GB2143519A

GB2143519A - Optical fibre preform manufacture

Info

Publication number: GB2143519A
Application number: GB08319551A
Authority: GB
Inventors: James Pinkstone; Norman Denton; Christopher Robert Smith
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-07-20
Filing date: 1983-07-20
Publication date: 1985-02-13
Also published as: GB8319551D0; AU3071384A; GB2143519B

Abstract

In the manufacture of optical fibre preform by a method involving vapour deposition upon the bore of a substrate tube fluffy accumulations of waste deposit can produce random fluctuations in working pressure and hence in deposition. A servo controlled gas feed is suppled to delivery pipe (14) inserted into an enlarged diameter portion (3) of the exhaust end of substrate tube (2) to stabilise the working pressure, act as a scraper to dislodge the accumulations, and provide increased gas flow to assist propulsion of the dislodge fluff out of the system. <IMAGE>

Description

SPECIFICATION Optical fibre preform manufacture This invention relates to the manufacture of optical fibre preforms, and in particular to the manufacture of such preforms by vapour deposition using the internal processing route.

The internal processing route first involves deposition upon the bore of a substrate tube in a localised hot zone repetitively traversed along the tube, and then the use of further traversals at a hotter temperature to bring about progressive collapse of the bore under the effects of surface tension forces.

Uniformity of the deposition and control of the collapse are adversely affected by fluctuations inside the substrate tube. During the deposition process the reaction conditions are normally chosen so that the reaction proceeds at least predominately as a gas phase reaction producing particles which fuse substantially immediately on landing on the substrate, but in practice a proportion of the reaction product is not collected in this way but passes down towards the exhaust end of the tube where it can accumulate as a sort of fluff liable to impede the gas flow and hence produce uncontrolled pressure variations.

According to the present invention there is provided a method of optical fibre preform manufacture in which a vapour reaction is used to deposit upon the bore of a substrate tube in a hot zone repetitively traversed along the tube as it is rotated about its axis, and in which the coated bore is subsequentiy progressively collapsed under the influence of surface tension in further traversals of the hot zone, wherein during the deposition, and/or during the initial stages of bore collapse, the pressure within the substrate tube is stabilised by means of a servo controlled loop controlling the supply of gas to a delivery pipe inserted into an exhaust chamber directly connected with the exhaust end of the substrate tube.

Preferably the delivery pipe rests against the bottom of an expanded diameter outlet portion of the substrate tube so that rotation of the tube causes it to assist the dislodging of accumulated 'fluff' by a mechanical scraping operation.

It is also preferred for the gas flow from the delivery pipe to be large compared with the flow entering the substrate tube from its upstream end, so that the former significantly assists the propulsion of the dislodged 'fluff' out of the substrate tube and the exhaust chamber.

There follows a description of a method of optical fibre preform manufacture embodying the invention in a preferred form. The description refers to the accompanying drawings in which: Figure l is a schematic representation of the apparatus, and Figure 2 depicts the gas delivery pipe resting inside the expanded diameter outlet portion of the substrate tube.

A vapor train, deposited schematically by a bubbler 1, delivers the requisite reagent mixture entrained in a carrier gas to the input end of a substrate tube 2 having an expanded diameter outlet portion 3 at the exhaust end.

This substrate tube is mounted horizontally in a lathe (not shown) between synchronously driven head and tail stocks (not shown), and so rotary gas seals 4 and 5 are provided at the two ends of the substrate tube. The expanded diameter outlet portion 3 of the substrate tube vents into an outlet chamber 6 having an outlet orifice 7. A pressure transducer 8 monitors the gas pressure either in outlet chamber 6 or at the input end to the substrate according to whether valve 9 is open or valve 10. The output of transducer 8 drives a feedback control unit 11 which regulates a valve 1 2 in a gas line 1 3 feeding a delivery pipe 14 inserted into the expanded diameter portion 3 of the substrate tube 2.

This is operated as a servo-controlled loop to stabilise the gas pressure within the substrate tube. As can be seen from Fig. 2 the delivery pipe 14 has a stopped end and the gas emerges from vents 1 5 directed to the rear.

Preferably the delivery pipe is allowed to rest under its own weight against the bottom of the expanded diameter portion of the substrate tube so that the rotation of the tube causes it to assist the dislodging of accumulated 'fluff' by a mechanical scraping action.

The additional gas flow provided by the delivery pipe serves to propel the dislodged 'fluff' put of the substrate tube, and the exhaust chamber, into some form of extraction system (not shown).

In a typical example in which deposition was performed in a substrate tube with a 1 6 mm diameter opening into an enlarged portion having a bore diameter of 36 mm, the pressure regulation was adjusted to provide, in the substrate tube, a pressure of 1.4 millebars in excess of atmospheric pressure. Under conditions providing a flow rate in the region of 3 litres per minute from the vapour train 1, this excess pressure was achieved with a nitrogen flow of about 35 litres per minute issuing from the delivery pipe 14. During deposition it was found preferable to monitor the pressure at the inlet end of the substrate tube so as to avoid the risk that the dislodging of accumulations of 'fluff' from the enlarged portion creates spurious flunctuations in the pressure registering in the control line. During the initial stages of the bore collapse process it may be preferred to monitor instead the pressure in the chamber 6 since the progressive reduction in bore diameter creates increased resistance in gas flow and hence a greater pressure drop across its ends. In the final traverse that entirely closes the bore there can of course be no gas flow down the tube, and hence the method of this invention is not used for that last traverse.

Claims

1. A method of optical fibre preform manufacture in which a vapour reaction is used to deposit upon the bore of a substrate tube in a hot zone repetitively traversed along the tube as it is rotated about its axis, and in which the coated bore is subsequently progressively collapsed under the influence of surface tension in further traversals of the hot zone, wherein during the deposition, and/or during the ini tiai stages of bore collapse, the pressure within the substrate tube is stabilised by means of a servo controlled loop controlling the supply of gas to a delivery pipe inserted into an exhaust chamber directly connected with the exhaust end of the substrate tube.

2. A method as claimed in claim 1 wherein the delivery pipe rests against the inside surface of the substrate tube in a region downstream of the deposition zone.

3. A method as claimed in claim 1 or 2, wherein the gas flow from the delivery pipe is large compared with the flow entering the substrate tube from its upstream end.

4. A method of optical fibre preform manufacture substantially as hereinbefore described with reference to the accompanying drawings.

5. An optical fibre preform made by the method claimed in any preceding claim.

6. An optical fibre drawn from a preform as claimed in claim 5.