CA1128739A

CA1128739A - Method of making large diameter optical waveguide preforms

Info

Publication number: CA1128739A
Application number: CA325,707A
Authority: CA
Inventors: Arnab Sarkar
Original assignee: Corning Glass Works
Current assignee: Corning Glass Works
Priority date: 1978-06-08
Filing date: 1979-04-18
Publication date: 1982-08-03
Also published as: JPS5851892B2; BE876882A; ES489422A0; BR7903533A; SU1068028A3; NO791909L; AU4722679A; FR2428011B1; CH642336A5; SE434149B; YU132379A; NL174539C; GB2023129A; ATA409179A; NO147948B; GB2023129B; DK228879A; IT7923341A0; NL174539B; FI791843A

Abstract

Sarkar ?

METHOD OF MAKING LARGE DIAMETER
OPTICAL WAVEGUIDE PREFORMS

Abstract of the Disclosure A glass optical waveguide filament preform is prepared by chemical reaction of vapor ingredients within a glass bait tube. As the reactants flow through the bait tube, a hot zone traverses the tube to cause the deposition of sooty reaction products in the region of the hot zone. A baffle tube extends into that end of the bait tube into which the reactants flow. The baffle tube, which traverses the bait tube along with the burner, ends just short of the hot zone so that no soot is deposited thereon. A gas flowing from the baffle tube creates a gaseous mandrel which confines the flow of reactant vapors to an annular channel adjacent the bait tube wall in the hot zone, thereby increasing deposition rate and efficiency.

Description

-Back~3round of the Invention - ~ The pr~sent invention relate~ to optical waveguide filaments, and more particularly to an improved ~ethod of forming bl~nks fr~m which such filame~ts are drawn~
Optical waveguides, w~ich are the most promising medi~n for use in optical communication ~ys~em~ operatirlg in the visible or near visiblP spectra, normally con`sist o an op~ical ~ilament ha~ving a transparen~ core surrollnded by a transparen~ cladding material having a refractive index lower ~han thal: of ~he eore.
The stringent optical requi rements placed on ~he transmisslon medium to be em~loyed in optical commNnlcations systems has negated the use of convention~l gla~s ~iber, ~l28739 op~ics, since attenuation therein due ~o both scattering and impurity absorption is much too high. Thus, unique methods had to be developed ~or preparing very high puriky glasses in filamentary form. Certain glass making processes, particu-larly vapor deposition processe~, have been co~monly employed in the formation of optical wa~eguide blanks. In one such process, the source material vapor is directed into a heated tube wherein it reacts to orm a material which is deposited in successive layers. The combination of deposited glass and tube is collapsed to form a draw blank which can be later heated and drawn into an optical waveguide fila~ent.
In order to obtain uniform deposition along the length of the substrate tube, a serial deposition process has been employed. Th~at is, reactants are fed into the end o~ the tube, but deposition occurs only in a narrow section of the tube which is heated by a flame. The flame moves up and down the tube to move the reaction and thus the region of glass deposition serially along the tube.
One of the limitations of such a process is a comparatively low ef~ecti~e mass deposition rate. To increase the deposition rate it appears to be necessary to incxease the inside diameter of the substrate tube to provide a greater collec~ion surface area. However, since heat is supplied from the outside of the tube, a larger tube diameter results in a lower vapor temperature at the axis of the tube. Moreover, the flow profile across the tube is such that maximum flow occurs axially within the tube. As tube diameter increases, a smaller portion of the reactant ~apor flows in that region of the tub~ adjacent the wall where reaction temperature is higher and where the resultant soot~ reaction products are more readily collected on the heated region of the tube.

~8739 I~ is therefore an object of the present invention to improve the deposition efficiency of a process whereby a reactant vapor ~lows into and reactq withln a he~ted t~be to form a layer therein.

Summary of the In~ention Briefly, the present invention relates to a method and apparatus for manufacturing a preform which is intended to be subsequently drawn into an optical filament. This method is of the type that includes the steps of flowing a lû vapor mixture including at least one compound, glass- forming pre-cursor, which is a gas ~ ich when heated forms glass particles, together with an oxidizing medi~n, through a hollaw, cylindrical substrate, and heating the substrate and contained ~apor mixture with a heat source that moves relative to the substrate in a longitudinal direction, wherPby a moving hot zone is established within the substrate, such that a suspension of particulate, oxidic reaction product material is produced within the hot zone. The particulàte material travels downstream where at least a portion thereof co~es to rest on the inner surface of the substrate where it is fused to form a continuous glassy deposit. The ~mprovement of the pres-ent invention comprises confining the flow of the vapor mixture to an annul~r channel adjacent the subs~rate sur-face in the hot zone whereby-the deposition efficiency of the vapor mixture reaction is increased.
In accordance with a preferred embodiment of the present in~ention, a gas condu~ting baffle tube is disposed in one end of the cylindrical substrate, one end of the baff~e tube terminating adjaciPnt the hot zone. Means is ~L128~3~

provided for moving ~he tube longitudinally with respect to the substrate in synchronism with the movement oE the heating means whic~ generates the moving hot zone, Gas emanating from the baf~le tube ~orms a gaseous mandrel ~n the hot zone which confines the vapor mixture to an ann~lar channel adjacent the substrate surface.

Brief Description o~ the ~rawings Figure 1 is a schematic representation of a prior art apparatus for depositing a glass layer within a tube.
Figure 2 shows a.section of the tube of Figure 1 depicting obser~ed conditions during processing.
Figure 3 is a schematic representation of an apparatus suitable for practice of the deposition process in accordance with the present invention.
Figures 4 and 5 are cross-sectional vie~s of the apparatus of the present in~ention depicting conditions oc.curring during processing.
Figure 6 shows the end of a modified baffle tube that can be employed in the apparatus o the present invention.

Descri tion of the Preferred Embodiment P ~

Figures 1 and 2 show a prior art system comprising a substrate tube 10 having handle tube 8 affixed to.the upstream end thereo and exhaust tube 12 affixed to the downstream end thereof. Tubes 8 and 12 are chucked in a conventional glass tusning lathe (not shown~, and the combination i5 rotated as indicated by the arrowD The ~andle tube, w~ich may be omitted, is an lnexpensive glass tube ha~ing the same diameter as the substrate tube, and it does not form a part of the resultant optieal wa~eguidP. A

~873~

hot zone 14 is caused ~o traverse tube 10 by moving heating means 16 as schematically depicted by arrows 18a and 18~.
Heating means 16 can consis~ o~ any suitable source o heat such as a plurality o burners encircl~ng tube 10. Reac~ants are introduced into tube 10 via inlet tube 20, which i5 connected to a plurality of sources of gases and vapors. In Figure 1, flow meters are represented by a circle having the letter "F" therein. A source 22 of oxygen is connected by flow meter 24 to inlet tube 20 and by flow meters 26, 28 and 30 to reservoirs 32, 34 and 36, respectively. A sour~e 38 of boron trifluoride is connected to tube 20 by a flow meter 40. Reservoirs 32, 34 and 36 contain normally liquid reactant materials which are introduced into tube 10 by bubbling oxygen or other suitable carrier gas therethrough.
Exiting material is exhausted through exhaust tube 12. Not shown is an arrangement of mixing valves and shutoff valves which may be utilized to meter flows and to make other necessary adjustments in composition.
Burner 16 initially moves at a low rate of speed rela-tive to tube 10 in the direction of arrow 18bS the same direction as the reactant flow. The reactants react in hot zone 14 to produce soot,-i.e., a powdery suspension of particulate o~idic material, which is carried downstream to region 42 of tube 10 by moving gas. In general, between twenty and seventy percent of reaction product produced in the vapor str~am removed from the substrate surface and results in deposited soot having the desired glass composi-tion.
It is noted that essentially no soot is for~ed in region 46 of tube 10 upstream rom hot zone 14. As burner 16 continues to move in the direction of arrow 18b, hot zone 14 moves downstream so tha~ a part of soot buildup 44 extends into the hot zone and is consolidated thereby to form a unitary, homogeneous glassy layer 48. Such process parameters as temperatures, flow rates, reactants and t~e like can be found in the publications J. B. MacC~esne~ et al., Proceedings of the IEEE, 1280 (1974) and W. G. French et al., Applied Optics, 15 (1976~. Reference is also made to the text V~
Deposition Edited by C. F. Powell et al. John Wil~y and Sons, Inc. (1966).
When burner 16 reaches the end of tube 10 adjacent to exhaust tube 12, the temperature of the flame is reduced and the burner returns in the direction of arrow 18a ~o the input end of tube 10. T~erea~ter, additional layers of glassy material are deposited ~ithin tube 10 in the manner described above. A~ter suitable layers have been deposited to serve as the cladding andlor core material of the resultant optical wa~eguide filament, the temperature of the glass is increased to about 2200G ~or hig~ silica content glass to cause tube lO to collapse. This can be accomplished by reducing the rate of traverse o the hot zone. The resultant draw blank is then d~a~n in accordance ~ith well known tec~niques to form an optical waveguide filament having the desired diameter.
To optimize the process from the standpoint of reaction, high temperatures are utilized. -For the usual silica based system~ temperatures at the substrate wall are generally maintained betweèn about 1400 and 1900C at the moving position co~responding with the hot ~one. Indicated tem-peratures axe those measured by a radlation pyrometer focused at the outer tube surface.

~Zt~73~

It îs co~monly known that one of the ~actors whlch limits deposition rate is the rate of sintering deposited soot to form a transparen~ glass layer. For a given com-position of glass to be deposited, there is a ma~imum layer thickness of glass that can be sintered using the optimum combination of hot zone width, peak temperature of the hot zone and burner traverse rate. If the thickness o the sintered glass layer can be kept to the maximum value for different tube diameters, deposition rate increases propor-tionately with tube inside diameter because of increased surfaee area. However, because of the nature of 10w dynamics of the reactant vapor stream and soot particle dynamics, the percentage vf soot produced which deposits in the substrate tube decreases with increased tube diameter, thereby causing an effecti~e decrease of deposition rate.
In accordance with the present i~vention means is provided for confining the flow of reactants to an annular channel adjacent the wall of the substrate tube in the hot zone. As show~ in Figure 3 a portion of gas conducting tube 50 extends into that end o~ substrate or bait tube 52 into which the reactants a~e introd~ced. That portion of tube 50 within tube 52 terminates just prior to the hot zone 54 created by mo~ing heat sou~ce 56. Tube 50 is mechanically coupled b~ means represented by dashed line 58 to burner 56 to ensure that tube 50 is maintained the proper distance upstream of the hot zone 54. Alternatively, the heat source and gas feed tube may b~ kept stationary, and the rotating substrate tube may ~e tra~ersed. ~he input end of tube 52 is connected to tube S0 by a collapsible member 60, a rotating seal 62 being disposed between member 60 and tube 52. As shown in Figure 4, which is a cross-sectional view of the hot zone and adjacent regions of tube 52, gas emanating ~r~m tube 50 provides an effective mandrel or barrier to the reactants ~lowing in the direction of the arrows between tubes 50 a~d 52, thereby con~ining those reactants to an annular channel adjacent the wall o~ tube 52 in hot zone 54.
For some distance downstream from hot zone 54, gas from tube 50 conti~ues to act as a barrier to soot formed in the hot zone, thereby enhancing the probability that such soot will deposit on the wall of tube 52 as shown at 44'. Dashed line 66 of Figure 5 represents the boundary between the gas emanating from tube 54 and the reactant vapor flowing in the hot zone 54.
The gas supplied to the hot zone by tube 50 may be any gas that does not detrimentally affect the resultant optical waveguide preform. Oxygen is p~eferred since it meets this requiremen~ and is reIatively inexpensive. Other gases such as argon, heIium, nitrogen and the like may also be employed As shown in Figure 4, the end of tube 50 is separated fr~m the center of the hot ZQne by a distance ~ which must be great e~ough to prevent the deposition of soot on tube 50. The distance x will vary depending upon such parameters as the width of the burner and the temperature of the hot - zone. The following findings were made for a deposition system wherein the outPr dia~eters of tubes 50 and 52 were 20 and 38 mm, espectiveIy, and the wall thicknesses thereof were 1.6 and 2 mm, respecti~ely. The burner ace orifices were located within a 45 mm diameter circle. In this syst~m it was found that soot will deposit on tube 50 if the distance x is about 13 mm. Mixing of the reactant vapor stream with`the gas flow through~the ba~fle tube increases with the longitudina~ distance from the baffle tube. The 3~

advantage derived by restricting reactant vapor to an annular region close to the wall of tube 52 may be obtained wlth a distance x up to about 15 cm. Best resul~s are obtained when the distance x is within the range of 25-75 mm.
The size and shape of tube 5~ should be such that a substantially laminar flow exists in the hot zone and in the region ~mmediately downstream therefrom~ Any turbulence which is introduced by tube 50 tends to pic~ up soot particles and carry them downstream to the exhaust tube.
In the prior art deposition process described in conjunction with Figures 1 and 2, deposition efficiency falls with an increase in tube diameter. An increase in deposition rate with increased ~ube diameter can be obtained by increasing tu~e dia~eter to about 30 mm. For tubes having diameters greater than 30 mm, deposition efficiency falls at a faster rate so that further increase in deposition rate is difficult to obtain. However, with the use of a ba~1e tube, the reactant vapor stream is restricted to a fixed distance fr~m the ins~de surace of the bait tube that ~0 produces opti~um deposition efficiency irrespective o bait tube di~meter. The maximNm size of the outer tube is Limi~ed by su~h consideratio~s as that size tube for which ~he inner hole can be closed to form an optical wa~eguide preform.
The wall thickne ses of the bai~ tube and the baffle tube are usually maintained reIativeIy small, i.e., a few milli-meters in thickness.
~ cylindrically shaped baffle tube such as that illus-trated in Figures 3 and 4 has been found to be easily con-structed and to function satisfactorily to supply a~mandrel of gas to the hot region of the bait tube without introducing an undue ~mount of turbulence. O~her shapes such as that _g_ ~2~739 shown in Figure 6 could also be ~mployed to perform this function. The direction of ~as ~low from tube 70 is s~own by arrow 72.
To illustrate the improvement in deposition rate and eficiency, a deposition sy~tem was opera~ed both with and without a baffle tube 50 therein, all other process parameters remaining unchanged. Apparatus similar to that shown in Figure l was employed to supply the reactant stream; however, only one reser~oir 32 was employed. Oxygen was flowed through reservoir or bub~ler 32 containing SiC14 maintained at 35C to provide a flow of about 2.5 g/m SiC14. The 10w rate of the BC13 was 92 sccm, and the 10w o ox~gen through flow meter 24 was 2.4 slm. The bait tube was a borosilicate glass tube having an outer diameter of 38 mm and a 2 mm wall thickness. A borosilicate glass having a composition of about 14 wt.% B203 and 86 wt.% SiO2 was deposited. From the flow rates of SiC14 and B~13, the rate of o~ide production was calculated to be Q.85 g/min SiO2 and 0.29 g/min B203.
The deposition rate was 0.251 glmin and the deposition efficiency was 26.2% w~en no baffle tube was employed. The system was then modiied by adding a fused silica baffle tube having an outside di~meter of 2Q mm and a wall thick-ness of 1 6 ~m. The end of the ba~fle tube was separated from the center of the hot zone by a distance of 50 mm. By employing the baffle tube, the deposition rate increased from 0.251 ~o 0.451 glmin and the efficiency increased from 26.2 to 43.~%.
TabIe I illustrates the effect of changing ~arious of the process parameters on deposition rate and efEiciency.

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2~3739 In Examples 1 through 6 of this Table the balt tubes consis~ed of 38 mm OD borosilicate tubes having a 2 mm wall ~hickness and the baffle tubes consisted of 20 mrn OD fused ~ilica tubes having a 1.6 mm wall thicknPss. In the course of these experiments, a plu~ality of layers o~ glass were deposited within the bait tube in the manner described above. Af~er 10 to 30 layers were deposited, the bait tubes ~ere broken, and the thicknPss of each of the layers was m~asured under a microscope. The deposition rate was calculated from the layer thickness, and the deposition efficiency was defined as the deposition rate in glmin di~ided by the total mass flow o~ soot entering the tube, assuming a 100% conver-sion to oxides. The best results obtained werP a deposition rate of 0.691 g/min, at 40.3% efficiency.
A tube of c~mmercial grade borosilicate glass havi~g a 38 mm outside di~meter and a 2 mm wall thickness is cleaned by sequential ~mmersion in hydrofluoric acid, deionized water and alcohol. This bait tube, which is about 120 cm long, is attached to a 90 cm length of exhaust tube having a 65 mm outside diameter on one end and a 60 cm handle tube of the same size as the bait tube on the other end. This combination is inserted into a lathe such ~hat the tubes are rota~ably supported. T~e fre end of the handle tube is provided with a rotatable seal through which a 1~0 cm long section of fused silica baffle tube having a 20 mm outside diameter and a 1.6 mm wall thickness is inserted. The baffle ~ube is supported at two dif~erent points along its length on a support which moves along with ~he burner. The burner tra~erses a 100 cm length of the bait ~u~e at a rate ~Q

~lZ873g of 25 cm/min. The burner is adjusted to provide a deposition temperature of 1800C at the outer surface of the bait tube.
After the burner reaches the end of its traverse during which a layer of glass is deposited, it returns to its starting point at a rate of 100 cm/min.
Oxygen flows into the ba~fle tube at the rate of 2.5 slm. Three reservoirs are provided containing SiC14, GeC14 ; and POC13, respectively, these reservoirs being maintained at a temperature of 32C. Oxygen flows through the first and third reservoirs at the rates of 0.3 lpm and 0.56 lpm, respectively thereby delivering constant amounts of SiC14 and POC13 to the bait tube during the en~ire deposition process. The rate at which oxygen is supplied to the second container increases linearly from 0 to 0.7 lpm so that during the first pass of the burner along the bait tube, no GeC14 is supplied to the bait tube, but the amount thereof is linearly increased during the remaining 49 passas of the burner. BC13 is supplied to the bait tube at the constant rate of 15 sccm, and bypass oxygen is supplied thereto at the rate of 2.4 slm.
After about 3 hours and 20 minutes, the time required for 50 burner passes, the rate of burner movement is decreased to 2.5 cm/min and the temperature increases to about 2200C
at the outer surface of the bait tube. This causes the collapse of the bait tube into an optical waveguide preform having a solid cross-section. The usable length of this preform is about 84 cm.

; 30 . The resulting preform or blank is then heated to a temperature at which the materials thereof have a low enough viscosity for drawing (approximately 2000C). This structure is then drawn to form about 25 km o optical waveguide filament having an outside diameter o about 110 ~m.

: 30

Claims

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

1. In a method of manufacturing an optical device, said method being of the type that includes the steps of flowing a vapor mixture including at least one gas which when heated forms glass particles, together with an oxidizing medium, through a hollow, cylindrical substrate, and heating said substrate and contained vapor mixture with a heat source that moves relative to said substrate in a longitudinal direction, whereby a moving hot zone is established within said substrate, such that a suspension of particulate material is produced, at least a portion of said particulate material traveling downstream where at least a portion thereof comes to rest on the inner surface of said substrate to form a deposit on said inner surface, the improvement which comprises flowing an unconfined stream of gas through the axial region of said substrate in the hot zone thereof so that said stream is the sole mechansim that acts to confine the flow of said vapor mixture to an annular channel adjacent the substrate surface, whereby the deposition efficiency of the reaction of said vapor mixture is increased.

2. The method according to claim 1 wherein the step of flowing comprises introducing a tube into said substrate that is coaxial therewith, the output end of said tube terminating just short of said hot zone and moving in synchronism therewith, said stream of gas emanating from that end of said tube adjacent said hot zone.

3. The method according to claim 2 wherein said gas comprises oxygen.

4. The method according to claim 3 further comprising the step of ceasing the flow of an unconfined stream of gas through the axial region of said substrate and thereafter heating said substrate to a temperature sufficiently high to close the aperture therein, thereby forming a draw blank.

5. The method according to claim 4 further comprising the steps of heating said draw blank to the drawing temperature of the materials thereof and drawing said blank to form an optical waveguide filament.

6. In an apparatus for manufacturing from a hollow, cylindrical substrate a preform which is intended to be subsequently drawn into an optical filament, said apparatus being of the type that includes means for supporting said substrate, means for heating an axial section of said substrate, thereby forming a hot zone within said substrate, means for providing relative longitudinal movement between said heating means and said substrate, and means for introducing into one end of said substrate a moving stream of a vapor mixture including at least one gas which when heated forms glass particles, together with an oxidizing medium, said vapor mixture being capable of reacting within said hot zone to form a suspension of particulate material which travels downstream where at least a portion thereof comes to rest on the inner surface of said substrate, the improvement comprising means for flowing an unconfined stream of gas through the axial region of said substrate in the hot zone thereof, the region within said substrate in the hot zone thereof being free from apparatus so that said gas stream is the sole mechanism for confining the flow of said vapor mixture to an annular channel adjacent the substrate wall in the hot zone whereby the reaction of said vapor mixture is confined to an annular region adjacent the substrate wall.

7. An apparatus according to claim 6 wherein said means for flowing comprises a tube disposed in the first end of said cylindrical substrate, an end of said tube terminating adjacent said hot zone, and means for moving said tube longitudinally with respect to said substrate in synchronism with the movement of said heating means, said stream of gas emanating from said end of said tube.

8. An apparatus in accordance with claim 7 wherein said stream of gas comprises oxygen.

9. In the method of manufacturing an optical waveguide preform which 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 second tube within said first tube while maintaining the end of said second tube, which is within said first tube, in spaced relation to said heat source and upstream of said heat source, said second tube having solid side walls and being open at the end thereof which is within said first tube, passing said gas between said first and second tubes, and passing another gas through said second tube, whereby said gas is confined by said another gas to an annular region adjacent the wall of said first tube in the region of said heat source.

10. The method of claim 9 wherein, after the deposition of said glass particles, said second tube is removed from said first tube, said method further comprising the step of collapsing said first tube by heating said first tube.

11. The method of claim 9 wherein the downstream end of said second tube is maintained between 25 mm and 75 mm upstream from said heat source.

12. In the method of manufacturing an optical waveguide preform which comprises the steps of passing through a tube a gas which, when heated, forms glass particles, and moving a heat source along the outside of said 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 tube, the improvement which comprises positioning a second tube coaxially within said first tube such that a first end of said second tube is within said first tube, said second tube having solid side walls and being open at the end thereof which is within said first tube, reciprocatingly moving said first end of said second tube, passing said gas between said first and second tubes, maintaining said first end of said second tube in axially spaced relation to and upstream of said heat source by a mechanical coupling, and passing a second gas through said second tube.

13. The method of claim 12 wherein said second gas is selected from the group consisting of oxygen, argon and nitrogen.

14. In an apparatus for manufacturing an optical waveguide preform which apparatus includes means for supporting a first tube, means for heating a portion of said first tube, and means for providing movement between said first tube and said heating means, the improvement which comprises a second tube, one end of said second tube being disposed within said first tube, said second tube having solid side walls and being open at the end thereof which is within said first tube, means for moving said one end of said second tube with respect to said first tube, and means for maintaining a relatively fixed relation between said heating means and said one end of said second tube, said one end of said second tube terminating short of said heating means, no apparatus being situated within said first tube in the region of said heating means, whereby a second gas flowing from said second tube functions to confine a first gas flowing between said first and second tubes to an annular region adjacent the wall of said first tube in the region of said heating means.

15. In a method of the type that includes the steps of flowing a vapor mixture including at least one reactant gas together with an oxidizing medium, through a hollow, cylindrical substrate, and heating said substrate and contained vapor mixture with a heat source that moves relative to said substrate in a longitudinal direction, whereby a moving hot zone is established within said. substrate, such that a suspension of particulate material is produced, at least a portion of said particulate material traveling downstream where at least a portion thereof comes to rest on the inner surface of said substrate to form a continuous deposit on said inner surface, the improvement which comprises flowing an unconfined stream of gas through the axial region of said substrate in the hot zone thereof so that said stream is the sole mechanism that acts to confine the flow of said vapor mixture to an annular channel adjacent the substrate surface, whereby the deposition efficiency of the reaction of said vapor mixture is increased.