GB1601005A

GB1601005A - Optical cable

Info

Publication number: GB1601005A
Application number: GB25171/78A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1978-05-31
Filing date: 1978-05-31
Publication date: 1981-10-21

Description

(54) AN IMPROVED OPTICAL CABLE (71) We, BICC Limited, a British Company, of 21 Bloomsbury Street, London WC1B 3QN, 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 optical cables for the transmission of the ultraviolet, visible and infra-red regions of the electromagnetic spectrum, which regions. for convenience, will hereinafter all be included in the generic term "light", and especially, but not exclusively, to optical cables for use in the communication field adapted for transmission of light having a wavelength within the range 0.8 to 1.3 micrometres.

In the Complete Specification of our Patent No. 1479427 B, one form of optical cable that is described and claimed comprises a central elongate reinforcing member; a plurality of tubes of rubber or plastics material arranged helically in one or more than one layer around the reinforcing member, each of some or all of which tubes loosely housing at least one optical bundle as hereinafter defined and/or at least one separate optical fibre; and an outer protective sheath surrounding the plurality of tubes, the reinforcing member being of such a material and of such a cross-sectional area having regard to the material or materials and cross-sectional area of the bundle or bundles and/or of the separate optical fibre or fibres that the strain otherwise imparted to the or each optical fibre when the cable is stressed in such a way as to tend to subject the or any optical fibre to a tensile force is eliminated or reduced at least to a substantial extent by the reinforcing member.

By the expression "optical bundle" as used in the specification of the aforesaid patent, and as subsequently used in this specification, is meant a group of optical fibres or a group of optical fibres including at least one optical fibre and at least one non-optical reinforcing fibre or other reinforcing elongate member.

By virtue of being housed loosely in a tube, limited relative movement between the or each optical bundle and/or the or each separate optical fibre and the tube can take place when the cable is flexed.

The present invention is an improvement in or modification of the invention forming the subject of the aforesaid patent, hereinafter referred to as the main patent, and provides an improved optical cable in which the optical fibres are protected against tensile and compression forces.

According to the present invention, in the aforesaid form of optical cable described and claimed in the main patent, each tube is made of molecularly oriented polyethylene terephthalate.

Molecular orientation of a tube of polyethylene terephthalate housing at least one separate optical fibre and/or optical bundle is achieved by extruding the tube around the or each optical fibre and/or bundle oversize and then drawing it down to final size at a temperature below the melting point of the polymer. A draw-down by a factor of four (longitudinally) is sufficient to confer a very low thermal expansion coefficient and a high Young's Modulus (in the longitudinal direction) on the polyethylene terephthalate.

Several advantages are obtained as a result of using tubes of oriented polyethylene terephthalate. Firstly, whilst a tube loosely housing at least one separate optical fibre and/or optical bundle is on a drum or reel waiting to be incroporated in an optical cable, there is negligible danger of the or each optical fibre being subjected to excessive strains due to large changes in temperature, e.g. arising from outdoor winter exposure during shipment. Secondly, the high Young's Modulus gives the tubes suffi cient strength to enable them to withstand normal stranding tensions during laying up of the tubes. Thirdly, the tubes of oriented polyethylene terephthalate have such an inherent strength that, when subjected to temperatures as low as -60 C, negligible contraction of the tubes occurs and there is little risk of buckling of the optical fibre or fibres; tubes of oriented polyethylene terephthalate are therefore especially suitable for use where the or each elongate reinforcing member is of a non-metallic material which offers little resistance to compression forces and where the cable sheath is made of a material, e.g. low density polyethylene, which exerts a longitudinal compressive force due to its thermal contraction.

One or more than one elongate reinforcing member may be arranged in the layer or in at least one of the layers of the tubes.

Alternatively, or additionally, a plurality of elongate reinforcing members may be embedded in the sheath at spaced positions, preferably uniformly spaced positions, around the sheath.

The length of the or each optical bundle and/or separate optical fibre may be greater than that of the tube in which it is loosely housed but. preferably, the or each optical bundle and/or optical fibre and the tube in which it is loosely housed are of equal or approximately equal lengths.

Preferably, as described and claimed in the Complete Specification of copending Application No. 23875/78 (Serial No.

1601004), the direction of lay of the helically wound tubes in the or each layer is reversed at spaced positions along the length of the cable.

The optical cable of the present invention may also include any or more than one of the optional features of the optical cable described and claimed in the Complete Specification of the main patent and/or any one of the features of the optical cables described and claimed in the Complete Specifications of our co-pending Patent Applications Nos. 3145/77 (Serial No.

1582851), 7572/77 (Serial No. 1572877) and 53166/77 (Serial No. 1601004).

The invention is further illustrated by a description, by way of example, of a preferred optical cable.

The optical cable comprises eight tubes of extruded molecularly oriented polyethylene terephthalate helically arranged around a central core comprising a fibrous rope constituting an elongate reinforcing member.

Each tube loosely houses a separate optical fibre. The assembly of central fibrous rope and tubes is surrounded by an extruded outer protective sheath of polyethylene.

The central fibrous rope has an overall diameter of 3.0 mm and each tube has an external diameter of 1.5 mm. The cable has an overall diameter of approximately 10.0 mm.

WHAT WE CLAIM IS: 1. A optical cable comprising a central elongate reinforcing member; a plurality of tubes of molecularly oriented polyethylene terephthalate arranged helically in one or more than one layer around the central core, each of some or all of which tubes loosely housing at least one optical bundle as hereinbefore defined and/or at least one separate optical fibre; and an outer protective sheath surrounding the plurality of tubes, the elongate reinforcing member being of such a material and of such a cross-sectional area having regard to the material or material and cross-sectional area of the bundle. or bundles and/or of the separate optical fibre or fibres that the strain otherwise imparted to the or each optical fibre when the cable is stressed in such a way as to tend to subject the or any optical fibre to a tensile force is eliminated or reduced at least to a substantial extent by the reinforcing member.

2. An optical cable as claimed in claim 1, wherein the central elongate reinforcing member is of non-metallic material.

3. An optical cable as claimed in any one of the preceding claims, wherein the direction of lay of the tubes is reversed at spaced positions along the length of the cable.

4. An optical cable as claimed in any one of the preceding claims, wherein one or more than one elongate reinforcing member is arranged in the layer or in at least one of the layers of tubes.

5. An optical cable as claimed in any one of the preceding claims. wherein a plurality of elongate reinforcing members are embedded in the sheath at spaced positions around the sheath.

6. An optical cable as claimed in any one of the preceding claims, wherein the or each optical bundle and/or optical fibre and the tube in which it is loosely housed are of equal or approximately equal lengths.

7. An optical cable substantially as hereinbefore described by way of example.

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Claims

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

cient strength to enable them to withstand normal stranding tensions during laying up of the tubes. Thirdly, the tubes of oriented polyethylene terephthalate have such an inherent strength that, when subjected to temperatures as low as -60 C, negligible contraction of the tubes occurs and there is little risk of buckling of the optical fibre or fibres; tubes of oriented polyethylene terephthalate are therefore especially suitable for use where the or each elongate reinforcing member is of a non-metallic material which offers little resistance to compression forces and where the cable sheath is made of a material, e.g. low density polyethylene, which exerts a longitudinal compressive force due to its thermal contraction.

One or more than one elongate reinforcing member may be arranged in the layer or in at least one of the layers of the tubes.

Alternatively, or additionally, a plurality of elongate reinforcing members may be embedded in the sheath at spaced positions, preferably uniformly spaced positions, around the sheath.

The length of the or each optical bundle and/or separate optical fibre may be greater than that of the tube in which it is loosely housed but. preferably, the or each optical bundle and/or optical fibre and the tube in which it is loosely housed are of equal or approximately equal lengths.

Preferably, as described and claimed in the Complete Specification of copending Application No. 23875/78 (Serial No.

1601004), the direction of lay of the helically wound tubes in the or each layer is reversed at spaced positions along the length of the cable.

The optical cable of the present invention may also include any or more than one of the optional features of the optical cable described and claimed in the Complete Specification of the main patent and/or any one of the features of the optical cables described and claimed in the Complete Specifications of our co-pending Patent Applications Nos. 3145/77 (Serial No.

1582851), 7572/77 (Serial No. 1572877) and 53166/77 (Serial No. 1601004).

The invention is further illustrated by a description, by way of example, of a preferred optical cable.

The optical cable comprises eight tubes of extruded molecularly oriented polyethylene terephthalate helically arranged around a central core comprising a fibrous rope constituting an elongate reinforcing member.

Each tube loosely houses a separate optical fibre. The assembly of central fibrous rope and tubes is surrounded by an extruded outer protective sheath of polyethylene.

The central fibrous rope has an overall diameter of 3.0 mm and each tube has an external diameter of 1.5 mm. The cable has an overall diameter of approximately 10.0 mm.

WHAT WE CLAIM IS: 1. A optical cable comprising a central elongate reinforcing member; a plurality of tubes of molecularly oriented polyethylene terephthalate arranged helically in one or more than one layer around the central core, each of some or all of which tubes loosely housing at least one optical bundle as hereinbefore defined and/or at least one separate optical fibre; and an outer protective sheath surrounding the plurality of tubes, the elongate reinforcing member being of such a material and of such a cross-sectional area having regard to the material or material and cross-sectional area of the bundle. or bundles and/or of the separate optical fibre or fibres that the strain otherwise imparted to the or each optical fibre when the cable is stressed in such a way as to tend to subject the or any optical fibre to a tensile force is eliminated or reduced at least to a substantial extent by the reinforcing member.
2. An optical cable as claimed in claim 1, wherein the central elongate reinforcing member is of non-metallic material.
3. An optical cable as claimed in any one of the preceding claims, wherein the direction of lay of the tubes is reversed at spaced positions along the length of the cable.
4. An optical cable as claimed in any one of the preceding claims, wherein one or more than one elongate reinforcing member is arranged in the layer or in at least one of the layers of tubes.
5. An optical cable as claimed in any one of the preceding claims. wherein a plurality of elongate reinforcing members are embedded in the sheath at spaced positions around the sheath.
6. An optical cable as claimed in any one of the preceding claims, wherein the or each optical bundle and/or optical fibre and the tube in which it is loosely housed are of equal or approximately equal lengths.
7. An optical cable substantially as hereinbefore described by way of example.