AU594532B2 - Optical fibre submarine cable - Google Patents

Description

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FOR OFFICE USE: Class Int. Class Application Number: 9**4 0 0 *0 6' 0 0 0 00 PH9029 Lodged: 18 November 1986 Accepted: Published: Priority: Related Art: Name and Address of Applicant: 0 0 The Overseas Telecommunications Commission (Australia) 32-36 Martin Place i EE STAMF To VALUE OF Sydney New South Wales 2000 T ATTACHED

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000 Actual Inventor: Address for Service: Peter Samuel Atherton, Donald Ross Nicol, Spruson Ferguson, Patent Attorneys, Level 33 St Martins Tower, 31 Market Street, Sydney, New South Wales, 2000, Australia Complete Specification for the invention entitled: OPTICAL FIBRE SUBMARINE CABLE The following statement is a full description of this invention, including the best method of performing it known to me/us 5815/2 L

ABSTRACT

Undersea optical fibre cable wherein optical fibres at the core of the cable are enclosed by an electrically conductive sleeve which in turn is encased by a layer of polymeric material the thickness of which is solely determined by the electrical field strength therein, and the tensile strength of the cable is determined principally by layers of helicaly wound, contra-rotating wires enclosing the polymeric material.

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This invention relates to optical fibre cable, and more particularly to such cables designed for deepsea use.

Submarine cables used for communications over long distances by means of fibre optics must be designed to: protoct the optical fibres from tensile forces, hydrostatic pressure, localised crushing forces, fishbite and chemical degradation; (ii) conduct electrical power to the optical signal regenerators (or repeaters); (iii) survive the laying and recovery processes in great depths of water (up to and possibly exceeding 8000 metres) without damage to the optical fibres.

In conventional designs of undersea optical fibre cables copper o is used as the principal electrical conductor and steel is used as the **09 J5 principal tensile strength member. Hydrostatic pressure resistance is o provided by both the copper and steel. An outer polyethylene layer is S provided which serves for electrical insulation and abrasion resistance. However, this outer layer does not provide much protection against fishbite which can seriously affect the function of the cable.

Although these cables comprise an optical fibre core and an outer covering of polyethylene between which are disposed a tubular copper

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conductor and helically wound wire strands, it has been proposed to modify this construction by incorporating the principal tensile strength member as an outer casing. That is to say, that the cable is provided with an external strength member, but such a proposal has been found unsatisfactory in the past in the case where the strength member is applied in the form of steel wires. This is mainly because with

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larger cables it is difficult to apply a suitably small amount of steel since manufacturing constraints impose a lower limit on the wire size.

In addition it is necessary to apply more than 1 layer of wires in order to torsionally balance the cable, and this leads to difficulties in coiling the cable for storage.

It is the main object of the invention to provide an optical fibre cable which will adequately meet the requirements for undersea service.

In accordance with the invention there is provided an undersea optical fibre cable comprising a core section including a plurality of optical fibre strands, an electrically conductive tube surrounding said core section, an annular layer of dielectric material about said GMG/272r 3conductive tube, and at least one layer of a tensile strength member enclosing said annular layer, said cable being characterised in that said annular layer is of extruded polymeric material having a thickness determined solely to ensure that the electric field strength therein does not exceed a maximum value.

In one embodiment of the present invention utilising 2 contra-rotating layers of steel wires the problems outlined above are overcome by reducing the cable diameter and hence the diameter at which the steel wires are applied. These diameter reductions are allowable largely as a result of recent improvements in optical fibre strength, permitting the fibres to undergo larger strains than in the past without undue damage.

The invention will be described in more detail with reference to the accompanying drawings, in which: Fig. 1 shows in cross-section a first form of conventional to optical fibre cable; Fig. 2 similarly represents a second form of conventional cable; and Figs. 3 and 4 show in cross-section constructions of optical fibre cables according to the present invention.

The known forms of optical fibre cables shown in Figs. 1 and 2 differ mainly in the relative disposition of their copper conductors and steel strengthening layers. The cable form shown in Fig. 1 incorporates a closed and welded copper tube 6 about the core 5 which 25' hermetically isolates and protects the cable's optical fibres 4. Nhen in service the tube 6 will conduct electrical power to undersea repeaters with a seawater return path. Tensile strength for the cable is provided by several layers of helically wound steel strands 7, which also conduct some of the electrical power and which may be of different gauges. The steel strands encircle the copper conductor 6. A polyethylene outer covering 8 is extruded over the layers of steel strands 7 to provide electrical insulation and for abrasion resistance.

Similar reference numerals are used with respect to the other known form of cable construction shown in Fig. 2, which differs from that described above only in that the layers of steel strands 7 are enclosed within the copper conductor tube 6. In both of these cable forms the thickness of the polyethylene layer 8 is determined principally to ensure that in addition to providing electrical insulation, a sacrificial outer layer is included to provide abrasion resistance.

GMG/272r 4 A first form of cable constructed in accordance with this invention is shown in Fig. 3 where, it will be noted, the core containing fibres 4 is enclosed by a copper tube 6 in a similar manner to the prior art form shown in Fig. 1. However, in this instance the extruded layer 8, of any suitable polymeric material, is applied directly to the copper tube 6 with a tensile strength member applied as two helically applied contra-rotating wire layers 7, as an external armour. More than two layers may be provided if desired. An important consideration in the design of this cable is that the layer 8 is maintained as thin as possible commensurate solely with the requirement for providing adequate electrical insulation. As in this instance the layer 8 is not required to provide abrasion resistance, such as with conventional cables, much less material need be used. Provision for a maximum electric field strength within the layer of approximately ;i 3000V DC/mm would be adequate.

'Studies have shown that an (pticel fibre cable constructed and designed according to this principle can be made servicable for long distance, high reliability communication systems, and, due to its smaller diameter, resulting from reduced thickness of the polymeric layer 8, it becomes possible to contruct cables of from 10mm to O.D. which are suitable for use in deep water in long distance, high s reliability communications systems. Some advantages which flow from this form of cable construction are good abrasion resistance and protection against fishbite. The electrical circuit to the repeaters 25" in any communication system utilising this form of cable will be closed by a seawater path.

Fig. 4 shows another form of cable construction where the tensile C strength member is applied as a first layer 7 of wires with an incomplete second outer 7ayer composed of only two wires 9, which may be diametrically opposed. Even a single outer wire 9 may be used with the primary purpose of the outer layer being to provide torsion balance for the inner complete layer 7.

Nhereas a preferred embodiment has been described in the foregoing passages it should be understood that other forms, modifications and refinements are feasible within the scope of this invention. It will, of course, be appreciated that the conductor tube 6 may be composed of any suitable material, such as aluminium or copper, and the outer tensile strengthening layer 7 to be composed of GMG/272r 5 any suitable material such as steel, aluminium or aramid fibres embedded in a suitable matrix. Furthermore, as an altenative to the layer, or layers, 7 of wires, the tensile strength member may be composed of a metallic tape or braiding forming one or both layers 7.

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Claims (6)

1. 2. A cable according to claim 1, wherein said tensile strength member includes two helicaly applied contra-rotating wire layers.
2. 3. A cable according to claim 1, wherein said tensile strength member includes a complete lVayer of helicaly wound wires, and an outer incomplete layer of one or more wires wound over said complete layer and deployed to provide torsion balance for said complete layer. a 4. A cable according to claim 2 or 3, wherein said wires are composed of metal. A cable according to claim 2 or 3, wherein said wires are composed of aramid fibres.
3. 6. A cable according to claim 1, wherein said tensile strength member is metallic tape, or braiding.
4. 7. A cable according to claim 6, wherein said metal tape is helicaly wound.
5. 8. A cable according to any one of the preceding claims, wherein said ft. maximum value of e'ectric field strength is 3,000V DC/mm.
6. 9. An undersea optical fibre cable substantially as hereinbefore described with reference to Fig. 3, or Fig. 4, of the o accompanying drawings. DATED this SIXTEENTH day of NOVEMBER 1987 OVERSEAS TELECOMMUNICATIONS COMMISSION AUSTRALIA Patent Attorneys for the Applicant SPRUSON FERGUSON GMG/272r -7-