GB1567176A

GB1567176A - Optical fibre waveguides and their manufacture

Info

Publication number: GB1567176A
Application number: GB31449/76A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1977-07-20
Filing date: 1977-07-20
Publication date: 1980-05-14

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL FIBRE WAVEGUIDES AND THEIR MANUFACTURE (71) We, THE GENERAL ELECrRIC 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 optical fibre waveguides of the kind comprising a core of vitreous silica, with or without a dopant for modifying the refractive index of the silica in a desired manner, and a cladding layer composed of a material having a lower refractive index than that of any part of the core, such that electromagnetic radiation in the visible and infra-red regions of the spectrum, incident upon one end of the waveguide, can be propagated along the waveguide and thus employed for the transmission of communication signals. The invention also relates to methods of manufacturing optical fibre waveguides of the form described.

It has been proposed to employ a polymeric plastic material for the cladding layer of an optical waveguide having a doped or undoped silica fibre core. Some polymeric materials have refractive indices appreciably lower than the refractive index of pure vitreous silica, which is 1.452 for radiation of wavelength of 900 nm: such polymers are therefore especially advantageous for use as cladding on undoped silica fibre cores. Thus a waveguide of this form can have a high numerical aperture, in the region of 0.3 to 0.4, due to the relatively large refractive index difference between the core and the cladding, as well as having a core with the advantageous optical properties characteristic of pure silica.

In view of the high numerical aperture which is obtainable in polymer-clad silica waveguides, the relatively high optical losses occurring in the propagation of radiation along these waveguides, as compared with those occurring in silica-clad waveguides, are acceptable for some applications of optical waveguides, for example in short and medium distance communication links required to carry information at low or medium rates. However, it is desirable that the losses in the core should be as low as possible. It has been found that silica fibres, although having relatively low losses for radiation of most wavelengths in the visible and infra-red regions of the spectrum, usually show an absorption peak at about 945 nm, in the near infra-red, which is close to one of the preferred wavelength bands for the operation of optical waveguides, and a further broad absorption peak at about 630 nm in the visible region, the latter resulting in undesirable losses in possible regions of operation. The 945 nm absorption peak is due to the presence in the fibre of hydroxyl groups derived from water with which the bulk silica, from which the fibre was drawn, has been in contact during its manufacture, such hydroxyl groups being located in tetrahedral sites in the silica lattice. The 630 nm absorption peak appears to be induced during drawing of the fibre from bulk silica.

It is an object of the present invention to provide an improved optical waveguide consisting of a silica fibre core with polymer cladding, in which the composition of the silica is such that the optical losses occurring in the core in operation of the waveguide, and in particular one or both of the absorption peaks referred to above, are reduced.

According to the invention, an optical fibre waveguide is composed of a core consisting of a fibre of vitreous silica, with or without dopant material for modifying the refractive index of the silica, which silica either is substantially free from hy droxyl groups or contains such groups in a proportion not exceeding 1000 parts per million by weight of the weight of silica, and in which silica the content. of oxygen atoms are not combined with hydrogen in the form of hydroxyl groups is of such proportion and distribution as to ensure that substantially all of the tetrahedral sites in the silica lattice, apart from any such sites which are occupied by hydroxyl groups, are occupied by said uncombined oxygen atoms; and a cladding layer at least ten micrometres thick consisting of a polymeric plastic material whose refractive index is lower than that of the core material and which is transparent to the radiation to be propagated along the waveguide in use.

We believe that absorption of radiation in the visible region of the spectrum.

especially that of wave lengths around 630 nm, which occurs in silica fibre waveguides, is due to local oxygen deficiencies in the silica lattice caused by the displacement of some oxygen atoms from sites in the tetrahedral SiO4 structure during drawing of the fibre from bulk silica, possibly as a result of viscous shear caused by drawing deformation of the silica. Some of the tetrahedral sites are thus left vacant, with the formation of SiO3+ complexes and free silicon bonds, giving the overall effect of oxygen deficiency in the silica lattice.

An optical waveguide in accordance with the invention, the silica fibre core of which has an oxygen content of such amount and distribution that substantially all of the tetrahedral sites in the lattice, other than those occupied by hydroxyl groups, are occupied by oxygen atoms uncombined with hydrogen, shows little or no absorption of visible radiation, in operation, and in particular the absorption peak at about 630 nm is nearly or entirely absent. The silica might contain only the stoichiometric proportion of oxygen atoms, provided that substantially all of these atoms occupy tetrahedral sites: at least some of the oxygen atoms displaced during drawing of the fibre can in some cases be restored to their original sites by diffusion before the drawn fibre has cooled to a temperature which is too low to permit of mobility of the said atoms. Preferably, however, in order to ensure that there is substantially no local oxygen deficiency in the fibre lattice, the silica contains a proportion of oxygen atoms in excess of the stoichiometric proportion, the additional oxygen atoms, which are incorporated - in the silica lattice during the process for the manufacture of the fibre, being present in sufficient quantity to ensure that any vacant tetrahedral sites produced during the fibre drawing process can be filled by subsequent diffusion of said excess oxygen atoms. Suitable proportions of oxygen, in excess of the stoichiometric proportion, are from 50 to 500 parts per million by weight of the weight of silica.

The content of hydroxyl groups in the silica fibre core should be as low as practicable, for the attainment of a reduction of the absorption peak at wavelength 945 nm, and is preferably not greater than 500 parts per million by weight. Ideally the silica is free from hydroxyl groups, so that the absorption of 945 nm radiation is eliminated as well as that of 630 nin radiation, but the complete absence of hydroxyl groups may be difficult to attain in practice, and for some applications the presence of a small proportion of hydroxyl groups, with the resulting relatively small absorption peak at 945 nm, is acceptable provided that there is little or no absorption of visible radiation.

The silica fibre core of the waveguide of the invention is preferably from 100 to 300 micrometres in diameter, and will usually be of homogeneous composition, preferably being of undoped silica.

Bulk vitreous silica which has been produced by some chemical processes, for example by the reaction between a silicon hydride and oxygen, or by a process involving the use of a hydrogen- or hydrocarbon-burning flame, contains hydroxyl groups derived from water generated during the reaction. Excess oxygen atoms in the silica fibre core of a waveguide may be derived from such hydroxyl groups initially present in the bulk silica, with or without dopant, from which the fibre is drawn, at least part of the hydrogen of such hydroxyl groups being removed during subsequent stages of the process for the manufacture of the fibre, so as to leave residual excess oxygen atoms in the silica lattice whilst having the additional desired effect- of reducing the hydroxyl content of the silica.

The bulk silica can, if desired, be subjected to heat treatment for the removal of hydrogen, before being drawn to fibre: for example, heating a silica rod 8 mm in diameter for 48 hours at 1700"C in vacuum results in the expulsion of the greater part of the hydrogen present. We have found, however, that a large proportion of the hydrogen can be removed during the drawing of-the fibre, by carrying out the drawing process in an atmosphere of dry flushing gas, and in some cases this treatment alone is sufficient to produce a fibre containing the required amount of excess oxygen and also having an acceptably low hydroxyl content, depending on the initial hydroxyl content of the bulk silica, which is preferably not greater than 1500 parts per million by weight.

Alternatively, excess oxygen can be introduced into the fibre by any of' the other methods described in the specification of cm pending Patent Application No. 45628/ 74 (Serial No. 1,497,066). Thus additional oxygen atoms may be introduced into inherently hydroxyl-free bulk vitreous silica during the manufacture thereof, by incorporating either water or excess oxygen gas in a gaseous mixture employed for the synthesis of the silica. For example, hydroxyl-containing bulk silica may be produced by carrying out the vapour phase reaction between silicon tetrachloride and oxygen in the presence of water, or silica containing excess oxygen can be produced by heating silicon tetrachloride in an oxygen r.f. plasma, in either case temperatures exceeding 2500"C being employed.

If water is used, the excess oxygen atoms derived therefrom will of course be present in the silica initially in the form of hydroxyl groups, so that hydrogen will have to be removed subsequently, by heat treatment of the bulk silica andlor during drawing of the fibre, as described above.

It will be understood that, whichever method is used for introducing excess oxygen atoms into the silica, the fibre is drawn under water-free conditions, preferably in a dry flushing gas atmosphere, to assist in removing any hydrogen present, and to ensure that no further hydroxyl groups can be introduced into the fibre.

The polymer cladding of an optical waveguide in accordance with the invention is required to be at least ten micrometers thick to ensure that, in operation, 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 transparent to the radiation concerned, in order to minimise loss due to absorption or scattering in the cladding. The cladding material must also be capable of making continuous, intimate contact with the surface of the silica 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 silica core is immaterial, since it will be compensated by the elasticity of the polymer cladding. The polymer chosen for use as the cladding material is preferably one which has a refractive index at least 0.01 lower than that of the silica of the core, in respect of the radiation to be propagated along the waveguide; for example, if the core is composed of undoped silica the refractive index of the polymer is preferably less than 1.44 for radiation of wavelength 900 nm.

The preferred materials for the cladding are silicone resins, since they readily wet silica fibres, possibly with some bonding, and also since the optical 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 we have found to be particularly suitable for use as cladding on an undoped 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 for radiation of wavelength 900 nm.

Some fluorocarbon polymers and copolymers have sufficiently low refractive indices to render them suitable for use as cladding on silica fibre cores, and can form transparent layers: compounds in this category include ppolyvinylidene fluoride, polychlorotrifluoroethylene, and a fluorinated ethylene-propylene copolymer. Other polymeric materials which are suitable for use as cladding materials include methyl cellulose and polyformaldehyde.

A preferred method of manufacturing an optical fibre waveguide in accordance with the invention includes the steps of drawing a fibre from a rod composed of vitreous silica with or without dopant material for modifying the refractive index thereof, and containing hydroxyl groups in a proportion not exceeding 1500 parts per million by weight, the drawing process being carried out in an atmosphere of dry flushing gas which is non-reactive with the silica, and immediately passing the drawn fibre, before it has come into contact with any solid surface, through a liquid composition consisting of a polymeric plastic 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, sa that the fibre is coated with said liquid composition, and passing the coated fibre through means for adjusting the thickness of the liquid coating and then through heating means to effect removal of any voltaile liquid medium present and curing or hardening of the polymeric material to form a continuous cladding layer on the silica fibre.

The silica fibre is coated with the cladding material immediately after drawing, and without being allowed to come into contact with any solid surface, 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 drawing and coating process is conveniently carried out by means of apparatus in which the fibre.is drawn vertically downward from a furnace into the top of which the silica rod is fed and which is continuously flushed with dry gas, and the drawn- fibre is pulled directly through a coating bath or around-guide means to which the liquid coating composition is fed, and thence through coating thickness adjustment means and through an elongated tubular oven maintained at a suitable temperature for curing or hardening the polymer cladding, the rate of drawing and travel of the fibre through the coating and curing system being maintained constant.

Most of the polymeric materials which are suitable for forming the cladding, for example silicones and some fluorocarbons, can be applied to the silica fibre core in solution in a suitable solvent, which is subsequently evaporated to leave a continuos -layer of polymer on the fibre surface. However, in the cases of some polymers which. are not soluble in any available solvent, for example polychlorotrifluoroethylene and fluorinated ethylenepropylene copolymer, the polymer can be applied to the fibre 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 solution or suspension of polymer may contain any hardener, cross-linking agent, or polymerisation catalyst which may be required to effect curing of the polymer. The concentration of the solution or suspension is adjusted to give a suitable viscosity, within the range specified above, for forming a cladding of the desired thickness.

For adjusting the thickness of the liquid coating, the coated fibre is suitably passed between felt pads or through a die, for removal of excess liquid before it enters the oven. A preferred form of coating thickness adjustment means consists of an elngated die formed with a capillary tube 20 to 30 mm long, suitably of glass, and having a bore diameter somewhat larger than the required diameter of the coated fibre after curing of the coating, such that the residual coating retained on the fibre after passage thereof through the capillary will, after shrinkage during curing, give a cladding layer of the desired thickness.

Pulling of the drawn silica fibre through the coating and heating means at a relatively high speed is preferred, since this facilitates the production of good quality, smooth coatings of suitable thickness and free from irregularities. The rate of travel of the fibre is adjsuted in relation to the duration of the heat treatment required to produce a fully cured cladding layer of the desired thickness.

The polymer cladding has the additional advantage that it provides protection for the silica fibre core, so that the application of a protective synthetic resin coating, which is usual in the case of wholly vitreous optical fibre waveguides, is not essentiaI for the waveguides of the present invention. However, if desired, the waveguides of the invention can be provided with one or more additional protective coatings in known manner; in particular, if the cladding is formed of a relatively soft material such as a silicone resin, a protective coat of a harder resin may be desirable.

A specific form of optical fibre waveguide in accordance with the invention, and a method of manufacturing the waveguide, will now be described by way of example.

The waveguide of the example consists of a fibre core composed of vitreous silica containing excess oxygen atoms and a small proportion of hydroxyl groups, and free from dopant, and a cladding layer 15 micrometres thick composed of a crosslinked dimethyl siloxane polymer, which is available commercially under the Registered Trade Mark "Sylgard", and which has a refractive index of 1.41 for radiation of wavelength 900 nm.

For the manufacture of this waveguide, an 8 mm diameter rod of commercially available vitreous silica containing 1200 parts per million, by weight, of hydroxyl groups is fed into the top of a vertically disposed electric resistance furnace comprising a tubular graphite heating element, maintained at a temperature of 2000"C to 2200"C and continuously flushed with dry nitrogen, and a fibre of 120 micrometers diameter is drawn from the rod vertically downwards and is passed directly to a guide pulley which has a peripheraI channel to which is fed a solution of the silicone resin, with hardener, in a solvent consisting of 1,1,l,-trichloroethane. The silica fibre is passed around the pulley, in the channel, so that it becomes coated with the silicone solution, and then passes through a die consisting of a glass capillary tube 20 mm long and having a bore diameter of 200 micrometres, for adjusting the thickness of the coating. The coated fibres then passes through a tubular oven 1.5 metres long, which is inclined upwards at an angle of 15 in the direction of travel of the fibre and is maintained at a temperature of 350"C to 5000 C; the rate of travel of the fibre through the system is one metre per second. Under these conditions, a 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. During the passage through the oven the coating is fully cured to form the silicone cladding layer, and on- emerg- iag from the oven the waveguide can either be wound directly on a drum, or preferably passed directly through a second coating system and oven for the application of a protective coat, for example of polyvinylidene fluoride, to prevent the possibihty of subsequent damage to the - silicone cladding. Additional coatings of synthetic resinous materials may be applied by similar procedures if desired; preferably at least one such additional coating contains an inorganic filler material, such as carbon powder, silica or titania, for increasing the breaking load of the coated waveguide.

The hydroxyl content of the silica fibre is reduced to less than 300 parts per million during the fibre drawing process, the hydrogen being removed from the remain der of the hydroxyl groups initially present in the silica, to leave over 400 parts per million of excess oxygen atoms dlstributed throughout the silica lattice. In operation of a specific waveguide produced by the method of the example, a small adsorption peak for radiation of 945 nm wavelength was observed, but an absorption at 630 nm was scarcely preceptible. The waveguide had a maximum numerical aperture of approximately 0.4.

WHAT WE CLAIM IS:- 1. An optical fibre waveguide composed of a core consisting of a fibre of vitreous silica, with or without a dopant material for modifying the refractive index of the silica, which silica either is substantially free from hydroxyl groups or contains such groups in a proportion not exceeding 1000 parts per million by weight of the weight of silica, and in which silica the content of oxygen atoms which are not combined with hydrogen in the form of hydroxyl groups is of such proportion and distribution as to ensure that substantially all of the tetrahedral sites in the silica lattice, apart from any such sites which are occupied by hydroxyl groups, are occupied by said uncombined oxygen atoms; and a cladding layer at least ten micrometers thick consisting of a polymeric plastic material whose refractive index is lower than that of the core material and which is transparent to the radiation to be propagated along the waveguide in use.

2. An optical fibre waveguide according ao Claim 1, wherein the proportion of oxygen atoms, uncombined with hydrogen, contained in the silica of the core is in excess of the stoichiometric proportion of oxygen atoms in silicon dioxide.

3. An optical fibre waveguide according to Claim 2, wherein the said proportion of oxygen atoms is- from 50 to 500 parts per million, by weight of the weight of silica, in excess of the stoichiometric proportion of oxygen atoms in silicon dioxide.

4. An optical fibre waveguide according to Claim 1, 2 or 3, wherein the silica of the core contains a proportion of hydroxyl groups not greater than 500 parts per million by weight of the weight of silica.

5. An optical fibre waveguide according to any preceding Claim, wherein the silica fibre core is from 100 to 300 micrometres in diameter.

6. An optical fibre waveguide according to any preceding Claim, wherein the cladding layer of polymeric plastic material is at least 15 micrometers in thickness.

7. An optical fibre waveguide according to any preceding Claim, wherein the polymeric material employed for the clading layer has a refractive index at least 0.01 lower than that of the silica of the core, in respect of the radiation to be propagated along the waveguide in use.

8. An optical fibre waveguide according to Claim 7, wherein the cladding layer is composed of a silicone resin.

9. An optical fibre waveguide according to Claim 8, wherein the cladding layer is composed of a cross-linked polysiloxane resin.

10. An optical fibre waveguide according to Claim 9, wherein the cladding layer is composed of a cross-linked polymer or copolymer of one or more of the compounds dimethyl siloxane, methyl vinyl siloxane, and methyl allyl siloxane.

11. An optical fibre waveguide according to Claim 7, wherein the cladding layer is composed of a fluorocarbon polymer or copolymer.

12. An optical fibre waveguide according to Claim 11, wherein the cladding layer is composed of polyvinylidene fluoride, or polychlorotrifluoroethylene, or a fluorinated ethylene-propylene copolymer.

13. An optical fibre waveguide according to Claim 7, wherein the cladding layer is composed of methyl cellulose.

14. An optical fibre waveguide according to Claim 7, wherein the cladding layer is composed of polyformaldehyde.

15. A method of manufacturing an optical fibre weveguide according to any preceding Claim, which includes the steps of drawing a fibre from a rod composed of vitreous silica with or without dopant material for modifying the refractive index thereof, and containing hydroxyl groups in a proportion not exceeding 1500 parts per million by weight, the drawing process being carried out in an atmosphere of dry flushing gas which is non-reactive with the silica, and immediately passing

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. after curing, of the required thickness of 15 micrometres. During the passage through the oven the coating is fully cured to form the silicone cladding layer, and on- emerg- iag from the oven the waveguide can either be wound directly on a drum, or preferably passed directly through a second coating system and oven for the application of a protective coat, for example of polyvinylidene fluoride, to prevent the possibihty of subsequent damage to the - silicone cladding. Additional coatings of synthetic resinous materials may be applied by similar procedures if desired; preferably at least one such additional coating contains an inorganic filler material, such as carbon powder, silica or titania, for increasing the breaking load of the coated waveguide. The hydroxyl content of the silica fibre is reduced to less than 300 parts per million during the fibre drawing process, the hydrogen being removed from the remain der of the hydroxyl groups initially present in the silica, to leave over 400 parts per million of excess oxygen atoms dlstributed throughout the silica lattice. In operation of a specific waveguide produced by the method of the example, a small adsorption peak for radiation of 945 nm wavelength was observed, but an absorption at 630 nm was scarcely preceptible. The waveguide had a maximum numerical aperture of approximately 0.4. WHAT WE CLAIM IS:-

1. An optical fibre waveguide composed of a core consisting of a fibre of vitreous silica, with or without a dopant material for modifying the refractive index of the silica, which silica either is substantially free from hydroxyl groups or contains such groups in a proportion not exceeding 1000 parts per million by weight of the weight of silica, and in which silica the content of oxygen atoms which are not combined with hydrogen in the form of hydroxyl groups is of such proportion and distribution as to ensure that substantially all of the tetrahedral sites in the silica lattice, apart from any such sites which are occupied by hydroxyl groups, are occupied by said uncombined oxygen atoms; and a cladding layer at least ten micrometers thick consisting of a polymeric plastic material whose refractive index is lower than that of the core material and which is transparent to the radiation to be propagated along the waveguide in use.

the drawn fibre, before it has come into contact with any solid surface, through a liquid composition consisting of a polymeric plastic 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 liquid coating and then through heating means to effect removal of any volatile liquid medium present and curing or hardening of the polymeric material to form a continuous cladding layer on the silica fibre.

16. A method according to Claim 15, wherein the silica fibre is drawn vertically downward from a furnace into the top of which the silica rod is fed, and which is continuously flushed with a stream of dry gas, and the drawn fibre is pulled directly through a coating bath or around guide means to which the liquid coating composition is fed, and thence through coating thickness adjustment means and through an elongated tubular oven maintained at a suitable temperature for effecting curing or hardening of the polymer cladding, the rate of drawing and travel of the fibre through the coating and curing system being maintained constant.

17. A method according to Claim 15 or 16, wherein the means for adjusting the thickness of the liquid coating on the silica fibre consists of an elongated die formed with a capillary tube 20 to 30 millimetres long and having a bore diameter larger than the required diameter of the coated fibre after curing of the coating, such that the residual coating retained on the fibre after passage thereof through the capillary tube will, after shrinkage during curing, give a cladding layer of the desired thickness.

18. An optical fibre waveguide according to any of the preceding Claims 1 to 14, which is provided with one or more protective coatings of synthetic resin.

19. An optical fibre waveguide according to Claim 1, substantially as hereinbefore described by way of example.

20. A method of manufacturing an optical fibre waveguide according to Claim 19, carried out substantially as hereinbefore described by yvay, of example.