GB1578439A

GB1578439A - Optical cables

Info

Publication number: GB1578439A
Application number: GB5350276A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1978-03-22
Filing date: 1978-03-22
Publication date: 1980-11-05

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL CABLES (71) We, BICC Limited, a British Company, of 21 Bloomsbury Street, London WClB 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 guides for the transmission of the ultra-violet, 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 waveguides for use in the communications 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 optical cables including one or more than one optical fibre. Such optical fibres 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 they may be 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 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 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 a transparent liquid having a higher refractive index than that of the cladding. All such optical fibres generally, but not necessarily, have diameters lying in the range 100 to 150 tm. They are usually of substantially circular cross-section but, in some circumstances, they may be of noncircular cross-section.

It is an object of the present invention to provide, in an optical cable, a flexible outer protective sheath of light weight which is highly resistant against crushing.

According to the invention an optical cable including one or more than one optical fibre has an outer protective sheath comprising at least one layer of closely wound helical turns of at least one elongate member, the helical turns being of such a form that limited relative movement between adjacent turns of said elongate member can take place when the cable is flexed.

Usually the closely wound helical turns will be formed by winding a single elongate member about the optical cable core but, in some circumstances, the closely wound helical turns may be in the form of a multistart helix made by helically winding about the optical cable core two or more elongate members.

Preferably the or each elongate member is of composite form and includes a plurality of resin bonded elongate reinforcing elements, preferably arranged in the form of a composite strip of substantially rectangular transverse cross-section, but the or each elongate member may be a single wire or a metallic strip. In the latter case adjacent turns of the metallic strip may inter-engage provided that limited relative movement between adjacent inter-engaged turns of the strip can take place when the cable is flexed.

Where the or each elongate member is a strip of composite form, the lay angle of the helical turns preferably lies in the range 65 to 75" to the longitudinal axis of the optical cable.

Adjacent turns of the helically wound elongate member or members may butt or, where the or each elongate member is of composite form or is in the form of a metallic strip, adjacent turns may partially overlap.

Where the outer protective sheath comprises two or more layers of helically wound elongate member or members, in each layer adjacent turns of the helically wound elongate member or members may butt or be spaced apart, the turns of one layer being of the same hand and overlying the butting edges of or gap between adjacent turns of the or each adjacent layer.

Preferably, the outer protective sheath also includes a continuous layer of plastics material or other fluid-impermeable material which overlies the layer of closely wound helical turns throughout the length of the optical cable. This continuous plastics layer is preferably applied by extrusion.

Where the or each elongate member is of composite form and includes a plurality of elongate reinforcing elements, these elements will generally, but not neccessarily, be of a non-metallic material and are preferably fibres of non-optical glass or aromatic polyamide but, in some circumstances, at least some of the reinforcing elements may comprise wires or tapes of other suitable reinforcing material or tapes on which, or on a surface of which, wires, fibres, filaments or other elongate reinforcing members are secured. Preferably, the helically wound composite elongate member is of substantially rectangular transverse cross-section.

To prevent boding between adjacent helical turns of the helically wound composite elongate member, a tape of polytetrafluoroethylene or of other material that will not bond to the resin-bonded reinforcing elements of the composite member may be introduced between adjacent turns and, if desired, between the turns and an underlying optical cable core and/or between the turns and the overlying continuous plastics layer, if present. Where a tape of polytetrafluoroethylene or other material is not provided between adjacent helical turns of the composite elongate member, curing of the helically wound turns will be so effected that no bonding will take place between adjacent turns of the composite member although some bonding is acceptable between the turns and an underlying optical cable core and/or between the turns and the overlying continuous plastics layer, if present.

The outer protective sheath is especially, but not exclusively, suitable for use in an optical cable of the kind comprising at least one separate optical fibre and/or at least one optical bundle as hereinafter defined loosely housed in a bore or other elongate compartment, or in one or more than one d at least two bores or other elongate compartments, extending along the length 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 loosely housed in a bore or other elongate compartment, limited movement of the or each optical fibre and/or of the or each optical bundle can take place when the cable is flexed and risk of fracture, or of attenuation increase by microbending, of the or an optical fibre or optical fibres is substantially reduced.

Preferably, two or more bores or other elongate compartments are arranged in and/ or at spaced positions around an elongate central core. Preferably, also, an elongate reinforcing member constitutes, or is embedded in, the central core, the elongate 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 separate optical fibre or fibres and/or of the optical bundle or bundles 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. The elongate reinforcing member is preferably of a non-metallic material, a plurality of fibres of aromatic polyamide sold under the Trade Mark 'Kevlar' or of non-optical glass, that may or may not be bonded with resin, being preferred.

In one aspect, bores may be formed at circumferentially spaced positions in an extruded elongate body of plastics material constituting the central core or elongate compartments may be formed between circumferentially spaced, radially outwardly extending ribs integral with an extruded elongate body of plastics material constituting the central core. In an alternative aspect, elongate compartments may be partially defined by elongate spacers that are separately formed with respect to and are arranged around the central core. Such spacers may comprise separately formed filaments of plastics material arranged at spaced positions around a central core, an examnle of an optical cable of this kind being the subject of our co-pending Patent Application No.

32719/76 (Serial No. 1,577,323). In a further aspect, the elongate compartments may be the bores of separately formed tubes, which tubes may be arranged at spaced positions around a central core, an example of an optical cable of this latter kind being the subject of our Patent No. 1479427. In all of these cases, preferably the bores or other elongate compartments each follow a helical path about the longitudinal axis of the cable.

In a further embodiment, the central core may comprise an elongate plastics body which has a central bore loosely housing at least one separate optical fibre and / or at least one optical bundle as hereinbefore defined and having at least two elongate reinforcing elements embedded in, and uniformly spaced around the bore of, the body.

The invention also includes a method of sheathing an optical cable core, which method comprises causing the optical cable core to travel in the direction of its length, and helically winding around the optical cable core at least one elongate member in such a way as to form around the core a layer of closely spaced helical turns, in which layer limited relative movement between adjacent turns can take place when the cable is flexed.

Preferably, the or each elongate member is of composite form and includes a plurality of resin-impregnated elongate reinforcing elements and in this case the layer of closely spaced helical turns is cured in such a way that adjacent turns are not bonded together so that limited relative movement between adjacent turns can take place when the cable is flexed. The elongate reinforcing elements of the composite elongate member may be pre-impregnated with resin before they are assembled together to form the composite elongate member or they may be impregnated with resin in travelling from a source of supply to the optical cable core.

To reduce the risk that adjacent turns of the closely wound helical turns will bond together when the layer of helical turns is cured, a tape of polytetrafluoroethylene or other material that will not bond to the resin when the resin is cured may be fed between adjacent turns of the composite elongate member as it is helically wound around the optical cable core.

The invention is further illustrated by a description, by way of example, of alternative forms of optical cable core, of a preferred form of optical cable, and of a preferred method of and apparatus for manufacturing the preferred optical cable, with reference to the accompanying drawings, in which: Figures 1(a) to (e) are transverse crosssectional views of five alternative forms of optical cable core; Figures 2 and 3 are diagrammatic representations of a further two alternative forms of optical cable core; Figure 4 is a sectional side view of the preferred optical cable having a core as shown in Figure l(b): Figure 5 is a diavrammatic representation of a preferred method of making the optical cable core shown in Figure 1(a); Figure 6 is a diagrammatic perspective view of detail of said preferred method, and Figure 7 is a diagrammatic representation of the preferred method of sheathing the optical cable core shown in Figure l(b).

Referring to Figures l(a) to (e), the optical cable core shown in Figure 1(a) comprises an extruded elongate body 1 of plastics material in which is embedded a central elongate reinforcing member 2 and which has, at circumferentially spaced positions around the reinforcing member 2, three bores 3, in each of which is loosely housed an optical fibre 4. The optical cable core shown in Figure l(b) differs from that shown in Figure 1(a) in that the extruded body 11 has two bores 13 circumferentially spaced around the central reinforcing member 12 and each loosely housing an optical fibre 14.

The optical cable core shown in Figure l(c) comprises an extruded elongate body 21 of plastics material in which is embedded a central elongate reinforcing member 22 and which has, at circumferentially spaced positions around its periphery, three recesses 23 in each of which is loosely housed an optical fibre 24. Figure l(d) shows an optical fibre core comprising an extruded elongate body 31 of plastics material having a transverse cross-section in the shape of a cross. A central elongate reinforcing member 32 is embedded in the body 31 and the four limbs of the body define recesses 33 in each of which is loosely housed an optical fibre 34.

In each of the optical cable cores illustrated in Figures 1(a) to (d) the bores and recesses extend helically about the central reinforcing member. The optical cable core shown in Figure 1(e) comprises an extruded elongate plastics body 41 of circular cross-section having a central bore 43 in which is loosely housed an optical fibre 44 and having two reinforcing members 42 embedded in, and circumferentially spaced around the bore of, the body.

The optical cable core shown in Figure 2 comprises a central core 72 of 'Kevlar' around which is helically wound a plurality of plastics tubes 73 (of which two only are shown), an optical fibre 74 being loosely housed in the bore of each tube. The optical cable core shown in Figure 3 comprises a central core 82 of 'Kevlar' around which is helically wound a plurality of circumferentially spaced plastics filaments 85 (of which one only is shown) which define between adjacent filaments elongate compartments in each of which is loosely housed an optical fibre 84 of a diameter smaller than that of the plastics filaments; a binding tape 86 is helically wound around the plastics filaments 85 and closes the elongate compartments housing the optical fibres.

The preferred optical cable shown in Figure 4 comprises an optical cable core 50 as illustrated in Figure l(b) and, surrounding the core, a composite sheath 51. The composite sheath consists of a layer 52 of closely wound helical turns of a composite strip 53 of rectangular transverse cross-section including a plurality of glass fibres 54 bonded by an epoxy resin 55, and an extruded layer 56 of plastics material which overlies the layer 52 of helical turns throughout the length of the cable. A tape 57 of polytetrafluoroethylene between adjacent turns of the layer 52 and between the turns and the core 50 and between the turns and the plastics layer 56 prevents bonding between adjacent turns and between the turns and the underlying core and overlying plastics layer. Limited relative movement between adjacent turns of the layer 52 can take place when the cable is bent or otherwise flexed with the result that the cable is highly flexible. The layer 52 of helical turns is also highly resistant against crushing.

An end of the cable may be sealed by a plastics cap 58 which has two holes 59 circumferentially spaced around an integral tubular boss 60. The cap 58 is applied over an end of the cable and is bonded to the plastics layer 56 and to the reinforcing member 12 of the core 50 protruding through the boss 60. The optical fibres 14 pass freely through the holes 59. The cap 58 may form, or may be shaped or otherwise modified for detachable connection to, one part of a two part optical cable connector.

Referring to Figures 5 and 6, in manufacturing the optical cable core shown in Figure 1(a) the plastics body 1 is extruded around the advancing reinforcing member 2 by an extruder 7 in such a way as to form the three bores 3 with slots 5 extending from the bores to the external surface of the body and optical fibres 4 are fed into the borcs, the slots 5 being temporarily widened to facilitate this operation by blades 61 (Figure 6).

The slots 5 are closed by the application of a plastics tape 6 (or by heat sealing) and the core so formed is cooled in an air cooling chamber 9 and wound on a take-up drum 10. During this operation the core is permanently twisted about its longitudinal axis by a twisting device 8 so that the bores 3 follow helical paths around the central reinforcing member 2.

As will be seen on referring to Figure 7, the layer 52 of closely wound helical turns of a composite strip 53 is formed around the optical cable core shown in Figure l(b) by causing the optical cable core 50 to pass upwardly from the drum 10 through a rotating bucket 62 containing liquefied resin 55, a plurality of glass fibres 54 passing downwardly through the resin and, when impregnated with resin, being helically wound around the advancing core. At the same time, the tape 57 of polytetrafluoroethylene is fed between adjacent turns as they are being formed so that the tape lies between the turns and the underlying core and between adjacent turns: a part of the tape also overlies the helical turns. The optical cable core with the layer 52 of helical turns of the composite strip 53 wound thereon then passes upwardly through a curing chamber 63 where the resin impregnated glass fibres of the helical turns are cured, bonding between adjacent turns being prevented by the tape 57. The plastics layer 56 may then be applied over the layer 52 of closely wound helical turns in tandem or as a separate operation.

WHAT WE CLAIM IS : 1. An optical cable including one or more than one optical fibre and, surrounding the optical fibre or fibres, an outer protective sheath comprising at least one layer of closely wound helical turns of at least one elongate member, the helical turns being of such a form that limited relative movement between adjacent turns of said elongate member can take place when the cable is flexed.

2. An optical cable including one or more than one optical fibre and, surrounding the optical fibre or fibres, an outer protective sheath comprising at least one layer of closely wound helical turns of at least one composite elongate member which includes a plurality of resin bonded elongate reinforcing elements, the helical turns being of such a form that limited relative movement between adjacent turns of said elongate member can take place when the cable is flexed.

3. An optical cable as claimed in Claim 2, wherein the or each composite elongate member is in the form of a composite strip of substantially rectangular transverse crosssection.

4. An optical cable as claimed in Claim 3, wherein the lay angle of the helical turns of composite strip lies in the range 65 to 750 to the longitudinal axis of the optical cable.

5. An optical cable as claimed in any one of Claims 2 to 4, wherein the elongate reinforcing elements of the composite elongate member are of a non-metallic material.

6. An optical cable as claimed in Claim 5, wherein the elongate reinforcing elements are fibres of non-optical glass or of aromatic polyamide.

7. An optical cable as claimed in any one of Claims 2 to 6, wherein a tape of polytetrafluoroethylene or of other material that will not bond to the resin-bonded reinforcing elements of the composite member lies between adjacent helical turns of the composite elongate member.

8. An optical cable as claimed in Claim 7, wherein said tape of polytetrafluoroethy

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. as illustrated in Figure l(b) and, surrounding the core, a composite sheath 51. The composite sheath consists of a layer 52 of closely wound helical turns of a composite strip 53 of rectangular transverse cross-section including a plurality of glass fibres 54 bonded by an epoxy resin 55, and an extruded layer 56 of plastics material which overlies the layer 52 of helical turns throughout the length of the cable. A tape 57 of polytetrafluoroethylene between adjacent turns of the layer 52 and between the turns and the core 50 and between the turns and the plastics layer 56 prevents bonding between adjacent turns and between the turns and the underlying core and overlying plastics layer. Limited relative movement between adjacent turns of the layer 52 can take place when the cable is bent or otherwise flexed with the result that the cable is highly flexible. The layer 52 of helical turns is also highly resistant against crushing. An end of the cable may be sealed by a plastics cap 58 which has two holes 59 circumferentially spaced around an integral tubular boss 60. The cap 58 is applied over an end of the cable and is bonded to the plastics layer 56 and to the reinforcing member 12 of the core 50 protruding through the boss 60. The optical fibres 14 pass freely through the holes 59. The cap 58 may form, or may be shaped or otherwise modified for detachable connection to, one part of a two part optical cable connector. Referring to Figures 5 and 6, in manufacturing the optical cable core shown in Figure 1(a) the plastics body 1 is extruded around the advancing reinforcing member 2 by an extruder 7 in such a way as to form the three bores 3 with slots 5 extending from the bores to the external surface of the body and optical fibres 4 are fed into the borcs, the slots 5 being temporarily widened to facilitate this operation by blades 61 (Figure 6). The slots 5 are closed by the application of a plastics tape 6 (or by heat sealing) and the core so formed is cooled in an air cooling chamber 9 and wound on a take-up drum 10. During this operation the core is permanently twisted about its longitudinal axis by a twisting device 8 so that the bores 3 follow helical paths around the central reinforcing member 2. As will be seen on referring to Figure 7, the layer 52 of closely wound helical turns of a composite strip 53 is formed around the optical cable core shown in Figure l(b) by causing the optical cable core 50 to pass upwardly from the drum 10 through a rotating bucket 62 containing liquefied resin 55, a plurality of glass fibres 54 passing downwardly through the resin and, when impregnated with resin, being helically wound around the advancing core. At the same time, the tape 57 of polytetrafluoroethylene is fed between adjacent turns as they are being formed so that the tape lies between the turns and the underlying core and between adjacent turns: a part of the tape also overlies the helical turns. The optical cable core with the layer 52 of helical turns of the composite strip 53 wound thereon then passes upwardly through a curing chamber 63 where the resin impregnated glass fibres of the helical turns are cured, bonding between adjacent turns being prevented by the tape 57. The plastics layer 56 may then be applied over the layer 52 of closely wound helical turns in tandem or as a separate operation. WHAT WE CLAIM IS :

1. An optical cable including one or more than one optical fibre and, surrounding the optical fibre or fibres, an outer protective sheath comprising at least one layer of closely wound helical turns of at least one elongate member, the helical turns being of such a form that limited relative movement between adjacent turns of said elongate member can take place when the cable is flexed.

lene or of other material also lies between the turns of composite elongate member and an underlying optical cable core and / or between said turns and an overlying continuous plastics layer.

9. An optical cable as claimed in any one of Claims 2 to 6, wherein the helically wound turns of the composite elongate member have been so cured that there is no bonding between adjacent turns.

10. An optical cable as claimed in claim 1, wherein the or each elongate member is a single wire.

11. An optical cable as claimed in Claim 1, wherein the or each elongate member is a metallic strip.

12. An optical cable as claimed in Claim 11, wherein adjacent turns of the metallic strip inter-engage in such a way as to permit limited relative movement between adjacent inter-engaged turns when the cable is flexed.

13. An optical cable as claimed in any one of the preceding Claims, wherein the helical turns are in the form of a multistart helix made by helically winding two or more elongate members.

14. An optical cable as claimed in any one of the preceding Claims, wherein the outer protective sheath includes a continuous layer of plastics material or other fluidimpermeable material which overlies the layer of closely wound helical turns throughout the length of the optical cable.

15. An optical cable as claimed in Claim 14, wherein the continuous plastics layer is an extruded layer.

16. An optical cable as claimed in any one of the preceding Claims, wherein at least one separate optical fibre and/or at least one optical bundle as hereinbefore defined is or are loosely housed in a bore or other elongate compartment, or in one or more than one of at least two bores or other elongate compartments, extending along the length of the cable.

17. An optical cable as claimed in Claim 6, wherein two or more bores or other elongate compartments are arranged in and/ or at spaced positions around an elongate central core.

18. An optical cable as claimed in Claim 17, wherein an elongate reinforcing member constitutes, or is embedded in, the central core, the elongate reinforcing member being of such a material and of such a crosssectional area having regard to the material or materials and cross-sectional area of the separate optical fibre or fibres and / or of the optical bundle or bundles that the strain otherwise imparted to the or each optical fibre when the cable is stressed in such a way as to tend to subiect the or any optical fibre to a tensile force is eliminated or reduced at least to a substantial extent by the reinforcing member.

19. An optical cable as claimed in Claim 18, wherein the elongate reinforcing member is a plurality of fibres of aromatic polyamide or of non-optical glass.

20. An optical cable as claimed in Claim 19, wherein the fibres of the elongate reinforcing member are bonded with resin.

21. An optical cable as claimed in any one of Claims 17 to 20, wherein the central core is an extruded elongate body of plastics material and the bores are at circumferentially spaced positions in the body.

22. An optical cable as claimed in any one of Claims 17 to 20, wherein the central core is an extruded elongate body of plastics material having circumferentially spaced ribs which are integral with and extend radially outwardly from the body and which define between them the elongate compartments.

23. An optical cable as claimed in any one of Claims 17 to 20, wherein the elongate compartments are partially defined by circumferentially spaced elongate spacers that are separately formed with respect to and are arranged around the central core.

24. An optical cable as claimed in any one of Claims 1 to 20, wherein the elongate compartments are the bores of separately formed tubes.

25. An optical cable as claimed in any one of the preceding Claims, wherein the bores or other elongate compartments each follow a helical path about the longitudinal axis of the cable.

26. An optical cable as claimed in Claim 16, which has a single bore, wherein the bore is the central bore of an elongate plastics body constituting a central core, which body has at least two elongate reinforcing elements embedded in, and uniformly spaced around the bore of, the body.

27. A method of sheathing an optical cable core, which method comprises causing the optical cable core to travel in the direction of its length, and helically winding around the optical cable core at least one elongate member in such a way as to form around the core a layer of closely spaced helical turns, in which layer limited relative movement between adjacent turns can take place when the cable is flexed.

28. A method as claimed in Claim 27, wherein the or each elongate member is of composite form and includes a plurality of resin-impregnated elongate reinforcing elements and wherein the layer of closely spaced helical turns is cured in such a way that adjacent turns are not bonded together.

29. A method as claimed in Claim 28, wherein a tape of polytetrafluoroethylene or of other material that will not bond to the resin when the resin is cured is fed between adjacent turns of the composite elongate member as it is helically wound around the optical cable core.

30. A method of sheathing an optical cable core substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.

31. An optical cable having an outer protective sheath substantially as hereinbefore described with reference to and as shown in Figure 4 of the accompanying drawings.