GB2105484A

GB2105484A - Optical fibre cables

Info

Publication number: GB2105484A
Application number: GB08126499A
Authority: GB
Inventors: James Garrard Titchmarsh; John Nigel Russell; Melvin Murray Ramsay
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-09-01
Filing date: 1981-09-01
Publication date: 1983-03-23
Also published as: GB2105486A; NZ201636A; AU8774182A; GB2105486B; AU553588B2

Abstract

An optical cable has a central core 11 surrounded by a layer which includes both optical fibres 12 and longitudinal strength members 13, e.g. formed from strands of a high strength plastics material. This layer may be surrounded in turn by a water barrier 15 and by sheathing layers 16, 17, 18. The core 11 may be an optical fibre. As shown layer 14 comprises wrapping tape. <IMAGE>

Description

SPECIFICATION Optical cables The invention relates to optical cables.

A typical optical cable incorporates one or more coated optical fibres, strength members to give structural strength and stability to the cable, a water barrier to protect the fibre or fibres against ingress of water and sheathing. The strength members can be strands of steel or other high strength material which may be wrapped helically round the optical fibres. The helical wrapping provides stability for the strenth members within the cable but results in a radial compressive load on the part of the cable within the helix when the cable comes under tensile load. This compressive load may be unacceptable, particularly if it can tend to crush two or more fibres together and because it can cause microbending.

An object of the invention is to provide an optical cable in which strength members are arranged in favourable locations.

According to the invention an optical cable includes optical fibres and strength members arranged in a common layer of the cable.

Preferably the optical fibres and strength members are wound helically round a core. The core may itself be an optical fibre or an additional strength member.

Preferably the strength members are of high strength plastics material and preferably they are cords each formed from strands of this material.

Preferably the layer containing optical fibres and strength members is surrounded by a water barrier which is in turn surrounded by sheathing.

An embodiment of the invention will now be described with reference to the accompanying drawing which is a cross-section of an optical cable embodying the invention.

A mono-filament in the form of a secondary coated optical fibre 11 is arranged centrally and forms a core. The core 11 is surrounded by an annular layer made up of four coated secondary optical fibres 12 interspersed between four longitudinally extending strength members 1 3.

The strength members 13 are formed from strands of a high strength plastics material such as aromatic poly-amide fibres formed into cords. In this embodiment we used Kevlar (RTM) cords. The strength members 1 3 are formed initially with an approximated circular cross-section but take on a shape conforming generally to the spaces within the layer between two adjacent optical fibres 12 when they are assembled into the cable. The optical fibres 12 and strength members 1 3 are wound helically round the core 11.

A layer 14 of wrapping tape surrounds the layer of optical fibres 12 and strength members 13. This tape is wound in a helix of the optical fibres 12 and strength members 1 3 and confines the strength members. The tape function to hold the cable together prior to sheathing.

An aluminium-polymer laminate wrapped round the layer of tape 14 provides a water barrier 1 5 to prevent ingress of water into the region of 'the optical fibres.

The cable as thus far described can be sheathed in any convenient way. In the example shown in the drawing, there is an inner cable sheathing layer 16 of black polyethelene. This is surrounded by a paper tape layer 1 7 made up of longitudinal paper tapes. The paper tape layer 1 7 is itself surrounded by an outer sheathing layer 1 8 of black polyethelene.

The provision of the strength members near the centre of the cable provides good flexibility for the cable without building up high local compressive loads. Also, when the cable and in particular the helically wound strength members 13 are subjected to tension, the tendency for the helix of the strength members to close up and thereby produce a compressive load only produces such a load directly on the central core 11. There is no question of compressing one optical fibre against another or against any other hard and possibly damaging component, and the possibility of microbending is minimised if not eliminated.

In a typical example the diameter of the core is 2 mm, the optical fibre is 125 mm, the O.D.

diameter of the secondary coating on the fibre is 2 mm and the initial diameter of the strength members 13 is 1.1 mm. The overall diameter of this cable, including sheathing is 13 mm. The wrapping tape and the polymer of the water barrier may both be polyethelene.

The central core 11 is rigid in relation to the soft flexible strength members 1 3.

In an alternative construction in which the capacity requirement for the cable does not require four secondary optical fibres, one or more of these optical fibres may be replaced by a filler member of similar size and mechanical properties to the optical fibres.

1. An optical cable which includes optical fibres 5 and strength members arranged in a common layer of the cable.

2. An optical cable as claimed in claim 1 wherein the optical fibres and strength members are wound helically round a core.

3. An optical cable as claimed in claim 2 wherein the core is itself an optical fibre.

4. An optical cable as claimed in any preceding claims wherein the strength member is of a high strength plastics material.

5. An optical cable as claimed in claim 4 wherein the strength members are formed as cords from strands of the material.

6. An optical cable as claimed in any of the preceding claims wherein the layer containing the optical fibres is surrounded by a water barrier

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical cables The invention relates to optical cables. A typical optical cable incorporates one or more coated optical fibres, strength members to give structural strength and stability to the cable, a water barrier to protect the fibre or fibres against ingress of water and sheathing. The strength members can be strands of steel or other high strength material which may be wrapped helically round the optical fibres. The helical wrapping provides stability for the strenth members within the cable but results in a radial compressive load on the part of the cable within the helix when the cable comes under tensile load. This compressive load may be unacceptable, particularly if it can tend to crush two or more fibres together and because it can cause microbending. An object of the invention is to provide an optical cable in which strength members are arranged in favourable locations. According to the invention an optical cable includes optical fibres and strength members arranged in a common layer of the cable. Preferably the optical fibres and strength members are wound helically round a core. The core may itself be an optical fibre or an additional strength member. Preferably the strength members are of high strength plastics material and preferably they are cords each formed from strands of this material. Preferably the layer containing optical fibres and strength members is surrounded by a water barrier which is in turn surrounded by sheathing. An embodiment of the invention will now be described with reference to the accompanying drawing which is a cross-section of an optical cable embodying the invention. A mono-filament in the form of a secondary coated optical fibre 11 is arranged centrally and forms a core. The core 11 is surrounded by an annular layer made up of four coated secondary optical fibres 12 interspersed between four longitudinally extending strength members 1 3. The strength members 13 are formed from strands of a high strength plastics material such as aromatic poly-amide fibres formed into cords. In this embodiment we used Kevlar (RTM) cords. The strength members 1 3 are formed initially with an approximated circular cross-section but take on a shape conforming generally to the spaces within the layer between two adjacent optical fibres 12 when they are assembled into the cable. The optical fibres 12 and strength members 1 3 are wound helically round the core 11. A layer 14 of wrapping tape surrounds the layer of optical fibres 12 and strength members 13. This tape is wound in a helix of the optical fibres 12 and strength members 1 3 and confines the strength members. The tape function to hold the cable together prior to sheathing. An aluminium-polymer laminate wrapped round the layer of tape 14 provides a water barrier 1 5 to prevent ingress of water into the region of 'the optical fibres. The cable as thus far described can be sheathed in any convenient way. In the example shown in the drawing, there is an inner cable sheathing layer 16 of black polyethelene. This is surrounded by a paper tape layer 1 7 made up of longitudinal paper tapes. The paper tape layer 1 7 is itself surrounded by an outer sheathing layer 1 8 of black polyethelene. The provision of the strength members near the centre of the cable provides good flexibility for the cable without building up high local compressive loads. Also, when the cable and in particular the helically wound strength members 13 are subjected to tension, the tendency for the helix of the strength members to close up and thereby produce a compressive load only produces such a load directly on the central core 11. There is no question of compressing one optical fibre against another or against any other hard and possibly damaging component, and the possibility of microbending is minimised if not eliminated. In a typical example the diameter of the core is 2 mm, the optical fibre is 125 mm, the O.D. diameter of the secondary coating on the fibre is 2 mm and the initial diameter of the strength members 13 is 1.1 mm. The overall diameter of this cable, including sheathing is 13 mm. The wrapping tape and the polymer of the water barrier may both be polyethelene. The central core 11 is rigid in relation to the soft flexible strength members 1 3. In an alternative construction in which the capacity requirement for the cable does not require four secondary optical fibres, one or more of these optical fibres may be replaced by a filler member of similar size and mechanical properties to the optical fibres. CLAIMS

6. An optical cable as claimed in any of the preceding claims wherein the layer containing the optical fibres is surrounded by a water barrier which is itself surrounded by sheathing.

7. An optical cable substantially as described with reference to and as illustrated by the accompanying drawings.