GB2159978A

GB2159978A - Optical fibre cables

Info

Publication number: GB2159978A
Application number: GB08512961A
Authority: GB
Inventors: Dennis Lawrence Lewis
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1984-05-23
Filing date: 1985-05-22
Publication date: 1985-12-11
Also published as: GB8413189D0; GB8512961D0; GB2159978B

Abstract

An optical fibre cable, within an outer sheath accommodating at least one channel containing one or more optical fibres, at least one further channel which is purged (eg. by passing non-hydrogen containing gas therealong) of hydrogen and into which hydrogen from the optical fibre channel or channels can diffuse, thereby reducing the amount of hydrogen present in the latter which could result in deterioration of the fibre or fibres. A hydrogen trapping material (eg. Maganese dioxide, Ti, Zr, Pa) may also be present within the purged channel. <IMAGE>

Description

SPECIFICATION Optical Fibre Cables This invention relates to optical fibre cables, that is to say cables incorporating one or more optical fibres, such fibres commonly being accommodated in channels or grooves formed in extrusions of plastics material, and surrounded by an outer sheath.

One problem which has been experienced with optical fibre cables is a gradual increase in transmission losses for certain waveiengths with age, and it has been found that this is caused, at least in part, by the gradual absorption of hydrogen by the fibres even where they are protected by coatings of synthetic plastics material, the hydrogen being initially present within the cable or being gradually evolved within the cable during its life.

An object of the invention is to provide a way of alleviating this problem.

According to the invention in an optical fibre cable having at least one optical fibre located in a channel or groove formed in an extrusion of plastics material permeable to hydrogen and surrounded by an outer sheath, there is provided within the sheath at least one channel the interior of which is or has been purged of hydrogen.

Due to diffusion of hydrogen through plastics material some of the hydrogen will tend to migrate from the channel or channels containing the optical fibre or fibres into the purged channel during the life of the cable, and as a result the amount of hydrogen available in the former channel or channels, which could result in deterioration of the fibre or fibres contained therein, will be correspondingly reduced.

Purging of the channel which may also be formed of plastics material is conveniently carried out by the passage therethrough of a gas or gas mixture, containing no, or a negligible amount of, hydrogen, either continuously or at suitable intervals.

The hydrogen purged channel may be provided by a channel or groove containing one or more optical fibres, or by a channel devoid of optical fibres and having hydrogen permeable walls.

The or each channel that contains one or more optical fibres may be provided by the bore of a simple extruded plastics tube, and the purged channel in such a case may also be in the form of a tube, the tubes conveniently being wound helically around a central strength member.

In an alternative form of cable the channels may be formed as longitudinally-extending openings in a common extrusion of plastics material, one of the channels being purged of hydrogen. The channels can be of helical or periodically- reversed helical form around a central strength member.

In accordance with a further aspect of the invention, an optical fibre cable comprises at least one tube of hydrogen permeable material containing one or more optical fibres, and surrounded by an outer sheath, the space between the tube or tubes and the wall of the sheath forming a channel which is purged of hydrogen.

The purged channel of any one of the above forms of cable may contain a quantity of hydrogen-trapping material distributed along it. The expression "hydrogen-trapping material" means a material capable of collecting free hydrogen, either by absorption or by chemical or other action, and may, for example, consist of finely divided manganese dioxide, preferably in an activated form, that is to say one in which its action is enhanced by the inclusion of an appropriate catalyst such as copper, certain copper salts, cobalt and silver either individually or in combination.

Other hydrogen-trapping materials that could be used are titanium zirconium and palladium.

The hydrogen trapping properties of some materials tend to be increased at higher temperatures and the purged channel may in such cases be associated with means for heating it either continuously or periodically, for example by means of an electric heating element contained within or surrounding the channel.

In addition, the reaction of some hydrogen-trapping materials with hydrogen produces water, and where this would react with components of the cable to give rise to the generation of more hydrogen or otherwise have a detrimental effect on the cable, means must be provided to prevent this occurring, as by the provision of suitable waterabsorbing or - blocking substance such as a suitable molecular sieve material. This can be in a finely divided form and mixed with the hydrogen-trapping material, which is also conveniently in a finely divided form.

The hydrogen-trapping material, and the molecular sieve material, where provided, could be incorporated within a paste - or jelly-like substance such as petroleum jelly or silica gel.

Two cables in accordance with the invention will now be described by way of example with reference to Figures 1 and 2 of the accompanying schematic drawing, which illustrate transverse crosssections of the two forms of cable.

Referring first to Figure 1, the cable comprises a strength member 1 having a plurality of tubes 2 of synthetic plastics material disposed helically around it, and enclosed within a metal tape 3 folded longitudinally around the assembly, and surrounded, in turn, by an extruded sheath 4 of synthetic plastics material. The tubes 2 and sheath 4 may, for example, consist of polypropylene or polyethylene, although other suitable plastics materials could alternatively be employed. All the tubes 2 except one contain one or more optical fibres as at 5 disposed loosely within the respective tubes and having a length which is greater than the length of the tubes in which they are accommodated.

The tube shown as 2' which does not contain any optical fibres has been purged of hydrogen by the passage through it of either nitrogen or air substantially free from hydrogen.

Purging can take place either prior to or subsequent to the manufacture of the cable and/or during subsequent use. In use hydrogen within the fibre containing tubes 2 will tend to diffuse through the walls of tubes and into the tube 2', until the partial pressure of hydrogen is substantially equalised within the cable, the amount of hydrogen available to produce a detriment effect on the fibres 5 being accordingly reduced.

In a modification, the space surrounding the tubes 2 and encompassed by the metal tape 3 is purged of hydrogen, and all the tubes in such a case can contain one or more optical fibres. In a further modification the central strength member 1 is hollow and is formed of a hydrogen permeable material, and is itself purged of hydrogen.

The second cable is illustrated in Figure 2, and comprises an extrusion of plastics material having a core 6 in which is embedded a central strength member 1, and four ribs 7 extending outwards to join an outer wall 8, so as to form four channels 9.

The extrusion is enclosed by a metal tape 3, either wrapped longitudinally around the extrusion or wound helically around it with overlap, and is surrounded in turn by an extruded plastics sheath 4.

As in the previous embodiment, all the channels 9 except one accommodate one or more optical fibres, and the exception 9' is purged of hydrogen in a similar manner to the tube 2' of the first embodiment, so as to reduce the amount of hydrogen available in the other channel 9.

It will, however, be appreciated that the invention can be applied to other constructions of optical fibre cable apart from those described above.

Moreover the tube 2' bf the first embodiment and/or the space around the tubes, and the chan nel 9' of the second embodiment, may contain a hydrogen trapping material as above described.

Where palladium is used for such a purpose it can be provided by palladised carbon, that is to say particles of charcoal coated with palladium, distrib uted along the cable.

Claims

1. An optional fibre cable having at least one optical fibre located in a channel or groove formed in an extrusion of plastics material permeable to hydrogen and surrounded by an outer sheath, wherein there is provided within the sheath at least one channel the interior of which is or has been purged of hydrogen.

2. An optical fibre cable according to Claim 1 wherein the hydrogen purged channel is provided by a channel or groove containing one or more op tical fibres.

3. An optical fibre cable according to Claim 1 wherein the hydrogen purged channel is provided by a channel devoid of optical fibres and having hydrogen permeable walls.

4. An optical fibre cable according to Claim 1 wherein the hydrogen purged channel is provided by the space or spaces between the extrusion or extrusions and the outer sheath.

5. An optical fibre cable according to Claim 1 wherein the or each channel that contains one or more optical fibres is provided by the bore of an extruded plastics tube, and the purged channel is also in the form of a tube, the tubes being wound helically around a central strength member.

6. An optical fibre cable according to Claim 1 incorporating a plurality of channels formed as longitudinally-extending openings in a common extrusion of plastics material, at least one of which contains one or more optical fibres, and at least one of which provides said purged channel.

7. An optical fibre cable according to Claim 6 wherein said channels are of helical or periodicallyreversed helical form.

8. An optical fibre cable according to any preceding Claim wherein the hydrogen purged channel contains a quantity of hydrogen-trapping material distributed along it.

9. An optical fibre cable according to Claim 8 wherein the hydrogen-trapping material consists of finely divided manganese dioxide, titanium, zirconium or palladium.

10. An optical fibre cable according to any preceding Claim incorporating water-absorbing or water-blocking material.

11. In an optical fibre cable according to any preceding Claim the purging of hydrogen is carried out by the passage through the respective channel of a gas or gas mixture containing no, or a negligible amount of, hydrogen.

12. In an optical fibre cable according to Claim 11 wherein purging is effected by passing said gas or gas mixture through the respective channel either continuously or at intervals whilst the cable is in use.

13. An optical fibre cable substantially as shown in and as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawing.