MANUFACTURE OF OPTICAL FIBRE PREFORMS

Abstract: In a method of manufacturing optical fibre preforms by causing a chemical reaction to take place in a gaseous reac­ tion mixture (5) so as to form a coating (19) on a glass sub­ strate (1) wherein the reaction is promoted by the production of a plasma in the mixture by means of a microwave cavity (12) powered by a microwave generator (15) and conductive tube (16) positioned around the reaction tube system, it has been found that if the conductive tube is made of metal, metal ions diffuse through to the substrate and contaminate it. Therefore, the conductive tube according to the invention is made of graphite.

Description: Manufacture of Optical Fibre Preforms This invention relates to the manufacture of glass preforms, from which optical fibre waveguides can be produced by drawing, by a method of the type (hereinafter referred to as the type specified) in which a chemical reaction is caused to take place between oxygen and the vapour or vapours of one or more compounds such as halides, which reaction results in the formation of a coating composed essentially of one or more oxides on a glass substrate surface. The substrate surface on which the coating is formed may be the interior surface of a glass tube or the exterior surface of a glass core. This method, and forms of apparatus used for carrying out this method, are disclosed in our co-pending Patent Applications Nos. 8204914 (Publication No. 2093829) and No. 8330821, and in Patent No. 2079267, to which reference should be made for further details. Methods which have been proposed for promoting a said chemical reaction include the application of heat, and the production of a plasma in the gaseous mixture. It will be seen from the above mentioned patents and applications that one form of a plasma-exciting device is a microwave cavity located around one end of the substrate tube, abutting an electrically conductive tube positioned coaxially around the glass tube system, and supplied with sufficient power to produce a plasma column of the required length, for example by means of a microwave generator. In a different embodiment an electrically conductive tube is positioned coaxially around the glass tube system where the chemical reaction is to take place, and actually forms part of the microwave cavity extending along the tube system for a distance corresponding to the desired length of plasma column. The arrangements are such that the combination of the conductive tube and the plasma column produced in operation constitute a coaxial waveguide by which power from the generator is coupled to the plasma, and a progressive electromagnetic wave is launched along the waveguide, promoting increased extension of the plasma column for a given power input to the cavity. Hitherto the conductive tube has been constituted either by the inner wall of a tubular furnace employed for external heating of the system or by a separate metal tube, made of, for example, Inconel. However, it has been found that metal ions from this metal tube tend to diffuse through to the substrate and/or coating and contaminate the preform, which can have the effect of impairing the performance of optical fibre formed from it. It is thus an object of the present invention to provide a method of manufacturing optical fibre preforms in which contamination from metal ions has been removed or at least significantly reduced. Accordingly, the invention provides a method of the type specified in which the chemical reaction is promoted by the production of a plasma in the gaseous mixture by means of a microwave cavity powered by a microwave generator and having a conductive tube positioned coaxially around said substrate so as to form a waveguide wherein said conductive tube is made of conductive carbon. Preferably the tube is made of graphite. One method in accordance with the invention will now be described by way of example with reference to the drawing which shows, in part-sectional elevation, apparatus employed for carrying out the method. In the drawing, a vitreous silica substrate tube 1, supported vertically, has an inlet for gas which may be a side tube 2 adjacent to its upper end, or alternatively (not shown) a tube inserted through a seal at its upper end, and an inner tube 3 of vitreous silica is sealed through a cap 4 closing the upper end of the tube 1, forming an annular space 5 between the tubes 1 and 3. The tube 3 is located coaxially within the substrate tube by means of three silica projections 6 attached to its lower end and abutting against the substrate tube wall, and a short length of closely fitting silica tube 7 with similar projections 8 which is placed over the tube 3 near its upper end. The open upper end 9 of the tube 3 is connected to means (not shown) for supplying the required mixture of carrier gas and reactant vapour or vapours to this tube, the lower end 10 of the tube 3 is closed, and the wall of the tube 3 is pierced by a multiplicity of performations 11 (shown greatly enlarged) to permit egress of the reactant vapours and carrier gas into the space 5 which constitutes the reaction zone. The lower end of the substrate tube 1 is connected to a vacuum pump (not shown), for exhaustion of the tube as indicated by the arrow. A microwave cavity 12, formed of an outer cylinder 13 and an inner cylinder 14 of predetermined height, is located adjacent to the gas exit end of the tube 1, the lower end of the tube being inserted through the inner cylinder 14. Power is supplied to the cavity from a microwave generator 15. A waveguide tube 16 of circular cross-section, formed of graphite, is also positioned around the substrate tube, extending from the top of the microwave cavity nearly to the gas inlet tube 2, and the tube 16 is surrounded by a tubular electric furnace 17 When the gaseous mixture is passed into the annular space (for further details of the composition of the mixture see the first above mentioned patent application) and power is supplied to the microwave cavity, a plasma column 18 is established in space 5 causing continuous deposition of a glass coating 19 on the substrate tube 1. It has been found that by making the conductive tube 16 of graphite, the problem of contamination of the preform due to diffusion of metal ions from a metal conductive tube disappears. For this reason, the upper part 20 of the cylinder 14 is also made of graphite and is made to reach a point near to the lowest set of perforations 11 in the tube 3, so that as little of the plasma column 18 as possible is wasted. The invention is clearly not limited to the particular form of apparatus described but may be used to advantage in any method of manufacturing optical fibre preforms which uses microwave power to excite a plasma for promoting a chemical reaction used to deposit glass on a substrate. For example, the invention could be equally well used in the apparatus described in the second above mentioned patent application in which there is disclosed apparatus used for manufacturing optical fibre preforms having two microwave cavities and generators, one at each end of the substrate tube and where both ends thereof are being exhausted. It will be understood that the invention includes within its scope apparatus for carrying out the method above described in which a graphite tube is used to form a waveguide into which microwave energy is arranged to be fed from a microwave generator located at one or each end. It will also be understood that the term glass preform11 includes within its scope glass tubes which may be used in the production of optical fibre waveguides, but which need to undergo further treatment or processing or to be combined with other glass structures before being drawn to form such waveguides

Claims: CLAIMS 1. A method of manufacturing glass preforms in which a chemical reaction is caused to take place between oxygen and the vapour or vapours of one or more compounds such as halides, which reaction is promoted by the production of a plasma in the gaseous mixture (5) by means of a microwave cavity (12) powered by a microwave generator (15) and a conductive tube (16) positioned coaxially around a glass substrate (1) so as to form a waveguide and which results in the formation of a coating (14) composed essentially of one or more oxides on said glass substrate (1) characterised in that said tube (16) is made of conductive-carbon. 2. A method according to Claim 1 characterised in that said tube (16) is made of graphite.

Inventors: CUNDY, STEVEN LANGFORD, EVANS, RONALD ALBERT, GREENHAM, ADRIAN CLEMENT, NICHOLS, BRUCE ARMSTRONG
Applicants: THE GENERAL ELECTRIC COMPANY, P.L.C.
Application year: 1984
Main classification: C03B37
IPC codes: C23C16, C03B37