GB2169094A

GB2169094A - Optical cables

Info

Publication number: GB2169094A
Application number: GB08526682A
Authority: GB
Inventors: Ronald Yaxley Gill
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1984-12-19
Filing date: 1985-10-30
Publication date: 1986-07-02
Also published as: GB8526682D0; GB8432124D0; GB2169094B

Abstract

An overhead optical cable is made up of any known optical cable unit 2 parallel to a strain member 1. The optical unit 2 is longer than the strain member 1 and only connected to it at intervals 7 along their length. The connection 7 may be achieved by running both parts through a Figure 8 extruder head so that both the unit 2 and the member 1 are sheathed by an extrudate, the two sheaths being joined together by a web of extrudate at predetermined intervals. <IMAGE>

Description

SPECIFICATION Optical cables This invention relates to optical cables and more particularly to optical cables which are to be suspended overhead from pylons or poles.

When such cables are suspended, any external loads cause tensile forces to act on the cable and these forces may be transmitted through to the optical fibres which may be damaged in consequence.

It is thus an object of the present invention to provide an optical cable which may be suspended between attachment links so that the part of the cable containing the fibres is not subjected to tensile forces.

Accordingly, the invention provides an optical cable comprising a first unit having a longitudinally extending strain member and a second slightly longer unit, containing at least one optical fibre and extending parallel to the first unit but only being attached thereto at intervals along their lengths.

In a preferred embodiment, the two units are both sheathed by an extrudate, such as a suitable plastic, the two sheaths being connected to each other by a web of extrudate at intervals along the length of the cable.

The second unit may be of any desired configuration but preferably consists of a plurality of tubes, each loosely containing one or more optical fibres, the tubes being stranded together around a strength member and held together by a helically wound tape.

The invention further provides a method of manufacturing an optical cable comprising the steps of feeding a strain member and an optical unit into an extruder in parallel, the optical unit being fed at a slightly greater rate than the strain member, extruding a plastic sheath around each of the strain member and the optical unit, and intermittently extending a web between the two sheaths so as to join them together.

One embodiment of an optical cable according to the invention will now be more fully described, by way of example, with reference to the drawings of which: Figure 1 shows a diagrammatic side elevation of an optical cable according to the invention and cross-sectional views at two longitudinally-spaced points along the cable as indicated; and Figure 2 shows schematically apparatus used for manufacturing the optical cable of Fig. 1.

Thus, as shown in Fig. 1, an optical cable comprises a strain member 1 and an optical unit 2. The optical unit 2 consists of several tubes 3, each loosely containing an optical fibre 4, the tubes being stranded together around a central strength member 5. The stranded tubes are held together by means of a tape 6 wound around them and the interstices may incorporate a water blocking medium (not shown) which may alternatively or additionally be provided within each of the tubes. As is shown in Fig. 1, the optical unit 2 is longer than the strain member 1 and is looped up at intervals and attached to the strain member at the joint regions 7.

This is achieved by coating each of the strain member 1 and the optical unit 2 with plastic sheaths 8 and 9. The two plastic sheaths are made separate from each other except at the joint regions 7 where they are joined together by a web 10 of plastic material.

As shown in Fig. 2, the cable is manufactured by feeding an optical unit 12 from a supply drum 14 in parallel with a strain wire 11 from a second supply drum 13 to an extruder 15 via an input capstan 16. Both the strain wire 11 and the optical unit 12 are fed through a "Figure 8" point and die assembly in the extruder die head 18. The die is fitted with a device to open and close the joining aperture of the "Figure 8" section. When the device is operated to open the aperture, the strain wire 11 and the optical unit 12 are joined together by a "Figure 8" plastic sheath, but when the aperture is closed the strain wire 11 and the optical unit 12 are plastic sheathed as separate items.The sheathed cable 17 then passes through a cooling trough 19 to an output capstan 20 through which the plastic coated strain wire 11 passes, but the plastic sheathed optical unit 12 travels outside the capstan 20. The complete cable is then taken up by a take-up drum 21.

During the plastic sheathing process, the output capstan 20 is maintained at a constant speed but the input capstan speed is cyclicly varied to be equal to or greater than the output capstan speed. During the period of equal throughput speeds the aperture of the "Figure 8" die is opened, thus joining the strain wire and the optical unit with a "Figure 8" sheath.

The aperture is then closed and the input capstan speed increased and decreased in a cyclic manner until both throughput speeds are again equal and the process continuously repeated.

Thus the strain wire and the optical unit are intermittently joined together at suitable intervals and length, with the optical unit forming a catenary between the joining "Figure 8" sections and the plastic sheathed strain wire running straight.

It will be appreciated that although the cable is described with a stranded-tube construction forming the optical unit, any other construction may be used if desired. For example the optical unit may comprise an extruded plastics core having either closed or open channels therein containing the fibres.

Claims

1. An optical cable comprising a first unit having a longitudinally extending strain member and- a second slightly longer unit, containing at least one optical fibre and extending parallel to the first unit but only being attached thereto at intervals along their lengths.

2. An optical cable according to Claim 1, wherein the two units are both sheathed by an extrudate, the two sheaths being connected to each other by a web of extrudate at intervals along the length of the cable.

3. An optical cable according to Claim 2, wherein the extrudate is a plastics material.

4. An optical cable according to Claim 1, 2 or 3, wherein the second unit consists of a plurality of tubes, each loosely containing one or more optical fibres, the tubes being stranded together around a strength member and held together by a helically wound tape.

5. A method of manufacturing an optical cable according to Claim 2, comprising the steps of feeding a strain member and an optical unit into an extruder in parallel, the optical unit being fed at a slightly greater rate than the strain member, extruding a plastic sheath around each of the strain member and the optical unit, and intermittently extending a web between the two sheaths so as to join them together.

6. An optical cable substantially as herein described with reference to the accompanying drawings.