GB1571110A - Optical cables - Google Patents

Abstract

The optical-fibre cable contains a reinforcing element formed from an aromatic polyester braided fibre (see figure) which extends in the axial direction in the cable. A number of optical fibres are arranged along the reinforcing element. A plastic cladding is now extruded onto this structure in order to ensure protection against unfavourable (atmospheric, mechanical) environmental influences. <IMAGE>

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL CABLES (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, W.C.2., England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to cable strain members and in particular to a flexible plastics strain member for an optical fibre cable.

A commonly used basic design of optical fibre cable comprises an axial strain member surrounded by one or more layers of plastics coated optical fibres bound with polyester tape and plastics sheathed overall. Location of the strength member along the cable axis provides the maximum of flexibility together with minimal tensile stress on the optical fibres. Until recently, among primary objectives in optical cable design have been small diameter and minimum weight coupled with sufficient tensile rigidity to withstand normal handling and installation in modest lengths, typically up to 100 metres. For this purpose strength members formed from oriented polyester monofilaments have proved adequate. New requirements for longer, typically 500 to 1,000 metres, and correspondingly stronger cables have produced a need for improved strength members.

A basic principle in fibre-optical cable design is that when the cable is subjected to a tension equal to the maximum which will be encountered in practical use, the strain in the optical fibres will be limited to a safe value. When the fibres are helically disposed within the cable the fibre strain is less than that of the cable strain by an amount dependent upon the helix angle. However, for optical reasons the helix angle has to be kept small, and in practice the cable strain is substantially the same as the fibre strain under conditions of pure tension.

The breaking strain of glass and silica optical fibre is small, and may vary from 0.2% to more than 5% extension depending upon method of manufacture.

Choice of a design figure for the strain is therefore somewhat arbitrary, but a limiting value of 1% appears to be useful in determining a tensile capability. Irrespective of this choice it is an obvious objective that for a given size or weight of cable the ratio of tensile load to strain should be as high as possible, and this is achieved by the use of components of high elastic modulus. A second objective is however to achieve good flexibility, which is favoured by the use of low modulus materials.

According to the invention there is provided an optical cable, including an axial plastics multifibre strength member, a plurality of optical fibres substantially parallel to and disposed around the strength member, and one or more protective sheaths surrounding the fibres and strength member.

Embodiments of the invention will now be described with reference to the accompanying drawings in which Figs. 1 to 3 show three forms of plaited strength members for optical cables.

In a preferred embodiment an aromatic polyamide fibre material, such as that marketed under the trade name KEVLAR, is plaited into a rope such as that shown in the accompanying drawings. The rope or plait is preferably encased in a plastics material to provide a smooth surface for the subsequent stranding of the individual plastics coated optical fibres on to the strength member. The cable is finished by the application of one or more layers of polyester tape followed by the extrusion of an outer plastics sheath. The preferred material for the strength member of KEVLAR 49 fibre, but KEVLAR-29 fibre may also be employed. The physical properties of these materials in filamentary form, in comparison with a commonly used steel wire are summarised in the following table:- TABLE 1

Young's modulus Strain at break Sp. gravity 105 N/mm2 Steel wire 7.86 1.93 > 2 Kevlar 29 fibre 1.44 0.83 4 Kevlar 49 fibre 1.45 1.31 3.5 In plaited form the effective tensile modulus of Kevlar is less than that of individual filaments, but is still of a high order.

These materials have been widely employed for high tensile cables in general, and in a number of designs of fibre-optical cable in particular, in which their high tensile moduli have been exploited but without specific consideration of their flexibility. If a single steel wire of adequate strength is employed it is usually unacceptably stiff, arising from the fact that strength is proportional to the square of the diameter, but stiffness to the fourth power. The ratio of flexibility to strength is much improved on by employing an assembly of strands of smaller individual diameters. Individual fibres of Kevlar are of very small diameter (e.g. 12ism) and therefore have to be used as many-fibre composites to achieve sufficient strength for use as strength members. In consequence the flexibility to strength ratio is in principle very favourable. Maximum tensile modulus is achieved if the fibres are laid parallel and axially but they do not then form a coherent strand. Some degree of coherence has previously been achieved by the introduction of twist, and particularly by using a bonding resin to bind the fibres together, but this adversely effects the flexibility and gives rise to permanent kinking when the assembly is bent. We have found that these problems are substantially overcome by the use of a plaited configuration of the fibres, as shown in Figures 1 to 3.

A further advantageous feature of Kevlar plaits arises from the observation that their stress-strain curves are of a composite nature, comprising an initial low modulus region at low elongation, passing into a high modulus region as the strain is increased.

Depending upon the specific structure of the plait, the range of strain over which the modulus is low may be 0.03-0.4%, and typically 0.2%. The transition to high modulus at higher strain is rapid so that the effective modulus up to 1% is also high.

Some typically measured values for the 1% modulus are: Kevlar 29 0.33 x 10a NJmm2 Revlar 49 0.94X105 Nlmm2 The existence of the initial low modulus region implies a high degree of flexibility under zero or low tension. Flexibility of a plait is also enhanced by its characteristic structure, in which strands pass between inner and outer positions on a band, with a pitch of only a few millimeters.

To assess the relative merits of KEVLAR plaits as a main strength member, comparison may be made with a typical cable having a stranded steel central strength member, eight coated fibres, tapes and sheath. Assumptions made are that the strand packing density is constant throughout, and that in the steel cable the steel takes 80% of the cable tension. Cable strength is taken as the tension at 1% strain.

Specific gravities; steel 7.86, KEVLAR 1.45, other components 0.9. Strength and weight are normalised to that of the steel cable. The results are summarised in the following table:- TABLE 2

Overall Strength Weight SM SM diam. diatn. S 2 L S 'W 1.00 Steel 1.5 mm 6.5 mm 1.00 1.00 Kev. 49 1.5 mm 6.5 mm 0.62 0.75 0.83 Kev 29 1.5 mm 6.5 mm 0.35 0.75 0.47 Kev. 49 2.1 mm 7.1 mm 1.00 0.88 1. 11 Kev. 29 3.5 mm 8.5 mm 1.00 1.33 0.74 Equal Equal size Equal strength Suitable plait forms are shown in Figs. 1 to 3 of the accompanying drawings, although other plait forms may of course be employed. The following table summarises the physical details of these KEVLAR plaits.

TABLE 3

Pitch Substance- Area of fibres Fig. Plait Type ODmm mm g/m mm2 1 2 pairs S and Z 1.1 13 0.8 0.55 intertwined 2 8 strand 4S + 4Z 1.6 19 1.5 1.04 3 3 8 strand 4S + 4Z 0.7 7 0.38 0.27 In a further application the plaited KEVLAR fibre may additionally be employed with or without a plastics coating as a reinforcing filler, forming a spacer or cushion between optical fibres in a fibre optic cable. The low initial modulus allows the use of this material in such off-axis positions without undue loss of flexibility, while retaining the feature of providing a significantly high degree of tensile reinforce- ment.

WHAT WE CLAIM IS: 1. An optical cable, including an axial plastics multifibre strength member, a plurality of optical fibres substantially parellel to and disposed around said strength member, and one or more protective sheaths surrounding the fibres and strength member.

2. An optical cable, including an axial strength member comprising a rope plaited from plastics fibres, a plurality of optical fibres substantially parallel to and symmetrically disposed around said strength member, one or more layers of polyester tape surrounding the optical fibres, and an outer protective plastics sheathing.

3. A cable as claimed in claim 2, and in which said strength member is coated with a layer of plastics material so as to provide a smooth surface on which the optical fibres are laid up.

4. A cable as claimed in claim 2 or 3, and which further includes a reinforcing plastics fibre between the fibres of the cable.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. TABLE 2 Overall Strength Weight SM SM diam. diatn. S 2 L S 'W 1.00 Steel 1.5 mm 6.5 mm 1.00 1.00 Kev. 49 1.5 mm 6.5 mm 0.62 0.75 0.83 Kev 29 1.5 mm 6.5 mm 0.35 0.75 0.47 Kev. 49 2.1 mm 7.1 mm 1.00 0.88 1. 11 Kev. 29 3.5 mm 8.5 mm 1.00 1.33 0.74 Equal Equal size Equal strength Suitable plait forms are shown in Figs. 1 to 3 of the accompanying drawings, although other plait forms may of course be employed. The following table summarises the physical details of these KEVLAR plaits. TABLE 3 Pitch Substance- Area of fibres Fig. Plait Type ODmm mm g/m mm2 1 2 pairs S and Z 1.1 13 0.8 0.55 intertwined 2 8 strand 4S + 4Z 1.6 19 1.5 1.04 3 3 8 strand 4S + 4Z 0.7 7 0.38 0.27 In a further application the plaited KEVLAR fibre may additionally be employed with or without a plastics coating as a reinforcing filler, forming a spacer or cushion between optical fibres in a fibre optic cable. The low initial modulus allows the use of this material in such off-axis positions without undue loss of flexibility, while retaining the feature of providing a significantly high degree of tensile reinforce- ment. WHAT WE CLAIM IS:

1. An optical cable, including an axial plastics multifibre strength member, a plurality of optical fibres substantially parellel to and disposed around said strength member, and one or more protective sheaths surrounding the fibres and strength member.

2. An optical cable, including an axial strength member comprising a rope plaited from plastics fibres, a plurality of optical fibres substantially parallel to and symmetrically disposed around said strength member, one or more layers of polyester tape surrounding the optical fibres, and an outer protective plastics sheathing.

3. A cable as claimed in claim 2, and in which said strength member is coated with a layer of plastics material so as to provide a smooth surface on which the optical fibres are laid up.

4. A cable as claimed in claim 2 or 3, and which further includes a reinforcing plastics fibre between the fibres of the cable.

5. A cable as claimed in any one of claims 1 to 4, and in which the plastics

fibre strength member is formed from an aromatic polyamide.

6. An optical cable substantially as described herein with reference to Fig. 1, 2 or 3 of the drawings accompanying the provisional specification.