GB2304205A

GB2304205A - Optical fibre cable or ducting

Info

Publication number: GB2304205A
Application number: GB9616609A
Authority: GB
Inventors: Kenneth George Hodge; Peter John Whitesmith
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1995-08-12
Filing date: 1996-08-08
Publication date: 1997-03-12
Anticipated expiration: 2016-08-08
Also published as: GB9516560D0; GB9616609D0; GB2304205B

Abstract

An optical cable comprises: (a) a plurality of hollow tubes 1,2 that are laid together so that their axes define a planet 4, each tube having one or more optical fibres 12,14 loosely housed therein; (b) a jacket 6 that encloses the tubes; and (c) a pair of strength members 8 that are embedded in the jacket, the strength members being embedded in the jacket wall on opposite sides of the plane 4 defined by the axes of the tubes. The invention also provides ducting that is the same as the cable above except that the tubes 1,2 do not contain fibres, but fibres can be introduced therein at a later stage, for example by blowing.

Description

OPTICAL CABLE This invention relates to optical cables, and especially to ducting for optical fibres in which optical fibres are or can be loosely housed therein.

In many instances it is necessary to lead optical fibres from one piece of equipment to another over relatively short distances, for example from a few centimetres to 10 metres or so, in which case it will be most appropriate to house the fibres in ducting in order to protect them. It is also important to prevent the fibres being bent to such an extent that their bend radius falls below about 30mum or otherwise significant quantities of light will be lost from the fibres. In view of this it has been the practice to employ "bend-limiting" means of one form or another, for example rigid plastics clips of defined bend radius which may be clipped onto the duct, or corrugated bend limiting tube which can only be bent until adjacent corrugations on the inside of the bend touch one another.Such bend limiting means are, however, relatively expensive, and it is the object of the present invention to provide ducting for optical fibres and "cable" formed from such ducting of reduced cost which will provide some protection to the fibres against overbending.

According to one aspect, the invention provides an optical cable which comprises: (a) a plurality of hollow tubes that are laid together so that their axes define a plane, each tube having one or more optical fibres loosely housed therein; (b) a jacket that encloses the tubes; and (c) a pair of strength members that are embedded in the jacket, the strength members being embedded in the jacket wall on opposite sides of the plane defined by the axes of the tubes.

According to another aspect, the invention provides ducting for optical fibres, which comprises: (a) a plurality of hollow tubes that are laid together so that their axes define a plane, each tube being capable of receiving one or more optical fibres; (b) a jacket that encloses the tubes; and (c) a pair of strength members that are embedded in the jacket, the strength members being embedded in the jacket wall on opposite sides of the plane defined by the axes of the tubes.

Normally and preferably, the strength members will define a plane that is substantially perpendicular to the plane defined by the axes of the tubes.

The cable and ducting according to the invention has the advantage that it is relatively simple and inexpensive to manufacture, simply requiring extrusion operations, but will provide adequate protection to the fibres against overbending in most cases. The ducting provides resistance to overbending in both orthogonal directions through different means: the ducting resists bending in the plane of the tube axes due to the relatively large lateral dimension of the ducting in this direction (at least two tube diameters), and also resists bending in a direction normal to this plane due to the presence of the strength members.

The tubes are preferably formed from a plastics material as is the jacket.

Examples of materials from which the tubes and/or jacket may be formed include low, medium or high density polyethylene linear low density polyethylene, poly alpha olefins, eva or eea polymers, polyamides, polyesters e.g. polybutylene terephthalate halogenated polymers such as pvc and the like. The plastics material may include fillers, pigments, cross-linking agents, flame-retardants, uv stabilisers, antioxidants, fungicides and the like. The strength members may be metallic or nonmetallic and are preferably formed from an aromatic polyimide for example as sold under the registered trade mark "Kevlar".

The tubes will normally each have a diameter in the range of from 3 to 8mm and a wall thickness in the range of from 0.2 to 0.5mm.

The jacket material may be adhered to the tubes if desired, and this will increase the rigidity of the ducting in the plane of the axes of the tubes, although this is not necessary since there will usually be sufficient friction between the tubing and the jacket to resist bending of the ducting. The presence of the strength members will cause he ducting to resist bending out of the plane of the axes of the tubing since the strength members will normally be embedded in the jacket wall with the result that the strength member on the inside of the bend will be prevented from buckling or crumpling by the material forming the jacket wall.

One form of cable and ducting will now be described with reference to the accompanying drawing which is a cross-section through a cable according to the invention.

The cable comprises a pair of hollow tubes 1 and 2 of 3mm outer diameter and 0.35mm wall thickness that are located side-by-side so that their axes define a plane 4. The tubes are provided with ajacket 6 that is extruded onto the tubes and which contains a pair of aromatic polyimide strength members 8. The strength members 8 are located in opposite sides ofthejacket 6 so that they define a plane 10 (horizontal as shown in the drawing) that is perpendicular to the plane 4 of the tube axes.

Each tube 1 and 2 contains a 900 ,um jacketed optical fibre 12 and 14 which is loosely housed in the tube so that it can move laterally or longitudinally therein at least to some extent. The fibres may be introduced into the tubes during manufacture or they may be introduced subsequently, for example they may be "blown" into the tubes by the viscous drag of a flow of a gaseous medium. If desired, the tubes may contain additional optical fibres and/or additional polyimide strength members (not shown) loosely housed therein.

The sides of the jacket 6 may be generally flat as shown. However, the sides may be thickened slightly to give the jacket a substantially oval profile which will improve the bending resistance in the plane 10.

The cable and ducting will resist bending in the plane 4 of the tube axes due to the relatively large lateral dimensions in this direction which are at least twice the diameter of the tubes 1 and 2. Resistance to bending in the orthogonal plane 10 is provided by the strength members 8.

Claims

Claims:

1. An optical cable which comprises: (a) a plurality of hollow tubes that are laid together so that their axes define a plane, each tube having one or more optical fibres loosely housed therein; (b) a jacket that encloses the tubes; and (c) a pair of strength members that are embedded in the jacket, the strength members being embedded in the jacket wall on opposite sides of the plane defined by the axes of the tubes

2. An optical cable as claimed in claim 1, wherein the strength members together define a plane that is substantially perpendicular to the plane defined by the axes of the tubes.

3. An optical cable as claimed in claim 1 or 2, wherein the hollow tubes are formed from a plastics material.

4. An optical cable as claimed in any one of claims 1 to 3, wherein the strength members are formed from a polymeric material.

5. An optical cable as claimed in claim 4, wherein the strength members are formed from an aromatic polyimide.

6. An optical cable as claimed in any one of claims 1 to 5, wherein the tubes have an outer diameter in the range of from 3 to 8 mm.

7. Ducting for optical fibres, which comprises: (a) a plurality of hollow tubes that are laid together so that their axes define a plane, each tube being capable of receiving one or more optical fibres; (b) a jacket that encloses the tubes, and (c) a pair of strength members that are embedded in the jacket, the strength members being embedded in the jacket wall on opposite sides of the plane defined by the axes of the tubes.