GB2168824A - Optical fibre cables containing an electrical conductor - Google Patents

Abstract

When a water-blocking medium such as petroleum jelly is provided in a an optical cable 1 of the type comprising one or more optical fibres loosely enclosed within at least one of a plurality of longitudinally extending channels 6 formed in an extrudate (an integral cable) the jelly tends to restrict the freedom of movement of the fibres. By enclosing the fibre(s) 8 within a tube 9 in the channel(s) 6 the fibre(s) 8 are allowed to move freely even when the channels 6 are filled with jelly. One or more other channels 6 include electrical conductors 11 therein so that an electrical/optical cable is provided which has a smooth circular outer profile. <IMAGE>

Description

SPECIFICATION Optical cables This invention relates to optical fibre cables and more particularly to cables of the type in which one or more optical fibres are accommodated within at least one of a plurality of longitudinally extending open or closed channels formed in an extrudate.

Such cables are known from, for example, our co-pending patent application No. 2123978A in which the optical fibres are loosely accommodated within the channels, the fibres being introduced into the channels during the extrusion process and having a greater natural length than the corresponding channel.

It is often desired to fill the channels with a water-impermeable medium such as petroleum jelly but this restricts the freedom of movement of the fibres. It is thus an object of the invention to provide an optical cable wherein such freedom of movement of the fibres is not lost.

There is now also a requirement for optical ca bles capable of carrying electrical conductors and in view of the disparity in size between, for exam ple copper quads, and optical fibres, there is a difficulty in manufacturing cables in which the conductors and fibres are stranded together.

It is thus an object of the present invention to provide an optical cable which may be easily man ufactured so as to include electrical conductors as well as optical conductors.

Accordingly, the invention provides an optical cable comprising an extrudate formed with a plu rality of longitudinally extending channels, with an optical fibre unit, comprising at least one optical fibre loosely accommodated within a tube, or at least one optical fibre ribbon, extending along at least one of the channels, and at least one electrical conductor extending along at least one other channel.

By electrical conductor, it is meant any electrical conducting member whether consisting of a single conducting wire or a plurality of conductors within an outer coating of insulant. The insulated conductors may be singles or twisted to form pairs of quads. Thus in a preferred embodiment, the electrical conductor comprises four insulated electrical conducting wires formed into a star quad.

By optical fibre ribbons is meant an elongate element of width substantially greater than its thickness, and comprising a plurality of optical fibres secured side-by-side in the form of a ribbon or tape.

Preferably, the channels are filled with a waterimpermeable compound such as petroleium jelly, although they could alternatively be partially or completely filled with a powder which may be either water-blocking or hydrogen-trapping.

The tube made also be filled with a water-repellent fluid.

One embodiment of an optical cable according to the invention will now be more fully described by way of example with reference to the drawing.

The optical cable 1 is composed of an extrudate 2 having a centre portion 3 extruded around a central strength member 4, a plurality of fins 5 extending raidally away from the centre portion 3, so as to form channels 6 between them and an outer portion 7 closing off the channels 6. Thus, the extrudate 2 has a cross-section in the shape of a cartwheel with channels formed in the sectors between the fins. In this embodiment there are four such channels 6 but of course any desired number may be provided, as is more fully explained in GB 2123978. In the case of the open channel version the outer portion 7 is removed or formed with slots.

A plurality of optical fibres 8, in this embodiment two fibres, are accommodated within a plastics tube 9 so as to form an optical fibre unit. The optical fibres are of greater length than the tube so that they are loose within the tube. An optical fibre unit is provided in at least one of the channels 6 (in this embodiment two of the channels) by feeding them in.

A plurality of electrical conductors 11, each in an insulating sheath 12, is provided in at least one of the other channels 6 (again two channels in this embodiment).

As can be seen, the tube 9 can be considerably smaller than the electrical conductor element without affecting the mechanical performance of the cable or its physical size or shape.

Each of the channels contains a water blocking or hydrogen absorbing compound 13 which does not impare the relationship between the fibre protected within its tube and mechanical stresses outside the cable sheath which may be encountered during manufacture, installation or the working life of the cable.

The tube 9 may itself be filled with a water impermeable fluid if desired.

The extrudate 2 may be surrounded by a protective sheath 14 if desired over which any of the conventional forms of armouring or protection including protection against magnetic or electrostatic interference may be applied.

By accommodating the optical fibres within tubes in this way, there is no loss of freedom of movement of the fibre, when the channel is filled with petroleum jelly as happens when the fibres are directly embedded in the jelly.

Further, by accommodating both the electrical conductors and the optical conductors within a cable of this type, the disparity in size does not matter and a cable with a smooth circular outer diameter is provided.

In place of one or more of the tubed optical fibres a channel may contain an optical fibre ribbon, comprising, for example, six, eight or ten, plastics coated optical fibres secured side-by-side, for example as described in copending patent application No. 8506499. Such a fibre ribbon may be of undulating form within the respective channel, in order to reduce the risk of the fibres being damaged if the cable is subjected to a tensile force or is bent.

Claims (10)

1. An optical cable comprising an extrudate formed with a plurality of longitudinally extending channels, with an optical fibre unit, comprising at least one optical fibre loosely accommodated within a tube, or at least one optical fibre ribbon, extending along at least one of the channels, and at least one electrical conductor extending along at least one other channel.

2. An optical cable according to Claim 1, wherein at least one of the longitudinally extending channels is filled with a water-impermeable compound.

3. An optical cable according to Claim 2, wherein the water-impermeable compound is petroleum jelly.

4. An optical cable according to Claim 1, wherein at least one of the longitudinally extending channels is partially or completely filled with powder.

5. An optical cable according to Claim 4, wherein the powder has water-blocking or hydrogen-trapping properties or both.

6. An optical cable according to Claim 1, wherein the electrical conductor consists of either a single conducting wire or a plurality of conductors within an outer coating of insulant.

7. An optical cable according to Claim 6, wherein the insulated conductors are either singles or twisted to form pairs or quads.

8. An optical cable according to Claim 7, wherein the electrical conductor comprises four insulated electrical conducting wires formed into a star quad.

9. An optical cable as claimed in any preceding claim, wherein the extrudate is surrounded by a protective sheath.

10. An optical cable substantially as herein described with reference to the accompanying drawing.