GB2226151A - Optical fibre cable - Google Patents

Abstract

An optical fibre cable (14), for use in a submarine cable (Figure 3, not shown) is made by dip-coating fibres from bobbins (2) together with a filler member (6), the cable passing through a guide plate (3), coating station (4), U.V. or thermal curing oven (5) and optional pre-heater (15). <IMAGE>

Description

OPTICAL FIBRE CABLE This invention relates to an optical fibre cable.

Our co-pending patent application (SR Barnes et al 4-3-2) discloses an optical fibre submarine cable comprising a strain member tube surrounding an optical fibre package which includes a plurality of acrylate-coated optical fibres circularly disposed around a king wire, the king wire and the fibres having been passed through a plastics injection head to encapsulate the fibres and the king wire with a plastics extrusion.

The fibres have a straight lay i.e. a zero lay angle, and the preferred extrusion material is a plastics sold under the trade name Hytrel.

It is also known to provide an optical fibre package for a submarine cable using W (ultra-violet)-cured resins, as disclosed in the article 'Characteristics and long term stability of mass produced Optical Fibre Submarine Cables' pages 135 et seuentia, International Wire and Cable Symposium proceedings, 1987.

In this package each fibre is coated with W cured and nylon resins. A centre tensile strength member has the coated optical fibres stranded around it, and this composite structure is then filled and covered with UV res It is an object of the present invention to provide an alternative optical fibre package particularly but not exclusively for a submarine optical fibre cable in which the fibre packing density can be improved over the prior art discussed above.

According to the invention there is provided a method of making an optical fibre package for an optical fibre telecommunications cable, comprising drawing a plurality of optical fibres firstly through guide means to determine a desired mutual spacing of the fibres from one another, secondly through a coating station in which the fibres are coated with a liquid encapsulating material and pass through an exit die which determines their final pitch circle diameter, and thirdly through a curing station in which the encapsulating material is fully cured to hold the fibres embedded therein, said stations being one below the other in the order recited.

Preferably the encapsulating material is a UV-curable urethane, silicone or acrylate fibre coating material.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which : Fig. 1 shows schematically apparatus for manufacturing a submarine optical fibre package according to an embodiment of the present invention; Figs. 2 and 2A show in cross section respective optical fibre cable packages manufactured by the method and apparatus of Fig.l and; Fig. 3 shows a complete optical fibre submarine cable incorporating the package of Figs 1 and 2.

Referring to Fig. 1 of the drawings twelve colour coded acrylate coated fibres such as 1 carried on respective bobbins such as 2 are drawn vertically via a guide plate 3 through a coating station 4 and thence through an ultra violet lamp or thermal curing oven 5. The fibres are equally spaced by the guide plate on a constant pitch circle diameter and in the embodiment disclosed in Fig. 1, there is a central filler member 6.

In the coating station 4 the package is coated with a W or heat curable resin and hauled through a single exit die having an outer diameter of 1.8mm. The coating material was then cured by passing through an ultra violet lamp or curing oven in the curing station designated by reference 5, before encountering the haul off capstan 7 at the bottom of the line.

The fibre cable can be pre-heated in station 15 shown in broken line, if desired.

As shown this is a vertical process which enables good fibre concentricity in the final package and enables a high fibre packing density.

A preferred coating material is a W curable resin sold under the trade name DeSoto R1400 silicone acrylate. This enables easy access to the fibres in the finished package because they can be easily exposed from the package and any residual material stripped off between finger and thumb.

In order to ensure that the cured material has a suitable mechanical behaviour in bending, adjustment of the proportions of the coating material to include a harder acrylate is necessary. The Young's modulus at 2.5% strain is approximately 20 MPa and the tensile strength is 3 MPa.

Line speeds up to ten metres per minute are achievable, although higher speeds may be obtainable.

An alternative material made by ICI, their Neorad BXP00038 urethene acrylate enabled the line to be run at 20 metres per minute using a single ultra violet curing lamp.

We have noticed that if the line speed is increased the fibre alignment improves.

Instead of W curable resins it is possible to use a thermal curable resin, for example Sylgard 182 matrix. However, so far we have not been able to utilise as high a line speed as with the previous materials and the mechanical stability is inferior.

From the take up drum 7 the package is fed to 0 'ne optical cable production line for manufacturing the cable shown in Fig. 3.

Referring to Fig. 3 the cable package, comprising the central filler 6 the optical fibres 1 and the acrylate coating material 14, is located within a closed C-section copper extrusion 31 to form a tube around the package. A water blocking compound either completely longitudinally fills or intermittantly fills any gap between this tube and the outer copper tube 33 which is welded to provide an hermetic seal. The nominal outer diameter is about 6 mm.

Around this copper tube 33 is a first layer of strength member wires 34, in this embodiment there are 14.

Around this is a second layer of strength member wires of smaller diameter and larger in number, item 35. The combination of tubes 31 and 33 and the strength member wires 34 and 35 provides the tensile strength and resistance to hydrostatic pressure necessary for a submarine cable. A strand water blocking compound such as silicone rubber is incorporated to water block the strands, and an insulation jacket 36 of extruded natural polyethelene provides insulation to dc power supplied along the composite central conductor consisting of the copper tubes and the strength member wires. This polyethelene has a nominal outer diameter of about 24 mm.

In the absence of a requirement for DC power then the insulation thickness can be reduced, sufficient still for enabling electroding or other test procedures to be carried out.

Around the polythylene is an inner serving 38 on which is bedded a layer of armour wires 39 surrounded by an outer serving 40 consisting of two layers of polypropylene roving incorporating a coal tar compound.

The strength member layers 34 and 35 have opposite directions of lay so that they are torsionally balanced.

The armouring layer is optional dependant upon whether the cablç is to be laid in shallow water. In de water the armouring layer may not be required.

If the optical fibre package discussed with reference to Figs 1 and 2 is manufactured on line with the optical cable of Fig 3, then it is very desirable on a fully automated production line to be able to stop the production line for various reasons, normally to rectify a fault in the production.

We have found that with the apparatus shown in Fig. 1, a temporary halt in the process of up to 5 minutes leads to a slight bulge in the overall diameter of the package lying just below the coating station during the period of halt. The diameter increases from approximately 1.8 mm to 2 mm. During this period the ultra violet lamp in station 5 would need to be put on standby.

As an alternative to the coating station of Fig.

4, which is an open cup system, this could be replaced by a pressurised system. In the event of the production line halting, such a station would be able to successfully retain the uncured resin thus preventing it from running down the now stationary package between stations 4 and 5.

This is due to two features of a pressurised coating station. Firstly it has an adjustable exit nozzle, seen more clearly with reference to Fig. 4. This shows the exit die of the coating station 4, reference 41. Towards the end of the die is a variable diameter nozzle 42 which can be expanded to the position indicated by the broken line 42A. In this position the nozzle fits around the fibres and central filler member to safely retain the coating material within the pressurised chamber.

The second feature of a pressurised coating station is that it is possible to induce a negative pressure in the station thus retaining the liquid.

Although the apparatus for Fig. 1 and the cross section of Fig. 2 show a central filler 6, it is possible to manufacture the package without the central filler 6 so that apart from the fibres, the whole package consists of the coating material applied in coating station 4. A cross section of such a package is shown in Fig. 2A.

In Fig. 2 the central filler 6 is primarily used as a filler to minimise the amount of coating material required to reduce costs. It is not intended to contribute any significant strengh to the package although it can do so if required and if suitably designed.

In the package described the fibres are easy to separate from the package to enable individual splicing etc, yet the package provides the necessary protection for the fibres.

Claims (11)

CLAIMS:

1. A method of making an optical fibre package for an optical fibre telecommunications cable, comprising drawing a plurality of optical fibres firstly through guide means to determine a desired mutual spacing of the fibres from one another, secondly through a coating station in which the fibres are coated with a liquid encapsulating material and pass through an exit die which determines their final pitch circle diameter, and thirdly through a curing station in which the encapsulating material is fully cured to hold the fibres embedded therein, said stations being one below the other in the order recited.

2. A method as claimed in claim 1 wherein said package has a substantially circular cross section.

3. A method as claimed in claim 2 wherein a central filler member is drawn with the optical fibres through the sequential stations.

4. A method as claimed in any preceeding claim, wherein the exist die has a variable diameter orifice which can be tightened to prevent leakage when the process is halted.

5. A method as claimed in any preceeding claim wherein said coating station is a pressurised coating station.

6. A method as claimed in claim 5 wherein the method is halted and the pressure in the pressurised coating station is reduced to prevent leakage.

7. A method of making an optical fibre package substantially as herein before described with reference to Figs. 1,2,2A and 4 of the accompanying drawings.

8. An optical fibre package made by the method according to any preceeding claims.

9 An optical fibre cable comprising an optical fibre package according to claim 8, and incorporating a strain member tube surrounding said package, said tube providing said cable with tensile strength and with resistance to hydrostatic pressure, said cable being suitable for use as a submarine telecommuncations cable.

10. A cable as claimed in claim 9 wherein said package is glued to the strain member tube.

11. An optical fibre submarine cable substantially as herein before described with reference to Fig. 3 of the accompanying drawings.