GB2079267A - Manufacture of Optical Fibre Preforms - Google Patents

Abstract

In the manufacture of an optical fibre preform by forming a multilayer vitreous coating composed of one or more oxides, in the bore of a glass 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 plasma flashes occupying the whole of the region of the tube length in which deposition of the coating is required. The plasma flashes are produced by the application of electrical or electromagnetic power pulses of high field strength to the said region of the tube, the tube being substantially filled with unreacted gases, at a constant low pressure, before the application of each pulse. Several methods of producing the plasma flashes, and of effecting the required repeated fillings of the tube in synchronism with the flashes, are described. Fig. 1 shows silica tube 1, microwave cavity 2, pulse generator 3, and tubular electric furnace 4. In another embodiment (Fig. 3), tube 1 contains a tapered silica rod extending coaxially along the length of the deposition region. <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 suitable compounds, such as halides, while the mixture is passed through a glass substrate tube, which reaction results in the formation of a glassy coating composed of one or more oxides on the interior surface of the tube, this coating constituting at least the core of the optical fibre. The invention also relates tp apparatus for use in carrying out the method described.

Usually the glass tube is composed of vitreous silica, and the deposited material consists of silica with one or more dopant oxides for modifying the refractive index of the silica, in at least part of the thickness of the coating, so as to impart a desired refractive index profile to the optical fibre.

It has been proposed to promote the oxideforming reaction in the substrate tube by means of a plasma produced in a gaseous mixture flowing continuously through the tube, deposition of the oxide coating being caused to take place progressively along the tube bore by effecting relative longitudinal movement between the tube and the plasma-exciting device.In another method of effecting plasma activation of a said reaction in a flowing gas stream which method is described in the specification of our co-pending Patent Application No. 8102582, the plasma exciting device is maintained in a stationary position around a portion of the glass tube adjacent to the gas exit end thereof, and the electrical power input to the said device is continuously and progressively varied to cause the termination of the plasma column to be swept along the tube, again resulting in progressive deposition being achievedby variation of the power input to cause alternate extension and contraction of the plasma column.

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

According to the invention, in a method of manufacturing an optical fibre preform which includes the step of forming a multilayer vitreous coating composed essentially of one or more oxides on the interior surface of a glass substrate tube, by introducing into the said tube a gaseous mixture consisting of oxygen, the vapour or vapours of one or more compounds each capable of reacting with oxygen to form an oxide, and an ionisable carrier gas. and producing a plasma in the gaseous mixture within the tube to cause an oxide-forming chemical reaction to take place, a plasma column occupying the whole of the region of the tube length in which deposition of the said coating is required is produced in repeated flashes by the application of electrical or electromagnetic power pulses of high field strength to the said region of the tube length, while heat is applied to the exterior of the said region of the tube length, the tube being substantially filled with the unreacted gaseous mixture, at a constant pressure of less than 20 Torr, before the application of each pulse.

Each power pulse produces a plasma flash resulting in the formation of a continuous layer of vitreous oxide material of uniform composition over the whole of the tube bore in the region occupied by the plasma column, without the preliminary formation of particulate material.

In a preferred method of producing the plasma flashes, employing electromagnetic pulses, the whole region of the tube length in which deposition of the oxide coating is required is enclosed in a microwave cavity to which high power pulses are applied from a microwave generator. The cavity is preferably so designed that the electromagnetic field pattern produced therein in operation will include one or more standing waves, at intervals of half the cavity wavelength along the length of the cavity, in order to increase the field strength obtainable for a given magnitude of power input.

Alternatively, plasma flashes can be generated by electrical pulses of high field strength. In one method of producing such pulses, an electrode is inserted into each end of the substrate tube, the electrodes being coated with a chemically inert refractory material such as silica, to prevent the introduction of metallic impurities into the glass of the preform, a unidirectional voltage, pulsed at the required frequency, is applied between the electrodes, and the initial ionisation of the gas required to produce a plasma flash with each pulse is effected by suitable auxiliary means such as, for example, a laser or a Tesla coil located outside the substrate tube, suitably at a point mid-way between the electrodes.

In one method of carrying out the invention, the glass tube is filled with the required gaseous mixture at a suitable pressure and then closed, a single power pulse is applied to the closed tube to produce a plasma flash therein, then the tube is re-opened and exhausted, this cycle of operations being repeated for the deposition of the required number of vitreous layers in the tube bore. For this procedure, the glass tube is fitted at one or both ends with valves or stopcocks for enabiing the tube to be filled with gas, closed, and exhausted.The gaseous mixture is of uniform composition and pressure throughout the tube, giving a deposited layer of uniform composition and thickness, and the gas pressure and concentration of the reactants in the gas mixture are maintained substantially constant for each filling of the tube, that is to say constant except for possible variations in the small concentration of a dopant-producing compound, so that all the deposited layers are of substantially equal thickness.

In another method of carrying out the invention, the gaseous mixture is caused to flow continuously through the substrate tube while the power pulses are applied, the pulse repetition frequency being so adjusted, in relation to the rate of flow of the gas mixture, that the deposition region of the tube is filled with a fresh supply of unreacted gases before the application of each pulse, This method, while permitting more rapid operation and easier controi than the static gas filling method described above, has the disadvantage that it is liable to result in a greater deposition rate near the gas entry end than the exit end of the tube. This effect can be minimised by using a substrate tube of wide bore, and can also be countered by periodically reversing the direction of gas flow through the tube.

Alternatively, substantially uniform thickness of the deposit along the length of the tube bore, in the flowing gas method, can be promoted by inserting into the tube a tapered glass rod which extends coaxially along the length of the deposition region of the tube and the circumference of which is greatest at the end nearest to the gas entry end of the tube and progressively decreases to a minimum at the end nearest to the gas exit end of the tube. Part of the vitreous reaction product will then be deposited upon the rod, the dimensions of which are so adjusted that the ratio of the total surface area of glass available for deposition in the cross-section of the tube-rod assembly, to the partial pressure of the reactant gases, is constant along the whole length of the deposition region.Thus deposition on the rod will be greatest at the gas entry end and least at the gas exit end, so that deposition on the tube wall will be of substantially uniform thickness along the deposition region.

In another method of carrying out the invention, which is intermediate between the static gas filling method and the continuous gas flow method described above, the gas mixture is allowed to flow continuously into the substrate tube, but its egress from the tube is interrupted intermittently for the production of each plasma flash.Thus a valve or other closure means is fitted to the gas exit end of the tube, the gas mixture is caused to flow through the tube at a suitable low pressure, for example approximately 1 Torr, and a cycle of operations is carried out which consists in closing the valve, allowing a short time to elapse to permit the pressure to become uniform throughout the tube but not to be increased to an undesirably high level, then applying a power pulse to produce a plasma flash, then opening the valve to allow the residual gases from the reaction to be replaced by a fresh supply of incoming gas, the removal of the residue being assisted by pumping if necessary, to ensure that there is no mixing of the residue and fresh gas and that the pressure is restored to its initial low level.The valve is reclosed when all the residue has been removed, and the cycle is repeated until the required thickness of glass has been deposited in the tube. This method results in uniform deposition along the region of the tube occupied by the plasma column.

It will be appreciated that the thickness of the deposited layer of glass resulting from each plasma flash, in each of the methods described above, will be dependent upon the concentration of the reactants in the tube, and hence upon the gas pressure within the tube: the lower the partial pressure of the reactant gases, the thinner will be each deposited layer, and hence the larger the number of layers required to produce a desired coating thickness, and the longer the time required to complete the deposition process.

However, it is desirable that the gas pressure should be as low as possible, since with increased pressure, increased power is required to strike the plasma, and low pressure assists in ensuring that the deposited material is in the vitreous, nonparticulate state. In practice, therefore, the gas pressure is chosen to provide a compromise between the requirements for a long plasma column and non-particulate deposition and for an acceptable thickness of each deposited layer.

The glass substrate tube is heated to a sufficiently high temperature to ensure that any dissolved gases in the deposited glass are expelled, and to render the deposited glass sufficiently fluid to build up a crack-free coating.

The heating is conveniently effected by means of a tubular electric furnace placed around the microwave cavity in which the glass tube is enclosed, (or around the tube itself if a cavity is not used) and co-extensive in length with the deposition region of the tube. When a silica substrate tube is employed, and the deposited material is composed of silica with one or more dopant oxides, the furnace is operated at such a temperature that the interior surface of the silica tube will be maintained at about 1 0000C.

After completion of the deposition process, the tube bore is collapsed in known manner to form a rod preform, which is then drawn to fibre by a conventional process.

Some specific methods of manufacturing an optical fibre preform, in accordnace with the invention, will now be described by way of example, with reference to the accompanying diagrammatic drawings, in which Figure 1 shows, in part-sectional elevation, apparatus employed for carrying out the static gas filling method, Figure 2 shows, in part-sectional elevation, one form of apparatus employed for carrying out the flowing gas filling method, and Figure 3 shows, in sectional elevation, another form of apparatus employed for the flowing gas method.

Like parts in the different figures of the drawings are indicated by the same reference numerals.

Referring to Figure 1 of the drawings, a silica substrate tube 1 is supported vertically and is surrounded, over nearly the whole of its length, by an elongated microwave cavity 2, which is connected to a microwave pulse generator 3, and the cavity is surrounded by a tubular electric furnace 4. The silica tube is closed at the lower end 5, and the upper end 6 is fitted with a head 7 into which pass gas inlet and outlet tubes, 8 and 9 respectively, provided with stopcocks 10, 11.

The inlet tube 8 is connected to means (not shown) for supplying a gaseous reaction mixture consisting of oxygen, silicon tetrachloride, with or without one or more dopant halides, and an ionisable carrier gas, thereto, and the outlet tube 9 is connected to a vacuum pump (not shown).

Both ends of the microwave cavity 2 are closed by metal discs or plates, 12, 13, having central apertures in which the tube 1 fits closely: these closures produce reflections of the electromagnetic waves propagated in the cavity in operation, to set up a standing wave pattern in the cavity.

In operation of the apparatus shown in Figure 1, the silica tube 1 is evacuated and then heated to 1 0000C, and the stopcock 10 is opened to admit the required gas mixture into the tube 1 until the tube is filled to the desired pressure, then the stopcock is closed. A single high power microwave pulse is applied to the cavity 2, causing a plasma to strike in the gas filling and to flash along substantially the whole of the length of the tube 1 within the cavity, as shown at 14.

The plasma flash causes a reaction to take place in the gas mixture, which results in the formation of a thin film of glass composed of silica, with or without a dopant oxide or oxides, over the whole of the interior of the silica tube along the length thereof occupied by the plasma. The stopcock 11 is then opened and the residual gas mixture and gaseous reaction products are withdrawn from the tube 1. The cycle of operations comprising filling the silica tube, applying a microwave pulse, and exhausting the silica tube, is repeated, while the tube is maintained at 1 0000C, until the number of layers required to give a desired total weight of vitreous oxide or oxides has been deposited in the tube bore.

In a specific example of the method, using the apparatus described above with reference to Figure 1, the silica tube 1 has a bore diameter of 16 mm and a wall thickness of 1 mm, and the microwave cavity 2 and furnace 4 are approximately one metre in length. The output power of the microwave generator is suitably 10 kilowatts at a frequency of 2.45 GHz.For the deposition of silica doped with phosphorus pentoxide in the bore of the tube 1, the gaseous mixture employed is composed of four gas streams respectively consisting of argon, oxygen, argon carrying silicon tetrachloride vapour, and argon carrying phosphorus oxychloride vapour, the chloride vapours each being entrained in argon by bubbling the latter through the respective liquids, and the four gas streams are adjusted to give pressures, in each filling of the tube 1, of 4 Torr of argon 5 Torr of oxygen, 1 Torr of silicon tetrachloride, to provide suitable relative proportions of these constituents, and a very small pressure of phosphorus oxychloride which is increased slightly for the deposition of each successive layer of glass, to give a deposit of graded composition.Each cycle of filling the silica tube, producing the plasma flash, and evacuating the tube can be carried out in 5-6 seconds.

For the production of a preform from which can be drawn a 5 kilometre length of optical fibre of 120 microns overall diameter, including a 50 microns diameter core composed of phosphorusdoped silica and having a graded refractive index profile, 22 grams of vitreous silica, together with phosphorus pentoxide in a concentration ranging from less than 0.5 mol.% in the first deposited layer to 1 5 mol.% in the final layer, will be required to be deposited in the bore of a silica tube of the dimensions specified above: the silica substrate tube will constitute a cladding or support layer surrounding the doped core of the fibre.At a temperature of 1 0000C and a total gas pressure of 10 Torr together with the small pressure of phosphorus oxychloride 2.9x 104 litres of the gaseous mixture specified will be required to produce the above amount of deposited glass. Since the volume of the bore of a one metre length of 1 6 mm diameter silica tube is 0.2 litre, the number of fillings of the tube, that is to say the number of operating cycles of the deposition process, required will be 1 .45x 105, forming the same number of deposited layers having an average thickness of 1 .4x 10-3 microns.

For the production of preforms for shorter lengths of optical fibre, substrate tubes of shorter length may be used, the procedure in other respects being similar to that described above.

The apparatus shown in Figure 2 includes a substrate tube 1, microwave cavity 2, and tubular furnace 4, similar to those described with reference to Figure 1, but the tube 1 is open at both ends and T-tubes 15, 16, and 17, 18, fitted with stopcocks 19, 20, 21, 22, are attached to the respective ends of the tube 1. In operation, the gaseous mixture of reactants and ionisable carrier gas is caused to flow continuously through the silica tube while the tube is maintained at 1 0000C and microwave pulses are applied to the microwave cavity to produce plasma flashes 14, the direction of flow of the gas mixture through the tube 1 being reversed intermittently by operation of the stopcocks. Thus the T-tube portions 1 5 and 17 are each connected to a sourcs of supply of the gases, and the portions 16 and 18 are each connected to a vacuum pump.

The apparatus may be initially operated, for example, with stopcocks 19 and 22 open and stopcocks 20 and 21 closed, so that the gases flow down the tube 1, and after a predetermined number of microwave pulses have been applied the stopcocks 1 9 and 22 are closed and stopcocks 20 and 21 are opened to permit the gases to flow up the tube, for the duration of a similar number of pulses, this procedure being repeated until the required number of vitreous oxide layers have been deposited in the bore of tube 1. The repetition frequency of the pulses is so adjusted as to ensure that there is sufficient time for the tube 1 to be filled with a fresh supply of unreacted gas mixture between successive pulses.For example, with a gas flow rate of 500 standard cubic centimetres per minute through a 1 6 mm bore tube one metre long, at a pressure of approximately 1 Torr, the gas filling of the tube is changed approximately 31 times per second, so that a suitable pulse rate will be 20 to 25 pulses per second.

In the arrangement shown in Figure 3, which is drawn on a larger scale than Figures 1 and 2, a tapered silica rod 23 is disposed coaxially in the bore of the silica tube 1, the wider end of the rod being located at the upper, gas entry, end of the tube and the narrower end of the rod terminating near to the lower, gas exit, end of the tube. In operation, the mixture of gaseous reactants and ionisable carrier gas, in the relative proportions specified above with reference to Figure 1, is caused to flow continuously through the silica tube, the total gas pressure being maintained at 10 Torr by means of a vacuum pump connected to the lower end of the tube, while the tube is maintained at 1 0000C and microwave pulses are applied to the cavity 2. The pulse repetition rate is suitably adjusted in relation to the rate of flow of the gas mixture through the silica tube, in the manner described with reference to Figure 2. By virtue of the presence of the rod 23, as explained above, each vitreous oxide layer deposited on the interior surface of the silica tube is of substantially uniform thickness throughout the length of the deposition region of the bore within the microwave cavity.

Claims (12)

Claims

1. A method of manufacturing an optical fibre preform which includes the step of forming a multilayer vitreous coating composed essentially of one or more oxides on the interior surface of a glass substrate tube, by introducing into the said tube a gaseous mixture consisting of oxygen, the vapour or vapours of one or more compounds each capable of reacting with oxygen to form an oxide, and an ionisable carrier gas, and producing a plasma in the gaseous mixture within the tube to cause an oxide-forming chemical reaction to take place, wherein a plasma column occupying the whole of the region of the tube length in which deposition of the said coating is required is produced in repeated flashes by the application of electrical or electromagnetic power pulses of high field strength to the said region of the tube length, while heat is applied to the exterior of the said region of the tube length, the tube being substantially filled with the unreacted gaseous mixture, at a constant pressure of less than 20 Torr, before the application of each pulse.

2. A method according to Claim 1 wherein, for the production of the plasma flashes by the application of electromagnetic pulses, the whole region of the tube length in which deposition of the said coating is required is enclosed in a microwave cavity to which power pulses are applied from a microwave generator.

3. A method according to Claim 2, wherein the microwave cavity is so designed that the electromagnetic field pattern produced therein in operation will include one or more standing waves, at intervals of half the cavity wavelength along the length of the cavity.

4. A method according to Claim 1 wherein, for the production of the plasma flashes by the application of electrical pulses, an electrode coated with chemically inert refractory material is inserted into each end of the substrate tube, and a pulsed unidirectional voltage is applied between the electrodes, the initial ionisation of the gas required to produce a plasma flash with each pulse being effected by auxiliary means located outside the substrate tube.

5. A method according to any preceding Claim, wherein the substrate tube is filled with the required gaseous mixture at a pressure below 20 Torr and then closed, a single power pulse is applied to the closed tube to produce a plasma flash therein, then the tube is reopened and exhausted, and these operations are repeated for the depositioon of each of the required number of vitreous layers in the tube bore, the gas pressure and concentration of the reactants in the gas mixture being maintained substantially constant for each filling of the tube.

6. A method according to any of the preceding Claims 1 to 4, wherein the gaseous mixture is caused to flow continuously through the substrate tube while the power pulses are applied, the pulse repetition frequency being so adjusted, in relation to the rate of flow of the gaseous mixture, that the deposition region of the tube is filled with a fresh supply of unreacted gases before the application of each pulse.

7. A method according to Claim 6, wherein the direction of flow of the gaseous mixture through the substrate tube is periodically reversed.

8. A method according to Claim 6, wherein a tapered glass rod is inserted into the substrate tube so as to extend coaxially along the length of the deposition region of the tube, the circumference of the rod being greatest at the end nearest to the gas entry end of the tube and progressively decreasing to a minimum at the end nearest to the gas exit end of the tube, and the dimensions of the rod being so adjusted that the ratio of the total surface area of glass available for deposition in the cross-section of the tube-rod assembly, to the partial pressure of the reactant gases, is constant along the whole length of the deposition region of said assembly.

9. A method according to any of the preceding Claims 1 to 4, wherein the gaseous mixture is caused to flow continuously into the substrate tube, and its egress from the tube is interrupted intermittently while each power pulse is applied.

10. A method according to any preceding Claim, wherein the substrate tube is heated by means of a tubular electric furnace which is coextensive in length with the deposition region of the substrate tube, and which is located around the said tube or around a microwave cavity in which the tube is enclosed.

11. Apparatus for the manufacture of an optical fibre preform by the method according to Claims 2 and 5, substantially as shown in, and as hereinbefore described with reference to, Figure 1 of the accompanying drawings.

12. Apparatus for the manufacture of an optical fibre preform by the method according to Claims 2, 6 and 7, substantially as shown in, and as hereinbefore described with reference to, Figure 2 of the accompanying drawings.

1 3. Apparatus for the manufacture of an optical fibre preform by the method according to Claims 2 and 8, substantially as shown in, and as hereinbefore described with reference to, Figure 3 of the accompanying drawings.

1 4. A method of manufacturing an optical fibre preform, carried out substantially as hereinbefore described by way of example with reference to Figure 1 or Figure 2 or Figure 3 of the accompanying drawings.

1 5. An optical fibre preform manufactured by a method according to any of the preceding Claims 1 to 10 and 14.

1 6. An optical fibre produced by drawing an optical fibre preform according to Claim 1 5.