GB2156858A

GB2156858A - Coating optical fibres with hydrogen-impermeable material

Info

Publication number: GB2156858A
Application number: GB08431496A
Authority: GB
Inventors: Ronald Albert Evans; Adrian Clement Greenham; John Stephen Mccormack; Bruce Armstrong Nichols
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-01-16
Filing date: 1984-12-13
Publication date: 1985-10-16
Also published as: GB2156858B; GB8401089D0; GB8431496D0

Abstract

An optical fibre is coated with a hydrogen-impermeable material which is either a metal e.g. indium or tin, or a dielectric, e.g. silicon nitride, or possibly carbon by a CVD process as the fibre moves continuously through the deposition zone. The optical fibre may comprise a glass core and a cladding layer made of a glass with a lower refractive index than that of the core, and the coated optical fibre may have a layer of abrasion resistant plastics material on the coating. In the drawing, as illustrated in the figure an optical fibre 6 is positioned along the longitudinal axis of an inner perforated silica tube 1 which is coaxially within an outer silica tube. Feed tubes 3 for the introduction of the different reactive constituents are arranged between the two silica tubes so as to allow the vapours to pass through the perforations in the inner tube 1 into the deposition zone where the optical fibre is positioned. <IMAGE>

Description

SPECIFICATION Coating optical fibres This invention relates to a method of coating optical fibres, particularly to a method of coating optical fibres with a hydrogen-impermeable material and to an optical fibre coated with such a material.

It has long been known that the provision of a moisture impermeable abrasion resistant coating on optical fibres is an essential requirement for maintaining the high inherent fibre strength. Protection is currently achieved by applying a heat or UV cured polymer coating immediately after the fibre has been drawn. All of the currently used plastic coating materials are known to have small but finite permeabilities to moisture and the provision of a totally impermeable coating has always been an attractive goal.

Recently it has been realised that hydrogen diffusing through the fibre cladding glass and reaching the core produces deleterious effects on the transmission of the fibre by increasing the attenuation at certain wavelengths. Thus the transmission of the fibre, for example in undersea cables, where hydrogen may be generated, is consequently reduced with age.

Accordingly the invention provides an optical fibre comprising a glass core, a cladding layer made of a glass with a lower refractive index than that of the core, and a coating of a material which is substantially impermeable to the diffusion of hydrogen. The use of such a coating prevents or at least markedly reduces the amount of hydrogen diffusing into the core of the fibre thereby effectively prolonging the useful life of the fibre.

The invention further provides a method of coating an optical fibre with a hydrogen-impermeable material by causing a chemical reaction to take place in a gaseous mixture such that a hydrogen-impermeable coating is deposited on the fibre passing through a zone where said chemical reaction is taking place.

One such method of coating elongate moving rods or fibres is known from patent applications such as GB 2117754 to which reference should be made for further details.

The coating is preferably of a material with a high adhesion to glass and may be either a metal such as tin or indium or a dielectric such as silicon nitride, silicon oxynitride or silicon carbide, or possibly carbon.

The gaseous mixture consisting of suitable reactants is preferably fed into the zone via one or more perforated feed tubes, the reactants being caused by means of a plasma to react in said zone and form the required material which is deposited on the fibre, the remainder of the reaction products being removed by exhaustion of the zone.

The plasma is preferably excited by a microwave generator.

After the application of the hydrogen-impermeable coating a layer of abrasion-resistant plastics material may be applied to the coated fibre, for example by a known technique.

One specific process in accordance with the invention will now be described by way of example, with reference to the drawing which shows, in part sectional elevation, apparatus designed for coating optical fibres with a thin film coating.

In the drawing, an optical fibre 6 is positioned along the longitudinal axis of an inner perforated silica tube 1 which is coaxially within an outer silica tube 2. A plurality of individual perforated feed tubes 3 for the introduction of different constituents of the gaseous reaction mixture are arranged between the two silica tubes so as to allow the different vapours to pass from the feed tube, through the perforations in the inner silica tube 1 and into the deposition zone where the optical fibre is positioned.

The silica tube arrangement described above is surrounded by a conductive tube 4 and furnace 5; exhaust means (not shown) are provided at each end of the inner tube section, and a microwave cavity (not shown) is located around one end of the inner tube.

Only two feed tubes 3 are shown in the drawing, but any number required may be employed. For example one such tube may be used for the introduction of an appropriate volatile metal halide entrained in a carrier gas such as argon. The numbers, distributions and diameters of the perforations in the respective feed tubes and in the inner tube 1 may be varied along the lengths of the tubes in such a manner that the flow rates of the gaseous constituents are controlled to give the required thin film coating on the fibre, which may be continuously moving through the apparatus and on into either another coating zone or onto a drum for storage.

Coating of the fibre is effected by allowing the gaseous constituents to flow into the inner zone and establishing a plasma throughout the space in the inner tube 1, by means of the microwave cavity and the conductive tube. The gaseous mixture reacts and deposits the desired film on the surface of the fibre while the other reaction products and unused reactants are exhausted from the tube.

In order to ensure that a plasma is produced only in the inner annular space between the fibre and thinner tube and not in the space between the two silica tubes, the gas pressure in the space between the outer and inner tubes must be maintained considerably higher than that in the inner space, the relative pressures being controlled by the flow rates of the gases, the arrangement of the perforations in the inner tube, and in individual gas/vapour feed tubes, and by continuous exhaustion of the said inner space.

The thickness of the coating required to prevent any significant diffusion of hydrogen into the fibre core will depend upon the coating material and in the case of metals such as indium, aluminium or tin is preferably at least 0.1 microns. The most satisfactory coating thickness for any particular coating material can, however, readily be found by trial.

In a different embodiment of the apparatus in accordance with the invention, the inner perforated tube 1 is removed and the feed tubes 3 are so arranged and perforated that the gas/vapour is passed directly into the deposition zone and directed at the fibre. In this case, it will be readily apparent that the pressure within the feed tubes must be adjusted, as compared to the pressure in the deposition zone, so that the plasma remains only within the deposition zone in the same way as described above.

Claims

1. An optical fibre comprising a glass core, a cladding layer made of a glass with a lower refractive index than that of the core, and a coating of a material which is substantially impermeable to the diffusion of hydrogen.

2. An optical fibre according to Claim 1 wherein said coating material has a high adhesion to glass.

3. An optical fibre according to either Claim 1 or Claim 2 wherein said coating material is a metal.

4. An optical fibre according to Claim 3 wherein said coating material is either tin or indium.

5. An optical fibre according to either Claim 1 or Claim 2 wherein said coating material is a dielectric.

6. An optical fibre according to Claim 5 wherein said coating material is either silicon nitride, silicon oxynitride or silicon carbide.

7. An optical fibre according to either Claim 1 or Claim 2 wherein said coating material is carbon.

8. An optical fibre according to any preceding Claim having a layer of abrasion resistant plastics material on said coating.

9. A method of coating an optical fibre with a hydrogen-impermeable material by causing a chemical reaction to take place in a gaseous mixture such that a hydrogen-impermeable coating is deposited on the fibre passing through the zone where said chemical reaction is taking place.

10. A method of coating an optical fibre according to Claim 9 wherein said gaseous mixture consisting of suitable reactants is fed into said zone via one or more perforated feed tubes, the reactants being caused by means of a plasma to react in said zone and form the required material which is deposited on said fibre, the remainder of the reaction products being removed by exhaustion of said zone.

11. A method of coating an optical fibre according to Claim 10 wherein said plasma is generated in said zone by means of a microwave generator.

12. A method of coating an optical fibre according to any one of Claims 9-11 further comprising the step of applying a layer of abrasion-resistant plastics material to the coating fibre.

1 3. An optical fibre substantially as here it before described.

14. A method of coating an optical fibre substantially as hereinbefore described with reference to the drawing.