GB2091903A

GB2091903A - Optical fibre cable

Info

Publication number: GB2091903A
Application number: GB8202301A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1981-01-27
Filing date: 1982-01-27
Publication date: 1982-08-04
Also published as: GB2091903B

Abstract

A method of manufacturing an optical cable with a fibre loosely fitting inside which comprises folding an advancing tape (5) transversely in such a way that adjacent longitudinal edges of the tape butt or overlap to form a tube (8); feeding into the tube an optical fibre (1) at a speed greater than the speed of the tube whereby the length of the fibre within the tube is greater than that of the tube, the relative cross- sectional sizes of the tube and the optical fibre being such that the optical fibre is loosely housed in the tube; and extruding around the tube an elongate plastics body (11) which bonds to the outer surface of the tube. <IMAGE>

Description

SPECIFICATION Optical fibre cable This invention relates to the manufacture of optical cables for the transmission of the ultraviolet, visible and infra-red regions of the electromagnetic spectrum, which regions for convenience, will hereinafter all be included in the generic term "light", and especially, but not exclusively, to optical cables for use in the communication field adapted fortransmis- sion of light having a wavelength within the range 0.8 to 1.9 micrometres.

In one form of optical cable that has been proposed one or more than one separate optical fibre and/or one or more than one optical bundle is or are loosely housed in a bore of a tube or other extruded elongate body of rubber or plastics material, the arrangement being such that when the cable is flexed limited relative movement can take place between the or each separate optical fibre and/or between the or each optical bundle and the tube or other extruded elongate body.

By the expression "optical bundle" is meant a group of optical fibres or a group of fibres including one or more optical fibres and one or more nonoptical reinforcing fibres or other reinforcing elongate elements.

When the aforesaid form of optical cable is wound onto a drum for storage, the optical fibre or bundle tends to settle within the tube in such a manner that, for a given number of turns of the cable, its length is less than that of the tube. Thus, as the cable is subsequently unwound, tensile stress can build up within the or each fibre risking mechanical distortion (which can affect its optical properties) and, if no relief is provided, ultimately breakage.

According to the present invention, we provide an improved method of manufacturing an optical cable, which method comprises causing at least one tape to travel in the direction of its length; folding the advancing tape or tapes transversely or otherwise arranging the tape or tapes in such a way that adjacent longitudinal edges of the tape or tapes butt or overlap to form a tube; feeding into the tube at least one optical fibre at a speed greater than the speed of the tube whereby the length of the fibre/s within the tube is greater than that of the tube, the relative cross-sectional sizes of the tube and the optical fibre/s being such that the optical fibre/s is or are loosely housed in the tube; and extruding around the tube so formed an elongate body of rubber or plastics material which bonds to the outer surface of the tube.

Preferably the fibre is fed into the tube at a speed greater than that of the tube by between 0.1 and 1%, particularly between 0.3 and 0.5%, especially 0.4%.

As the optical cable made by this method has a length of fibre within the tube greater than the length of the tube itself, even when the cable is wound onto a drum stress during subsequent unwinding is minimised. Thus, when the cable is being installed, it can be stretched without imparting tensile forces on the optical fibre.

The tape may comprise metal or metal alloy; alternatively, the tape may comprise a plastics material of low elasticity.

Preferably, the or each tape of metal or metal alloy has a layer of rubber or plastics material on at least one of its major surfaces and the or each tape is so transversely folded or otherwise arranged that the layer of rubber or plastics material of the or each tape is outermost so that, when the elongate body of rubber or plastics material is extruded around the tube, a firm bond is achieved between the materials of the layer on the or each tape and the elongate body. The or each tape may also have a layer of rubber or plastics material on its inwardly-facing surface so that the tube formed therefrom will have a lining of rubber or plastics material. The tape may be a polyethylene and aluminium laminate.

Important advantages which may be obtained by bonding the extruded elongate body to an underlying metal tube are that the tube serves to prevent substantial longitudinal shrinkage of the extruded elongate body whilst it is cooling which might otherwise cause an optical fibre or optical fibres loosely housed in the bore of the elongate body to wrinkle or kink and perhaps fracture; the risk that an optical fibre will stick to the internal surface of the extruded elongate body is eliminated; feeding of an optical fibre into the tube as it is being formed substantially facilitates manufacture of the optical fibre cable; and the tube constitutes a circumferentially continuous moisture-proof barrier.

Preferably, the tube is formed by causing a single tape to advance in a direction substantially parallel to the direction of travel of the advancing optical fibre or fibres and folding the tape transversely so that its longitudinal edges abut or overlap.

Where the optical cable is to include two or more separate optical fibres, the optical fibres may be secured side by side on or within at least one substantially flat flexible support member or metallic or non-metallic material, the supported fibres being fed into the tube as it is being formed and the relative cross-sectional sizes of the tube and the supported fibres being such that the supported fibres are loosely housed in the tube.

Usually, but not necessarily, the or each optical fibre will have a protective coating of enamel, lacquer or other suitable hardened material.

To facilitate feeding of the or each separate optical fibre into the tube, preferably the leading end of the optical fibre or of the tape supporting the optical fibres is lightly bonded to the inside surface of the tube by a suitable adhesive.

Where it is desired to fill the interstices in the tube with a greasy filling medium, for instance petroleum jelly, to reduce the risk that the or any optical fibre may be scratched, scored or otherwise damaged as a result of sliding contact with another optical fibre or other component of the cable when the cable is flexed, the greasy filling medium may be introduced into the tube during manufacture of the cable, preferably at the die or other means by which the tube is formed.

The or each tape forming the tube is preferably of aluminium or aluminium alloy or copper or copper alloy. In some circumstances, to enhance the flexibil ity of the cable, the tube may have transversely extending corrugations and the or each tape may be formed with transversely extending corrugations as it is advancing towards the die or otherwise arranged to form a tube.

At least one separate elongate reinforcing member may be fed into and embedded in the body as it is extruded.

The optical fibre/s may constitute the component part of an optical bundle which is incorporated in a cable embodying the present invention.

The invention also includes an optical cable made by the method hereinbefore described.

The invention is further illustrated by a description, by way of example, of the preferred method of manufacturing an optical cable with reference to the accompanying drawing, in which: Figure lisa schematic representation of the preferred method; and Figure2 is a transverse cross-sectional view of an optical cable manufactured by the method shown in Figure 1, drawn on an enlarged scale.

Referring to Figure 1, an enamel-coated optical fibre 1 under a controlled tension is fed over guide pulley 3 to a tube-forming die 4where a polyethylene-coated aluminium tape 5 (fed from supply reel 6 over guide pulley 7) is folded transversely around the optical fibre in such a way that the longitudinal edges of the tape butt and that a polyethylene coating 8 (Figure 2) forms an outer plastics coating of the tube 9 so formed. Fibre 1 is fed into die 4 at a speed 0.4% greater than the speed at which the tube 9 exits from die 4, so that the length of fibre 1 in tube 9 is 0.4% greater than the length of the tube 9 itself.

On emerging from the die 4, the tube 9 passes to an extruder 10 where a sheath 11 of plastics material is extruded around the tube, the plastics material of the sheath bonding firmly with the plastics coating 8 on the outer surface of the tube. The rate of production of the optical cable is about 15 metres per minute.

The greater length of fibre 1 within the tube 9 upon manufacture ensures that the length of the fibre is not less than the mean length of the tube once the cable is wound onto a drum; when the cable is being unwound from the drum for installation, tensile stress (which can cause distortion or breakage) is minimised or prevented altogether.

As will be seen on referring to Figure 2, the optical cable manufactured by the method described with reference to Figure 1 comprises an optical fibre 1 loosely housed in a composite tubular body of shorter length than the fibre and comprising the aluminium tube 9 formed by the transversely folded tape 5 and the plastics sheath 11 bonded firmly to the plastics coating 8 on the outer surface of the tube 9.

Claims

1. A method of manufacturing an optical cable which comprises causing at least one tape to travel in the direction of its length; folding the advancing tape or tapes transversely or otherwise arranging the tape or tapes in such a way that adjacent longitudinal edges of the tape or tapes butt or overlap to form a tube; feeding into the tube at least one optical fibre at a speed greater than the speed of the tube whereby the length of the fibre/s within the tube is greater than that of the tube, the relative cross-sectional sizes of the tube and the optical fibre/s being such that the optical fibre/s is or are loosely housed in the tube; and extruding around the tube so formed an elongate body of rubber or plastics material which bonds to the outer surface of the tube.

2. A method of manufacturing an optical cable which comprises causing at least one tape having a layer of rubber or plastics material on at least one of its major surfaces to travel in the direction of its length; folding the advancing tape ortapestrans- versely or otherwise arranging the tape or tapes in such a way that adjacent longitudinal edges of the tape or tapes butt or overlap to form a tube; feeding into the tube at least one optical fibre at a speed greater than the speed of the tube whereby the length of the fibre/s within the tube is greater than that of the tube, the relative cross-sectional sizes of the tube and the optical fibre/s being such that the optical fibre/s is or are loosely housed in the tube; and extruding around the tube so formed an elongate body of rubber or plastics material which bonds to the outer surface of the tube.

3. A method as claimed in Claim 1 or 2, wherein a single tape advancing in a direction substantially parallel to the direction of travel of the advancing optical fibre/s is folded transversely so that its longitudinal edges butt.

4. A method as claimed in any one of the preceding Claims, wherein the or each tape has a layer of rubber or plastics material on its inwardlyfacing surface so that the tube formed therefrom has a lining of rubber or plastics material.

5. A method as claimed in any one of the preceding Claims, wherein at least two separate optical fibres are secured side by side on or within at least one substantially flat flexible support member of metallic or non-metallic material, the supported fibres being fed into the tube as it is being formed and the relative cross-sectional sizes of the tube and the supported fibres being such that the support for the fibres is loosely housed in the tube.

6. A method as claimed in any one of the preceding Claims, wherein a greasy filling medium is introduced into the tube during manufacture of the cable at the die or other means by which the tube is formed.

7. A method as claimed in any one of the preceding Claims, wherein the or each tape is formed with transversely extending corrugations as it is advancing towards the die or other means by which it is transversely folded or otherwise arranged to form a tube.

8. A method as claimed in any one of the preceding Claims, wherein the or each tape is of aluminium or aluminium alloy or of copper or copper alloy.

9. A method as claimed in any one of the preceding Claims, wherein at least one separate elongate reinforcing member is fed into and embedded in the body as the body is extruded.

10. A method of manufacturing an optical cable substantially as hereinbefore described with reference to the accompanying drawing.

11. An optical cable when manufactured bythe method claimed in any one of the preceding Claims.