GB2184863A

GB2184863A - Optical fibre cable

Info

Publication number: GB2184863A
Application number: GB08626773A
Authority: GB
Inventors: Alan James Peacock; Terence Hall
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1985-12-18
Filing date: 1986-11-10
Publication date: 1987-07-01
Also published as: GB8626773D0; GB8531148D0

Abstract

An optical fibre cable has within an outer sheath a central strength member 1 surrounded by a cushioning layer 3, for example of resilient foamed plastics material, and between the cushioning layer 3 and the outer sheath 8 at least one elongate spacer member 4 extending helically around the cushioning layer 3, the optical fibres being disposed in bundles 5 each of which is encased in a further cushioning layer 6 and wound helically around the first cushioning layer 3 in the space between it and the surrounding sheath 8. Instead of the cushioning layers 6, the fibre bundles may be enclosed by a tape of resilient foamed plastics material. In a modification, the spacers 4 may be omitted and the fibre bundles confined between cushioning layer 3 and an outer tape of resilient foamed plastics material. <IMAGE>

Description

SPECIFICATION Optical Fibre Cables This invention relates to optical fibre cables of the kind comprising a plurality of bundles of one or more optical fibres disposed within an outer sheath, and incorporating also at least one longitudinally extending strength member for reducing or preventing tensile forces on the fibres.

According to the invention such an optical fibre cable comprises, within the outer sheath, a central longitudinally extending strength member surrounded by a cushioning layer and the latter is spaced from the outer sheath by at least one elongate spacer member of relatively harder material extending helically around the cushioning layer, the fibres being disposed in a plurality of bundles each of which is also encased within a cushioning layer and wound helically around the central cushioning layer within the space between it and the surrounding sheath.

The presence of the helically extending spacer or spacers, which may be formed of solid polyethylene or PVC, protects the cushioned fibre bundles against crushing.

Each bundle of fibres may be individually surrounded by a separate cushioning layer, but a common cushioning layer, which may be in the form of a tape of suitably resilient material, may be associated with a plurality of fibre bundles the layer conveniently being wound helically around the fibre bundles after they have been wound around the central cushioning layer.

It will be understood that in the specification and appended claims the word helically" includes within its scope a configuration in the form of a helix in which the direction of winding changes periodically, known generally as an s/z winding as well as one which is wound continuously in one direction.

Both the central cushioning layer and the cushioning layers around the fibre bundles preferably consist of a resilient foamed plastics material, and the cushioned fibre bundles preferably have a radial dimension which is slightly less than the radial dimension of the spacers.

The strength member preferably has a coating of plastics material. The central cushioning layer may be in the form of a foamed plastics tape helically wound or longitudinally wrapped around the coated strength member.

The composite structure comprising the strength member, spacer or spacers and the encase fibre bundles may be surrounded by a helically wound or longitudinally wrapped tape prior to the application of the surrounding sheath, the latter is conveniently formed of an extruded plastics material with a barrier of plastic coated metal foil.

The interstices between the coated fibre bundles and the spacers may contain a water blocking and/or hydrogen trapping substance.

These may be in the form of powder introduced by dusting the surfaces of the coated fibre bundles prior to wrapping them around the strength member, One optical fibre cable in accordance with the invention will now be described by way of example with reference to the accompanying schematic drawing, which represents a transverse section through the cable.

The cable comprises a central longitudinally extending strength member 1, which may be metallic or non-metallic having a coating 2 of plastics material extruded around it.

A tape 3 of resilient foamed plastics material is wrapped helically around the coated strength member, and around this tape are wound two circular-sectioned extruded spacers 4 of solid plastics material. A plurality of fibre bundles 5 each encased in a surrounding cushioning layer 6 of resilient foamed plastics material are wound round the foamed plastics tape between the spacers 4, the fibre bundle structures 5, 6 having a diameter slightly less than the diameter of the spacers. A tape 7 is wound helically around the spacers 4 and the fibre bundle structures 5, 6, and the whole assembly is surrounded by an extruded outer sheath 8 which may, if desired, incorporate a moisture barrier laminate (not shown) in known manner.

A water blocking substance and/or a hydrogen trapping substance, conveniently in powder form may be introduced into the cable prior the application of the outer sheath, for example by dusting the exterior of the fibre bundle structures with such a powder. In some cases however the interstices may be completely filled.

The spacers 4 together with the cushioning layers 6 serve to protect the fibre bundles 5 against crushing, and as the space between the fibre bundle structures 5, 6 is restricted only a limited amount of water-blocking and/or hydrogen trapping material is required.

Instead of the central tape 3 being wound helically around the coated strength member, it may be folded longitudinally around it. Other modifications are clearly possible.

Thus instead of the fibre bundles 5 being provided with separate cushioning layers, the fibre bundles, which may individually comprise aplurality of fibres secured together by a suitable bonding medium, may first be wound helically around the central cushioning layer provided by the tape 3, and a further tape of resilient foamed plastics material wound around the outside of, so as to enclose, a plurality of fibre bundles. For example two such tapes may be employed in the cable illustrated, each enclosing all the fibre bundles on a respective side of the two spacers 4.

In a modification of this form of cable the spacers may be omitted, the fibre bundles ef fectively being confined between two cushioning layers provided by the inner tape 3, and an outer helically wound or longitudinally wrapped tape of resilient foamed plastics material. In some cases the fibre bundles may be sandwiched between two tapes of resilient foamed plastics material or other suitable cushioning material and the composite fibre sandwich wound helically or wrapped longitudinally around the central strength member.

Claims

1. An optical fibre comprising, within an outer sheath, a central longitudinally extending strength member surrounded by a central cushioning layer, said central cushioning layer being separated from the outer sheath by at least one elongate spacer member extending helically around the cushioning layer, and a plurality of bundles of optical fibres, each of which is encased within a fibre cushioning layer and wound helically around the central cushioning layer within the space between the central cushioning layer and the outer surrounding sheath.

2. An optical fibre cable, as claimed in Claim 1, wherein the or each helically extending spacer member is formed from plastics material.

3. An optical fibre cable, as claimed in Claim 1 or 2, wherein the other sheath is formed of an extruded plastics material.

4. An optical fibre cable, as claimed in Claim 3, wherein the outer sheath has a barrier of plastic coated metal foil.

5. An optical fibre cable, as claimed in Claim 1, 2, 3 or 4, wherein the cushioning layers are formed from a resilient foamed plastics material.

6. An optical fibre cable, as claimed in any preceding claim, wherein each bundle of fibres is surrounded by a separate cushioning layer.

7. An optical fibre cable, as claimed in any one of Claims 1 to 5, wherein a plurality of optical fibre bundles are surrounded by a common cushioning layer.

8. An optical fibre cable, as claimed in Claim 7, wherein the common cushioning layer is a tape of resilient material.

9. An optical fibre cable, as claimed in Claim 8, wherein the common cushioning layer is wound helically around the fibre bundles.

10. An optical fibre cable, as claimed in any preceding claim, wherein the central strength member has a coating of plastics material.

11. An optical fibre cable, as claimed in any preceding claim, wherein the cushioned fibre bundles have a radial dimension which is slightly less than the radial dimension of the spacers.

12. An optical fibre cable, as claimed in any preceding claim, wherein the central cushioning layer is in the form of a foamed plastics tape, helically wound or longitudinally wrapped around the central strength member.

13. An optical fibre cable, as claimed in any preceding claim, wherein the composite structure which comprises the strength member, the spacer or spacers and the encased fibre bundles is surrounded by a helically wound or longitudinally wrapped tape within the surrounding sheath.

14. An optical fibre cable, as claimed in any preceding claim, wherein the interstices between the fibre bundles and the spacers contain a water blocking substance or a hydrogen trapping substance or both of these substances.

15. An optical fibre cable, as claimed in Claim 14, wherein at least one of the water blocking and hydrogen trapping substances is in the form of a powder on the surfaces of the fibre bundles.

16. An optical fibre cable comprising, within an outer sheath, a central longitudinally extending strength member, surrounded by a central cushioning layer and a plurality of optical fibre bundles confined between the central cushioning area and an outer helically wound or longitudinally wrapped tape or resilient foamed plastics material.

17. An optical fibre cable comprising, within an outer sheath, a central longitudinally extending strength member and a composite optical fibre tape, wound helically or wrapped longitudinally around the central strength member, wherein the composite optical fibre tape is made by sandwiching optical fibre bundles between two cushioning layers.

18. An optical fibre cable as hereinbefore described and illustrated in the accompanying drawing.