GB2179072A

GB2179072A - Optical fibre cables

Info

Publication number: GB2179072A
Application number: GB08609773A
Authority: GB
Inventors: Mahesh Kumar Ramniklal Vyas
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-08-16
Filing date: 1986-04-22
Publication date: 1987-02-25
Also published as: GB2179072B; JPS6255611A; GB8609773D0

Abstract

A strength member, for an optical fibre cable, comprises, for example, a matrix of silica glass fibre and aromatic polyamide yarn bound together by a resin. The uniaxial compressive strength of silica glass is substantially higher than its tensile strength, whereas aromatic polyamide has a very high tensile strength but zero compressive strength. By combining these materials or other materials with similar properties an all-dielectric strength member having high tensile strength and adequate compressive strength can be achieved.

Description

SPECIFICATION Optical fibre cables This invention relates to optical fibre cables and in particular to strength members for use therein.

A commonly used basic design of optical fibre cable, the so-calied "tight cable design", comprises an axial strain member surrounded by one or more layers of plastics coated optical fibres bound thereon with polyester tape and a plastics sheath overall. Location of the strength member along the cable axis provides the maximum of flexibility together with minimal tensile stress on the optical fibres.

Different types of application call for different requirements on the cable. In many land line applications no special dielectric or corrosion resistance requirements exist and the cable can, therefore, include metallic components, such as a metallic strength member.

For certain military and other applications, however, there is a need for an all dielectric optical fibre cable. An all dielectric cable is immune to electro-magnetic interference and does not attract lightning. The strength member used in such cables must therefore be non-metallic. Very few dielectric materials have been found with a high tensile strength and low elongation to qualify them as strength members to be used in optical cables. There are only a few commercially available strength members, which are based on glass reinforced polymer. However, they sometimes lack dimensional control and the tensile modulus is around 50GPa which is too low for some specific applications.

According to one aspect of the present invention there is provided a strength member for an optical fibre cable, the strength member comprising a combination of first and second dielectric materials, the first material being of relatively high compressive strength and relatively low tensile strength and the second material being of relatively high tensile strength and relatively low compressive strength.

According to a further aspect of the present invention there is provided a method of manufacturing a strength member for an optical fibre cable comprising forming a matrix of glass fibres and aromatic polyamide yarns, impregnating the matrix with a resin and shaping the resin impregnated matrix to a predetermined strength member profile by drawing it through a suitably cross-sectioned die and curing the resin whereby to maintain the predetermined profile.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates an arrangement of apparatus for producing a dielectric strength member according to the present invention, and Figure 2 to 11 illustrate cross-sections through different cable designs incorporating strength members according to the present invention.

The basic requirements of an all dielectric strength member are high tensile modulus, good compressive strength, good dimensional stability over long lengths and cost effective manufacture in long lengths.

The uniaxial compressive strength of silica glass is substantially higher than its tensile strength. Aromatic polyamidq, such as KEVLAR manufactured by DuPont, has a very high tensile modulus but virtually zero compressive modulus. By combining these two materials a material with both of their properties can be achieved, that is a composite strength member can be made which fulfills the requirements listed above for an all dielectric strength member.

The need for good compressive strength for the strength member arises because in a cable which is exposed to fairly high temperatures the sheath may contract appreciably, thereby inducing compressive forces in the cable core.

The strength member has therefore to be rigid enough to absorb this compression and avoid buckling of the whole cable structure. The magnitude of compressive stiffness of the strength member will depend on the type of cable structure in which it is employed. In a tight cable design where the strength member occupies the neutral axis and there are no other reinforcements, the strength member should be fairly rigid.

A key parameter of cable components is the product of the resultant Young's modulus E, in the axial direction and the cross-sectioned area A, of the component. The tensile or compressive capability of the overall cable being SE,A,. For the strength member to have sufficient compressive strength, the EA product of the strength member in compression should be at least five times the sum of the EA products of the other cable components in compression. In addition, to provide adequate ten siie strength the EA product of the strength member in tension should be about thirty times the sum of the EA products of the other cable components in tension.

We have found that by using a glass/aromatic polyamide composite strength member with at least 15-20% glass, a suitable all dielectric strength member can be achieved. A composite strength member formed from a straight layed glass/aromatic polyamide matrix bound together by a resin provides adequate tensile and compressive strength for most cable requirements. Whereas 15-20% glass in the composite matrix provides adequate strength, other amounts can be used with a maximum of the order of 60% by volume.

Fig. 1 illustrates one possible arrangement for manufacturing a composite strength member using a Pultrusion continuous manufacturing process. Rovings of glass and aromatic polyamide, for example KEVLAR 49, are disposed in racks 1. By means of reciprocating gripper-pullers 2 the glass fibres and polyamide yarns are pulled through a resin bath 3 and a die 4. The resin in bath 3 may be either thermally or UV radiation curable and may comprise, for example, urethane acrylate, epoxy acrylate or polyesters. The die 4 has an internal configuration to conform to the desired strength member profile. While passing through the die 4 the composite structure can be subjected to heat, as a result of heating the die, to initiate thermal curing for a resin requiring thermal cure.In order to ensure a straight lay of the fibres is achieved if so required the fibres may be threaded through suitably apertured lay plates 5. If a constant velocity infeed is required two capstan rollers suitably placed may be employed (not shown).

The mechanical properties of the strength member manufactured by this process depend on the combined effect of the amount of glass reinforcement used and its arrangement in the finished composite. When all the glass strands are laid parallel to each other, maximum strength and modulus are obtained in the filament direction. The resin is only used to bind the composite strength member elements together and does not occupy an appreciable section of the overall member.

Whereas the all-dielectric strength member described above comprises a matrix of glass fibre and aromatic polyamide yarn bound together by a resin, other combinations of first and second dielectric materilas, where the first material is of relatively high compressive strength and relatively low tensile strength and the second material is of relatively high tensile strength and relatively low compressive strength, may be employed. For example, a glass and self-reinforcing thermoplastic combination can be used. A self-reinforcing thermoplastic (SRP) is basically a thermotropic liquid crystal and by regulating the orientation one can obtain a required modulus. Thus an alldielectric strength member may be comprised by a series of parallel glass filaments coated with a highly orientated SRP or by another glass and SRP composite structure.Another possibility is a combination of glass and amorphous carbon or boron.

Such a composite strength member is particularly suitable for military and pressurised deep sea type cables. Figs. 2 to 6 show examples of different cable constructions where such a composite strength member can be used. The Fig. 2 construction comprises a central composite strength member 10 surrounded by a layer of UV acrylate on-line primary coated optical fibres 11 with a polyester or KEVLAR tape wrap 12 and a sheath 13.

The Fig. 3 construction comprises a central composite strength member 20, a layer of UV acrylate primary coated fibres 21 with parallel lay and a polyester or KEVLAR tape wrap 22 within a closed aluminium C-section 23 and thus suitable for insertion in submarine cable.

The constructions of Figs. 2 and 3 are based on conventional tight cable design but in view of the use of a composite strength member they have potential improvements in microbending loss in comparison with structures where the fibres are stranded around a steel strength member.

Fig. 4 illustrates a single packaged optical fibre cable. A jacketed fibre 30 is embedded in a sheath 31 in which are disposed symmetrically placed four KEVLAR/glass composite strength members 32. Such a cable structure could, for example, be used in naval or military applications where additional requirements such as gas or pressure blocking are essential.

Fig. 5 illustrates a construction with only one UV acrylate on-line coated fibre 41, either a single hard coating or a double on-line coat soft/hard, that is a relatively soft inner coat and a relatively hard outer coat. Around the fibre 41 is a cushion layer 42. A plurality of composite strength members 43 are disposed on the cushion layer 42 with parallel lay, and a sheath 44 is provided. The construction of Fig. 5 may be considered to be a miniaturised version of the Fig. 4 construction.

A variant of the Fig. 2 structure is shown in Fig. 6 where the fibres 51 are secondary coated to 0.85mm or 1mm using NYLON or another suitable polymeric coating, and thus packaged. The packaged fibres 51 are disposed around a composite strength member 52. A cushioning layer 53 is provided between the fibres 51 and a sheath 54.

A particular glass and SRP composite that can be envisaged comprises SRP loaded with glass, which composite is extruded to required cross-sections, for example circular to provide strength members as illustrated in Figs. 2 to 6. A slotted strength member structure 70 as illustrated in Fig. 7 is particularly suitable for aerial cables. An optical fibre 71 may be retained in a single slot 72 in member 70 by means of a suitable tape wrapping 73, and a sheath 74 provided therein. Alternatively the glass and SRP composite may be extruded to form a slotted core structure 80 as illustrated in Fig. 8. An optical fibre 81 may be disposed in each slot such as 82 and retained therein by a suitable tape wrapping 83, with a sheath 84 provided thereon. Figs. 9 and 10 illustrate gas blocking structures in which fibres 90 and 100, respectively, are disposed in bores within an extruded composite 91 and 101, respectively. If desired some of the structures, Figs. 8 and 9 in particular, may also include a metallic strength member (indicated by a dotted circle) in cases where an all dielectric structure is not required. Another possible composite strength member structure merely comprises an extruded tube 110 (Fig. 11) of, for example, SRP loaded with glass, in which optical fibres 111 are loosely arranged.

Claims

1. A strength member for an optical fibre cable, the strength member comprising a combination of first and second dielectric materials, the first material being of relatively high compressive strength and relatively low tensile strength and the second material being of relatively high tensile strength and relatively low compressive strength.

2. A strength member as claimed in claim 1 and comprising a matrix of glass fibre and aromatic polyamide yarn bound together by a resin.

3. A strength member as claimed in claim 2 wherein the glass fibres and aromatic polyamide yarns are straight layed in the matrix.

4. A strength member as claimed in claim 2 or claim 3 and including at least 15 to 20% glass fibre in the matrix.

5. A strength member as claimed in any one of claims 2 to 4 wherein the resin is urethane acrylate or epoxy acrylate or polyesters.

6. A strength member as claimed in any one of the preceding claims and manufactured by a pultrusion process.

7. A strength member as claimed in claim 1 and comprising a series of parallel glass filaments coated with a highly orientated selfreinforcing thermoplastic material.

8. A strength member as claimed in claim 1 and comprising a glass and self-reinforcing thermoplastic material composite.

9. A strength member as claimed in claim 8 and extruded to a cross-section appropriate to a cable construction in which it is to be used.

10. A strength member as claimed in claim 1 and comprising a combination of glass and amorphous carbon or boron.

11. A method of manufacturing a strength member for an optical fibre cable comprising forming a matrix of glass fibres and aromatic polyamide yarns, impregnating the matrix with a resin and shaping the resin impregnated matrix to a predetermined strength member profile by drawing it through a suitable crosssectioned die and curing the resin whereby to maintain the predetermined profile.

12. A method as claimed in claim 11 wherein the resin is thermally curable and the die is heated to effect the curing.

13. A method of manufacturing an all-dielectric strength member for an optical fibre cable substantially as herein described with reference to Fig. 1 of the accompanying drawings.

14. An all-dielectric strength member for an optical fibre formed by a method as claimed in any one of claims 11 to 13.

15. An optical fibre cable incorporating a strength member according to any one of claims 1 to 10 or 14.

16. An optical fibre cable incorporating a strength member and substantially as herein described with reference to any one of Figs. 2 to 11 of the accompanying drawings.