furnace. As the drying gas encounters the soot preform, that gas flowing between the preform and the furnace wall acts as a buffer to prevent the gas which has contacted the wall from entering the preform. In accordance with a modification of this last mentioned embodiment, a "guard flow" of inert gas is introduced at the bottom of the furnace near the vertical wall, this "guard flow" tending to increase the shielding of the preform from impurities in the furnace refractories.
The invention will be further described with reference 3~ to specific embodiments thereof which are set forth in the following examples. In these examples, which pertain to the 110~

manufacture of optical waveguides, the inside diameter ofthe furnace muffle is 3 1/4 inches and the length thereof is 50 inches. In all examples a flushing gas mixture comprising 20 l/min helium and 500 ml/min oxygen was supplied to the bottom of the furnace as shown in Figure 4.

Example 1 A tubular starting member of fused quartz, approximately 0. 6 cm in diameter and about 50 cm long is secured to a handle. Liquid SiC14, liquid GeC14 and BC13 are maintained at 35C, 47C and 20C in first, second and third containers, respectively. Dry oxygen is bubbled through the first container at 2000 cc per minute and through the second container at 200 cc per minute. BC13 is metered out of the third container at 60 cc per minute. The resultant vapors entrained within the oxygen are combined and passed through a gas-oxygen flame where the vapor is hydrolyzed to form a steady stream of particles having a composition of 16% by weight GeO2, 3% by weight B203 and 81% by weight SiO2. The stream is directed to the starting member and a soot coating comprising particles of this composition is applied up to about 2.5 cm in diame~er. A second coating of 86 wt.% SiO2 and 14 wt.% B2O3 is then applied over the first soot coating by terminating the flow of oxygen to the liquid GeC14 and adjusting the flow of BC13 out of the third container to 300 cc per minute while maintaining the flow of oxygen through the first container at 2000 cc per minute. This cladding soot is applied until an outside diameter of approximately 5 cm is attained. The starting member is pulled from the soot preform, thereby leaving a soot preform weighing 450 g and having a diameter of 5 cm and a length of 50 cm. The drying 1100~1 gas tube 58 of Figure 4 is inserted into the preform aperture which has a diameter of about 0.6 cm. Platinum wire is employed to attach ,he upper end of the preform to a tubular support. - `
The gases and vapors constituting the drying gas flow into the preform aperture at the following rates: 30 ml/min oxygen, 3 ml/min SiC14 vapor, 3.7 ml/min GeC14 vapor and 1500 ml/min helium. This mixture is obtained by main-taining the SiC14 and GeC14 at 25C and by bubbling helium through the SiC14 at 6 ml/min and oxygen through the GeC14 at 30 ml/min and flowing helium at a rate of 1.5 l/min.
through bypass line 78. At 25C the vapor pressure of the SiC14 is about 240 Torr so that the helium picks up about 3 ml/min of SiC14 vapor, and the vapor pressure of the GeC14 is about 85 Torr so that the oxygen picks up about 3.7 ml/min of GeC14 vapor.
As the drying gas mixture flows into the preform aper-ture, the preform is lowered into the furnace at about 0.5 - cm per min, the maximum furnace temperature being about 1380C.
The preform is completely consolidated in about 90 min.
The resultant dense glass body is withdrawn from the furnace and cooled. The resultant structure is drawn at a temperature of about 1800C to collapse the central hole and decrease the outside diameter thereof. Drawing is continued until the final waveguide diameter of 125 ~m is achieved, the core diameter being about 62 ~m. Waveguide attenuation at standard measurement wavelengths of 630, 800, 8~0, 900 and 1060 nm is 8.7, 3.4, 3.2, 2.7 and 1.6 dB/km, respecti~ely. The excess absorption due to water at 950 nm is estimated to be about 17 dB/km. In this example and in some of the examples set 1 100 Q~ 1 forth in Table I below the attenuation at 950 nm is estimated by measuring the attenuation at 820 nm and 900 nm. The estimated attenuation at 950 nm is then determined by the equation A950 = 33(Agoo~A820 ~ l. ) There is a small spike in the refractive index profile at the center of the fiber due to the formation of a small excess of GeO2 at the center of the fiber during the drying process.
The specific drying agent employed in Example 1 is effective in the drying of optical waveguides due to the occurrence of several chemical reactions. First, the SiC14 and GeCl4 react with oxygen to form chlorine according to the equations:

SiC14 + 2 ~ SiO2 + 2 C12 GeC14 ~ 2 ~ GeO2 + 2 C12 The chlorine formed in these reactions removes hydroxyl groups from the glass according to the reactions:

2~SiOH + C12 ~SiOSi- + 2 HCl + 12 2 where ~SiOH denotes that the silicon atom is connected to three other parts of the glass network.

~xam~les 2-9 Gas flow rates and optical waveguide attenuation values for Examples 2-9 are set forth in Table I. Each of these examples employs the same type of optical waveguide as Example 1 except Examples 5 and 6 in which graded index fibers are formed. The fibers in these two examples have the same cladding and axial compositions as the waveguide of Example 1. However, in Examples 5 and 6 the amount of GeO2 gradually decreases between the fiber axis and the cladding.
Also, Examples 5 and 6 employ the same soot preform consoli-dation process as described in Example 1 except for the drying gas mixture. Example 11 of Table I refers to an untreated fiber.

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Q~l It is to be noted that attenuation curve 38 of Fi~ure

3 pertains to the optical waveguide formed in accordance with Example 6.
As indicated hereinabove a balance of SiCl4 and GeC14 is required in the drying and consolidation of germania doped optical waveguides to prevent distortion of the refrac-tive index profiles thereof. An excess of SiC14, for example, is believed to cause leaching of GeO2 from the soot preform according to the reactions:

SiC14 + 2 ' SiO2 + C12 C12 + GeO2 (glass) ) GeOC12 or GeC14 or other volatile germa-nium products or SiC14 + GeO2 (glass) ~ SiO2 (glass) + GeC14 On the other hand, excessive amounts of GeC14 cause a deposition of a thin layer (50-100 ~m thick) of a glass rich in GeO2. This thin layer of glass has a higher expansion than the bulk of the blank. On cooling cf ~he bLank, this layer goes into tension ~nd often causes blank breakage because of unbalanced stresses. For example, several blanks were dried during consolidation by applying to the preform aperture a gas-vapor mixture obtained by bubbling about 8 ml/min oxygen through GeC14 at 25C and mixing the resultant oxygen-vapor mixture with 1.5 l/min helium. Most of the resuitant optical waveguide blanks broke because of the high stress introduced therein during the dryin~ process.
Without special care, such breakage approaches 80%. If the blank is not cooled between the consolidation process and the process of drawing the resultant blank into a fiber, 1 l~QQ~ i breakage is not a problem, but this technique is very inconvenient.
The amount of water remaining in the consolidated blank is a function of the amount of water initially present in the soot preform. This variable can be eliminated by con-- trolling the humidity of the atmosphere to which the soot preform is subjected from the time of soot deposition until the consolidation is completed. Then, by varying the amount of chlorine or other drying agent present in the atmosphere flowing rom tube 58 to preform aperture 54, the amount of water removal can be controlled. Also, by controlling the amount of dopant in such atmosphere, the adverse effect of the drying process on the refractive index profile of the resultant optical waveguide fiber can be minimized.

Claims (16)

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

1. In the method of forming a glass article comprising the steps of depositing on a starting member a coating of flame hydrolysis-produced glass soot particles to form a porous soot preform, and consolidating said soot preform to form a dense glass layer free from particle boundaries, said consolidation step being characterized in that it comprises disposing said preform in a furnace wherein it is subjected to a temperature within the consolidation temperature range for a time sufficient to cause said soot particles to fuse and form a dense glass layer, and flowing a stream of an atmosphere containing a drying agent through the interstices of said porous soot preform while preventing that portion of said stream which contacts said preform from previously contacting said furnace.

2. The method of claim 1 further comprising the step of removing said starting member to form an aperture in said soot preform prior to the step of consolidating said preform, and wherein the step of flowing comprises flowing said stream into said aperture so that at least a portion of said stream flows outwardly from the center of said preform through the interstices therein to the outer surface thereof.

3. The method of claim 2 wherein said drying agent comprises chlorine.

4. The method of claim 2 wherein the inner portion of said soot preform comprises glass particles doped with a material which increases the refractive index thereof to a value greater than that of the soot particles of the outer portion of said preform, and wherein said stream comprises a component which reacts during said consolidation step to form said dopant material.

5. The method of forming a glass article comprising the steps of depositing on a starting member a coating of flame hydrolysis-produced glass soot to form a porous soot preform, removing said starting member to form an aperture in said preform, heating said soot preform to a temperature within the consolidation temperature range for a time sufficient to cause said soot particles to fuse and form a dense glass layer, and simultaneously flowing into said aperture a stream of an atmosphere containing a drying agent, at least a portion of said stream flowing outwardly from the center of said preform through the interstices therein to the outer surface thereof.

6. The method of claim 5 wherein said drying agent comprises chlorine.

7. The method of claim 5 wherein the inner portion of said soot preform comprises glass particles doped with a material which increases the refractive index thereof to a value greater than that of the soot particles at the outer portion of said preform, and wherein said stream comprises a component which reacts during said consolidation step to form said dopant material.

8. The method of claim 5 wherein the step of depositing comprises depositing on said starting member a coating of flame hydrolysis-produced glass soot comprising SiO2 and an amount of a dopant oxide to increase the refractive index of said glass soot particles to a value greater than that of SiO2 alone, and depositing on said first coating at least one additional coating of glass soot particles having refractive index lower than that of the particles of said first coating, and wherein said stream comprises a component which reacts during said consolidation step to form SiO2 and a component which reacts to form said dopant oxide and wherein said method further comprises the step of drawing the resultant dense glass body to form an optical fiber.

9. The method of claim 8 wherein the refractive index of said first coating is constant throughout the radius thereof.

10. The method of claim 8 wherein the refractive index of said first coating decreases with. increasing radius.

11. The method of claim 8 wherein said drying agent is selected from the group consisting of C12, SiC14, GeC14, POC13, PC13, A1C13, BC13, TiC14, Br2, I2 and mixtures thereof.

12. The method of claim 5 further comprising the step of flowing a flushing gas over the outside surface of said preform to remove said stream as it emerges from said surface.

13. The method of forming a glass article comprising the step of depositing on a starting member a coating of flame hydrolysis-produced glass soot to form a porous soot preform, removing said starting member to form an aperture in said preform, disposing said preform in a furnace, heating said soot preform to a temperature within the consolidation temperature range for a time sufficient to cause said soot particles to fuse and form a dense glass layer, and simultaneously flowing into said aperture a stream of chlorine-containing atmosphere, at least a portion of said stream flowing outwardly from the center of said preform through the interstices therein to the outer surface thereof, and flowing a gas through said furnace to flush said stream from said furnace as it emerges from said surface.

14. The method of claim 13 wherein the inner portion of said soot preform comprises glass particles doped with a material which increases the refractive index thereof to a value greater than that of the soot particles at the outer portion of said preform, and wherein said stream comprises a component which reacts during said consolidation step to form said dopant material.

15. The method of claim 13 wherein the step of depositing comprises depositing on said starting member a coating of flame hydrolysis-produced glass soot comprising SiO2 and an amount of a dopant oxide to increase the refractive index of said glass soot particles to a value greater than that of SiO2 alone, and depositing on said first coating at least one additional coating of glass soot particles having refractive index lower than that of the particles of said first coating, and wherein said stream comprises a component which reacts during said consolidation step to form SiO2 and a component which reacts to form said dopant oxide and wherein said method further comprises the step of drawing the resultant dense glass body to form an optical fiber.

16. Method according to claim 1, 5, or 13, wherein said atmosphere continue to flow into said aperture until said soot preform is completely consolidated.