GB2068359A

GB2068359A - Manufacture of optical fibre preforms

Info

Publication number: GB2068359A
Application number: GB8102582A
Authority: GB
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1980-01-29
Filing date: 1981-01-28
Publication date: 1981-08-12
Also published as: GB2068359B

Abstract

In the manufacture of an optical fibre preform by forming a vitreous coating, preferably composed of silica and one or more dopants, in the bore of a glass (preferably silica) substrate tube, from a gaseous reaction mixture passed through the tube, the reaction mixture is carried by an ionisable gas and the reaction is activated by a plasma produced in the gas by means of a plasma-exciting device maintained in a stationary position around the gas exit end (3) of the tube. The plasma column is swept along the tube by varying the power input to the device (7), while the tube is heated and the gas pressure is kept below 10 Torr, to effect direct formation of a glass layer (12) along the swept and heated region of the tube. Preferred apparatus includes an asymmetrical microwave cavity (4), and a conducting tube (8) surrounding the substrate tube (1), the arrangement being such that the conducting tube and the plasma column (10) form a coaxial waveguide along which a progressive electromagnetic wave, preferably in the H10 mode, is launched. <IMAGE>

Description

SPECIFICATION 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 in which a chemical reaction is caused to take place in a gaseous mixture including oxygen and the vapour or vapours of one or more compounds such as halides, while the mixture is passed through a glass substrate tube, which reaction results in the formation of a vitreous coating composed essentially of one or more oxides on the interior surface of the tube, this coating constituting at least the core material of the optical fibre. The invention also relates to apparatus for use in carrying out the method described.

The chemical reaction of the gas mixture in the tube can be promoted by traversing a heat source along the exterior of the tube, the solid reaction product being formed as fine particles which are deposited progressively along the surface of the tube bore and converted into a glass layer by further heating, the process being repeated as required td form a multi-layer coating. It has also been proposed to promote the said reaction by means of a plasma produced in the gaseous mixture within the tube, formation of the coating being caused to take place progressively along the tube bore by effecting relative longitudinal movement between the tube and the plasma-exciting device.

It is an object of the present invention to provide an improved method of the type referred to above, for the manufacture of an optical fibre preform, in which method plasma activation of the said reaction is employed.

According to the invention, in a method of manufacturing a glass optical fibre preform which includes the step of forming a vitreous coating composed essentially of one or more oxides on the interior surface of a glass substrate tube, by causing a chemical reaction to take place in a gaseous mixture within the tube, a gaseous mixture consisting of oxygen, the vapour or vapours of one or more compounds capable of reacting with oxygen to produce the desired vitreous coating material, and an ionisable carrier gas, is passed continuously through the glass tube while a plasma column for activating the reaction is produced within the tube by means of a plasma-exciting device maintained in a stationary position with respect to the tube, around at least a portion of the tube adjacent to the gas exit end thereof, heat is applied to the exterior of the region of the tube length in which the formation of said coating is required, to maintain the tube wall at a sufficiently high temperature to ensure vitrification of the coating without causing distortion of the tube, the gas pressure within the tube is maintained not greater than 10 Torr, and the termination of the plasma column is swept along the said heated region of the tube by effecting continuous and progressive variation of the electromagnetic power input to the plasma-exciting device, whereby a continuous layer of vitreous material resulting from the plasma-activated reaction of the gas mixture is formed on the interior surface of the tube progressively from one end to the other of the said heated region.

The conditions of temperature and pressure under which the reaction is effected are such that the vitreous reaction product is formed directly on the tube wall, without preliminary formation of particulate material either on the wall or in the gas out of contact with the wall. The formation of the vitreous coating occurs within the region of the tube occupied by the plasma column, and only in the vicinity of the termination of the plasma column: hence sweeping of the termination of the plasma column along the tube results in progressive formation of the glass coating along the tube between the limiting positions of the plasma column termination, preferably from the vicinity of the plasma-exciting device to the vicinity of the gas entry end of the tube.The formation of a multilayer coating is achieved by sweeping the plasma column termination backwards and forwards along the glass tube. Thus the power input to the plasma-exciting device is continuously oscillated between minimum and maximum levels, resulting in alternate extension and contraction of the plasma column, each cycle of gradually increasing or decreasing power resulting in the deposition of a single layer of glass along the predetermined region of the tube. The length of the optical fibre preform produced, which corresponds to the length of the deposition region in the tube, is determined by the maximum length of plasma column produced at maximum power.

It will be understood that, in accordance with the normal practice in the manufacture of optical fibre preforms, the glass substrate tube is of circular cross-section.

The method of the invention is particularly applicable to the manufacture of an optical fibre composed of silica with one or more dopants for imparting a desired refractive index profile to the fibre.

Thus the substrate tube is preferably formed of vitreous silica, and the gaseous reaction mixture suitably consists of oxygen, a silicon tetrahalde and, for at least part of the process, a halide or halides from which the required dopant or dopants is or are derived, the halide vapours being entrained in streams of the carrier gas which are bubbled through the liquid halides: the halides employed are usually chlorides, which react with the oxygen to produce silica and dopant oxides, but if it is desired to introduce fluorine as a dopant in the silica coating silicon tetrafluoride may be included in the reaction mixture in addition to, or instead of, silicon tetrachloride.The silica tube may form the cladding of an optical fibre the core of which is formed of the coating material, or the tube may constitute a support for an optical waveguide structure wholly formed by the coating material. In a multilayer process, the composition of the gas mixture may be maintained constant for the formation of all the coating layers, to produce a step refractive index fibre, or may be varied as desired for successive coating layers or groups of layers to produce a graded refractive index fibre or a fibre composed of two or more concentric regions with and without dopants, or with different dopants, which regions may constitute, for example, cladding and/or buffer layers and core, each region being of either uniform or graded composition as required.

The plasma-exciting device employed in the method of the invention preferably comprises a microwave cavity supplied with power from a microwave generator, the gas exit end portion of the glass substrate tube being inserted within the cavity. When the device is operated at low power, the plasma column extends only a short distance along the tube beyond the portion thereof lying within the cavity, and as the power is increased the plasma column extends along the tube towards the gas entry end.

Preferably the microwave cavity is of the asymmetrical type, formed of two concentric circular cylinders, the inner cylinder being located around a portion of the glass tube adjacent to the gas exit end thereof, the outer cylinder extending further along the glass tube than the inner cylinder, and the cavity being so designed that, in operation, a region of high electrical field strength is produced at the inner end only of the inner cylinder (that is to say the end of the inner cylinder remote from the gas exit end of the glass tube) so that, in any given gas conditions, the extension of the plasma column from the portion of the glass tube within the cavity, as the power input to the cavity is increased, will take place only in the desired direction along the tube, towards the gas entry end, and not in the reverse direction.It is further preferred to surround the glass tube by an electrically conductive tube extending from the microwave cavity substantially to the gas inlet end of the glass tube, such that a coaxial waveguide is formed by the said conductive tube and the plasma column.

The power input to the microwave cavity is suitably varied over a range within 50 to 1000 watts, but in some cases the maximum power input may, if desired, be considerably higher, for example up to 20 kilowatts, the upper limit depending upon the required length of the deposition region of the glass tube, and on other conditions of operation. The sweeping of the plasma column termination along the tube may be effected either by starting with the minimum power and progressively increasing the power to the maximum desired value, or by starting with maximum power and decreasing to the minimum. It will usually be necessary to employ auxiliary means, such as a Tesla coil, to initiate the plasma.

A preferred form of apparatus for carrying out the method of the invention includes an asymmetrical microwave cavity as described above, located around the gas exit end portion of the glass substrate tube, a circular cylindrical electrically conductive tube positioned concentrically around the glass tube, abutting the microwave cavity and extending substantially to the gas inlet end of the glass tube, the arrangement of said conductive tube and microwave cavity being such that the combination of the conductive tube and the plasma column produced in the glass tube in operation constitutes a coaxial waveguide, and heating means located outside the said conductive tube. In operation of this arrangement, power from the cavity is coupled to the plasma, and a progressive electromagnetic wave is launched along the waveguide.

In the said preferred form of apparatus, the dimensions of the microwave cavity are preferably such that the cavity will support an electromagnetic wave in the transverse electromagnetic (TEM) mode, and the length of the inner cylinder of the cavity is preferably adjusted so that the power reflected at the termination of the inner cylinder within the outer cylinder (that is to say the inner end of the inner cylinder) sets up a standing wave pattern within the cavity, with the maximum voltage at the level of the said inner cylinder termination. Such a standing wave pattern is produced when the length of the inner cylinder is approximately equal to (2n + 1) 4 where n is an integer and A is the free space wavelength of the operating frequency applied to the cavity. A progressive wave is transmitted from the cavity along the aforesaid coaxial waveguide.

Preferably the conductive tube (hereinafter referred to as the "waveguide tube") has an internal diameter which is such, in relation to the diameter of the plasma column in the glass tube, that the wave propagated along the waveguide is in the H10 coaxial mode: the reason for the preference for the Hro mode will be explained below.

The region of the substrate tube swept by the plasma column termination is suitably heated to a temperature of about 10000 C, for the formation of a silica-based coating in a silica tube, to ensure the formation of a smooth, continuous glass coating. If the glass tube is not surrounded by a waveguide tube, the heating may be effected either by supporting the whole of the deposition region of the glass tube within a long stationary furnace throughout the deposition process, or by moving a small furnace or a flame (or other suitable heat source) along the tube in synchronism with the sweeping of the plasma termination. The heating means employed in the above-described preferred form of apparatus is preferably a tubular electric furnace surrounding the waveguide tube and extending along the whole of the length of the glass tube which is swept by the plasma column.

The ionisable carrier gas suitably consists of argon, krypton or xenon. The maximum length of plasma column attainable with a given maximum power level varies according to the ionisation potential of the gas. Krypton and xenon both have a lower ionisation potential than argon, and therefore produce longer plasma columns than argon; however, an argon plasma column is usually adequate, although the length of the column is reduced by the presence of oxygen and the reactant vapour or vapours in the ionisable gas.

The maintenance of low gas pressure within the glass tube is favourable to the production of a plasma column of optimum length, and is essential for achieving the direct deposition of glass, with the absence of discrete particle formation during the reaction. The low pressure is maintained by means of a suitable pump connected to the gas exit end of the glass tube, through which gaseous reaction products and any residual gaseous reaction mixture are removed in the stream of carrier gas.

The length of the plasma column is also influenced by the ratio between the flow rates of the carrier gas, oxygen, and reactant vapour or vapours, at given overall pressure and temperature, increases in the oxygen and reactant vapour flow rates causing contraction of the column. For example, in the case of a gas mixture consisting of argon, oxygen and silicon tetrachloride, with or without one or more dopant-producing halides, at a pressure of less than 10 Torr and with the tube maintained at 10000 C, a typical ratio of flow rates of argon :oxygen :silicon tetrachloride is 20:10:1 or less. Any dopant-producing vapour included is usually present in such a small proportion that it has little effect on the length of the plasma column.

We have further found that, when the aforesaid preferred form of apparatus is employed, with any given conditions of power input and gas composition and pressure the maximum length of the plasma column is critically dependent upon the dimensions of the coaxial waveguide formed, more specifically upon the relationship between the diameter of the plasma column and the internal diameter of the surrounding waveguide tube. Thus, for a given plasma column diameter (which corresponds to the internal diameter of the glass tube), there is a critical minimum internal diameter of the waveguide tube: the maximum length of the plasma column is considerably greater if the internal diameter of the waveguide tube exceeds this critical magnitude than if it is less than the critical magnitude.The apparent reason for this effect is that, under given gas and power conditions, the maximum length of the plasma column obtainable depends upon the mode of the wave propagated in the waveguide: with a waveguide tube diameter less than the critical magnitude, only the TEM mode can exist in the waveguide, but with a waveguide tube of internal diameter greater than the critical magnitude the wave is propagated in the H10 mode, the increased length of the plasma column being associated with the latter mode.The minimum waveguide tube internal diameter required, to ensure propagation of the H10 mode, is close to that derived from the following relationship:- A=Z(x+y) (1) where A is the free space wavelength of the operating frequency, xis the internal radius of the glass tube (the radius of the plasma column), and y is the internal radius of the waveguide tube.

For ensuring satisfactory operation of the microwave cavity/coaxial waveguide arrangement, the total impedance of this arrangement should match that of the combination of the microwave generator and the coaxial cable connecting the generator to the cavity. The impedance of the cavity/waveguide arrangement is affected by the internal diameter of the waveguide tube, and can be conveniently adjusted to the optimum value by altering the ratio of the diameters of the inner and outer cylinders of the cavity.

For carrying out the process of the invention, the glass substrate tube is usually disposed vertically, the gas mixture usually being introduced into the upper end of the tube, and the plasmaexciting device being disposed around the lower end of the tube, although this arrangement may be reversed if desired. Alternatively the process can be carried out with the glass tube disposed horizontally. In either case it may be desirable to rotate the tube continuously about its longitudinal axis, in order to avoid the possibility of non-uniformity of deposition of the glass coating over the whole circumference of the tube bore which might occur, for example, if the electromagnetic field produced, especially in a coaxial waveguide, is not radially symmetrical.

Some specific methods in accordance with the invention which we have carried out, for the manufacture of optical fibre preforms composed of a doped silica core and silica cladding, will be described in the examples below, with reference to the accompanying diagrammatic drawing, which shows, in sectional elevation, the apparatus employed in carrying out the methods of the examples.

Referring to the drawing, a vitreous silica tube 1 is supported vertically with its upper end 2 connected to means (not shown) for supplying the required gas mixture to the tube, and its lower end 3 connected to a vacuum pump (not shown). A microwave cavity 4, formed of an outer cylinder 5 and an inner cylinder 6 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 6: the cylinder 6 is formed in two portions enabling its height to be adjusted telescopically. Power is supplied to the cavity from a microwave generator 7.A waveguide tube 8 of circular cross-section, formed of oxidation resistant steel, is also positioned around the silica tube, extending from the top of the microwave cavity nearly to the gas inlet end 2 of the silica tube, and the tube 8 is surrounded by a tubular electric furnace 9.

In operation of the apparatus shown in the drawing, a gas mixture consisting of ionisable.carrier gas, oxygen, silicon tetrachloride vapour, and one or more dopant-producing halide vapours is passed through the tube 1, a Tesla coil (not shown) is applied initially to the silica tube close to the microwave cavity to generate ionisation and thus initiate a plasma in the incoming gas stream, and then the power input to the microwave cavity is varied continuously between the desired minimum and maximum values, so that the plasma column is swept up and down the tube 1.With the power at the minimum level the length of the plasma column is substantially as shown at 10, and as the power is increased the termination 11 of the plasma column moves up the tube 1, ideally to the position 11' when the maximum power is applied, the actual maximum extension of the plasma column obtainable being dependent upon the gas composition and pressure, the maximum power input, and the internal diameter of the tube 8 in relation to that of the tube 1, as explained above.

Reaction between the oxygen and the halides in the gas mixture takes place within, and adjacent to the termination of, the plasma column, so that as the plasma is swept along the silica tube a continuous layer of glass composed of silica and the dopant or dopants is formed on the region of the silica tube wall surrounded by the waveguide tube and the furnace, over a distance determined by the maximum extension of the plasma column. The process is continued until a glass coating 12 of the desired thickness has been formed on the bore of the silica tube.

In a specific form of the apparatus described above with reference to the drawing, which form was employed for carrying out the processes described in Examples 1 and 2; below, the silica tube 1 has an internal diameter of 16.5 mm and an external diameter of 19 mm, the waveguide tube 8 has an internal diameter of 6.1 cm, and the tubular furnace 9 has an internal diameter of 7.5 cm. The frequency of the power input to the microwave cavity is 2.45 GHz (wavelength A = 12.2 cm): the height of the inner cylinder 6 of the cavity is therefore approximately 9.15 cm, that is to say 3/4 A, the optimum value being slightly less than this and being found by tuning, which is effected by adjusting the height telescopically.The outer diameter of the inner cylinder, and the inner diameter of the outer cylinder, of the cavity, are respectively 32 mm and 57 mm; these dimensions are found to make the overall impedance of the cavity/waveguide system match that of the microwave generator, when the system is operated under the conditions described in the following examples.

EXAMPLE 1 For the production of a step refractive index preform consisting of an undoped silica cladding layer constituted by the silica substrate tube 1, and a core of silica doped with phosphorus pentoxide, the gas mixture passed into the tube 1 was formed of four streams respectively composed of argon, oxygen, argon with entrained silicon tetrachloride, and argon with entrained phosphorus oxychloride, these gas streams being mixed together immediately before entering the tube 1. The flows of the four gas streams were controlled to give constant flow rates of 200 standard cubic centimetres per minute (sccm) of total argon, 100 sccm of oxygen, 6.6 sccm of silicon tetrachloride, and 2 sccm of phosphorus oxychloride.The gas pressure in the silica tube was maintained at approximately 1 Torr throughout the process, by means of the vacuum pump and by controlling the input pressure, suitably by means of a capillary restriction incorporated in the supply line. The silica tube was maintained at a temperature of 1 0000C by the furnace 9.

The power input to the microwave cavity 4 was varied between 100 watts and 250 watts, and the duration of each complete power cycle was 45 seconds, the power being raised slowly from minimum to maximum over 44 seconds, and then dropped rapidly to the minimum level: thus the power increasing part of each cycle caused the plasma column to rise slowly to its maximum extension, resulting in the deposition of a continuous layer of glass, composed of silica and phosphorus pentoxide, in the bore of the silica tube, deposition occurring during the rapid decrease in power being negligible.

After the required number of layers had been deposited, to form the glass coating 12, the coated silica tube was mounted horizontally in a glass-working lathe, and the tube bore was progressively collapsed by a few passes of a flame along the exterior of the tube while the latter was rotated about its axis, the temperature of the tube being raised to 1800--1900"C, to produce a rod preform. The preform was subsequently drawn in known manner, to produce an optical fibre.

In the manufacture of a specific preform by the process of Example 1, the process was continued for 5.3 hours, in which time 424 layers of glass were deposited along 21 cm of the length of the silica tube bore, the total weight of deposited core material 12 being 5.6 grams. The deposited core material contained approximately 1 mol.% of phosphorus pentoxide: it was found that only a minor proportion of the phosphorus pentoxide formed in the reaction taking place in the heated region of the silica tube was incorporated in the glass deposit, a major proportion being distilled to the outlet end of the tube: this effect is believed to be due to the high vapour pressure of phosphorus pentoxide combined with the high drift velocity, caused by the low pressure, of the gases in the tube.

The preform obtained by collapsing the coated silica tube had an overall diameter of 10.3 mm, with a core diameter of 3.9 mm. The 21 cm length was drawn to give 1.6 Km of optical fibre of overall diameter 120 microns with a 45 microns diameter core.

EXAMPLE 2 The preform produced by the method of this example consisted of a core of silica doped with germania and phosphorus pentoxide, and silica cladding constituted by the silica substrate tube 1. The gas mixture employed was similar to that described in Example 1, with the addition of a stream of argon containing entrained germanium tetrachloride. The flow rates of the components of the gas mixture were 200 sccm for the total argon, 100 sccm for oxygen, 3.3 sccm for silicon tetrachloride, 0.1 75 sccm for germanium tetrachloride, and 1.0 sccm for phosphorus oxychloride. The temperature and pressure conditions were as described in Example 1.

The power input to the microwave cavity was varied between 60 watts and 240 watts, the duration of each power cycle being 45 seconds, but in this case the power increasing and decreasing parts of each cycle were of equal duration, so that two layers of the vitreous reaction product were deposited in the silica tube bore during each complete power cycle, these layers being thinner than the single layer formed in each cycle in the process of Example 1.

In a specific example of the procedure of Example 2, the glass coating 12 was formed along 32 cm of the length of the silica tube bore, and the total weight of core material deposited in 5.3 hours was approximately 3 grams, this material consisting of silica containing 5.3 mol.% of germania, representing 100% conversion of the germanium tetrachloride to oxide incorporated in the deposited glass, and approximately 1 mol.% of phosphorus pentoxide, the conversion rate for phosphorus oxychloride to deposited phosphorus pentoxide being low, as explained in Example 1. The coated silica tube was collapsed in the manner described in Example 1, to produce a rod preform of overall diameter 10.3 mm with a core diameter of 2.4 mm.

In view of the above-mentioned effect occurring with phosphorus pentoxide, germania is the preferred dopant for incorporation in silica (for increasing the refractive index) by the method of the invention, for obtaining an optical fibre waveguide having an acceptable numerical aperture. However, it is desirable to include a small proportion of phosphorus pentoxide in addition to the germania, to improve the flow of the deposited glass and thus produce a vitreous coating of good quality.

If desired, the processes described in the above examples can be modified by gradually increasing the rates of flow of the dopant halides, in known manner, starting with very low rates, to produce preforms having graded refractive index cores. Further if desired, the dopant halides may be initially excluded from the gas mixture introduced into the silica tube, or compounds producing different dopants may be used initially, so that the first few deposited layers of glass consist either of silica alone, or of silica with a different dopant or dopants from those incorporated in the core material, to constitute a cladding or buffer layer around the core of the optical waveguide.

The optimum diameter of the waveguide tube 8 for use in the processes of Examples 1 and 2, above, using a silica tube of 1 6.5 mm diameter bore, was determined by carrying out a series of experimental runs using waveguide tubes of different internal diameters ranging from 2.5 cm to 6.7 cm, and with a gas mixture consisting of argon and oxygen only flowing through the silica tube, under temperature and pressure conditions as described in Example 1 above, and with the power input to the microwave cavity varied between 50 watts and 300 watts. The results of these experiments are summarized in the following Table.

TABLE Internal Maximum diameter length of Argon Oxygen of wave- plasma flow rate flow rate guide tube column 200 sccm 10 sccm 2.5-5.5 cm 33 cm 200 sccm 10 sccm 6.1-6.7cm 65 cm 200 sccm 33 sccm 2.5-5.5 cm 23 cm 200 sccm 33 sccm 6.1-6.7 cm 45 cm It was apparent from these results that the progressive wave propagated along the coaxial waveguide changed from the TEM mode to the H10 mode at a waveguide tube internal diameter of 6.1 cm, and therefore a tube of this diameter was employed in the processes of the examples. An increase in the oxygen flow rate, and the introduction of the halide reactant vapours into the gas mixture, result in a reduction in the extension of the plasma co!umn, as indicated in the above examples.

We have also carried out similar experiments using silica tubes of internal diameter 8 mm and 12 mm respectively, under the same conditions as the above experiments and omitting silicon tetrachloride and phosphorus or germanium chloride from the gas mixture. We have found that the minimum waveguide tube internal diameter for propagation of a wave in the H10 mode wave 7.0 cm with the 8 mm bore silica tube and 6.6 cm with the 12 mm bore silica tube, these figures being approximately consistent with equation (1) above. The maximum length of the plasma column is substantially independent of the diameter of the silica tube bore, with the waveguide propagating in the Hlo mode.

The process of the present invention is advantageous as compared with previously proposed processes for manufacturing optical fibre preforms, in which sweeping of a plasma along the substrate tube is achieved by relative longitudinal movement betwen the tube and the plasma-exciting device, in that the present technique of generating the plasma from a stationary device and varying the length of the plasma column by varying the power input to the device enables the plasma to be swept along the tube much more rapidly. Hence the number of glass layers deposited in the substrate tube in a given time, and with a given gas flow rate, can be greatly increased, so that for the production of a graded index preform it is possible to increase the concentration of the dopant compound in the gas mixture continuously, rather than step by step, and thus the dopant concentration in the deposited glass coating can be increased almost continuously, i.e. by very small increments, from layer to layer. The core of an optical fibre obtained from the preform so produced will thus have a refractive index profile in the form of a substantially smooth curve.

Claims

1. A method of manufacturing a glass optical fibre preform which includes the step of forming a vitreous coating composed essentially of one or more oxides on the interior surface of a glass substrate tube, by causing a chemical reaction to take place in a gaseous mixture within the tube, wherein a gaseous mixture consisting of oxygen, the vapour or vapours of one or more compounds capabie of reacting with oxygen to produce the desired vitreous coating material, and an ionisable carrier gas, is passed continuously through the glass tube while a plasma column for activating the reaction is produced within the tube by means of a plasma-exciting device maintained in a stationary position with respect to the tube, around at least a portion of the tube adjacent to the gas exit end thereof, heat is applied to the exterior of the region of the tube length in which the formation of said coating is required, to maintain the tube wall at a sufficiently high temperature to ensure vitrification of the coating without causing distortion of the tube, the gas pressure within the tube is maintained not greater than 10 Torr, and the termination of the plasma column is swept along the said heated region of the tube by effecting continuous and progressive variation of the electromagnetic power input to the plasma-exciting device, whereby a continuous layer of vitreous material resulting from the plasma-activated reaction of the gas mixture is formed on the interior surface of the tube progressively from one end to the other of the said heated region.

2. A method according to Claim 1, wherein a multi-layer coating is formed by continuously varying - the power input to the plasma-exciting device between minimum and maximum levels, to cause the plasma column termination to be swept backwards and forwards along the glass tube.

3. A method according to Claim 1 or 2, wherein the substrate tube is formed of vitreous silica, and the gaseous mixture consists of oxygen, a silicon tetrahalide and, for at least part of the process, a halide or halides from which the required dopant or dopants is or are derived, the halide or halides being entrained in streams of the carrier gas.

4. A method according to Claim 1,2 or 3, wherein the plasma-exciting device employed comprises a microwave cavity supplied with power from a microwave generator, the gas exit end portion of the glass substrate tube being inserted within the cavity.

5. A method according to Claim 4, wherein the microwave cavity is of the asymmetrical type and is so designed and located that the extension of the plasma column, from the portion of the glass tube within the cavity, as the power input to the cavity is increased in operation, will take place only in the direction along the glass tube towards the gas entry end thereof.

6. A method according to Claim 5, wherein the glass tube is surrounded by an electrically conductive tube extending from the microwave cavity substantially to the gas inlet end of the glass tube, such that a coaxial waveguide is formed by the said conductive tube and the plasma column.

7. A method according to Claim 4, 5 or 6, wherein the power input to the microwave cavity is varied over a range within 50 to 1000 watts.

8. A method according to Claim 3, wherein the region of the silica tube swept by the plasma column termination is heated to a temperature of approximately 100000.

9. A method according to any preceding Claim, wherein the ionisable carrier gas consists of argon, krypton or xenon.

10. A method according to Claim 8 wherein the gaseous mixture consists of argon, oxygen and silicon tetrachloride, with or without one or more dopant-producing halides, and the ratio of the flow rates of argon :oxygen :silicon tetrachloride through the glass tube is 20:10:1 or less than 1.

11. A method according to any preceding Claim, wherein the glass substrate tube is disposed vertically.

12. Apparatus for carrying out the method according to Claim 6, which includes an asymmetrical microwave cavity, formed of two concentric circular cylinders, the inner cylinder being located around a portion of the glass substrate tube adjacent to the gas exit end thereof, the outer cylinder extending further along the glass tube than the inner cylinder, and the cavity being so designed that, in operation, a region of high electrical field strength is produced only at the end of the inner cylinder remote from the gas exit end of the glass tube, a circular cylindrical electrically conductive tube positioned concentrically around the glass tube, abutting the microwave cavity and extending substantially to the gas inlet end of the glass tube, the arrangement of said conductive tube and microwave cavity being such that the combination of the conductive tube and the plasma column produced in the glass tube in operation constitutes a coaxial waveguide along which a progressive electromagnetic wave is launched, and heating means located outside the said conductive tube.

13. Apparatus according to Claim 12, wherein the dimensions of the microwave cavity are such that the cavity will support an electromagnetic wave in the transverse electromagnetic mode, and the length of the inner cylinder of the cavity is adjusted so that the power reflected at the termination of the inner cylinder within the outer cylinder sets up a standing wave pattern within the cavity, with the maximum voltage at the level of the said inner cylinder termination.

14. Apparatus according to Claim 12 or 13, wherein the internal diameter of the said conductive tube is such, in relation to the diameter of the plasma column produced within the glass tube, that the wave propagated along the said waveguide in operation is in the H1o coaxial mode.

1 5. Apparatus for carrying out the method according to Claim 1, substantially as shown in, and as hereinbefore described with reference to, the accompanying drawing.

1 6. A method according to Claim 1, for the manufacture of a silica-based optical fibre preform, substantially as hereinbefore described in Example 1, or Example 2, with reference to the accompanying drawing.

17. An optical fibre preform manufactured by a method according to any of the preceding Claims 1 to 11,and 16.

18. An optical fibre waveguide produced by drawing a preform according to Claim 17.