OPTICAL FIBRE CABLE

Abstract: An optical fibre cable especially for submarine use has a core (1) surrounded by a layer (5, 6, 7) of strength members which include both wires (6) and laser-welded metallic tubes (7) containing the optical fibres (5}.

Description: This invention relates to optical fibre cables, and especially to such cables to be used in underwater applications. Underwater cables are required to have high strength and be resistant to high pressures, the action of the sea and damage from vessels. Conventionally, such cables are constructed with a fibre package protected by a seamless metal tube and layers of armouring wires. The metallic elements are insulated and protected by a thick polythene sheath. Where repeaters are needed, the power to the repeaters is supplied from the metallic elements. If the polythene sheath is damaged, there is a risk of the metallic elements being earthed. Thus, in order to provide satisfactory insulation of the wires, a thick sheath is used to encase the cable. The combined effect of the armouring wires and sheath is to produce a heavy and unwieldy cable. One optical fibre underwater cable has been disclosed in EP-A-0203538, the cable comprising a central cylindrical core with a layer of metallic members, including at least one tubular member containing an optical fibre, wound helically around the core, and surrounded by an outer sheath. In that cable the central core provides main tensile strength of the cable. It is an object of this invention to provide an alternative form of cable which is smaller and lighter than conventional cables, but in which the optical fibres are adequately protected against damage. According to the present invention, there is provided an optical fibre underwater cable comprising a central cylindrical core, with a layer of metallic members, including at least one tubular member containing an optical fibre, wound helically around the core, and surrounded by an outer sheath, characterised in that the layer of metallic members comprises a plurality of steel wires having interspersed between them at least one laser-welded steel tube within which is encased at least one said optical fibre, the steel wires and the steel tube or tubes providing a substantial part of the tensile strength of the cable. Preferably a second layer of strength members in the form of steel wires is provided around said layer of strength members, where higher mechanical strength is required. Preferably the two layers of strength members are wound helically in opposite directions, however both layers may be wound in the same direction. An additional insulating or bedding sheath layer may be provided between the two layers, making the cable easier to coil and handle. The core may comprise a central conducting member surrounded by an inner sheath, preferably formed of low density polythene, the conducting member then providing the conductor which carries current for operating the repeaters. This means that the thickness of the outer sheathing means, commonly of extruded plastics material, can be reduced, compared with known submarine cables, as it merely serves to provide electical insulation. Alternatively, to provide a cheaper cable, if no power feeding is required the core may be solid and non-conducting. Conveniently, there is a water blocking compound between the two layers of strength members and a layer of water-blocking compound between the optical fibres and the laser welded tube. This compound in the laser welded tubes may be a thixotropic grease. One embodiment of the invention will now be described by way of example only, with reference to Figure 1 which shows a section of the cable. The central copper conductor 1 is surrounded by a layer of low density polythene which constitutes the inner sheath 3. A first layer of high tensile steel wires 6 is wound helically about the inner sheath 3, a number of optical fibres 5 encased in laser welded stainless steel tubes 7 being interspersed between the wires 6. The steel tubes 7 and the wires 6 have the same diameter. A second layer of high tensile steel wires 11 is wound helically around the first layer in the opposite direction with a quantity of silicone water blocking compound 9 between the layer. The cable is encased in an outer sheath 13 formed of medium density polythene. The stainless steel tubes 7 protect the respective optical fibres 5 from damage; each tube replaces an armouring wire. Thus there is no need for a separate optical fibre package within the tube, enabling a lighter and more compact cable to be produced. Typically, stainless steel tubes of 1mm to 6mm diameter are used. These are filled with thixotropic grease and laser welded longitudinally to enclose the optical fibre. The tubes provide moderately hard, hermetically sealed fibre packages. The length of the helical layer of the first layer may be varied depending on the fibre strain relief required in the cable.

Claims: An optical fibre underwater cable comprising a central cylindrical core (1), with a layer of metallic members, including at least one tubular member containing an optical fibre, wound helically around the core, and surrounded by an outer sheath, characterised in that the layer (6, 7) of metallic members comprises a plurality of steel wires (6) having interspersed between them at least one laser-welded steel tube (7) within which is encased at least one said optical fibre (5), the steel wires and the steel tube or tubes providing a substantial part of the tensile strength of the cable. A cable according to Claim 1, in which the or each steel tube has the same diameter as the other strength members of the said layer. A cable according to Claim 1 or 2, comprising a second layer (11) of strength members in the form of steel wires around the said layer (6, 7) of strength members, the sheath (13) surrounding both layers. A cable according to Claim 3, in which the two layers are wound helically. A cable according to Claim 4, in which the two layers are wound helically in opposite directions. A cable according to Claim 3, 4 or 5, comprising a water-blocking compound (9) between the two layers of strength members. A cable according to any preceding claim, in which the central cylindrical core is a central conducting member surrounded by an inner sheath. A cable according to any preceding claim, in which a layer of water-blocking compound is provided between the optical fibre or fibres and the respective metallic tube.

Inventors: SMITH, LAWRENCE IAN
Applicants: TELEPHONE CABLES LIMITED
Application year: 1988
Main classification: G02B6
IPC codes: G02B6