GB2164469A - Optical fibre cables - Google Patents

Abstract

Strain relief is provided in an optical fibre cable by housing an optical fibre package (A, A1, B, C) in a gapped hollow metallic tube (E), heating the metallic tube to about 100 DEG C at the point where the optical fibre package enters the metallic tube during manufacture, closing the gap, and subsequently applying a strength member layer around the tube, whereby an excess length of optical fibre package is provided and maintained within the cable due to differential thermal expansion between the core member and the optical fibre package. <IMAGE>

Description

SPECIFICATION Optical fibre cables This invention relates to optical fibre cables.

Optical fibre cables, such as those designed for undersea telecommunications or overhead power cables incorporating optical fibres, need to isolate as far as possible from the optical fibres the tensile stresses involved in laying the cable on the sea bed and possibly recovering the cable for repair should the cable become damaged during service, in the case of a submarine cable, and the similar tensile stresses involved in supporting an overhead cable from pylons where the cable may undergo significant temperature changes during summer and winter months.

Our earlier patent specification 1461151 proposed an optical fibre cable in which the optical fibres are arranged within a strain member tube and the fibres are longer than the tube with the tube being a loose fit about the fibres so that the fibres and the tube are capable of relevant movement, the tensile and bending stresses on the cable being borne substantially by the strain member tube. In this patent specification it is proposed that any potential strain on the fibres is avoided by making the fibres longer than the strain member tube so that when the strain member tube is in fact strained and stretches somewhat, the excess length of fibres within the tube ensures that the fibres themselves do not become strained.One way of achieving this excess length of fibres proposed by this earlier patent specification, is to feed the collection of fibres into the tube at a greater speed than that at which the tube is manufactured.

Another earlier patent specification 1487464 proposes optical fibre elements for use in the manufacture of optical fibre cables, in which each element comprises an optical fibre housed loosely within a tubular sheath individual to the fibre or a number of the fibres, the optical fibre or fibres occupying a longer length within the tubular sheath than the length of the tubular sheath itself. Two techniques are envisaged to accomplish this.

Firstly it is envisaged that the sheath can be elongated during manufacture by applying tension to it and then allowing it to relax; alternatively thermal treatment can be used such as by maintaining the sheath sufficiently hot when it issues from a cooling tank after extrusion, such that when it cools to room temperature it shortens considerably owing to the difference in coefficient of thermal expansion between the sheath material and the fibre or fibres. However, these optical fibre elements incorporate a sheath which is merely intended to facilitate handling of the optical fibres in the manufacture of a cable. The sheath itself does not provide any significant tensile strength.

Any tensile strength required of the cable is provided by a separate tensile strength member which in the embodiment shown in the patent, consists of a strong wire rope at the centre of the cable around which the optical fibre elements are laid. With this arrangement it is difficult to predict what if any strain relief is provided in the finished cable having regard to the stresses inevitably applied to the optical fibre elements during laying up in the manufacture of the cable. For example the sheaths may become stretched during cable manufacture in which case there could be no excess length of fibre in the finished cable.Thus some fibres may end up with significant strain relief whereas other fibres may have little or no strain relief and it would require complex and sensitive equipment to ensure that the excess length of optical fibre in its loose sheath is maintained in the finished cable.

It is an object of the present invention to provide an optical fibre cable with strain relief of the optical fibres in a controlled and predictable manner.

According to the present invention there is provided a method of making an optical fibre cable in which an optical fibre or package of fibres is housed within a metallic tube, the method including feeding the fibre or fibre package into the metallic tube before the tube is closed and providing a temperature differential between the closing core and the fibre or package wherein the tube is hotter than the fibre or package, such as to provide at least 0.1% strain relief.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings in which: Fig. 1 is a cross-section of an optical fibre cable manufactured according an embodiment of the present invention, Fig. 2 shows an overhead earth wire conductor manufactured according to an embodiment of the present invention and, Fig. 3 shows schematically a manufacturing arrangement for manufacturing the cables off Fig. 1 or Fig. 2, according to the embodiment of the invention.

Referring to Fig. 1 the cable comprises an optical fibre package consisting of a package strength member A made of high tensile steel wire which may, if desired, be copper plated to improve its electrical conductivity, and coated with a plastics coating Al. Around the package strength member A are eight single mode secondary coated optical fibres B which are held in a package with the strength member A by means of a fibre package whipping C. We prefer a fibrous Kevlar ribbon which in the particular embodiment described has about 200 denier.

The fibre package is ioosely housed in a tubular core formed by tube E which in this embodiment is aluminium. The tube E has a single split El whereby the optical fibre package is introduced into the tube before the tu bular member is closed. The tubular core has an internal bore D.

The optical fibre package and the annular gap between the fibre package and the internal bore D of the tubular member E, are impregnated and filled with a water blocking compound.

Around the core member E is a laminate sealing strip or alternatively a thin copper tape, F. This strip or tape is sealed to provide an hermetic seal around the optical fibres.

Around the strip or tape F are applied two layers of tensile strength elements for the cable, G and H respectively. The first layer G comprises a smaller number of larger high tensile steel wires which in conjunction with a larger number of smaller outer diameter wires, support the core and bridge against one another so that there is formed a crush-resistant electrically conductive tensile strength member which provides the tensile strength the cable is required to have. For a submarine cable this could be of the order of 18 tons breaking strength.

Extruded around the outside of and directly into the outer layer strength member wires is a polyethelene sheath J which forms an electrical insulation between the outside and any electrical power or electrical signal which is, in use of the cable, transmitted along the conductive central core E.

Fig. 2 shows in cross-section an overhead earth wire incorporating optical fibres.

It has a longitudinal strength member which comprises a first layer of high tensile steel wires 13 arranged around and surrounding a split aluminium tube 11, and this is surrounded by a further electrical conductor of tubular configuration comprises a layer of larger diameter aluminium wires 14.

Within the aluminium tube 11 are a number of optical fibres 10 which are held together and supported in a package by a Kevlar whipping 9, similar to that used in the embodiment of Fig. 1.

In both the cables described in Fig. 1 and Fig. 2 it is important to incorporate strain relief to the optical fibres because this will extend the fibre lifetime by reducing or removing tensile stresses from the fibres, thereby reducing or eliminating stress corrosion, and this will also increase the safe load on the cable during installation or repair by reducing the actual strain experience by the fibres.

Referring to Fig. 3 there is shown schematically an arrangement for manufacturing the cables of Fig. 1 or Fig. 2, to incorporate strain relief.

Referring to Fig. 3 the optical fibre package for the cable of Fig. 1 or Fig. 2 is stored on reel 20 and the metallic "C' '-section extrusion (E or 11) is stored on reel 21. The fibre package enters the open slot in the extruded "C"section at a point where the two components enter an induction heater 22. This heats the "C"-section to approximately 100 C and closure rollers and closure dye are incorporated at station 23 to close the C-section slot. The closed "C"-section and fibre package 24 then cools to ambient temperature and can continue through to the first stranding operation or alternatively can be stored temporarily on a drum.Package 24 enters a stranding machine 25 wherein the first layer of wires G or 13, are applied around the "C"-section. In the case of the cable of Fig. 1 the tape or strip F would be applied from bobbin F1 before the first layer of strength members are laid up.

This is shown at F2, in Fig. 3. Station 25 applies the first layer of wires in a conventional manner and the package 24 or 24A with first layer of wires (G or 13) proceeds, indicated by reference numeral 26, to the second stranding machine 27 where either aluminium alloy wires 14 or applied in the case of the cable of Fig. 2 or alternatively a second layer of high tensile steel strength member wires H in the case of the cable of Fig. 1.

In the case of the cable of Fig. 1 a subsequent extrusion for the polyethelene J and an outer armouring layer K are applied in conventional manner.

By maintaining a positive temperature differential between the metal C-section and the fibre package, subsequent thermal shrinkage of the metal relative to the fibres creates excess fibre length within the overall structure.

Indeed, due to the difference in coefficients of thermal expansion between silica glasses and metals (approximately 10 6 and 10 5 per degree centigrade respectively), both units could be mated together at the elevated temperature and subsequently, and strain relief would still be obtained at the lower operating or ambient temperature. However, the strain relief effect would be somewhat smaller than by maintaining a positive temperature differential at stations 22 and 23. We have found that 0.1% relief is the minimum which will proivde useful protection of the fibre or fibres.

For example, consider an aluminium tube formed around the fibres, i.e. the C-section tube. The coefficient of thermal expansion of aluminium is approximately 25 X 10 6 per degree centigrade. A strain relief of 0.25% could be obtained by forming the tube at the temperature stated above of a 100"C above ambient or service temperature.

In the embodiments described above the aluminium C-section tube acts as the "shrink" member and the shrinkage is then effectively "fixed" by the application around the C-section member of a tensile strength member.

Since the fibres lie on the neutral axis of the cable, intermediate storage on a drum of the C-member housing the fibres will not substantially affect the strain relief already achieved.

The optical fibre package in both embodiments has a package "whipping" around it and this provides limited thermal insulation layer which assists in maintaining a positive temperature differential at stations 22 and 23 in Fig. 3. Thus the optical fibre package, although subject to some extent to the temperature of the aluminium tube when it is heated by the induction heater 22, nevertheless does not reach 100 C because of the insulation layer.

A better thermal insulation could be provided by applying to the optical fibre package silicone rubber impregnation. Thus referring to Fig. 3, a thermal insulation applicator stage indicated by the dotted line box 30 could impregnate the package, or in the case of a single fibre, simple coat the fibre, with e.g.

silicone rubber.

Although not shown in Fig. 3 station 22 incorporates an injector for injecting a water blocking material into the "C"-section prior to closure thereof, in order to prevent water which may enter the C-section at a point of damage to the cable during subsequent use, penetrating longitudinally along the cable. This water blocking could be at discrete intervals along the C-section. Very preferably the water blocking material has a very high viscosity at ambient service temperature e.g. ten million cPs at around 25"C but which is easily injectable at 100"C. This effectively "glues" the fibre package to the C-section tube but allows relative creepage over longer periods e.g. days.

We have found that these properties of the water blocking material are important to relieve strain on the fibre which may not always be exactly on the cable neutral axis owing to the looseness within the cable.

Claims (11)

1. A method of making an optical fibre cable, in which an optical fibre or package of fibres is housed within a metallic tube, the method including feeding the fibre or fibre package into the metallic tube before the tube is closed and providing a temperature differential between the closing core and the fibre or package, wherein the tube is hotter than the fibre or package, such as to provide at least 0.1% strain relief.

2. A method as claimed in claim 1, wherein a tensile strength member is applied around the tube when the tube has cooled to ambient temperature or when the temperature of the strength member is greater than that of the tube.

3. A method as claimed in claim 1 or claim 2, wherein the tube is heated during injection of a water blocking compound into the tube and heating is provided also at a subsequent stage when the tube is closed.

4. A method as claimed in claim 3, wherein the subsequent stage comprises a set of rollers which are heated and a closure dye which is heated.

5. A method as claimed any preceding claim wherein the tube is a C-section extrusion which is closed to a circular cross section tube.

6. A method as claimed in claim 5 wherein the tube is of aluminium.

7. A method as claimed in any preceding claim wherein a layer of strength member wires are heated just prior to being laid up over the tube.

8. A method as claimed in any preceding claim wherein an insulating layer is extruded over the strength member wires.

9. A method as claimed in any of claims 1 to 7, wherein a layer of highly conductive metallic e.g. copper or aluminium, wires are laid up over the strength member wires.

10. A method of making a cable substantially as hereinbefore described with reference to the accompanying drawings.

11. A cable made by a method according to any preceding claim.