Optical waveguides – Optical transmission cable – Tightly confined
Patent
1991-04-02
1992-01-14
Healy, Brian
Optical waveguides
Optical transmission cable
Tightly confined
385 99, 385105, 174 70R, 174 70S, 174 75R, G02B 644, H02G 300
Patent
active
050816950
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a bend restrictor for optical fibre cable, and in particular to a bend restrictor for preventing a submarine optical fibre cable bending too much at a joint.
A submarine optical fibre cable has a plurality of optical fibres at its centre, the optical fibres being surrounded by a copper tube, which in turn is surrounded by two layers of helically-wound steel wires which are encapsulated in plastics material. The steel wires carry longitudinal cable forces, and the copper tube carries electrical power to components such as repeaters spaced along the cable. The optical fibres are subject to a bend restriction, namely that they should not bend with a radius of curvature of less than about one meter, and so the cable itself is also limited in this way. This gives rise to problems at cable joints (a cable joint is a connection from cable to cable) and at termination joints (a termination joint connects a cable to a repeater, via what is known as a pig-tail). For example, a cable joint has a steel sleeve surrounding the joint, and the cables leaving this sleeve tend to bend sharply when subject to transverse forces, particularly as the cable is being laid or recovered around the forward or aft sheaves of a cable ship or around a cable engine drum. Similar problems arise with termination joints.
A known bend limiter is constituted by a boot made of artificial rubber. The boot is tapered sleeve-like member, which is about one meter in length, the boot having a steel coupling ring moulded into its wider end. The coupling ring is externally threaded for connection to the steel sleeve of a cable joint. The wider end of the boot is about 5 to 6 inches (127 to 152 mm) in diameter, and the bore of the boot is about 1.5 inches (38 mm), the dimensions being chosen to take standard commercial cables of diameter between 1 and 1.25 inches (25 and 31 mm). The known type of boot tapers so that the boot has a varying rigidity along its length. The conical shape of the boot also acts as a smooth contour profile to allow the diameter discontinuity of the cable-to-joint interface to pass through or around ship's machinery with much reduced risk of fouling. The boot rigidity is arranged to vary from a very high value at its wider end (to match the rigidity of the steel sleeve of the cable joint) to a relatively low value at its narrower end (to match the rigidity of the cable).
The disadvantage of this type of bend limiter is that different cables have different structures, and so have different flexibilities. This implies that they will have different allowable minimum bend radii, so that different boots are required for different cables. This is particularly disadvantageous for ships which are used to repair submarine optical fibre cables, as these ships will need to carry a large inventory of boots to suit the many different types of submarine cable presently in use. This problem is compounded by the fact that the British, French, Japanese and Americans all use different cable structures and different joints.
SUMMARY OF THE INVENTION
The present invention provides a bend restrictor for an optical fibre cable, the bend restrictor comprising an elongated sleeve-like member having an axial through bore for accommodating the cable, and a coupling ring for fixing the sleeve-like member to a rigid housing from which the cable extends, wherein the sleeve-like member is provided with a plurality of longitudinal bores, and wherein at least one of the bores is provided with a stiffening rod.
Advantageously, the sleeve-like member is made of a plastics material such as a cast polyurethane rubber, preferably a two-component, ambient temperature curing polyurethane elastomer having a Shore A Hardness, after 7 days' curing, of at least 72. Conveniently, the sleeve-like member has a length of substantially 1 m, the axial through bore of the sleeve-like member having a diameter of substantially 1.5 inches (38 mm), and the sleeve-like member tapering away from the coupling ring end thereof
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Furukawa et al., "Structural Design . . . Optical Fiber Cable Joint Box" J. of Lightwave Tech., vol. LT-2, No. 4, 8/84, pp. 550-555.
Niiro, "Optical Fiber Submarine Cable . . . " IEEE Journal, vol. SAC-1, No. 3, 4/83, pp. 467-478.
Gould, "Cable-to-Repeater Connection . . . " British Telecom Eng., vol. 5, 7/86, pp. 104-108.
British Telecommunications public limited company
Healy Brian
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