GB2157018A

GB2157018A - Optical fibre cables

Info

Publication number: GB2157018A
Application number: GB08508124A
Authority: GB
Inventors: Ronald Yaxley Gill
Original assignee: Telephone Cables Ltd
Current assignee: Telephone Cables Ltd
Priority date: 1984-04-02
Filing date: 1985-03-28
Publication date: 1985-10-16
Also published as: GB8508124D0; GB2157018B

Abstract

An optical fibre cable is made by extruding a sinusoidal-shaped flexible plastics strip (2) having sinusoidal longitudinal grooves (9) in one side and wedge shaped grooves (11) in the other side. The strip is wrapped longitudinally around a central strength member (1) with the sinusoidal grooves (9) outermost. These contain optical fibres (3) and possibly a water repellent medium and are covered by a flexible plastics tape (4) sealed to the grooved surface of the strip. <IMAGE>

Description

SPECIFICATION Optical fibre cables This invention relates to optical fibre cables of the type where one or more optical fibres are contained in one or more discrete sections of the cable.

The invention provides a new construction of such a cable and a method of manufacturing the cable.

Thus an optical fibre cable, according to the invention, comprises an extruded flexible plastics strip of generally rectangular cross-section having a plurality of longitudinally-extending grooves in one surface, which strip is wrapped longitudinally around a central strength member with the grooves outermost, one or more optical fibres in at least one of said grooves, and a cover member which covers the grooves and serves to retain the fibre or fibres within the respective groove or grooves.

The strip is conveniently made of polythene or polypropylene and preferably the strip and the grooves are of sinusoidal shape in the longitudinal direction, and the parallel sinusoidal edges of the strip meet along the central strength member.

The cover may be in the form of a flexible plastics tape sealed to the grooved surface of the strip.

The fibre or fibres is/are preferably loosely accommodated in the respective groove or grooves which may be filled with a water repellent medium if desired.

The cover member preferably has short longitudinal slits therein to allow access to the grooves.

Preferably, also, the strip has longitudinal grooves on the opposite surface to the first-mentioned surface, which grooves may also be sinusoidal in the longitudinal direction, and facilitate wrapping the strip around the central strength member during manufacture.

The invention also provides a method of manufacturing an optical fibre cable comprising the steps of: a) extruding a flexible plastics strip of generally rectangular cross-section having a plurality of longitudinally-extending grooves in one surface thereof; b) feeding one or more optical fibres into at least one of said grooves; c) covering said grooves with a cover member to retain the fibre or fibres within the respective groove or grooves; and d) wrapping said flexible strip longitudinally around a central strength member.

Preferably the cover member, which may be a flexible plastics tape, is welded to the ridges between the grooves of the strip and may be perforated by means of short longitudinal slits disposed at intervals along it at positions which register with the grooves.

The strip holding the fibres and with the tape welded thereto is preferably stretched and then allowed to relax, before wrapping around the strength member in order to make sure that the fibres are not under tension as the strip is wrapped around the strength member or during subsequent handling of the cable.

The cable may have further strengthening tapes wrapped around it and may also have a plastic sheath extruded around the tapes if desired.

The invention will now be more fully described by way of example with reference to the drawings in which: Figure 1 shows a cross-sectional view through one embodiment of an optical fibre cable according to the invention; Figure 2 shows a diagrammatic plan view of the strip, fibres and tape before wrapping around the central strength member; Figure 3 shows a cross-sectional view of part of the strip shown in Figure 2 along line X-X; Figure 4 shows a similar view to that shown in Figure 3 of the strip as it is being bent; and Figure 5 shows a diagrammatic view of apparatus used to manufacture the cable shown in Figure 1.

Thus the optical fibre cable as shown in Figure 1 consists of a plurality of optical fibres 3 loosely enclosed within grooves 9 on the outside surface of a plastic section 2. As can best be seen from Figures 2-4 the plastic section 2 is extruded as a strip having a substantially rectangular cross-section, being subsequently formed with edges of a sinusoidal shape in the longitudinal direction of the extrusion.

The strip has rectangularly shaped grooves 9 in one surface thereof having ridges 10 between them of a depth approximately half that of the strip, the grooves being of sinusoidal shape and parallel to the edges of the strip. Wedge-shaped grooves 11 are also provided in the opposite surface of the strip, which grooves 11 are in line with the centre of each ridge 10 in the first surface and of a depth slightly less than half the thickness of the strip 2.

The grooves each contain one or more optical fibres 3 (two being shown in the drawings) and are closed off by means of a plastics tape 4 which is also sinusoidally shaped and which is welded to the tops of the ridges 10. The tape 4 contains short longitudinal slits 8 at intervals along its length aligned with the grooves 9 so as to permit access to the grooves containing the optical fibres thus allowing them to be filled with a medium to prevent the passage of water in the cable if required.

The assembly of the plastic strip 2, the optical fibres 3 and the plastic tape 4 is shown in Figure 3 at the time when it has first been assembled and, as can be seen, the slits 8 are still very narrow, However, as the assembly is bent so as to be wound around a strengthening member 1, the slits 8 widen due to the flexibility of the tape 4, as can best be seen from Figure 4. As this occurs, the rectangularly-shaped grooves 9 widen out to form a truncated wedge shape and the wedge-shaped grooves10 close up.

When the assembly is fully wound onto the central strength member 1, which may be metallic or non-metallic and may first be plastic coated if desired, the two sides of the plastic strip 2 meet as shown in Figure 1. Strengthening tapes 5 and 6 are tighly spirally wound around the plastic tape 4 and these are preferably paper but could be plastic or thin metal. The grooves 9 may be filled with a water-impermeable medium (not shown) to provide a water barrier for the fibres.

Finally the cable is coated with a plastic sheath 7 including a moisture or other metallic barrier if required.

A method of manufacturing a cable as described above will now be described with reference to Figure 5.

A plastic strip 2 of rectangular shaped section is first extruded from a plastic extrusion machine 12 and is removed from the extruder die face by a continuous belt 14 driven at a speed so as to further control the dimensions of the strip, and to allow some cooling to occur before the strip is passed between two water-cooled rolls 16. The rolls 16 each have a pattern engraved on the roll surface so as to produce the pattern of sinusoidal grooves and ridges on both sides of the strip 2 as described above and also to cut it into a sinusoidal shape. The strip continues around the circumference of one of the two water-cooled rolls to allow the formed strip to harden. It is then removed around an idling roller and travels towards a capstan unit where it is stretched.

Before reaching the capstan unit, however, the optical fibres 3, fed from tension controlled supply stands 18, are guided singly or in groups into the patten of grooves 9 in the strip 2 via a guiding device 20. The optical fibres are pulled into the grooves from the supply reels relative to the stretching of the plastic strip by the capstan unit.

Between the capstan unit and the point at which the optical fibres are inserted into the grooves, a plastic tape 4 is fed by a roller 24 from a supply reel 22 onto the plastic strip 2 so as to enclose the fibres in the grooves and the tape is suitably welded onto the ridges between the grooves by passing the assembly between heated rollers 26.

The roller 24 also perforates the tape 4 providing short longitudinal slits in the tape aligned with the grooves 9.

The assembly which now consists of the plastic strip 2, the optical fibres within the grooves of the plastic strip, and the plastic tape 4 welded to the top of the plastic strip is now passed through a set of three rollers 28 one of which is biassed towards the other two and includes a transducer so as to keep the assembly under tension whilst it is being stretched by the capstan unit. The transducer senses the tension of the assembly and adjusts the tension device 30 on the capstan unit accordingly.

The asembly passes through the capstan unit which comprises several rollers, 36, 38, arranged horizontally and vertically alternately, the horizontal rollers 38 being driven by a motor 32 via the tension device 30 and the vertical rollers 36 being free to turn. By this means the assembly is stretched and the optical fibres 3 are pulled from the supply reels 18 into the grooves as mentioned above. The assembly then contracts again as it leaves the capstan unit under a lesser tension than the stretching load and thus the optical fibres are "overfed" to the assembly so that they are loose within the grooves and not under tension.

After leaving the capstan unit, the assembly is wrapped longitudinally around a central strength member 1, supplied from a reel 34, by a forming head 40. As described above, this causes the slits 8 in the tape 4 to widen and the grooves 9 are filled with a water-impermeable medium at a filling head 42.

The construction is immediately tightly spirally wrapped with tapes 5 coming from feed reels 44.

The tapes may be metallic or non-metallic as desired. The cable is then wound on to a suitable drum 46 or fed into an in-line sheathing extruder (not shown) to provide a final layer of protection.

It should be understood that although the cable is described with reference to optical fibres only, electrical conductors may take the place of some of the optical fibres so that the cable is both optical and electrical. Also, the central strength member may itself be of conducting material and used as a conductor if desired.

Claims

1. An optical fibre cable comprising an extruded flexible plastics strip of generally rectangular cross-section having a plurality on longitudinally-extending grooves in one surface, which strip is wrapped longitudinally around a central strength member with the grooves outermost, one or more optical fibres in at least one of said grooves, and a cover member which covers the grooves and serves to retain the fibre or fibres within the respective groove or grooves.

2. An optical fibre cable according to Claim 1 wherein the strip and the grooves are of sinusoidal shape in the longitudinal direction and the parallel sinusoidal edges of the strip meet along the central strength member.

3. An optical fibre cable according to either Claim 1 or Claim 2 wherein said strip is made of polythene or polypropylene.

4. An optical fibre cable according to any preceding Claim wherein said cover member is a flexible plastics tape sealed to the grooved surface of the strip.

5. An optical fibre cable according to any preceding Claim wherein said strip also has longitudinal grooves on the opposite surface to the firstmentioned surface.

6. An optical fibre cable according to Claim 5 wherein said grooves on said opposite surface are sinusoidal in the longitudinal direction.

7. An optical fibre cable according to any preceding Claim wherein said cover member has short longitudinal slits therein to allow access to the grooves.

8. An optical fibre cable according to any preceding Claim wherein the fibre or fibres is/are loosely accommodated within the rspective groove or grooves.

9. An optical fibre cable according to any preceding Claim wherein said groove or grooves con tain a water repellent medium.

10. A method of manufacturing an optical fibre cable comprising the steps of extruding a flexible plastics strip of generally rectangular cross-section having a plurality of longitudinally-extending grooves in one surface thereof, feeding one or more optical fibres into at least one of said grooves, covering said grooves with a cover member to retain the fibre or fibres within the respective groove or grooves and wrapping said flexible strip longitudinally around a central strength member.

11. A method according to Claim 10 wherein the cover member is welded to the ridges between the grooves of the strip.

12. A method according to either Claim 10 or Claim 11 wherein said cover member is a flexible plastics tape.

13. A method according to any one of Claims 10 to 12 wherein the cover member is perforated by cutting short longitudinal slits therein at intervals along it at positions which register with the grooves.

14. A method according to any one of Claims 10 to 13 including the step of stretching the strip with the fibres within the grooves and with the cover member covering the grooves and allowing it to relax before wrapping it around the central strength member so that the fibres are not under tension within the grooves as the strip is wrapped around the central strength member.

15. An optical cable substantially as hereinbefore described with reference to Figure 1 of the drawings.

16. A method of manufacturing an optical fibre cable substantially as hereinbefore described with reference to Figures 2 to 5 of the drawings.