OPTICAL CABLES

Abstract: An optical fibre cable, especially an aerial cable, has, within an outer sheath (3), four or five strength members (1) with at least one optical fibre package (6, 6.1) accommodated in an interstitial space formed by the strength members (1). Such a cable can be constructed to have sufficient tensile strength for installation between towers up to 1000m apart.

Description: This invention relates to optical cables, and especially to such cables of the kind known as aerial cables which are designed to be freely suspended from spaced supports which may be up to 1000 metres apart. It is desirable for such aerial cables to be relatively light in weight, to reduce loading when so suspended. It has been proposed to incorporate optical fibres in overhead conductors, for example as in GB-A-2064163, the fibres in such a case being accommodated in tubes within a polyethylene sheath containing strength members, and the sheath being surrounded by layers of helically wound conductor wires. The conductor wires, in addition to carrying electrical current serve as a protective covering for the optical fibre unit and also provide a substantial degree of tensile strength. It is also known from EP 0060180 to locate optical fibres in the interstitial spaces between adjacent generally metallic strength members and a surrounding sheath, in order to provide protection of the fibres against high pressures, but in such a case there is a risk of the fibres being entrapped between the respective strength members. This document does not however suggest that similar construction could be used to provide a completely metal-free aerial cable. According to the present invention an optical aerial cable comprises a cylindrical outer sheath, at least three longitudinally extending cylindrical strength members accommodated within and forming interstitial spaces with said outer sheath, with each strength member bearing against only two other strength members and against the inner surface of said outer sheath, and a plurality of optical fibres accommodated in an interstitial space between adjacent strength members, characterised in that said plurality of optical fibres in a said interstitial space are loosely accommodated in a protective tube to form an optical fibre package, the strength members, the outer sheath and the protective tube are of non-metallic material, and the radius of the optical fibre package is substantially smaller than that of the strength members such that part of the interstitial space separates the package from the inner surface of the outer sheath. Preferably the tube containing the fibres is filled with a water blocking substance such as grease. A water blocking substance may also be incorporated in the interstices between adjacent strength members, and between the strength members and the outer sheath. The strength members must, of course, provide an adequate resistance to stretching of the cable to prevent an undue stress being applied to the fibres when the cable is suspended between two supports spaced apart the maxiumum distance for which the cable is designed to be used. Moreover it is particularly desirable for aerial cables to have the minimum overall diameter for a given total cross-sectional area of the strength members, but at the same time there should be adequate interstitial space for the optical fibre package or packages. We have found that with strength members of circular cross-section this is conveniently achieved with four or five strength members, these preferably being formed of glass reinforced plastic or other suitable high modulus material stranded together in a helical configuration. In the case of an aerial cable incorporating four circular-sectioned strength members, optical fibre packages are conveniently stranded around the strength members in the interstitial channels formed between adjacent members and the inner surface of the surrounding sheath, the depth of the channels being such that the optical fibre packages are spaced from the inner surface of the sheath. This protects the fibres against external pressure during installation and use of the cable. The strength members may, for example have a diameter of about 5 millimetres, this permitting a tube of approximately 2 millimetres external diameter, and containing, say, four optical fibres, to be accommodated in the said interstitial channels without contacting the inner surface of the sheath. A cushioning member may be provided in the central channel between the four stranded strength members in such a case. One form of aerial cable in accordance with the invention will now be described by way of example with reference to the accompanying drawing, which represents a transverse section through the cable. The cable comprises four circular-sectioned strength members 1 of glass-reinforced plastics material, each having a diameter of approximately 5mm stranded together around a central filler 2 of polyethylene having a diameter of approximately 2mm, and surrounded by a close fitting polyethylene or other polymer sheath 3 having a wall thickness of approximately 1.5mm. Within the four channels 4 formed between adjacent pairs of strength members 1 there are located four optical fibre elements 5, each of which comprises a bundle of four single mode optical fibres 6 within a 2mm diameter tube 7 of a suitable polymer having a filling of grease or other water blocking compound which allows a degree of transverse movement of the fibres within the tube. The depth of the channel 4 is such that the optical fibre elements 5 are spaced from the inner surface of the surrounding sheath 3, and thereby protected against external pressure. The interstitial spaces between the strength members 1, optical fibre elements 5 and the sheath 3 may also be filled with a suitable water-blocking compound to prevent water penetration along the cable. The sheath may contain a suitable filler to provide an adequate level of resistivity. The strength members may, in some cases, be coated with a plastics material to give added mechanical and/or environmental protection.

Claims: An optical aerial cable comprising a cylindrical outer sheath (3), at least three longitudinally extending cylindrical strength members (1) accommodated within and forming interstitial spaces with said outer sheath, with each strength member bearing against only two other strength members and against the inner surface of said outer sheath, and a plurality of optical fibres (6) accommodated in an interstitial space (4) between adjacent strength members, characterised in that said plurality of optical fibres (6) in a said interstitial space are loosely accommodated in a protective tube (7) to form an optical fibre package (5), the strength members (1), the outer sheath (3) and the protective tube (7) are of non-metallic material, and the radius of the optical fibre package is substantially smaller than that of the strength members such that part of the interstitial space separates the package from the inner surface of the outer sheath. An optical cable according to Claim 1 characterised in that each tube (7) is filled with a water blocking substance. An optical cable according to Claim 1 or 2 characterised in that a water blocking substance is incorporated in the interstitital spaces between adjacent strength members (1), and between the strength members (1) and the outer sheath (3). An optical cable according to Claim 2 or 3 characterised in that the water blocking substance is a grease substantially filling the tube (7). An optical cable according to any preceding claim characterised in that it has four or five strength members (1) of circular cross-section. An optical cable according to any preceding claim characterised in that the strength members (1) are formed of a glass reinforced plastics material stranded together in a helical configuration. An optical cable according to Claim 6 characterised in that it has four circular-sectioned strength members (1), incorporating a plurality of said optical fibre packages (6) stranded around the strength members (1) in the interstitial channels formed between adjacent members (1) and the inner surface of the surrounding sheath (3). An optical cable according to Claim 8 characterised in that the strength members (1) surround a cushioning member (2).

Inventors: PEACOCK, ALAN JAMES
Applicants: TELEPHONE CABLES LIMITED
Application year: 1987
Main classification: G02B6
IPC codes: G02B6