GB2124205A

GB2124205A - Method of fabricating optical fiber preforms

Info

Publication number: GB2124205A
Application number: GB08319566A
Authority: GB
Inventors: Herman Melvin Presby
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1982-07-26
Filing date: 1983-07-20
Publication date: 1984-02-15
Also published as: FR2530613B1; FR2530613A1; GB2124205B; JPS5935036A; GB8319566D0; CA1218270A; DE3326928A1; NL8302641A

Abstract

An improved method of forming a soot form (from which fiber preforms are made) involves focussing the stream of particulate precursor materials, and directing the focussed stream downward onto a rotating supporting member (11). Core (31) and cladding (33) may be deposited simultaneously. <IMAGE>

Description

SPECIFICATION Method of fabricating optical fiber preforms This invention relates to the Axial Vapor-phase Deposition (AVD) method of fabricating optical fiber preforms.

In the typical prior art method of fabricating optical fiber preforms by the AVD process (alternatively referred to as Vapor-phase Axial Deposition VAD1), a porous soot form is grown while being pulled in an upward axial direction. See, for example, the paper by T.

Izawa et al, entitled "Material and Processes for Fiber Preform Fabrication-Vapor-Phase Axial Deposition" published in the October 1 980 issue of the Proceedings of the IEEE, Vol. 68, No. 10, pp. 1184-1187. As illustrated in this article, the flames are directed upward and, hence, the soot is deposited in an upward direction. While this is consistent with the direction of convection flow, due to the hot gasses produced by the torch, it is opposite to the downward pull of gravity.

Thus, two of the parameters controlling the efficiency with which soot is deposited are tending to operate in opposite directions. For a discussion of the effect of gravity on this process see "Influence of Gravity on Chemical Vapor Deposition Processes" by G. Wahl Prog. Astronaut Aeronut 52 (Mater. Sci.

Space Appl. Space Processes,) 461-482, 1977.

In addition to the opposing influences of convection flow and gravity on the deposition efficiency, the convection flow carries "fluff" (i.e., random density particles) upward toward the growing soot form and deposits it about the outer surface of the rotating form. This can have an adverse effect upon the refractive index profile of the resulting preform made from the soot form.

According to the present invention there is provided a method of forming a glass soot form, comprising forming, from a stream of precursor materials, a soot capable of being consolidated into a glass; focussing said stream of precursor material; and directing said focussed stream in a downward direction onto a rotating member to produce said soot form.

The various disadvantages and limitations in the prior art AVD method of fabricating soot forms for consolidation into optical fiber preforms are mitigated, in an embodiment of the present invention, by the downward deposition of the precursor materials. While the effects of gravity and convection flow still tend to operate in opposite directions, the natural tendency of the heated gas to flow upward can be minimized by focussing the gas flow onto the growing soot form. However, it has been found that enough of the convection flow away from the downward directed gas stream remains so as to minimize the accumulation of fluff.

For a better understanding of the invention, reference is made to the accompanying drawings in which: Figure 1 shows an embodiment arrangement for fabricating soot forms; Figure 2 shows the effect of convection upon a downward directed flame produced by a torch of uniform diameter; Figure 3 shows an arrangement for simultaneously depositing core and cladding layers; and Figures 4 and 5 illustrate the use of tapered adapters for focussing the gas flow from a conventional torch.

Referring to the drawings, Fig. 1 shows an arrangement 10 for fabricating soot forms employing the Downward Axial Vapor-phase Deposition (DAVD) method in accordance with an embodiment of the present invention. The form is grown on a silica starting member 11 which is rotated about its vertical axis by a motor 1 2 which is connected to member 11 by means of a shaft 9. A second motor 1 3 causes the starting member to move in a downward direction as the soot form grows, so as to maintain the growing surface at a fixed location relative to the focal point of the flame.

Raw materials, such as SiCI4, GeCI4, POCL3, oxygen and hydrogen, are fed into the base chamber 1 5 of torch 14, which produces fine glass particles by the flame hydrolysis reaction. The particles are, initially, deposited onto the end of starting member 11. As the soot form grows, the glass particles are deposited onto the upper surface of the downward drawn, axially growing form.

If one attempts to practice the DAVD process employing a conventional torch 20 of uniform cross section, the situation depicted in Fig. 2 is produced. In this case, the convection effect is so pronounced as to cause the flame 21 to bend upward and completely away from the starting member 11. As a result, deposition is erratic and totally unsatisfactory if at all. To avoid this, the gas flow must be focussed in the manner produced, for example, by the tapered torch disclosed by applicant in his copending application Serial No. 251,259, filed April 6, 1981. When the torch is tapered, as shown in Fig. 1, the flame configuration is substantially independent of orientation and, hence, the torch can be directed substantially downward at an angle Id to the vertical where O < c90 degrees.In addition to focussing the gas flow, a further advantage of the use of the tapered torch is that it provides a means for controlling the diameter of the soot form. The smaller the torch diameter at the output end, the smaller the diameter of the resulting form. As an example, a 3/4" diameter form was grown in one embodiment using a 3/8" diameter torch. The resulting form is considerably smal ler than the typical 2" to 3" diameter forms produced by the upward AVD process using conventional torches. In addition, the form grew with a flat upper surface, was of uniform diameter, and free of fluff.

The use of a focussing torch further permits the simultaneous deposition of one or more cladding layers using additional torches. Fig.

3 shows a soot form 35 comprising a core region 31 being deposited by a first torch 32, and a single cladding region 33 being deposited by a second torch 34. The latter can be directed perpendicular to the vertical (i.e., = = 90 degrees). Experience has shown that the precise location of the focal point is not critical. Fig. 3 also shows the well controlled manner in which the soot form grows with clean vertical lines and a flat upper surface.

Additional torches can be similarly employed to simultaneously deposit additional cladding layers.

After the soot form is fabricated, it is consolidated by heating to form the optical fiber preform. The fiber is then drawn from the preform.

As noted in connection with Figs. 1 and 3, the resulting soot forms have well defined upper and side boundaries. This is the result of the focussing action of the tapered torch.

The latter generates a converging gas flow whose focal point is advantageously located near the center of the upper surface of the growing form. This tends ta produce a well defined temperature gradient across the upper surface of the form and a well defined cutoff temperature below which deposition does not occur. This, plus the fact that the convection flow carries the nondeposited particles away from the growing soot form, accounts for the well defined boundaries.

A further advantage of a focussed flame is that the cladding flame operates substantially independently of the core producing flame thus permitting their simultaneous use. The simultaneous deposition of a cladding layer is not normally practical with the upward AVD process.

Figs. 4 and 5 illustrate the use of tapered adapters for focussing the gas flow from a conventional torch. The tapered tip 41, illustrated in Fig. 4, is a single tapered tube which fits over the end of the torch 40. In the embodiment of Fig. 5, the tapered focussing tip 43 comprises a plurality of concentric cylindrical sections 44, 45 and 46 which serve to preserve the separate flow of the constituent materials. Tip 43 can be made in the manner described in the above cited copending application.

Claims

1. A method of forming a glass soot form, comprising forming, from a stream of precursor materials, a soot capable of being consolidated into a glass; focussing said stream of precursor materials; and directing said focussed stream in a downward direction onto a rotating member to produce said soot form.

2. The method according to claim 1, wherein the focal point of said stream is located on the upper surface of said soot form.

3. The method according to claim 1, or 2 wherein said member rotates about its vertical axis and is translated in a downward direction at a rate equal to the growth rate of said soot form.

4. The method according to claim 1, 2, or 3, wherein said soot is formed by means of a hydrolysis torch.

5. The method according to claim 4, wherein the diameter of said torch tapers from a first diameter along the input end of its length to a second smaller diameter at its output end.

6. The method according to claim 4 or 5, wherein the longitudinal axis of said torch makes an angle of Z degrees to the vertical, where O < 690 degrees.

7. The method according to claim 1, including the further step of: forming, from at least one other stream of precursor materials, a second soot capable of being consolidated into a glass; focussing said at least one other stream of precursor materials; and directing said focussed stream onto the side of the soot form produced by said first precursor materials.

8. An adapter for use with the method according to claim 4, and serving to focus the gases discharged from the torch, said adapter comprising a tapered section of hollow tubing adapted to fit over the output end of said torch.

9. An adapter according to claim 8, wherein the tapered section comprises an assembly of coaxially aligned tubes adapted to fit over the end of said torch and to align with a corresponding assembly of coaxially aligned tubes forming said torch.

10. A method of forming a glass soot form, substantially as hereinbefore described with reference to any one of Figs. 1, 3, 4 or 5, of the accompanying drawings.