GB2180665A - Optical cables and the manufacture thereof - Google Patents

Abstract

An optical cable in which a strip of thermoplastics material has a plurality of longitudinally extending grooves 3 in one surface, at least one of which loosely accommodates one or more optical fibres 4 having a length greater than the strip, and a cover strip 5 bonded to the grooved surface so as to enclose the grooves, the strip being wrapped around a central elongate element (not shown). The strip 3 is manufactured by continuous extrusion and the optical fibres are then fed into the grooves (Figure 3). <IMAGE>

Description

SPECIFICATION Optical cables and the manufacture thereof This invention relates to optical cables, that is to say cables of the kind incorporating one or more optical fibres accommodated within a protective cover, and relates also to the manufacture of such cables.

According to one aspect of the invention an optical cable comprises at least one strip of thermoplastics material having a plurality of longitudinallyextending grooves in onesurface, at least one of which loosely accommodates one or more optical fibres having a length greater than the length ofthe strip, and a cover strip bonded to said surfacetoen- close the grooves, the strip assembly being wrapped helically around a central elongate element.

The strip assembly is preferably wrapped around the central elongate element, which may be a strength member,for example of stranded metal wires, with the cover strip facing outwards, the thickness of the cover strip being small compared with that ofthe grooved strip to provide ready access to the fibres, either by the action of slitting the material of the strip at the appropriate position or by the provision of a series of slits disposed along the length of the strip over each of the grooves.

The ribsthatareformed attheedgesofthe grooved strip and between the grooves, may be formed with transverse slots spaced along the strip to facilitate the bending ofthe strip when winding it around the central elongate element.

The optical fibres may be in the form of individual fibres, or multi-fibre units such as optical fibre ribbons, and conveniently follow an undulating path within the respective grooves, so that they are longer than the length of the grooves in which they are accommodated,thereby avoiding the risk of the fibres being subjected to excessive tensile stress when the cable is stretched or bent.

The strip assembly may be surrounded in turn buy a cover comprising one or more outerwindings or sheaths.

In the manufacture of such a cable in accordance with a further aspect ofthe invention the grooved strip is formed by a continuous extrusion process, the fibres being fed into the grooves at a speed grea terthanthatofthe strip in order to impart a degree of overfeed to the fibres.

Ina modification ofthisprocessthestrip is initially extruded with a rectangular cross-section and is fed to a machine designed to impart a grooved pattern to a surface of the strip before the thermoplastics material has hardened, the fibres then being fed into the grooves ata speed sufficientto provide a required degree of overfeed as in the previous process. In each case the cover strip is preferably bonded to the grooved surface of the strip, either by an adhesive or by heating, following the introduction of the optical fibres.

Where the strip is initially extruded with a rectangular cross-section and the grooves subsequentlyformed in it, this can be achieved by feeding the extrudate either directly or by means of a suitable conveyor unit to the nip of a pair of rollers designed to impart a rib and groove pattern to the semi-plastic material. One roller in particular may be watercooled and the grooved plastic strip maintained in contact around part ofthe circumference until the plastics material has sufficiently hardened.

In a modification of either of the processes as above described the fibres are pulled into the respective grooves by the stretching of the grooved strip, which is subsequently allowed to relax, this resulting in the fibres within the formed strip having a length in excess of the relaxed length of the strip. The cover strip could be applied to the grooved strip prior to the stretching operation, or alternatively to the stretched part ofthe grooved strip, also in this case in a stretched condition.

The invention will be further explained by describing byway of examplewith referenceto Figures 1 to 5 ofthe accompanying schematic drawings one form of optical cable in accordance with the invention and various methods of manufacturing the cable.

In the drawings, Figure 1 illustrates part of a grooved strip employed in the cable, Figure2 shows part of a strip as illustrated in Figure 1 containing a number of optical fibres in ribbon form, and fitted with a cover strip, Figure 3 illustrates a method of introducing the fibre ribbons into the grooves, and Figures 4 and 5 illustrate different methods of winding a strip assembly around a central member.

Referring first to Figure 1, a strip 1 ofthermoplastics material is extruded with a plurality of ribs 2 on one surface defining between them a series of grooves 3. Into each of the grooves 3 is fed an optical fibre ribbon as at4in Figure 2, and a thin strip 5of thermoplastics material of the same width as the grooved strip 1 is bonded to the tops ofthe ribs 2to enclose the grooves 3 and retain the fibre ribbons within them, the fibre ribbons having a length greaterthan the length ofthe strip in which they are accommodated.

This is conveniently achieved by feeding the fibre ribbons 4 into the grooves 3 by means of nip rollers driven at a speed slightly greaterthan the throughput speed ofthe strip, so asto imparttothe ribbonsa degree of overfeed.

Alternatively the fibre ribbons 4 may be pulled into the grooved strip 1 while the latter is stretched between pairs of nip rollers 5 downstream of the point of introduction ofthefibres, and following the bonding of the cover strip 5 thereto, as shown in Figure 3, the strip assembly 7 then being allowed to relax, so that the fibre ribbons then have a length in excess of that of the strip.

The completed strip assembly, containing the fibre ribbons 4, is then wrapped helically around an elongate central core, which can be a simple strength member, for example of stranded wires, or a compound member comprising a strength membersurrounded by an intermediate cushioning layer.

The wrapping process is conveniently effected, as shown in Figure 4, as a continuation of the stripforming process, the strip assembly 7, comprising the strips 1 and 5 and optical fibre ribbons 4, being fed at an angletothe central core 8,thesupply reel 9 forth core, and the take up reel 11 for the formed cable, being rotated about the core and cable axes at an appropriate rate to cause the strip assembly7to be wrapped helically around the core.

One or more tapes as at 12 may be applied around thestripassembly7 by means of suitable lapping heads 13, and in some cases the assembly may be surrounded by an outer protective sheath (not shown).

In an alternative method of forming cable the completed strip assembly 7 as shown in part in Figure 2 may befirstwound on to a reel 14 (Figure 5). The core 8 is then arranged to be fed from a supply reel 15 through the bore ofthe reel 1 4which is then rotated around the core, in order to wind the strip assembly7 heiically around it, one or more tapes as at 16 being wound around the wound strip assembly, if desired, as in the previous embodiment. The supply reel 15 forthe central core 8 and the cable take-up stand 17 do not need to rotate about the core and cable axes in this embodiment.

Sets of transverse slots, one set of which is shown at 17 in Figure 1, may be formed at spaced positions along the ribs 2 to facilitate the bending of the strip 1 during the winding process.

Singlefibres orother multi-fibre units may be employed in place of one or more of the fibre ribbons 4 if desired, and the grooves 2 may in some cases contain a hydrogen trapping and/or a water blocking substance.

Morethan one strip assembly containing optical fibres may, in some, cases be wound around the central core. These may be disposed side by side, in the form of a multi-start helix, or be wound one on top of the other, preferably in the latter case with alternate layers wound in opposite directions.

Claims (18)

1. An optical cable comprising at least one strip of thermoplastics material having a plurality of longitudinally-extending grooves in one surface, at least one of which loosely accommodates one or more op ticalfibres having a length greaterthanthe length of the strip, and a cover strip bonded to said surface to enclose the grooves, the strip assembly being wrapped helically around a central elongate element.

2. An optical cable according to Claim 1, wherein the strip assembly is wrapped around the central el ongate element,with the coverstrip facing outwards.

3. An optical cable according to Claim 2, wherein the thickness ofthe coverstrip is small compared with that of the grooved strip.

4. An optical cable according to Claim 3, wherein the ribs that are formed atthe edges of the grooved strip and between the grooves are formed with transverse slots spaced along the strip to facilitate the bending ofthe strip when winding it around the central elongate element.

5. An optical cable according to any preceding claim, wherein the central elongate element is a strength member of stranded metal wires.

6. An optical cable according to Claim 5, wherein the optical fibres are in the form of individual fibres or multi-fibre units such as optical fibre ribbons or bundles.

7. An optical cable according to Claim 6, wherein the optical fibresfollowan undulating path within their respective grooves.

8. An optical cable according to any preceding claim, wherein the strip assembly is in turn surrounded by a cover comprising one or more outerwindings or sheaths.

9. The manufacture of an optical cable, according to any preceding claim, in which the grooved strip is formed by a continuous extrusion process, the fibres being fed into the grooves at a speed greaterthan that of the strip in order to impart a degree of over feed tothefibres.

10. The manufacture of an optical cable according to Claim 9, wherein the strip is initially extruded with a rectangular cross-section and is fed to a machine designed to impart a grooved pattern to a surface ofthe strip before the thermoplastic material has hardened.

11. The manufacture of an optical cable according to Claim 10, wherein the rectangular extrudate is fed, either directly or by means of a suitable conveyor unit, to the nip of a pair of rollers designed to impart a rib and groove pattern to the semi-plastic material.

12. Themanufactureofan optical cable according to Claim 1 1,wherein one roller is water cooled and the grooved plastic strip maintained in contact around part of the circumference until the plastics material has sufficiently hardened.

13. The manufacture of an optical cable according to any of Claims 9to 12, wherein the cover strip is bonded to the grooved surface ofthe strip, either by an adhesive or by heating,following the introduction of the optical fibres.

14. The manufacture of an optical cable according to any of Claims 1 to 8, wherein the fibres are pulled into the respective grooves by the stretching of the grooved strip, which is subsequently allowed to relax, to cause the fibres within the formed strip to have a length in excess ofthe relaxed length ofthe strip.

15. An optical cable according to Claim 14, wherein the cover strip is applied to the grooved strip priorto the stretching operation.

16. An optical cable according to Claim 14, wherein the cover strip is applied in a stretched condition to the stretched part of the grooved strip.

17. An optical cable substantially as herein described with reference to Figures 1 and 2 of the accompanying drawing.

18. The manufacture of an optical cable substan- tally as herein described with reference to Figures 3 to 5 ofthe accompanying drawing.