NL8302641A - METHOD FOR MANUFACTURING PREFORMED OPTICAL FIBER BODIES - Google Patents

Description

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A method of manufacturing preformed optical fiber bodies.

The invention relates to an axial vapor phase deposition method (AVD method) for the manufacture of preformed optical fiber bodies.

In the typical known method of manufacturing preformed optical fiber bodies by the AVD process (also referred to as axial direction precipitation from the vapor phase [VAD]), their porous soot body is grown while being pulled in an upward axial direction . For example, reference is made to the article by T. Izawa and others, entitled "Material and Processes for Fiber Preform 10 Fabrication-Vapor-Phase Axial Deposition", published in the October 1980 issue of the Proceedings of the IEEE, vol-68, No. 10, pages 1184-1187. As indicated in the article, the flames are directed upwards and therefore the soot is deposited in an upward direction. Although this is consistent with the convection flow direction, which is a result of the torches generated by the torch, it is opposite to the downward gravity. Therefore, two of the parameters controlling the efficiency with which carbon black is precipitated attempt to act in opposite directions. For a discussion of the influence of gravity on this process, reference is made to "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 counteracting influences of convection current and gravity on the deposition barrier, the convection current "fluff" 25 (i.e., particles of arbitrary density) ascends to the growing carbon black body and is deposited about the outer surface of the rotating body. This can adversely affect the refractive index profile of the resulting preformed body made from the carbon black body.

According to the invention, the various drawbacks and limitations of the known AVD method for manufacturing carbon black bodies for consolidation into preformed optical fiber bodies are reduced by precipitating down the precursor materials. Although the influence of gravity and the convection current still tend to work in opposite directions, the natural tendency of the heated gas to move upwards , are reduced to a minimum by focusing the gas stream on the growing carbon black body. However, it has been found that sufficient of the convection current remains from the downward gas flow to minimize the accumulation of "fluff".

The invention will be explained in more detail below with reference to the drawing. In the drawing: figure 1 shows an apparatus for manufacturing soot bodies according to the invention; Figure 2 shows the influence of convection on a downward facing flame formed by a torch of uniform diameter; Figure 3 shows a device for simultaneously depositing 15 core and cladding layers; and Figures 4 and 5 use beveled adapters to focus the gas flow from a conventional torch.

As can be seen from the drawing, Figure 1 shows an apparatus 10 for producing soot bodies using the downward vapor phase axial precipitation method (DAVD-r method) according to the invention. The body is grown on a silicon oxide starting member 11, which is rotated about its vertical axis by the motor 12, which is connected to the member 11 by an axis 9. A second motor 13 causes the output member 25 to move downward as the soot body grows to keep the growth flat in a fixed position relative to the focal point of the flame.

Raw material, such as SiCl 4, GeCl 4, POCl 4, oxygen and hydrogen, is fed to the base chamber 15 of the torch 14, which produces fine glass particles through the flame hydrolysis reaction. The particles are initially deposited at the end of the blank 11. As the soot body grows, the glass particles are deposited on the top surface of the downwardly drawn axially growing body.

If one tries to apply the DAVD process using a conventional torch 20 of uniform cross section, the situation shown in Figure 2 is obtained. In this case, the convection effect is so pronounced that the flame 21 is moved upwards and completely away from the starting part 11. As a result, the precipitate is erratic and, if present, totally insufficient. To avoid this, the gas stream must be focused in the manner obtained, for example, with the beveled torch described in U.S. Patent Application

Serial no.251.259. When the torch is beveled, as shown in Figure 1, the flame configuration is substantially independent of the orientation and therefore the torch can be angled downward at any angle $ 10 relative to the vertical, where 0 <CiZ $ ^ 90 ° . A further advantage of using the beveled torch is that it provides a means for controlling the diameter of the soot body. The smaller the torch diameter at the exit end, * the smaller the diameter of the resulting body. By way of example, a 19 mm diameter body according to the invention was grown using a 9 mm diameter torch. The resulting body is significantly smaller than the typical 5-7.5 cm diameter bodies obtained by the upward AVD process using conventional torches. In addition, the body grew with a flat top surface, the body had a uniform diameter, and the body was free from "fluff".

The use of a focusing torch further makes it possible to deposit one or more coating layers simultaneously using further torches. Figure 3 shows a carbon black body 35 having a core region 31 deposited by a first torch 32 and a single cladding region 33 deposited by a second torch 34. The latter can be perpendicular to the vertical (ie j = 90 ° ) are addressed. Experience has shown that the precise location of the focal point is not critical. Figure 3 also shows the well-controlled manner in which the carbon black body grows with purely vertical lines and a flat top surface. Further torches can be used in a similar manner for the simultaneous precipitation of further coatings.

After the carbon black body has been manufactured, it is consolidated by heating to form the preformed optical fiber body. The fiber is then pulled out of the preformed body.

As noted with reference to Figures 1 and 3, the resulting carbon blacks have well defined upper and side boundaries.

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This is the result of the focusing action of the beveled torch. The latter generates a converging gas stream, the focus of which may preferably be at the center of the top surface of the growing body. This creates the tendency for a well-determined temperature gradient to occur over the top surface of the body and a well-determined pinch-off temperature, below which no precipitation occurs. This, plus the fact that the convection current drains the non-deposited particles from the growing soot body, contributes to the well-defined boundaries.

A further advantage of a focused flame is that the coating flame operates essentially independently of the core-forming flame, so that it can therefore be used simultaneously.

The simultaneous deposition of a coating layer is normally not possible on the upward side. AVD process.

Figures 4 and 5 show the use of beveled adapters to focus the gas flow from a normal torch. The beveled end 41, shown in Figure 4, consists of a single beveled tube that fits over the end of the torch 40. In the embodiment of Figure 5, the beveled focusing end 43 comprises a plurality of concentric, cylindrical sections 44, 45 and. 46, which serve to maintain the separate flow of the constituent materials.

The end 43 can be manufactured in a manner as described in the aforementioned U.S. Patent Application Serial No. 251,259.

8302641

Claims (4)

1. A method for forming a glass carbon black body, wherein a carbon black is formed from a stream of precursor materials which can be consolidated in a glass, and the stream of precursor materials is focused, characterized in that the focused stream descends in downward direction. direction is directed at a support member to obtain the carbon black body. 2. A method according to claim 1, characterized in that the support member is rotated relative to the focused current and the support member is vertically translated at a speed proportional to the growth rate of the soot body. Method according to claim 2, characterized in that the focused current is oriented to make an angle of φ with the vertical, where ΟΚφ <90 °. 15 Method according to claim 1, characterized in that at the same time a second carbon black is formed from at least one other stream of precursor materials, which can be consolidated in a glass, the at least one other stream of precursor materials is focused and the focused current on the side of the carbon black body, which is provided by the first precursor materials. 8302641