AERIAL OPTICAL FIBRE CABLE

Abstract: An all-dielectric optical fibre cable comprises a strength member rod (1) having a roughened surface (1A) formed by a moulding process during manufacture of the rod using a woven tape imprint, which serves to improve adhesion between an extruded outer sheath (7) and the surface (1A) of the rod (1). The cable is suitable as an all dielectric aerial cable for large unsupported spans eg 500M, alongside a high voltage electricity distribution system.

Description: This invention relates to an aerial optical fibre cable, particularly but not exclusively one that has no metallic components. Our published European patent application No EP-A-0 216 548 discloses an all-dielectric aerial optical fibre cable in which the strength member comprises a rod-like member of pultruded glass reinforced plastics material and having a slot accommodating the optical fibres. This strength member is tightly surrounded by an extruded plastic sheath and there may be an intermediate winding of tape or cord around the strength member prior to extrusion of the sheath. In use, this cable is supported from pylons along with but spaced from the high tension electrical conductors for the purpose of providing a telecommunications link. The cable has the advantage of being able to be installed without the need for shutting down the high tension transmission because it is an all-dielectric cable. Our European Application EP-A-0 278 648 discloses an all dielectric aerial optical fibre cable similar to our earlier patent discussed above and wherein the slot is filled with a water blocking compound and a slot cap is applied over the slot after it has been filled with water blocking compound, to close the slot, and the excess of water blocking compound is wiped cleanly away from the surface of the pultruded glass reinforced plastics rod. We have found it is important to remove as far as possible any excess water blocking compound as this tends to prevent the outer sheath firmly gripping the rod-like strength member. As shown in this patent specification we have provided a binding around the strength member and the slot cap both to hold the slot cap in position and also to provide a frictional interface between the sheath and the strength member. However we have experienced a certain amount of decoupling between the strength member and the sheath. A known arrangement for providing adhesion between an extruded covering and an electrical conductor is shown in US patent US-A-4 610 909. Here unplasticised particles of PVC resin are coated onto the conductors and the portions of the particles contacting the surfaces of the conductor are melted and degraded and adhere to the surfaces of the conductors, forming a discontinuous coating of adhesion sites. A plasticised PVC resin is extruded over the conductors and a controlled degree of adhesion to the adhesion sites is claimed to occur. In this arrangement the conductor and strength member are the same component, made of copper-coated steel and therefore very robust. However the dielectric cable of the above-mentioned European patent applications has a dielectric strength member which houses the optical conductors and which would be damaged by the process of degrading particles on the surface. It is an object of the present invention to provide an alternative solution to this problem, without degrading the strength member. According to the present invention there is provided an optical fibre cable, comprising a dielectric rod-like strength member (1) having a longitudinal slot (2) which contains an optical conductor (3), said conductor being protected by the strength member against longitudinal and transverse stress applied to the strength member, said strength member being encased in an extruded sheath (7) in contact with the surface of the strength member, characterised in that the surface of the strength member is roughened by moulding an imprint (1A) in the surface of the strength member in order to improve the grip between the sheath and the strength member. Preferably the roughened surface of the strength member is produced during manufacture of the strength member while the strength member is in a soft condition, by impressing a rough surface mould onto the soft strength member so that when the strength member hardens, the rough surface contour becomes imprinted upon the surface of the strength member. Preferably this is provided by laying a woven tape around the outside of the strength member during manufacture and removing it after the strength member has hardened. Preferably the strength member comprises glass rovings impregnated with resin to produce a glass reinforced resin strength member with an imprinted roughened surface. According to another aspect of the invention there is provided a method of making an optical fibre cable, comprising providing a dielectric strength member rod (1) having a longitudinal slot (2), providing an optical conductor in the slot, and encasing the strength member in an extruded sheath (7) in contact with the strength member rod, characterised in that the surface of the strength member rod is roughened while the rod is in a softened condition and that the rod is subsequently allowed to harden so that the surface roughening remains imprinted in the hardened surface. Where the strength member is made by a glass reinforced resin rod by a pultrusion process, the surface is effectively "abraded " without damaging the glass rovings. If the glass rovings were damaged, this would seriously impair the strength of the glass reinforced resin rod. In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which:- Fig 1 shows in cross-section an all dielectric optical fibre aerial cable according to an embodiment of the present invention; and Fig 2 shows a method of making the strength member of the cable shown in Fig 1. Referring to Fig 1, a non-electrically conductive rod 1 of homogenous material made from glass-fibre reinforced resin by a pultrusion or similar process, has a rectangular slot 2 with convexly radiused edges 2A and 2B, containing an optical fibre ribbon element 3 in the slot 2. The rod 1 acts as the cable strength member and the cable armour (having high tensile strength and being highly crush-resistant) and is resilient with a modulus of at least 40,000Nmm⁻². The slot 2 runs straight along the rod 1 and is relatively shallow, that is to say it is always located to one side of the neutral axis of the cable which is at or close to the geometric centre referenced 1A. The slot 2 is closed by a cap 5 made of extruded plastics material and having concavely radiused edges 5A and 5B which exactly fit onto the edges 2A and 2B of the slot 2. The slot 2 is filled with a grease-like material 8, for example one sold under the brand name SYNTEC Type FCC210S and which is a soft thixotropic water blocking material. Around the rod 1 and the cap 5 is a binder 6 which is helically wound around and serves to locate the cap 5 in place on the slot 2 prior to extrusion of an outer sheath 7 and during the induction of an excess length of fibre around a capstan as described in our published specification NO. GB-B-2,201 008 and EP-A-0216548. The rod 1 has a roughened surface 1A which improves the adhesion between the sheath 7 and the rod 1. It is pointed out that Fig 1 is not to scale and the binder 6 is helically applied so that most of the surface 1A of the rod 1 is in direct contact with the sheath 7. The configuration of the roughened surface 1A is in the form of moulded dimples produced by a woven tape which has been laid against the outside of the rod during manufacture and while the rod is in its soft state and removing the woven tape after the glass/resin rod substrate has been cured, thereby leaving a roughened surface produced by a "moulding" process. The shape and configuration of the woven tape becomes imprinted in the surface of the rod. Fig. 2 shows in detail how this is achieved. Referring to Fig 2 the strength member rod 1 is produced by glass rovings 10 carried on bobbins 11 which are drawn into a resin bath 12 beneath a guide roller 13. The rovings 10, coated with resin, are drawn through a die 14 together with a peel-ply tape 15 drawn from a supply reel 16. The tape is applied longitudinally around the impregnated glass rovings as they pass through the die 14 and into a curing oven 17 where the resin impregnating the glass rovings is cured. The rod 1 thus produced emerges from the oven covered with the peel-ply tape 15 and is wound onto a take up drum 18. The peel-ply tape can either be removed prior to winding onto the take up drum 18 or alternatively can be left on the rod to be removed when the aerial optical fibre cable shown in Fig. 1 is manufactured. It is thus possible to draw the rod from the take up drum 18 during manufacture of the cable and peel off the peel-ply tape 15 before the optical fibres and the water blocking compound are introduced into the slot 2. The mesh size of the peel-ply tape which moulds the roughened surface onto the rod is preferably about 0.1mm. However a range of sizes would be suitable from 0.01mm up to 1.0mm for a rod having a diameter of around 8 to 10mm as in the present embodiment. The peel-ply tape 15 is so constructed that it does not adhere strongly to the outer surface of the rod 1 and can therefore be easily peeled away from it. This is accomplished by making the tape of woven or non-woven mono filament which does not have interstices into which the resin could penetrate and form a strong adhesive bond. We have found that the roughened surface 1A on the rod 1 significantly improves adhesion between the extruded sheath 7 and the rod 1 so that during use and over long spans, eg 500m between pylons, relative longitudinal movement between the rod and the sheath is substantially eliminated.

Claims: An optical fibre cable comprising a dielectric rod-like strength member (1) having a longitudinal slot (2) which contains an optical conductor (3), said conductor being protected by the strength member against longitudinal and transverse stress applied to the strength member, said strength member being encased in an extruded sheath (7) in contact with the surface of the strength member, characterised in that the surface of the strength member is roughened by moulding an imprint (1A) in the surface of the strength member in order to improve the grip between the sheath and the strength member. A cable as claimed in claim 1, characterised in that the roughened surface comprises a moulded imprint formed during manufacture of the rod. A cable as claimed in claim 2, characterised in that the moulded imprint has a mesh size in the range 0.01mm to 1.0mm. A cable as claimed in claim 3, characterised in that the moulded imprint has a mesh size of approximately 0.1mm. A method of making an optical fibre cable, comprising providing a dielectric strength member rod (1) having a longitudinal slot (2), providing an optical conductor in the slot, and encasing the strength member in an extruded sheath (7) in contact with the strength member rod, characterised in that the surface of the strength member rod is roughened while the rod is in a softened condition and that the rod is subsequently allowed to harden so that the surface roughening remains imprinted in the hardened surface. A method as claimed in claim 5, characterised in that said surface roughening is provided by applying a woven tape against the surface of the rod during manufacture with the rod in its softened condition the tape being removed prior to application of the extruded sheath.

Inventors: SUTEHALL, RALPH
Applicants: NORTHERN TELECOM LIMITED
Application year: 1990
Main classification: G02B6
IPC codes: G02B6