GB1584457A

GB1584457A - Optical cables

Info

Publication number: GB1584457A
Application number: GB1683677A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1977-04-22
Filing date: 1977-04-22
Publication date: 1981-02-11

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL CABLES (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.

According to the present invention an improved optical cable comprises a tube of rubber or plastics material in which is or are loosely housed at least one optical bundle as hereinafter defined and/or at least one separate optical fibre; a substantially moisture-impermeable barrier surrounding the tube and formed of at least one tape of metal or metal alloy having a layer of rubber or plastics material on at least one, and preferably each, of its major surfaces; and, surrounding the moisturebarrier, an outer protective sheath of rubber or plastics material in which elongate reinforcing members are embedded at spaced positions around and extend substantially lengthwise of the sheath, some or all of the reinforcing members each having, in the member, undulations, preferably radially extending undulations, that serve to improve the mechanical bonding of the member within the sheath and the flexibility of the cable.

By the expression "optical bundle" as used in this specification 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 the tube, limited relative movement between the or each optical bundle and/or between the or each separate optical fibre and the tube can take place when the cable is flexed.

Where two or more optical bundles and/ or separate optical fibres are loosely housed in the tube, they may be secured side-by-side on or within at least one substantially flat flexible support member which is housed loosely in the tube. In one embodiment, the tube may loosely house two or more substantially flat flexible support members, each of which has two or more optical bundles and/or separate optical fibres secured sideby side on or within it.

Since the or each substantially flat flexible support member on or within which the or each optical bundle and/or separate optical fibre is secured is housed loosely in the tube, limited relative movement between the flexible support member and the tube-and hence 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.

By virtue of the fact that the position of each optical bundle and/or each separate optical fibre secured on or within the or each flexible support member with respect to the other optical bundle or bundles and/ or other separate optical fibre or fibres is constant throughout the length of the cable, any optical bundle and/or separate optical fibre can be readily identified at any transverse cross-section of the cable. Furthermore, since each optical bundle and/or separate optical fibre is secured on or within a flexible support member, feeding of the optical bundles and/or optical fibres into the tube during manufacture of the cable, and especially when initially introducing the optical bundles and/or optical fibres into the tube, is facilitated.

Preferably the or each substantially flat flexible support member is in the form of at least one tape, for instance of paper or plastics material, of glass or of metal or metal alloy. In some circumstances, it is preferred that the or each tape is of a material having a coefficient of thermal' expansion approximating to that of the material or materials of the optical fibres. For example, where the optical fibres are of a silica-based material, the or each tape may be of steel.

Where the or each flexible support member consists of a single tape, the optical bundles and/or optical fibres may be secured by adhesive, preferably a flexible adhesive, to one surface of the tape; viewed in transverse cross-section the tape may be corrugated so that it has a plurality of troughs extending along its length, in each of some or all of which an optical bundle or separate optical fibre may be secured. Where the or each flexible support member consists of two tapes, one overlying the other, the optical bundles and/or separate optical fibres may be sandwiched between the two tapes and secured by adhesive to the adjacent surfaces of the tapes; one or each of these two tapes may be transversely corrugated as described above. Where one or each of two tapes is transversely corrugated, the two tapes may be so bonded together that the optical bundles and/or separate optical fibres are secured within the flexible support member so formed but are capable of limited movement within the troughs in which they lie. Where optical bundles and/ or separate optical fibres are secured within a flexible support member, the flexible support member may comprise a single tape of plastics material with the optical bundles and/or separate optical fibres wholly or partially embedded in it.

As a means of initially identifying any optical bundle and/or separate optical cable secured side-by-side on or within at least one substantially flat flexible support member, the support member may carry a longitudinally extending datum mark on its surface and/or at least one of the optical bundles and/or optical fibres may be assymmetrically positioned with respect to the or each other optical bundle andlor optical fibre so that it constitutes a longitudinally extending datum.

The or each optical bundle and/or separate optical fibre and the tube are of equal or approximately equal lengths.

The interstices between the optical bundle or bundles and/or the separate optical fibre or fibres and the tube throughout the length of the tube may be filled with a greasy water-impermeable medium. A preferred water-impermeable medium consists of, or comprises as a major constituent, petroleum jelly.

Preferably, the -moisture-impermeable barrier surrounding the tube is formed of a single tape of metal or metal alloy which is applied longitudinally to, and is folded transversely around, the tube so that its longitudinal edges overlap Preferably, also, the or each layer of rubber or plastics material on the tape is bonded to the tube or outer protective sheath, as the case may be.

Each elongate reinforcing member embedded in the outer protective sheath is 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 eliminated or reduced at least to a substantial extent by the reinforcing members.

The elongate reinforcing members having undulations therein may each comprise a single wire or a plurality of wires stranded together, the wire or strand being crimped at spaced positions along its length to form undulations but, with a view to maintaining the weight of the cable as low as possible, preferably each of these elongate reinforcing members comprises a bunch of compacted yarns of glass fibre or other inorganic reinforcing material.

Preferably, each undulating elongate reinforcing member extends substantially parallel to the axis of the cable but, in some circumstances, each member may extend helically around the axis of the cable, if desired the direction of lay of the helically extending undulating reinforcing member being reversed at spaced positions along the length of the cable.

The optical cable of the present invention is especially, but not exclusively, suitable for use as a self-supporting aerial cable because, when the cable is suspended, the elongate members having undulations therein provide means for taking up the tensile load. Moreover, since limited relative movement between the or each optical bundle and/or between the or each separate optical fibre and the tube can take place when the cable is flexed, the optical cable of the present invention can be suspended with negligible risk that fracture of optical fibres will occur when the cable is installed or whilst it is in service.

The outer protective sheath of the optical cable is preferably an extruded tube of rubber or plastics material and the undulating elongate reinforcing members embedded therein are preferably at uniformly spaced positions around the sheath.

Suitable materials of which the tube and/ or outer protective sheath may be made include polyethylene, for example high density polyethylene, a modified polyethylene such as ethylene vinyl acetate, or hard polyolefin such as polypropylene or- a modified polypropylene, for example propylene ethylene copolymer. The outer protective sheath may have an oversheath, for example an oversheath of a material having a low friction coefficient, such as nylon, and/or an undersheath may be provided around the moisture-barrier and under the outer protective sheath. Where an oversheath is provided, the outer protective sheath and the oversheath may be separated by an armouring layer.

The or each optical fibre of the optical bundle or bundles and/or the or each separate optical fibre may be 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 surface of the fibre or it may be of composite form comprising a core of transparent material clad through out 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 form ing 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.

Each optical fibre may be bare but preferably each fibre has a coating of lacquer, enamel, metal or other protective material which provides protection against abrasion and/or absorption of moisture into the material of the fibre. In all cases, each optical fibre is usually of substantially circular cross-section but, in some circumstances, it may be of non-circular crosssection.

The invention is further illustrated by a description by way of example, of two preferred forms of optical cable with reference to the accompanying drawing in which Figures 1 and 2 show, on an enlarged scale, transverse cross-sectional views of the two cables, and Figures 3 to 7 show diagrammastic transverse cross-sectional views of five alternative optical fibre arrays for incorporation in an optical cable.

7 Referring to Figure 1, the optical cable comprises a tube 2 or polyethylene in which are loosely housed four separate optical fibres 1 each having a protective coating (not shown) which protects the fibre against abrasion. The tube 2 is surrounded by a moisture-impermenble barrier 3 consisting of a single aluminium tape 4 which carries plastics coatings 5 and 6 on each of its major surfaces and which is applied longitudinally to, and is transversely folded around, the tube with an overlap. The plastics coating 5 is bonded to the tube 2 and the plastics coating 6 is bonded to a surrounding outer protective sheath 7 of polyethylene. Embedded in the outer protective sheath 7 of polyethylene. Embedded in the outer protective sheath 7 at uniformly spaced positions are reinforcing members 8 each comprising a bunch of compacted yarns of glass fibre having, in the bunch, radially extending undulations that serve to improve the mechanical bonding of the bunch within the sheath and the flexibility of the cable.

Limited relative movement between the optical fibres 1 and the tube 2 can take place when the cable is flexed.

The optical cable shown in Figure 2 differs from that shown in Figure 1 only by the manner in which optical fibres are loosely housed in the tube and, consequently, components of the optical cable shown in Figure 2 that are similar to those of the optical cable shown in Figure 1 have been given references larger by ten than the corresponding references in Figure 1. In the optical cable shown in Figure 2, the tube 12 loosely houses an optical fibre array 20 comprising three optical fibres 21 secured by adhesive in longitudinally extending troughs 24 of a corrugated plastics tape 22, the troughs being closed by a flat plastics tape 23 which is bonded to the peaks 25 of the corrugated tape.

Since the optical fibre array 20 is loosely housed in the bore of the tube 12, limited relative movement between the tape-supported optical fibres 21 and the tube can take place when the cable is flexed. Further more, any of the optical fibres 21 can be identified at any transverse cross-section of the optical cable.

Each of the optical cables shown in Figures 1 and 2 can be used as a self supporting aerial cable because the strain that would otherwise be imparted to each optical fibre when the optical cable is suspended is substantially eliminated by the embedded radially undulating reinforcing members.

The optical fibre arrays shown in Figures 3 to 7 are drawn on a greatly enlarged scale to clarify their structure and can be used in an optical cable of the present invention as alternatives to the optical array shown in Figure 2. The array shown in Figure 3 comprises a flexible tape 32 of plastics material which has secured by adhesive to one of its major surfaces four optical fibres 31 which are spaced apart and extend substantially parallel to the axis of the tape. In the array shown in Figure 4, four optical fibres 41 are sandwiched between and secured by adhesive to the adjacent surfaces of two flexible tapes 42, 43 of plastics material. The tape 43 has a longitudinally extending rib 47 to provide for ready identification of any optical fibre 41. The array shown in Figure 5 comprises a flexible plastics tape 52 which, viewed in transverse cross-section, is so corrugated that it has a plurality of troughs 54 extending along its length in each of which an optical fibre 51 is secured by adhesive. In the array shown in Figure 6, optical fibres 61 are sandwiched between, and secured by adhesive in the troughs 64, 68 of, two flexible corrugated plastics tapes 62, 63 whose peaks 65, 69 are secured together by adhesive.

To provide for ready identification of any optical fibre 61, the tape 63 has a longitudinally extending rib 67. The array shown in Figure 7 comprises a plastics tape 72 in which four optical fibres 71 extending sideby-side are wholly embedded. The tape 72 has a longitudinally extending rib 77 to provide for ready identification of any optical fibre 71.

WHAT WE CLAIM IS:- 1. An optical cable comprising a tube of rubber or plastics material in which is or are loosely housed at least one optical bundle as hereinafter defined and/or at least one separate optical fibre; a substantially moisture-impermeable barrier surrounding the tube and formed of at least one tape of metal or metal alloy having a layer of rubber or plastics material on at least one, and preferably each, of its major surfaces; and, surrounding the moisturebarrier, an outer protective sheath of rubber or plastics material in which elongate reinforcing members are embedded at spaced positions around and extend substantially lengthwise of the sheath, some or all of the reinforcing members each having, in those member, undulations, that serve to improve the mechanical bonding of the member within the sheath and the flexibility of the cable.

2. An optical cable comprising a tube of rubber or plastics material in which is or are loosely housed at least one substantially flat flexible support member on or within which two or more optical bundles as hereinbefore defined and/or separate optical fibres are secured side-by-side; a substantially moisture-impermeable barrier surrounding the tube and formed of at least one tape of metal or metal alloy having a layer of rubber or plastics material on at least one of its major surfaces; and, surrounding the moisture-barrier, an outer protective sheath of rubber or plastics material in which elongate reinforcing members are embedded at spaced positions around and extend substantially lengthwise of the sheath, some or all of the reinforcing members each having, in the member, undulations, that serve to improve the mechanical bonding of the member within the sheath and the flexibility of the cable.

3. An optical cable as claimed in Claim 2, wherein the substantially flat flexible support member or at least one of the substantially flat flexible support members is a flexible tape to one surface of which said optical bundles and/or separate optical fibres are secured by adhesive.

4. An optical cable as claimed in Claim 3, wherein, viwed in transverse cross-section, the tape is corruglated so that it has a plurality of troughs extending along its length and an optical bundle or optical fibre is secured in each of some or all of said troughs.

5. An optical cable as claimed in Claim 2, wherein the substantially flat flexible support member or at least one of the substantially flat flexible support members is a pair of flexible tapes between which said optical bundles and/or spearate optical fibres are sandwiched and to the adjacent surfaces of which the optical bundles and/or separate optical fibres are secured by adhesive.

6. An optical cable as claimed in Claim 5, wherein, viewed in transverse crosssection, one or each of said two tapes is corrugated so that it has a plurality of troughs extending along its length and an optical bundle or optical fibre is secured in each of some or all of said troughs.

7. An optical cable as claimed in Claim 2, wherein the or each flexible support member comprises two overlying flexible tapes which are bonded together, one or each of which tapes, viewed in transverse cross-section, being so corrugated that it has a plurality of troughs extending along its length in each of some or all of which an optical bundle and/or separate optical fibre is housed and wherein the optical bundles and/or separate optical fibres are secured within the flexible support member but are capable of limited movement within the troughs in which they lie.

8. An optical cable as claimed in any one of Claims 3 to 7, wherein the or each tape is of a metal or metal alloy.

9. An optical cable as claimed in any one of Claims 3 to 7, wherein the or each tape is of paper or plastics material.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. as alternatives to the optical array shown in Figure 2. The array shown in Figure 3 comprises a flexible tape 32 of plastics material which has secured by adhesive to one of its major surfaces four optical fibres 31 which are spaced apart and extend substantially parallel to the axis of the tape. In the array shown in Figure 4, four optical fibres 41 are sandwiched between and secured by adhesive to the adjacent surfaces of two flexible tapes 42, 43 of plastics material. The tape 43 has a longitudinally extending rib 47 to provide for ready identification of any optical fibre 41. The array shown in Figure 5 comprises a flexible plastics tape 52 which, viewed in transverse cross-section, is so corrugated that it has a plurality of troughs 54 extending along its length in each of which an optical fibre 51 is secured by adhesive. In the array shown in Figure 6, optical fibres 61 are sandwiched between, and secured by adhesive in the troughs 64, 68 of, two flexible corrugated plastics tapes 62, 63 whose peaks 65, 69 are secured together by adhesive. To provide for ready identification of any optical fibre 61, the tape 63 has a longitudinally extending rib 67. The array shown in Figure 7 comprises a plastics tape 72 in which four optical fibres 71 extending sideby-side are wholly embedded. The tape 72 has a longitudinally extending rib 77 to provide for ready identification of any optical fibre 71. WHAT WE CLAIM IS:-

1. An optical cable comprising a tube of rubber or plastics material in which is or are loosely housed at least one optical bundle as hereinafter defined and/or at least one separate optical fibre; a substantially moisture-impermeable barrier surrounding the tube and formed of at least one tape of metal or metal alloy having a layer of rubber or plastics material on at least one, and preferably each, of its major surfaces; and, surrounding the moisturebarrier, an outer protective sheath of rubber or plastics material in which elongate reinforcing members are embedded at spaced positions around and extend substantially lengthwise of the sheath, some or all of the reinforcing members each having, in those member, undulations, that serve to improve the mechanical bonding of the member within the sheath and the flexibility of the cable.

10. An optical cable as claimed in any

one of Claims 3 to 7, wherein the or each tape is of a material having a coefficient of thermal expansion approximating to that of the material or materials of the optical fibres secured to the tape.

11. An optical cable as claimed in Claim 10, wherein the optical fibres are of a silicabased material and the or each tape is of steel.

12. An optical cable as claimed in Claim 2, wherein the substantially flat flexible support member or at least one of the substantially flat flexible support members is a flexible tape of plastics material in which said optical bundles and/or separate optical fibres are wholly or partially embedded.

13. An optical cable as claimed in any one of Claims 2 to 12, wherein the flexible support member or at least one of the flexible support members carries a longitudinally extending datum mark and/or at least one of the optical bundles and/or optical fibres is assymmetrically positioned with respect to the or each other optical bundle and/or optical fibre so that it constitutes a longitudinally extending datum.

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 are of equal or approximately equal lengths.

15. An optical cable as claimed in any one of the preceding Claims, wherein the interstices between the optical bundle or bundles and/or the separate optical fibre or fibres and the tube throughout the length of the tube are filled with a greasy waterimpermeable medium.

16. An optical cable as claimed in any one of the preceding Claims, wherein the moisture-impermeable barrier is a single tape of metal or metal alloy which is applied longitudinally to, and is folded transversely around, the tube so that its longitudinal edges overlap.

17. An optical cable as claimed in Claim 16, wherein the or each layer of rubber or plastics material on the tape is bonded to the tube or outer protective sheath, as the case may be.

18. An optical cable as claimed in any one of the preceding Claims, wherein the undulations in each undulating reinforcing member extend radially of the cable.

19. An optical cable as claimed in any one of the preceding Claims, wherein at least one of the undulating reinforcing members is a single wire or a plurality of wires stranded together, the wire or group of stranded wires being crimped at spaced positions along its length to form said undulations.

20. An optical cable as claimed in any one of Claims 1 to 18, wherein at least one of the undulating reinforcing members comprises a bunch of compacted yarns of glass fibre or other inorganic reinforcing material.

21. An optical cable as claimed in Claim 20, wherein the compacted yarns of each bunch of glass fibre or of other inorganic reinforcing material are not pre-impregnated with an agent for binding to the material of the outer protective sheath.

22. An optical cable as claimed in Claim 20 or 21, wherein in each bunch of compacted yarns of glass fibre or of other inorganic reinforcing material, there is substantially no air in the interstices between the elementary filaments of the bunch.

23. An optical cable as claimed in any one of the preceding Claims, wherein each undulating reinforcing member extends substantially parallel to the axis of the cable.

24. An optical cable as claimed in any one of Claims 1 to 22, wherein each undulating reinforcing member extends helically around the axis of the cable

25. An optical cable as claimed in Claim 24, wherein the direction of lay of the helically extending undulating reinforcing members is reversed at spaced positions along the length of the cable.

26. An optical cable as claimed in any one of the preceding Claims, wherein the undulating reinforcing members are embedded at uniformly spaced positions around the outer protective sheath.

27. An optical cable as claimed in any one of the preceding Claims, wherein the mutual spacing between adjacent undulating reinforcing members is at least twice the diameter of a reinforcing member.

28. An optical cable as claimed in any one of the preceding Claims, wherein the outer protective sheath is surrounded by an oversheath of a material having a low friction coefficient.

29. An optical cable as claimed in Claim 28, wherein the outer protective sheath and the oversheath are separated by an armouring layer.

30. An optical cable as claimed in any one of the preceding Claims, wherein an undersheath is provided around the moisture barrier and under the outer protective sheath.

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