GB2144556A - Optical fibre cable - Google Patents

Abstract

An optical fibre cable comprises optical fibres (6) stranded around a central member (7) before the assembly is encased in a cushioning material (4) and then three other layers (3, 2, 1) provided over the cushioning. These three layers comprise a tensile member (2) sandwiched between inner (3) and outer (1) extrusion layers and all three layers are able to slide relative to each other. <IMAGE>

Description

SPECIFICATION Optical fire cable Optical fibres covered by one or several plastic claddings, may be considered as the manufacturing basic units in the design of optical cables, in the same way as insulated metal wire art used in electrical conductors cables.

Mà y different designs have been employed in cables, but, in all of them, the most important constituent parts are: Cladding of the optical fibres Traction reinforcement Radial reinforcement or damping Cable sheaths and humidity barriers The present invention relates to an optical fibres cable structure for communications, particularly suitable for aerial laying from a messenger wire support. Fastening to such a support requires special technology that takes into account the suspension span of the messenger wire, and the mechanical strains of the messenger wire.

These strains, to which the messenger wire can be subjected are considerable in the case of large spans and metal messenger wires as they are subject to climatic variations of temperature, ice, wind, etc. An optical cable firmly attached to such a messenger wire is accordingly subjected to similar strains. The cable of the present invention is particularly useful for such applications and serves to protect the fragile optical fibres from all disturbances that the cable may receive from the exterior.

According to the invention there is provided an optical fibre cable comprising a central strength member, a layer of optical fibres stranded around the central strength member, a layer cushioning material around the layer of optical fibres and a tensile layer sandwiched between inner and outer extruded layers around the cushioning material which extruded and tensile layers are able to slide relative to each other.

The invention also comprises an optical fibre cable comprising a central strength member made of non metallic material, around which are cabled, in a single layer, and with a certain helix pitch or cabling, a series of optical fibre elements, jointly with filling elements or metallic conductors, in such a way that the complete assemble totally covers the central resistant element, the core formed by the central strength member,and the cladding elements, being covered by at least one cushioning layer; and in that the core cladded by the cushioning layers has over it a first layer of extruded plastic material, adjacent to the last cushioning layer, but not jointly fixed to the same; a second layer, consisting of several weaves of non metallic material, having a high resistance to traction, longitudinally disposed, and adjacent to the first layer of extruded plastic material, but not joined thereto; and a third layer of extruded plastic material, adjacent to the second layer of weaves of non metallic material, but joined thereto.

Installation of optical fibre cables to messenger wires is normally recommended to be effected in a discontinuous manner by using properly preformed elements, so that the strains on the messenger wire may be absorbed by installation means, without damage to the optical fibres of the cable. Such an installation can be employed with the cable of the present invention which relates to the cable itself and not the type of installation.

In order that the invention and its various other preferred features may be understood more easily, an embodiment thereof will now be described, by way of example only, with reference to the drawing, the single Figure of which illustrates the end of an optical cable with layers progressively stripped to illustrate its construction.

In the drawing the cable consists of a central strength member formed by a traction resistant central core 7, so that when the cable is subjected to high traction forces, up to 400 Kg., it may tolerate a small elongation of for example about 0.5% to 1%, having, at the same time, sufficient flexibility to allow bending with a small radius. This central strength member has the particular characteristic that being of non metallic material it provides, for equal dimensions, mechanical characteristics of strength to traction, higher than those of steel, and of course, much high'er flexibility, less. elongation and less weight than steel, all these qualities being very positive characteristics in the manufacture of optical fibre cables.

Around the central core 7, there are cabled in a single layer, and with a predetermined helical pitch or cabling, a series of elements all or some of which are optical fibres 6.

Some may be filling elements 5, or even metal conductors, but in such a way that the whole totally covers the central resistant element. The number of elements situated around the core depends on the core diameter.

It is important that the fibres are kept in a firm geometrical configuration in the cable, under any stress conditions, and also that these fibres are kept in lineal contact with adjacent components in order to avoid microbending effects.

Around the optical fibre element layer, one or rriore cushioning layers 4 are provided. A very simple technique is to employ one or several tapes of expanded material, in closed and overlaped helix. Alternative constructions may be employed however such as tubing the core or placement of longitudinal tapes.

As the cable described has been designed for aerial installation fastened to a messenger wire, it will be subject to the mechanical forces originated on the wire. In order to avoid such forces affecting the optical fibres, and to prevent their passing from the exterior to the inner core, an arrangement of sliding claddings is employed which absorb, due to their location and materials used, the traction forces to which the cable is subjected, due to expansion or contraction of the messenger wire, as well as those which result from vibration or other disturbances caused by atmospheric disturbance.

Firstly, a first cover of plastic material 3 is provided adjacent to the one or several cush ioning layers 4, which cover is not fixed to the cushioning layers. Over this first cladding a woven layer, or series of woven layers 2, are longitudinally disposed. The layers should be of material highly resistant to traction. The value of this resistance to traction depends on the number of woven layers employed, this being a design data to be calculated considering the conditions under which the cable is to be used. Similarly, the woven layers must be of a material with practically no elongation, or in any case, below 2%, since it forms the absorbing element of the tension to which the cable is submitted, which tension must not pass to the core of the fibres. Over layers 2, a second cover forming an exterior sheath 1 made of plastic material is provided adjacent to the woven layers but not fixed to them. The woven cladding, although adjacent to both the first cover 3 and the second cover 1, is not fixed to either covers, but may slide between them.

This cable may be unprotected or filled with a material which prevents ingress of humidity through the inner of the core of the fibres, e.g. petroleum jelly, thereby to prevent moisture from entering the cable due to an accident, or because of badly sealed joints.

The described cable does not normally have metallic elements, although, if desired, some conductors may be included between the optical fibres, as communication, signalling or feeding means, but not as mechanical or protective means.

Claims (8)

1. An optical fibre cable comprising a central strength member, a layer of optical fibres stranded around the central strength member, a layer cushioning material around the layer of optical fibres and a tensile layer sandwiched between inner and outer extruded layers around the cushioning material which extruded and tensile layers are able to slide relative to each other.

2. An optical fibre cable as claimed in claim 1, wherein the central strength member is non metallic.

3. An optical fibre cable as claimed in claim 1 or 2, wherein the tensile layer comprises at least one layer of woven non metallic material.

4. An optical fibre cable comprising a central strength member, made of non metallic material, around which are cabled, in a single layer, and with a certain helix pitch or cabling, a series of optical fibre elements, jointly with filling elements or metallic conductors, in such a way that the complete assembly totally covers the central resistant element, the core formed by the central strength member, and the cladding elements, being covered by at least one cushioning layer; and in that the core cladded by the cushioning layers has over it a first layer,of extruded plastic material, adjacent to the last cushioning layer, but not jointly fixed to the same; a second layer, consisting of several weaves of non metallic material, having a high resistance to traction, longitudinally disposed, and adjacent to the first layer of extruded plastic material, but not joined thereto; and a third layer of extruded plastic material, adjacent to the second layer of weaves of non metallic material, but joined thereto.

5. An optical fibre cable as claimed in claim 4, characterised in that the three layers of the cladding are slidable one on the other, without damage to any of them on being submitted to exterior stresses.

6. An optical fibre cable as claimed in any one of the preceding claims, filled with material which prevents ingress of humidity through the interior of the core of the fibres.

7. An optical fibre cable as claimed in claim 6, wherein the material which prevents ingress of moisture is petroleum jelly.

8. An optical fibre cable substantially as described herein with reference to the drawing.