GB1572877A

GB1572877A - Optical cables

Info

Publication number: GB1572877A
Application number: GB757277A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1977-02-23
Filing date: 1977-02-23
Publication date: 1980-08-06
Also published as: CA1098350A; AU509851B2; AU3352778A

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL CABLES (71) We, BICC Limited, a British Company, of 21 Bloomsbury Street, London WC I B 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.

For the purpose of transmitting light in an optical transmission system it has been proposed to employ optical guides in the form of cables including one or more than one optical fibre Each optical fibre is usually of substantially circular crosssection but, in some circumstances, it may be a non-circular cross-section.

Optical cables with which the invention is concerned include cables comprising one or more than one optical fibre of glass or other transparent material whose refractive index is arranged to decrease over at least a part of the radius of the fibre in a direction towards the outer surfaces of the fibre and cables including one or more than one optical fibre of composite form comprising a core of transparent material clad throughout its length with another transparent material of lower refractive index which, by total internal refraction of light being transmitted along the fibre, confines at least a major proportion of the light within the core. A composite optical fibre is generally, but not necessarily, made up of two glasses of different refractive indices, the glass forming the core having a higher refractive index than the glass forming the cladding; the refractive index of the glass of the core may gradually decrease in a direction towards the outer surface of the core over at least a part of the distance between the central axis of the core and its outer surface. In an alternative form of composite fibre the core may be made of a transparent liquid having a higher refractive index than that of the cladding.

The present invention provides an improved optical cable which by virtue of its constriction reduces the strain that would otherwise be imparted to the optical fibres of the cable if the cable should be stressed in such a way as to tend to subject each or any of the optical fibres to a tensile force.

According to the present invention the improved optical cable comprises a plurality of separately formed tubes of rubber or plastics material in each of some or all of which is or are loosely housed at least one optical bundle as hereinbefore defined and/or at least one separate optical fibre; at least one separately formed flexible elongate filler having at least a circumferentially continuous layer of compressible material; and, surrounding the assembly of tubes and flexible filler or fillers, an outer protective sheath, the compressible material of the or each flexible filler being substantially softer than material of the tubes and the radial thickness and compressibility of the compressible material being such that, when the cable is stressed in such a way as to subject an optical fibre or optical fibres to a tensile force, a tube or tubes can move transversely of the cable to compress the flexible material of the filler or of at least one of the fillers to such an extent as to reduce the strain that would otherwise be imparted to the optical fibre or fibres loosely housed therein.

By the expression "optical bundle" as used in this specification and in the claims forming part thereof is meant a group of optical fibres or a group of fibres including at least one optical fibre and at least one non-optical reinforcing fibre or other reinforcing elongate member. Each optical fibre of the optical bundle may be used independently as a separate light guide, each with its own modulated light source and detector, or a plurality of optical fibres of a bundle may be used together as a single light guide, with a single light source.

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 in which it is loosely housed can take place when the cable is flexed. The tubes themselves may be loosely housed in the sheath thereby permitting limited relative movement between the tubes when the optical cable is flexed.

Preferably, the flexible filler or each of some or all of the flexible fillers is made wholly of compressible material but, in some circumstances, it may comprise a core of substantially non-compressible material and an outer peripheral layer of compressible material or a core of compressible material and an outer peripheral layer of substantially noncompressible material.

The compressible material of the filler or fillers may be any material that will be compressed to the required extent when a fibre-containing tube or tubes moves or move transversely with respect to the optical cable, but preferably it comprises cellular plastics material or cellular rubber.

Where in the or a filler of composite form, an outer peripheral ;ayer of cellular plastics material or cellular rubber surrounds a core, the core may be of solid plastics material or solid rubber bonded to the outer peripheral layer; alternatively, the core may be an elongate reinforcing member of such a material and of such a cross-sectional area having regard to the material or materials and cross-sectional area of the optical 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 reduced at least to a substantial extent by the reinforcing member. Where the or a filler is of composite form including a core of cellular plastics material or cellular rubber, preferably the outer peripheral is of solid plastics material or solid rubber and is bonded to the core.

Where the tubes of the optical cable are arranged in one or more than one layer around an elongate central core, this core may be constituted by one or more than one flexible elongate filler having at least a circumferentially continuous layer of compressible material. Where the elongate central core consists of a single flexible filler having at least a circumferentially continuous layer of compressible material, the transverse cross-section of the filler may be of a shape complementary to the shapes of the tubes immediately overlying the filler.

Alternatively, or additionally, flexible fillers having at least a circumferentially continuous layer of compressible material may be arranged in the layer or at least one of the layers of tubes, preferably with the fillers alternating with the tubes; the tubes and fillers may be arranged with their axes substantially parallel to the axis of the cable or they may extend helically around the elongate central core.

One or more than one of the tubes of the optical cable may have loosely housed in the tube at least one elongate electric conductor. Alternatively or additionally at least one elongate bare or insulated electric conductor may be assembled with the plurality of tubes and, for example, may constitute an elongate central core around which the tubes are arranged.

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

The invention is further illustrated by a description, by way of example, of two preferred forms of optical cable with reference to the accompanying diagrammatic drawing which shows transverse cross-sectional end views of the two cables.

The optical cable shown in Figure 1 comprises six tubes 2 of extruded polyethylene which are arranged helically around a central flexible elongate filler 3 comprising a reinforcing steel wire 4 surrounded by a circumferentially continuous cushing layer 5 of cellular plastics material. Loosely housed in each tube 2 are two separate optical fibres 1 whose lengths approximate to the length of the tube in which they are housed. The assembly of filler 3 and tubes 2 is surrounded by an extruded outer protective sheath 6 of polyethylene in such a way that they are loosely housed in the sheath. When the cable is stressed in such a way as to subject an optical fibre or fibres 1 to a tensile force a tube or tubes 2 can move transversely of the cable to compress the cushioning layer 5 to such an extent as to reduce the strain that would otherwise be imparted to the optical fibre or fibres housed in the tube or tubes.

The optical cable shown in Figure 2 comprises a central flexible elongate filler 13 around which are helically lapped three tubes 12 of extruded polyethylene and three flexible elongate fillers 17, each tube being positioned between and in contact with two fillers. The filler 13 comprises a reinforcing steel wire 14 surrounded by a circumferentially continuous cushioning layer 15 of cellular plastics material and each filler 17 comprises a core 18 of solid plastics material surrounded by a circumferentially continuous cushioning layer 19 of cellular plastics material. In each tube 12 are loosely housed two optical fibres 11, each of a length approximating to that of the tube in which it is housed. The assembly of tubes 12 and fillers 13 and 17 is enclosed in an extruded outer protective sheath 16 of polyethylene. When the cable is stressed in such a way as to subject an optical fibre or fibres 11 to a tensile force a tube or tubes 12 can move transversely of the cable to compress the cushioning layers 15 and 19 to such an extent as to reduce the strain that would otherwise be imparted to the optical fibre or fibres housed in the tube or tubes.

WHAT WE CLAIM IS: 1. An optical cable comprising a plurality of separately formed tubes of rubber or plastics material in each of some or all of which is or are loosely housed at least one optical bundle as hereinbefore defined and/or at least one separate optical fibre; at least one separately formed flexible elongate filler having at least a circumferentially continuous layer of compressible material; and, surrounding the assembly of tubes and flexible filler or fillers, an outer protective sheath, the compressible material of the or each flexible filler being substantially softer than material of the tubes and the radial thickness and compressibility of the compressible material being such that, when the cable is stressed in such a way as to subject an optical fibre or optical fibres to a tensile force, a tube or tubes can move transversely of the cable to compress the flexible material of the filler or of at least one of the fillers to such an extent as to reduce the strain that would otherwise be imparted to the optical fibre or fibres loosely housed therein.

2. An optical cable comprising an elongate central core constituted by one or more than one flexible elongate filler having at least a circumferentially continuous layer of compressible material; a plurality of separately formed tubes of rubber or plastics material arranged in one or more than one layer around the central core, each of some or all of which tubes loosely houses at least one optical bundle as hereinbefore defined and/or at least one separate optical fibre; and, surrounding the assembly of flexible filler or fillers and tubes, an outer protective sheath, the compressible material of the or each flexible filler being substantially softer than the material of the tubes and the radial thickness and compressibility of the compressible material being such that, when the cable is stressed in such a way as to subject an optical fibre or optical fibres to a tensile force, a tube or tubes can move transversely of the cable to compress the compressible material of the filler or of at least one of the fillers to such an extent as to reduce the strain that would otherwise be imparted to the optical fibre or fibres loosely housed therein.

3. An optical cable as claimed in Claim 2 in which the central core consists of a single flexible elongate filler, wherein the transverse cross-section of the filler is of a shape complementary to the shapes of the tubes immediately overlying the filler.

4. An optical cable as claimed in Claim 2 or 3, wherein flexible fillers having at least a circumferentially continuous layer of compressible material are arranged in the layer or in at least one of the layers of tubes.

5. An optical cable comprising a plurality of separately formed tubes of rubber or plastics material in each of some or of all of which is or are loosely housed at least one optical bundle as hereinbefore defined and/or at least one separate optical fibre, the tubes being arranged in one or more than one layer around a central tube; a plurality of separately formed flexible elongate fillers having at least a circumferentially continuous layer of compressible material, the fillers being arranged in the layer or in at least one of the layers of tubes; and, surrounding the assembly of tubes and flexible fillers, an outer protective sheath, the compressible material of each flexible filler being substantially softer than the material of the tubes and the radial thickness and compressibility of the compressible material being such that, when the cable is stressed in such a way as to subject the optical fibre or fibres to a tensile force, the tube or tubes can move transversely of the cable to compress the compressible material of at least one of the fillers to such an extent as to reduce the strain that would otherwise be imparted to the optical fibre or fibres loosely housed therein.

6. An optical cable as claimed in Claim 4 or 5, wherein the flexible fillers in the or each layer of tubes alternate with the tubes.

7. An optical cable as claimed in any one of Claims 2 to 6, wherein the tubes extend helically around the central core or central tube.

8. An optical cable as claimed in any one of the preceding Claims, wherein the flexible filler or each of some or all of the flexible fillers is made wholly of compressible material 9. An optical cable as claimed in any one of Claims 1 to 7, wherein the flexible filler or each of some or all of the flexible fillers

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. fillers. The filler 13 comprises a reinforcing steel wire 14 surrounded by a circumferentially continuous cushioning layer 15 of cellular plastics material and each filler 17 comprises a core 18 of solid plastics material surrounded by a circumferentially continuous cushioning layer 19 of cellular plastics material. In each tube 12 are loosely housed two optical fibres 11, each of a length approximating to that of the tube in which it is housed. The assembly of tubes 12 and fillers 13 and 17 is enclosed in an extruded outer protective sheath 16 of polyethylene. When the cable is stressed in such a way as to subject an optical fibre or fibres 11 to a tensile force a tube or tubes 12 can move transversely of the cable to compress the cushioning layers 15 and 19 to such an extent as to reduce the strain that would otherwise be imparted to the optical fibre or fibres housed in the tube or tubes. WHAT WE CLAIM IS:

1. An optical cable comprising a plurality of separately formed tubes of rubber or plastics material in each of some or all of which is or are loosely housed at least one optical bundle as hereinbefore defined and/or at least one separate optical fibre; at least one separately formed flexible elongate filler having at least a circumferentially continuous layer of compressible material; and, surrounding the assembly of tubes and flexible filler or fillers, an outer protective sheath, the compressible material of the or each flexible filler being substantially softer than material of the tubes and the radial thickness and compressibility of the compressible material being such that, when the cable is stressed in such a way as to subject an optical fibre or optical fibres to a tensile force, a tube or tubes can move transversely of the cable to compress the flexible material of the filler or of at least one of the fillers to such an extent as to reduce the strain that would otherwise be imparted to the optical fibre or fibres loosely housed therein.

8. An optical cable as claimed in any one of the preceding Claims, wherein the flexible filler or each of some or all of the flexible fillers is made wholly of compressible material

9. An optical cable as claimed in any one of Claims 1 to 7, wherein the flexible filler or each of some or all of the flexible fillers

comprises a core of substantially noncompressible material and an outer layer of compressible material.

10. An optical cable as claimed in Claim 9, wherein the core of the flexible filler or of each of said flexible fillers is of solid plastics material or solid rubber and the outer peripheral layer is of cellular plastics material or cellular rubber and is bonded to the core.

11. An optical cable as claimed in Claim 9, wherein the core of the flexible filler or of each of said flexible fillers is an elongate reinforcing member of such a material and of such cross-sectional area having regard to the material or materials and cross-sectional area of the optical 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 submit the or any optical fibre to a tensile force is reduced at least to a substantial extent by the reinforcing member.

12. An optical cable as claimed in any one of the preceding Claims, wherein at least one of the tubes has loosely housed in the tube at least one elongate electric conductor.

13. An optical cable as claimed in any one of the preceding Claims, wherein at least one elongate electric conductor is assembled with the plurality of tubes.

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

15. An optical cable substantially as hereinbefore described with reference to and as shown in Figure 1 or Figure 2 of the accompanying drawing.