Optical fibre cable

Abstract

An optical fibre cable 10 comprises a plurality of optical fibres 24 enclosed within a protective coating 26, surrounded by woven aramid fibre 20, itself surrounded by a sheath of plastics material 12

Description

FIELD OF THE INVENTION The present invention relates to an optical fibre cable, especially one suitable for carrying communications data. BACKGROUND ART The design of optical fibre cabling requires a number of factors to be balanced. The fibre itself, which carries the data signal, is relatively fragile and needs to be protected from damage caused by the local environment and during handling, installation, and use. Thus, a certain degree of strength is needed, but for some installation contexts such cables are also required by telecommunications operators to have a break strength below an upper threshold so that in the event of an accident the cable fails prior to structural damage being caused to the physical infrastructure. For example, if an over-height vehicle strikes an overhead cable, the telecommunications operator will prefer to see the cable fail rather than allow it to also pull down the long line of supporting poles from which it is suspended. A maximum tensile strength permitted by one UK telecommunications operator is 2kN , for example. In addition, metallic content within the cable is ideally to be avoided as this limits the usability of the cable in close proximity to electrical power distribution networks. One of the advantages of optical communications methods is that - in principle - they can be used in electrically noisy environments and thus the inclusion of an electrically conductive longitudinal strengthening member (which will be susceptible to induced currents) would be undesirable. It is known to use aramid fibre reinforcement in an optical fibre cable, and optical fibres with strength members within them are shown (for example) in US2013 / 077922, JP2018-017797 and EP4212931. US2013 / 077922, for example, shows an ADSS optical fibre cable with a central strength member of rigid epoxy / fibreglass composite material, surrounded by multiple fibre bundles, in turn surrounded by a peripheral strength member in the form of an aramid yarn. US2013 / 077922 discusses options for increasing the tensile strength of the fibre as required for particular contexts, whereas this is of course undesirable SUMMARY OF THE INVENTION The present invention therefore provides an optical fibre cable, comprising a plurality of optical fibres enclosed within a protective coating, surrounded by woven aramid fibre, itself surrounded by a sheath of plastics material containing an embedded longitudinal aramid-fibre strength member. This provides a cable which need not contain any electrically conductive members, can be designed so that a desired upper strength limit can be complied with, and which can retain a small outer diameter and light weight, while being soft and flexible / easy to bend. We prefer that, subject to the desired maximum strength, that there are two strength members embedded within the sheath. Ideally these are embedded within the sheath in radially opposed locations either side of the woven aramid fibre. However, the number of embedded strength members is a useful and convenient way of tailoring the overall strength of the cable to the desired level. The or each strength member ideally has a thickness of at least 0.5mm, which we find to be straightforward to manufacture and provides the requisite strength. The woven aramid fibre layer ideally has a thickness of 1.4mm±0.1mm, i.e. between 1.3 and 1.5mm. The plurality of optical fibres can comprise a group of between 4 and 12 fibres. The protective coating can be partly or wholly in the form of a surrounding tube for each fibre; these can each be provided with a different external colour in order to aid identification of the fibres. The external colour for each tube can be provided by a coloured sheath carried by each tube. The tubes are ideally encased within a gel compound which may form a further or alternative element of the protective coating, in order to keep the tubes or fibres supported and prevent physical damage due to movement, etc. A layer of water-impermeable tape can be provided within the woven aramid fibre, ideally located between the fibres and the substrate and / or between the fibres and the sheath. A suitable aramid is poly-paraphenylene terephthalamide. Suppliers of aramid yarn include Dupont, Teijin, Kolon etc. Kevlar® (DuPont) is a widely used aramid yarn. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; Figure 1 shows a partially-exploded view of a cable according to the present invention; and Figure 2 shows a section through the cable of figure 1. DETAILED DESCRIPTION OF THE EMBODIMENTS Referring to figure 1, a cable 10 according to the present invention is made up of a number of layers and elements within it. Working from the outside inwards, these comprise the following. First, an outer sheath 12 of black UV-stabilised HDPE in accordance with BS EN 50290 2 24. This provides excellent abrasion resistance if the cable should rub against nearby structures such as trees. HDPE of this type is also free of hazardous substances according to RoHS 2002 / 95 / EG and there are knowns ways to add anti rodent protection. In this examples, the cable outer sheath 12 has a diameter of 5.0mm and a thickness of 1.3mm. Embedded in the outer sheath are two non-metallic strength members 14, 16 in the form of flexible elongate rods of poly-paraphenylene terephthalamide (Kevlar™), specifically Kevlar 49 Aramid Yarn 1580D. The two rods are located radially symmetrically within the outer sheath 12, i.e. 180° apart, one on either opposite side. The precise number of rods, their thickness and density, etc, can be tailored to provide the cable 10 with the required tensile strength. A first water-blocking tape layer 18 of is provided within the outer sheath 12, wound around the elements within. An aramid sheath 20, also in this example of a Kevlar yarn, is provided within the outer sheath 12. Layers of the aramid polymer fibre are laid up longitudinally over the optical fibre. This principally provides a basic level of strength to the cable 10, tailored by the strength members 14, 16 together with physical protection for the fibres within, but also provides a degree of water blocking to protect the fibres form environmental degradation. The sheath can be formed by moulding the aramid fibre layer around the elements within it; suitable mould sleeves can control the dimensional stability process of the sheath with different layers subjected to different release tensions and release forms as needed. A second water-blocking layer 22 of is provided within the aramid sheath 20, in the form of a water-swelling compound on the inner surface of the aramid sheath. At the core of the cable 10, a set of four optical fibres 24a, 24b, 24c, 24d are jacketed within tubes 26a, 26b, 26c, 26d. Each tube has a coloured sheath 28a, 28b, 28c, 28d on its exterior to aid in identification of the fibres at each end of the cable 10, or can be coated or infused with ink. These are embedded in a gel-filling compound 30 which fills the void around and between each fibre tube 26 up to the water block tape layer 22. A wide range of cablefilling gels are available from numerous suppliers in this field. In this example, the tubes are each 2.0mm and the fibres within are 250pm ITU-T G.657A2 fibres. In order to prevent direct contact between the optical fibre and the outside world, and further provide better protection for the fibre, it is advisable to protect the fibre in a coating including both gel (also known as fibre paste) and a tube of material such as polybutylene terephthalate (PBT). This therefore provides a round-profile drop cable containing four tight jacketed fibres for installation in network infrastructure. It is intended primarily for overhead (OH) installation but is equally suitable for underground (UG) application. It can be fitted with factory installed hardened connectors to interface with existing CBT equipment, ideally factory-fitted but potentially also field-fit connectors. The tensile load to break value can be tailored so that the same cable in an overhead application does not exceed 2kN but in an underground application has sufficient strength for direct pulled installation As the cable has a round profile, there is no need to apply anti-galloping twists as are required for previous figure-of-8 hybrid copper / fibre cable and other flat cable designs. The cable design detailed is of an all dielectric construction, ensuring compliance with electrical tests or other requirements for use in the vicinity of power lines, etc. 5 In this example, a 4-fibre arrangement is illustrated, but other numbers of fibres can be accommodated easily, such as 8, 12 or more fibres. It will of course be understood that many variations may be made to the abovedescribed embodiment without departing from the scope of the present invention.

Claims

1. An optical fibre cable, comprising a plurality of optical fibres enclosed within a protective coating, surrounded by woven aramid fibre, itself surrounded by a sheath of plastics material containing an embedded longitudinal aramid-fibre strength member.

2. An optical fibre cable according to claim 1 in which there are two strength members embedded within the sheath.

3. An optical fibre cable according to claim 2 in which the two strength members are embedded within the sheath in radially opposed locations either side of the woven aramid fibre.

4. An optical fibre cable according to any one of the preceding claims in which the or each strength member has a thickness of at least 0.5mm.

5. An optical fibre cable according to any one of the preceding claims in which the woven aramid fibre has a thickness of between 1.3 and 1.5mm.

6. An optical fibre cable according to any one of the preceding claims in which the plurality of optical fibres comprises a group of 4, 8 or 12 fibres.

7. An optical fibre cable according to any one of the preceding claims in which the protective coating comprises a plurality of tubes, each enclosing one of the plurality of optical fibres.

8. An optical fibre cable according to claim 7 in which the tubes each have a different external colour.

9. An optical fibre cable according to claim 8 in which the external colour for each tube is provided by a coloured sheath carried by each tube.

10. An optical fibre cable according to any one of the preceding claims in which the protective coating further comprises a gel compound surrounding the plurality of optical fibres.-7-11. An optical fibre cable according to any one of claims 7 to 9 in which the protective coating further comprises a gel compound within which the tubes are embedded.

12. An optical fibre cable according to claim 10 or claim 11 in which the gel compound substantially fills the space within the woven aramid fibre layer.5 13. An optical fibre cable according to any one of the preceding claims comprising a layerof water-impermeable tape within the woven aramid fibre layer.

14. An optical fibre cable according to claim 11 comprising a layer of water-impermeable tape located between the woven aramid fibre layerand the substrate.

15. An optical fibre cable according to claim 11 or claim 12 comprising a layer of water-10 impermeable tape located between the woven aramid yarn and the sheath.

16. An optical fibre cable according to any one of the preceding claims in which the aramid is poly-paraphenylene terephthalamide.

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