GB1590767A - Cable incorporating optical fibres - Google Patents

Description

(54) CABLE INCORPORATING OPTICAL FIBRES (71) We, LIGNES TELEGRAPHIQUES ET TELEPHONIQUES - 89 Rue de la Faisanderie - 75782 Paris Cedex 16 - France - a Body Corporate organised according to the laws of France, 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: The present invention relates to an improvement in cable structures incorporating optical fibres which are intended more particularly for use in telecommunications.

Many cable structures have already been described comprising optical fibres as a transmission element and more particularly structures consisting of a mechanically resistant core on which is extruded a dielectric structure defining helical slots in which the optical fibres are freely lodged, the said slots being closed by a wrap which is in turn surrounded by sheathings which protect the structure. More particularly it is stated in French Patent Specification No.

2,288,318 that in the absence of mechanical stress exerted on the cable element it is advantageous for the optical fibres to bear on the external wrapping or sheathing. Such a structure may have disadvantages when sheathing is performed through extrusion as due to the forces applied almost directly to the fibre, it may be shifted from its slot. Such a disadvantage is avoided in the structure forming the subject of French Patent Specification No. 2,312,788 wherein the slots are so dimensioned that a number of optical fibres can be located therein in a fluid filling medium which prevents adhesion between the fibres disposed in the same slot and contact thereof with the external sheathing or wrapping.

According to the present invention there is provided a cable element structure comprising a mechanical support surrounded by a profiled member of thermoplastic material having a plurality of slots running helically along the length of said cable element and along the outside surface of said thermoplastic material, each slot being adapted to receive a single optical fibre, characterized in that each slot has at least one lip forming a radial outward prolongation of an edge of the slot and extending longitudinally of the slot, said lip being constructed to be folded over the slot and being so dimensioned that, when folded, it closes the slot sufficiently to prevent dislodgement therefrom of a contained optical fibre.

The structures according to the invention can be readily processed through conventional apparatus employed in cable manufactures. They may undergo the operations of braiding, wrapping, sheathing, etc..., with much less danger of breakage of an optical fibre than structures of the prior art, since the fibre is protected at least partially against external stresses by the closing of the slot, even before the wrapping. Also, the possibilities of the fibres being shifted are limited. Indeed, in the prior art, the application of the optical fibres in helical form around the core is made difficult because they withstand substantially no tractive force. Since the fibres are not under tension in the slots in such prior art forms, they tend to leave them under the action of the rotation to which the cable element is subjected in the course of manufacture and under the action of unavoidable vibrations.

The present invention will now be described in greater detail with reference to the accompanying drawing given by way of illustration of structures according to the invention and in which: Figures 1A and 1B are two sectional views of the central core provided with its profiled structure; Figure 2 is a larger view of a fraction of figure 1 before the introduction of the optical fibre, Figure 3 is the same view after introduction of the optical fibre and closure of the slot; Figure 4 is a variant of figure 2; Figure 5 illustrates a variant of figure 3 after introduction of the optical fibre; and Figures 6 and 7 illustrate a third variant of the structure before and after introduction of the optical fibre, respectively.

In figure 1A and 2, there is shown at 1 the mechanical support of the structure. It may consist of a wire or a strand of metallic or insulating wires consisting of a relatively rigid material and having low elongation. Disposed around the said mechanical support 1 is a profiled extruded sheath of insulating thermo plastic material 2, which has a positioning groove 3 permitting the centering of the structure in the course of the manufacture of the cable element and a set of V-shaped slots, one of which is shown on a larger scale in the following figures. The lateral walls of the 'V' are prolonged in the radial direction by two lips 5 and 6 situated on either side of the opening of the slot. Figure 1B illustrates a variant very similar to that illustrated in figure 1 A. The only difference is the absence of the positioning groove 3 in the structure of figure 1 B. The angular location of the structure is then effected by other means. Referring to figures 2 and 3, it will be seen that the optical fibre 10 has been introduced into the slot 4 and the lips 5 and 6 are turned over towards one another so as to close the opening of the slot at least partially. The distance e between the ends of the folded over lips must remain smaller than the diameter d of the fibre 10, at least over a fraction of the length of the slot of more than 60%. Preferably, the length over which e would not obey the above condition must not exceed one quarter of the pitch of the helix constituting the slot.

Preferably, the depth of the slot 4 is between four and six times the diameter of the fibre 10 and the height of the lips 5 and 6 before they have been folded back is between three and four times the value of the diameter of the fibre 10. By way of illustration, the diameters of some optical fibres which can be used in the structures of the invention are in the neighbourhood of 100cm. Figures 4 and 5 illustrate a variant of figures 2 and 3 respectively, wherein the slot 4 has only one lip 7 on one of its edges. Under these conditions, the preferred value of the height of the lip is between five and six times the diameter of the optical fibre, the depth of the slot having the same values as before. Figures 6 and 7 relate to another variant of the structure according to the invention in which the slot 4 is partially closed, as of manufacture, by two lips which form a prolongation of the external periphery of the structure 2. The form of this slot can be obtained only by direct extrusion of the thermoplastic material constituting the profiled member 2. It is necessary for the tool to be such that the opening e between the edges of the slot at its periphery is less than the diameter d of the fibre 10, at least over the greater part of the length of the slot. While in the structures illustrated in figures 2 to 5 the optical fibre can be deposited lengthwise in the slot, its introduction into a slot in the profiled structure illustrated in figure 6 requires that the lips bounding the slot should be spaced apart.

This spacing may be effected with the aid of a guide introduced into each of the slots at the point of introduction of the corresponding optical fibre.

In a modified embodiment, the slots 4 have a profile terminated by an arc of a circle. By way of illustration, a cable element according to the invention has the following characteristics: - external diameter of the taping 4.8 millimeters - nature of the extruded dielectric 2 polypropylene - diameter of the extruded dielectric 2 4 millimeters - number of optical fibres 8 - diameter of the optical fibres 125 micrometers - pitch of the slots 0.20 meter -centering groove width 1 millimeter depth 1 millimeter - slots (8) width 0.5 millimeter depth 0.6 millimeter each bounded by two lips of width 0.3 millimeter and of thickness 0.2 millimeter - diameter of the central core 1 millimeter - nature of the central core : single wire of steel type XC 80.

In the above structure, the optical fibre is free in the slot. In some cables, a liquid fills the slots which serves both to damp the displacements of the fibre and to prevent contact with the walls of the slots.

WHAT WE CLAIM IS: 1. A cable element structure comprising a mechanical support surrounded by a profiled member of thermoplastic material having a plurality of slots running helically along the

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

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. Figure 4 is a variant of figure 2; Figure 5 illustrates a variant of figure 3 after introduction of the optical fibre; and Figures 6 and 7 illustrate a third variant of the structure before and after introduction of the optical fibre, respectively. In figure 1A and 2, there is shown at 1 the mechanical support of the structure. It may consist of a wire or a strand of metallic or insulating wires consisting of a relatively rigid material and having low elongation. Disposed around the said mechanical support 1 is a profiled extruded sheath of insulating thermo plastic material 2, which has a positioning groove 3 permitting the centering of the structure in the course of the manufacture of the cable element and a set of V-shaped slots, one of which is shown on a larger scale in the following figures. The lateral walls of the 'V' are prolonged in the radial direction by two lips 5 and 6 situated on either side of the opening of the slot. Figure 1B illustrates a variant very similar to that illustrated in figure 1 A. The only difference is the absence of the positioning groove 3 in the structure of figure 1 B. The angular location of the structure is then effected by other means. Referring to figures 2 and 3, it will be seen that the optical fibre 10 has been introduced into the slot 4 and the lips 5 and 6 are turned over towards one another so as to close the opening of the slot at least partially. The distance e between the ends of the folded over lips must remain smaller than the diameter d of the fibre 10, at least over a fraction of the length of the slot of more than 60%. Preferably, the length over which e would not obey the above condition must not exceed one quarter of the pitch of the helix constituting the slot. Preferably, the depth of the slot 4 is between four and six times the diameter of the fibre 10 and the height of the lips 5 and 6 before they have been folded back is between three and four times the value of the diameter of the fibre 10. By way of illustration, the diameters of some optical fibres which can be used in the structures of the invention are in the neighbourhood of 100cm. Figures 4 and 5 illustrate a variant of figures 2 and 3 respectively, wherein the slot 4 has only one lip 7 on one of its edges. Under these conditions, the preferred value of the height of the lip is between five and six times the diameter of the optical fibre, the depth of the slot having the same values as before. Figures 6 and 7 relate to another variant of the structure according to the invention in which the slot 4 is partially closed, as of manufacture, by two lips which form a prolongation of the external periphery of the structure 2. The form of this slot can be obtained only by direct extrusion of the thermoplastic material constituting the profiled member 2. It is necessary for the tool to be such that the opening e between the edges of the slot at its periphery is less than the diameter d of the fibre 10, at least over the greater part of the length of the slot. While in the structures illustrated in figures 2 to 5 the optical fibre can be deposited lengthwise in the slot, its introduction into a slot in the profiled structure illustrated in figure 6 requires that the lips bounding the slot should be spaced apart. This spacing may be effected with the aid of a guide introduced into each of the slots at the point of introduction of the corresponding optical fibre. In a modified embodiment, the slots 4 have a profile terminated by an arc of a circle. By way of illustration, a cable element according to the invention has the following characteristics: - external diameter of the taping 4.8 millimeters - nature of the extruded dielectric 2 polypropylene - diameter of the extruded dielectric 2 4 millimeters - number of optical fibres 8 - diameter of the optical fibres 125 micrometers - pitch of the slots 0.20 meter -centering groove width 1 millimeter depth 1 millimeter - slots (8) width 0.5 millimeter depth 0.6 millimeter each bounded by two lips of width 0.3 millimeter and of thickness 0.2 millimeter - diameter of the central core 1 millimeter - nature of the central core : single wire of steel type XC 80. In the above structure, the optical fibre is free in the slot. In some cables, a liquid fills the slots which serves both to damp the displacements of the fibre and to prevent contact with the walls of the slots. WHAT WE CLAIM IS:

1. A cable element structure comprising a mechanical support surrounded by a profiled member of thermoplastic material having a plurality of slots running helically along the

length of said cable element and along the outside surface of said thermoplastic material, each slot being adapted to receive a single optical fibre, characterized in that each slot has at least one lip forming a radial outward prolongation of an edge of the slot and extending longitudinally of the slot, said lip being constructed to be folded over the slot and being so dimensioned that, when folded, it closes the slot sufficiently to prevent dislodgement therefrom of a contained optical fibre.

2. A cable element structure according to Claim 1, wherein a lip is formed at each of the outer edges of each slot, the two lips of each slot being so dimensioned that, when folded over the slot, they coact to close the slot sufficiently to prevent dislodgement therefrom of a contained optical fibre.

3. A cable element structure according to Claim 1 or 2, wherein said slots are obtained by extrusion of a thermoplastic material.

4. A cable element structure according to Claim 1, wherein the depth of the slots is between four and six times the diameter of the optical fibres and the height of the said lip is between five and six times the diameter of the optical fibres.

5. A cable element structure according to any one of the preceding claims, wherein the slots are filled with a liquid medium in which the optical fibre is located.

6. A cable element structure as claimed in Claim 1 constructed substantially as herein described with reference to and as illustrated in the accompanying drawing.