GB1558672A

GB1558672A - Manufacture of optical fibre waveguides

Info

Publication number: GB1558672A
Application number: GB9761/76A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1976-12-01
Filing date: 1976-12-01
Publication date: 1980-01-09

Description

(54) IMPROVEMENTS IN OR RELATING TO THE MANUFACTURE OF OPTICAL FIBRE WAVEGUIDES (71) We, THE GENERAL ELECTRIC COMPANY LIMITED, of 1 Stanhope Gate, London W1A lEH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention relates to a method of manufacturing optical fibre waveguides of the kind comprising a core of vitreous material, that is to say of glass or vitreous silica, and a cladding layer, in intimate contact with the core, composed of a material having a lower refractive index than that of the core material and which is substantially transparent to the radiation to be propagated along the waveguide in use, such that electromagnetic radiation in the visible and infra-red regions of the spectrum, incident on one end of the waveguide, can be propagated along the waveguide and thus employed for the transmission of communications signals.

Both the core and cladding of optical fibre waveguides of the kind referred to have hitherto usually been formed of vitreous material the required difference between the refractive indices of the core and cladding being achieved by appropriate adjustment of the compositions of these components, suitable dopants being incorporated in one or both of the components.

Optical fibres of this form can be manufactured by drawing preforms produced by chemical vapour deposition techniques giving either a step refractive index profile in a graded refraction index profile the core diameter usually being from 50 to 100 micrometres. Pure vitreous silica is a preferred material for optical fibre cores, since it gives low attenuation of radiation along the waveguides and hence low optical loss, in operation.However, since silica has a low refractive index, the possibilities of providing a doped silica cladding material which has a lower refractive index than the pure silica core are limited, and furthermore since the difference in refractive indext between such a cladding material and pure silica is usually small, the resultant waveguides usually have a low radiation acceptance angle, the numerical aperture of such a waveguide typically being from 0.1 to 0.2.

It is an object of the present invention to provide a method of manufacturing an optical fibre waveguide of the kind referred to which can have a relatively large numerical aperture, a particular object being the manufacture of a waveguide comprising a core of pure silica and having a numerical aperture exceeding 0.2.

According to the invention, a method of manufacturing an optical fibre waveguide consisting of a core of vitreous material and, in intimate contact with the core, a cladding layer at least ten micrometres thick composed of a polymeric plastics material whose refractive index is lower than that of the core material and which is substantially transparent to the radiation to be propagated along the waveguide in use, includes the steps of drawing a fibre from a rod of vitreous material, to constitute the waveguide core, passing the drawn fibre, before it has come into contact with any solid surface, through a liquid composition consisting of a said polymeric plastics material in solution or in suspension in, pr in admixture with, a liquid medium, ånd ,having a viscosity in the range of 0.5 to 50 poises so that the fibre is coated with said liquid composition, and passing the coated fibre through means for adjusting the thickness of the coating and then through heating means to effect removal of any volatile liquid medium present and curing or hardening of the plastics material to form said cladding layer.

The drawn fibre constituting the vitreous core of the waveguide is preferably from 100 to 200 micrometres in diameter.

The rod from which the fibre is drawn will usually be of homogeneous composition, preferably being of pure silica, although if desired a dopant can be employed to give the core a graded refractive index profile.

A minimum thickness of ten micrometres for the plastics cladding is required to ensure that the radiation is confined within the waveguide, so that optical loss due to passage of radiation through the cladding is avoided; preferably the cladding thickness is greater than 10 micrometres, for example at least 15 micrometres. It is also essential that the cladding material is substantially transparent to the radiation concerned, in order to minimise optical loss due to absorption or scattering in the cladding.

A number of polymeric materials are available which have lower refractive indices than those of vitreous materials hitherto employed for the cladding of optical fibres, and in particular having refractive indices appreciably lower than that of pure vitreous silica, which is 1.452 for radiation of wavelength 900nm. Such polymers are thus advantageous for use as cladding materials, since they make possible the provision of optical fibre waveguides with a relatively large refractive index difference between the core and the cladding, and hence with a relatively large numerical aperture of the order of 0.3 to 0.4.An increased numerical aperture is advantageous in that it renders less critical the effect of any minor distortions in the vitreous fibre in giving rise to optical losses, known as microbanding losses and also in that it increases the radiation launching efficiency. In the case of an optical fibre waveguide manufactured in accordance with the invention having a core of pure vitreous silica, the polymer chosen for use as the cladding material is preferably one which has a refractive index less than 1.44 in respect of the radiation to be propagated along the waveguide.

The cladding material must be capable of making continuous, intimate contact with the surface of the vitreous fibre core, and preferably is capable of bonding to the core material. Any difference between the thermal expansion coefficient of the cladding material and that of the vitreous core is immaterial, since it will be compensated by the elasticity of the polymer.

The preferred materials for use as the cladding of waveguides in the method of the invention are silicone resins, since they readily wet core fibres of silica or high silica content glass, possibly with some bonding, and also since the attenuation losses of these resins are somewhat lower than those in some other types of polymers which in other respects are suitable cladding materials. One class of resins which for use as cladding on a pure silica fibre core consists of cross-linked polysiloxanes, for example cross-linked polymers and copolymers of dimethyl siloxane, methyl vinyl siloxane, and methyl allyl siloxane, which have refractive indices in the range of 1.40 to 1.43.

Some fluorocarbon polymers and copolymers have sufficiently low refractive indices to render them suitable for use as cladding on silica fibre cores, but some of these materials form opaque or crystalline layers after heat treatment, so would give rise to an undesirable degree of absorption or scattering of radiation and are therefore excluded from use in the waveguides prepared by the method of the invention. However some fluorocarbons can form transparent layers and have refractive indices of suitable values, so can be used as cladding: compounds in this category include polyvinylidene fluoride, polychlorotrifiuoroethylene, and a fluorinated ethylene-propylene copolymer.

Other polmeric materials which are suitable for use as cladding materials include methyl cellulose and polyformaldehyde.

The attenuation losses of the abovementioned polymeric materials are considerably higher than those of the vitreous materials hitherto employed for both the core and cladding of optical fibre wave guides: for example the attenuation of silicone resins is of the order of 105dB/km and that of fluorocarbon resins is of the order of 106dB/km whereas attenuation values as low as 2 to 5 dB/km can be obtained in optical fibre waveguides composed of high purity silica with one or more dopants. However with a relatively thick resin or other polymer cladding on a pure silica core of diameter 100 to 200 micrometres, the attenuation of a waveguide in accordance with the invention can be reduced to an acceptable value, for example to about 20 dB/km with a silicone resin cladding 20 micrometres thick.

Waveguides of this form are especially suitable for use in short and medium distance communication links involving low or medium information rates.

It is essential that the liquid composition containing the polymer is applied to the vitreous fibre core as soon as possible after drawing of the latter, and without allowing the fibre to come into contact with any solid surfaces, in order to ensure that the fibre is free from surface damage and therefore does not suffer any deterioration in tensile strength, and to prevent contamination of the interface between the fibre and the cladding with foreign matter. The coating is therefore applied in-line during the fibre drawing operation.

The process is preferably carried out by drawing the fibre downwardly from a furnace into the top of which the vitreous rod is fed, and passing the fibre, which solidifies immediately on emerging from the furnace, directly through a coating bath or around guide means to which the liquid coating composition is fed, and thence through coating thickness adjusting means and through an elongated tubular oven maintained at a suitable temperature for curing or hardening the coating, the rate of drawing and subsequent travel of the fibre through the whole coating and curing system being maintained constant.

Most of the polymeric materials which are suitable for forming the cladding, for example silicons and some fluorocarbons, can be applied to the vitreous core in solution in a suitable organic solvent, which is subsequently evaporated to leave a continuous layer of polymer on the fibre surface. Alternatively, silicone resins may be mixed with silicone oils, the latter functioning as diluents for forming liquid compositions of suitable viscosity, and also being capable of being cured so as to be incorporated in the silicone cladding layer.

In the cases of some polymers which are not soluble in any available solvents, for example Dolychlorotrifluoroethylene and fluorinated ethylene-propylene copolymer, the polymer can be applied as a powder suspension in water, and on subsequent heating the water is driven off and the polymer particles fuse and coalesce, or sinter, to form a continuous layer. The liquid composition may contain any hardener, cross-linking agent or polymerisation catalyst which may be required to effect curing of the polymer. The concentration of the polymer in the liquid medium is adjusted to give a suitable viscosity, within the range specified above, for forming a coating of the desired thickness.

The means for adjusting the thickness of the liquid composition coating, through which the coated fibre is passed before it enters the curing oven, is designed to remove excess coating material so as to leave a coating of the thickness required to give a final polymer cladding layer of the desired thickness after curing. Since the polymer cladding layer form part of the optical waveguide structure, it is desirable that the cladding, and therefore the liquid coating from which it is formed, should be of uniform thickness and concentric with the vitreous fibre core, and that the cross-section of both the liquid-coated fibre and the completed waveguide after curing should be circular.A preferred form of coating adjustment means, for obtaining the desired concentricity of the coating and core and circularity of cross-section of the coated fibre, is an elongated die formed with a capillary tube, suitably of glass. through which the coated fibre is passed; the capillary is suitably 20 to 30 millimetres long, and has a diameter slightly larger than the required diameter of the coated fibre after curing. A mathematical analysis of the flow conditions in such a die for a liquid flowing at a constant rate, has shown that the capillary diameter controls the cladding thickness, although the actual diameter of the coated fibre after curing is smaller than the diameter of the capillary itself, because the relevant controlling parameter is mass flow of the solvent and resin through the capillary.

The above-mentioned analysis has also shown that when a capillary die is used in the manner described, the rate of travel of the fibre and the viscosity of the coating liquid do not appreciably effect the final thickness of the coating. However, a relatively high rate of drawing of the fibre and passage thereof through the coating means, the die, and the curing oven, for example a rate of the order of one metre per second, is preferred, since this facilitates the drawing of fibre of constant diameter, and also facilitates the production of a good quality, smooth cladding layer free from irregularities. The curing oven must, of course, be of sufficient length, in relation to the rate of travel of the coated fibre therethrough, to ensure that complete curing of the cladding layer is achieved.

The above-described process for manufacturing an optical fibre waveguide consisting of a vitreous core with a polymeric plastic cladding is simpler, involving fewer steps, than the known processes for the production of optical fibre waveguides composed wholly of vitreous material with one or more dopants distributed in different proportions through the cross-section of the fibre, to give either a step refractive index profile, as in the wave guides of the present invention, or a graded index profile.The invention thus provides a relatively easy and economical method of manufacturing optical fibre waveguides of the step index profile type which, although subject to higher attenuation than that desirable in the highest quality waveguides required to be used over long distances, are, as indicated above, acceptable for use over relatively short distances and for relatively low signal rates.

The polymer cladding has the additional advantage that it provides protection for the vitreous fibre core, so that the application of a protective synthetic resin coating, which. is usual in the case of conventional, wholly vitreous, optical fibre waveguides, is not always essential for the waveguides of the present invention. However, if desired, the waveguides of the invention can be provided with an additional protective coating in known manner; in particular, if the cladding is formed of a reiativ ly soft material such as a silicone resin, a protective coat of a harder resin will be required.

A specific method of manufacturing an optical fibre waveguide in accordance with the invention, will now be described by way of example, with reference to the accompanying drawing which shows, in sectioned elevation, the means employed for adjusting the thickness of the polymercontaining coating on the vitreous fibre core of the waveguide.

The waveguide produced by the method of the example consists of a pure silica fibre core of diameter 150 micrometres, and a cladding layer 15 micrometres thick composed of a cross-linked dimethyl siloxane polymer, which is available commercially under the Registered Trade Mark "Sylgard", and which has a refractive index of 1.41.

For the manufacture of this waveguide, a pure silica rod of diameter 8 mm is fed into the top of a vertically disposed furnace comprising a tubular graphite heating element, maintained at a temperature of 2000"C, and a fibre is drawn from the rod vertically downwards and is passed directly to a guide pulley which has a peripheral channel to which is fed a solution of the silicon resin with hardener, in a solvent consisting of l,l,l,-trichloro- ethane. The silica fibre is passed around the pulley, in the channel. so that it becomes coated with the silicone solution.

The coated fibre then passes through a glass capillary die (as shown in the drawing, and to be described below) for adjusting the thickness of the coating, and then through a tubular oven 1.5 metres long, which is inclined upwards at an angle of 15C in the direction of travel of the fibre, and is maintained at a temperature of 3500C to 5000C; the rate of travel of the fibre through the system is one metre per second.

Referring now to the drawing. the die through which the coated fibre is passed for adjustment of the coating thickness consists of a glass tube 1 having a capillary bore 2, the capillary being 20 mm long and 250 micrometres in diameter: the diameters of the fibre and the capillary are shown on an enlarged scale in relation to the length of the capillary, for clarity.

The directlon of travel of the fibre through the die is shown by the arrows. Both ends of the glass tube are flared, as shown at 3, to facilitate insertion of the silica fibre 4 into the capillary, to effect removal of excess coating solution 5 from the fibre in such a manner as to promote smootn flow of phe residual coating 6 through the capillary, and to achieve flow separation on emergence of the coated fibre from the capillary.The die is supported at one point only in a cradle which allows the die to rotate in vertical and horizontal planes; thus the die is enabled to take up the same angle of inclination as that of the oven, so that the fibre can pass directly, in a straight line, from the coating pulley, through the die, to the oven, without any possibility of distoition, and the die is maintained balanced in the correct position so that the fibre is subjected to little force arising from the weight of the die.

Under the conditions described above, a silicone resin solution containing 70 parts by weight of the solid silicone to 100 parts of solvent and having a viscosity of approximately 30 poises will give a cladding layer, after curing, of the required thickness of 15 micrometres, the total diameter of the waveguide thus being 180 micrometres. During the passage of the coated fibre through the oven the coating is fully cured to form the silicone cladding layer, and on emerging from the oven the waveguide is passed directly through a second coating system for the application of a protective coat, to prevent subsequent damage to the silicone cladding. The protective coat may consist, for example, of polyvinylidene fluoride, which can be applied in the form of a solution of the polymer in a mixture of acetone and dimethyl formamide; after adjustment of the coating thickness, suitably by means of a capillary die similar to that described above, or by felt pads, the solvent mixture is evaporated at room temperature, no thermal curing of the polymer being necessary. The coated waveguide can be wound on to a drum, and if desired further protective coats of polyvinylidene fluoride or other suitable resin, such as polyurethane, may be subsequently applied.

The optical fibre waveguide manufactured by the method described in the example, has a maximum numerical aperture of approximately 0.4, and an attenuation of 25 dBkm-1 for radiation of wavelength 850nm.

Claims

WHAT WE CLAIM IS:

1. A method of manufacturing an optical fibre waveguide consisting of a core of vitreous material and, in intimate contact with the core, a cladding layer at least ten micrometres thick composed of a polymeric plastics material whose refractive index is lower than that of the core material and which is substantially transparent to the radiation to be propagated along the waveguide in use, which method includes the steps of drawing a fibre from a rod of vitreous material, to constitute the waveguide core, passing the drawn fibre, before it has come into contact with any solid surface, through a liquid composition consisting of a said polymeric plastics material in solution or in suspension in, or in admixture with, a liquid medium, and having a viscosity in the range of 0.5 to 50 poises, so that the fibre is coated with said liquid composition, and passing the coated fibre through means for adjusting the thickness of the coating and then through heating means to effect removal of any volatile liquid medium present and curing or hardening of the plastics material to form said cladding layer.

2. A method according to Claim 1, wherein the fibre is drawn downwardly from a furnace into the top of which the vitreous rod is fed, and the fibre is passed directly through a coating bath or around guide means to which the liquid coating composition is fed, and thence through coating thickness adjusting means and through an elongated tubular oven maintained at a suitable temperature for effecting curing or hardening of the coating, the rate of drawing and subsequent travel of the fibre through the coating and curing system being maintained constant.

3. A method according to Claim 1 or 2, wherein the said drawn fibre constituting the waveguide core is from 100 to 200 micrometres in diameter.

4. A method according to Claim 1, 2 or 3, wherein the said rod is composed of silica.

5. A method according to any preceding Claim, wherein the said liquid composition consists of a solution of the polymeric plastics material in an organic solvent.

6. A method according to any of the preceding Claims 1 to 4, wherein the said liquid composition consists of a suspension of the polymeric plastic material in water.

7. A method according to any preceding Claim. wherein the said plastics material formed for forming the cladding layer of the waveguide is a silicone resin.

8. A method according to Claim 7, wherein the said plastics material is a crosslinked polysiloxane resign.

9. A method according to Claim 8, wherein the said plastics material is a polymer or copolymer of one or more of the compounds dimethyl siloxane, methyl vinyl siloxane, and methyl allyl siloxane.

10. A method according to any of the preceding Claims 1 to 4, wherein the said liquid composition consists of a silicone resin and a silicone oil.

11. A method according to any of the preceding Claims 1 to 6, wherein the said plastics material employed for forming the cladding layer of the waveguide is a fluorocarbon polymer or copolymer.

12. A method according to Claim 11, wherein the said plastics material is polyvinylidene fluoride, or polychlorotrifluoroethylene, or a fluorinated ethylene-propylene copolymer.

13. A method according to any of the preceding Claims 1 to 6, wherein the said plastics material employed for forming the cladding layer of the waveguide is methyl cellulose.

14. A method according to any of the preceding Claims 1 to 6, wherein the said plastics material employed for forming the cladding layer of the waveguide is polyformaldehyde.

15. A method according to any preceding Claim, wherein the said means for adjusting the thickness of the liquid composition coating on the vitreous fibre consists of an elongated die formed with a capillary tube of diameter larger than the required diameter of the clad fibre after curing of the coating.

16. A method according to Claim 15, wherein the said capillary tube is 20 to 30 millimetres long.

17. A method according to any preceding Claim, wherein subsequently to the formation of the said cladding layer the optical fibre waveguide is covered with a protective coating of a synthetic resin.

18. A method of manufacturing an optical fibre waveguide, according to Claim 1, carried out substantially as hereinbefore described in the specific example and with reference to the accompanying drawing.

19. An optical fibre waveguide manufactured by a method according to any preceding Claim.