GB2164471A

GB2164471A - Optical fibre cables

Info

Publication number: GB2164471A
Application number: GB08521212A
Authority: GB
Inventors: Edward James Rodney Seville
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1984-09-14
Filing date: 1985-08-23
Publication date: 1986-03-19
Also published as: GB8521212D0; GB8423311D0; AU4711685A

Abstract

An optical fibre cable comprises a protective cover (1) in the form of an extrudate of thermoplastics material having a plurality of channels (5), and in at least one of the channels a plurality of optical fibres (8) disposed within a strip (6) of plastics material which may also be formed as a single extrusion or two strip-like sections secured together. Electrical conductors may also be incorporated in one or more of the channels (5), additional to, or in place of the optical fibre strips. <IMAGE>

Description

SPECIFICATION Optical fibre cables This invention relates to optical cables, that is to say cables of the kind incorporating one or more optical fibres accommodated within a protective cover, and relates also to the manufacture of such cables.

According to the invention an optical cable comprises a protective cover in the form of an extrudate of thermoplastics material having a plurality of longitudinally extending channels, and in at least one of said channels a plurality of optical fibres disposed within a strip of plastics material.

The optical fibres are preferably loosely accommodated in channels extending longitudinally within the enclosing strip, and preferably have a length which is greater than that of the channels so that they follow an undulating path within the channels, thereby reducing the risk of the fibres being subjected to an excessive tensile force during manufacture or subsequent use of the cable. The strip may also incorporate one or more strength members of any convenient material extending along it.

The strip may also be formed as an extrudate of thermoplastics material with the fibres introduced into respective channels in the strip during the extrusion process.

Alternatively the strip may be formed in two separate strip-like sections, at least one of which is formed with a plurality of grooves in which the fibres are laid, the two sections being secured together to enclose the grooves with the fibres accommodated therein.

The channels in the strip may incorporate a water-repellent or water-blocking medium, conveniently of a powder or jelly-like nature.

Conveniently a plurality of strips are located in respective channels in the protective cover, and more than one strip may be disposed in at least one of the channels.

The or each strip may be helically twisted within the respective channel of the protective cover.

The latter may be extruded with closed channels with the strip or strips introduced into the respective channels during the extrusion process.

Alternatively the cover may be formed with an outer wall having slots which provide access to the channels, the strip or strips being introduced into the channel or channels through the slots.

In either case the cover may be surrounded by at least one protective sheath.

The invention will be further explained by describing by way of example, with reference to Figs. 1 to 4 of the accompanying schematic drawing, in which Figure 1 represents a transverse cross-section of one form of cable in accordance with the invention, Figures 2 and 3 represent, on an enlargedscale alternative forms of optical fibre strips for use in the cable, and Figure 4 represents a cross-section of another form of cable.

Referring first to Figs. 1 and 2, the cable comprises a cover 1, formed as an extrusion of thermoplastics material, for example polyethylene or pvc, with a central core 2 having a plurality, in this case four, Jongitudinally-extending, uniformly spaced fins 3 radiating therefrom and joined at their outer ends to a circular-sectioned outer wall 4.

Each of the channels 5 formed between adjacent fins 3 contains, in accordance with the invention, a strip 6 of plastics material having a series of spaced channels 7 disposed sideby-side within it, and each of these channels accommodates an optical fibre 8, the fibres being loose within the respective channels, and having a length which is slightly greater than that of the channels to avoid them being subjected to any excessive tensile stresses.

Each strip 6 may also contain a elongate strength member of any suitable material, for example Kevlar or helically wound steel strands, the strength member preferably being disposed along the strip axis as shown at 9 in Fig. 2.-The core 2 of the cover 1 may also have a strength member (not shown) embedded within it.

The strips 6 may be formed as extrusions with the optical fibres 8 introduced into the respective channels during the extrusion process. Alternatively, as shown in Fig. 3, the strip may be formed as two separate sections 11, 12 with one of the sections provided with longitudinally-extending grooves 13 into which the optical fibres 8 are inserted and the other being secured to the first section over the grooves to form separate closed channels.

Each strip 6 may be helically twisted within the respective cover channel 5, or may follow an undulating path within it. In addition more than one strip may be accommodated within one or more of the channels.

Electrical conductors may also be incorporated in one or more of the channels 5, additional to, or in place of, the optical fibre strips 6.

It will be appreciated that although the cover 1 has been shown with four fins 3 and channels 5 other numbers of fins and channels may be employed if desired.

In a modification illustrated in Fig. 4 the wall 4 of the cover 1 is formed with longitudinally extending slots 14, which permits the optical fibre strips 6 to be introduced separately from the extrusion process.

The cover 1 in such a case can be initially extruded with the slots 14, or it may be extruded with closed channels and the slots formed subsequently.

In each of the examples illustrated in Fig. 1 and 4 the cover 1 may be surrounded by a water impermeable barrier layer, for example in the form of a metal strip folded longitudinally or wound helically around the cover, and this could be enclosed in turn by an extruded outer sheath.

Although in each of the embodiments above described the individual fibres are loosely accommodated in respective channels. In a modification of the cable illustrated in Fig. 3 the fibres are sandwiched between two tapes, which conveniently also have one or more strength members also sandwiched between them. The fibres may be arranged parallel to the edges of the tapes or may have an undulating path. Other modifications are clearly possible.

Claims

1. An optical cable comprises a protective cover in the form of an extrudate of thermoplastics material having a plurality of longitudinally extending channels, and in at least one of said channels a plurality of optical fibres disposed within a strip of plastics material.

2. An optical cable according to Claim 1 wherein the fibres are loosely accommodated in channels extending longitudinally within the enclosing strip.

3. An optical cable according to Claim 2 wherein the optical fibres have a length greater than that of the channels in which they extend so that they follow an undulating path within the channels.

4. An optical cable according to Claim 1, 2 or 3 wherein the strip incorporates one or more strength members.

5. An optical cable according to any preceding Claim wherein the strip is formed as an extrudate of thermoplastics material with the fibres introduced into respective channels in the strip during the extrusion process.

6. An optical cable according to any one of Claims 1 to 4 wherein the strip is formed in two separate strip-like sections, at least one of which is formed with a plurality of grooves in which the fibres are laid, the two sections being secured together to enclose the grooves with the fibres accommodated therein.

7. An optical cable according to Claim 2 wherein the channels in the strip incorporate a water-repellent or water-blocking medium.

8. An optical cable according to any preceding Claim wherein the or each strip is helically twisted within its respective channel in the cover.

9. An optical cable according to any one of Claims 1 to 7 wherein the or each strip follows an undulating path within its respective channel in the cover.

10. An optical cable according to any preceding Claim in which the channels in the cover are closed, the strips having been introduced into the channels during the extrusion of the cover.

11. An optical cable according to any one of Claims 1 to 9 wherein the cover is formed with an outer wall having slots which provide access to the channels.

12. An optical cable according to any preceding Claim wherein the cover is surrounded by at least one protective sheath.

13. An optical cable according to Claim 12 wherein the cover is surrounded by a metal strip folded longitudinally around the cover or wound helically around it, the sheath surrounding the metal strip.

14. An optical cable substantially as shown in and as hereinbefore described with reference to Fig. 1 or 4 of the accompanying drawing.

15. An optical cable as claimed in Claim 14 incorporating an optical fibre strip substantially as shown in and as hereinbefore described with reference to Fig. 2 or 3 of the accompanying drawing.