GB2180662A - Optical fibre cable having slotted core - Google Patents

Abstract

An optical fibre cable comprises an elongate core member of non-electrically conductive material having a modulus of at least 40,000 N/mm<2>, e.g. a glass-fibre-reinforced plastics core member (1), having a slot (2) accommodating loose tubed optical fibres (3,3A) and sheathed with e.g. a longitudinal tape (13) bound with a binder (15) and over-sheathed with a plastics extrusion (14). The cable may be self-supporting and completely non-metallic and is suitable for telecommunications and data transmission services alongside overhead power transmission systems. An alternative embodiment (Figure 2) is not self-supporting but is similar in design and adapted for support from a support wire. <IMAGE>

Description

SPECIFICATION Optical fibre cable This invention relates to optical fibre cables, particularly long span aerial cables incorporating optical fibres.

Various cable designs have been proposed for use as aerial cables incorporating optical fibres, forexample as earth wires in an overhead powertransmission system. When an overhead powertransmission is installed, it is convenientto usethe same route for purelytelecommunications purposes and it has already been proposed to provide a communication system via an earth wire of the power transmission system. British Patent 2029043 B is an example of an overhead earth wire cable for a power transmission system incorporating an opticalfibrefortelecommunication purposes.

On existing powertransmission routes which have not had an optical fibre cable installed in the earth wire, then three alternatives exists in order to install a fibre optical cable in an existing route. The first alternative isto replace the existing standard earth conductorwirewith a fibre optical earth wire as mentioned above in British Patent 2029043 B; another alternative would beto wrap a fibre optic cable around a power conductor ofthe system; and a third alternative would beto install a self-supporting optical fibre aerial cable by suspending it from the pylons which supportthe existing system.

Thefirsttwo options above are expensive and inconvenient, requiring as they do the complete shutdown of the power transmission sytem while the modifications are effected.

The third alternative offers the more satisfactory solution. However it is undesirable to install a cable which contains metallic elements because the presence of an additional electrically conductive cable in the vicinity of the power conductors of a powertransmission system adversely affects certain aspects of the existing system operation. It is therefore necessary to provide a non-metallicfibre optic cable and such a cable has already been proposed. This known aerial fibre-optic cable is made by Standard Electric Lorenz in Germany and comprises a helically-laid-upfibre optic package surrounded by a glass-fibre reinforced tube acting as the strength member and formed into position around the fibre optic bundle during manufacture ofthe glass fibre reinforced strength member.

Although such a cable is effective in providing a self-supporting telecommunications link in an existing powertransmission system, it nevertheless has certain disadvantages, not least being the cost of the cable and the limited amount of excess fibre which can be achieved in orderto minimise damagetothefibre under conditions of use.

It is an object of the present invention to provide a metal-free aerial optical fibre cable which is cheap to produce effective in its application.

According to the present invention there is provided an optical fibre cable comprising an elongate core member of non-electrically-conductive material defining at least one longitudinally-extending slot, said core member having a modulus of at least 40000 N/mm2 and forming the sole tensile strength member of the cable, there being at least one optical fibre located in said slot, and a sheath surrounding the core member, said core member also forming the sole crush-resistant armouring around the optical fibre.

According to another aspect of the present invention there is provided a method of making a fibre optic cable comprising providing an elongate core member of non-electrically-conducting material defining at least one longitudinally-extending slot, feeding at least one optical fibre into the slot, and sheathing the core member with a sheath to close the slot, wherein the core member is made of a non-metallic material having an elastic modulus of at least 40000 N/mm2.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings, wherein: Figure 1 shows in crosssection a self-supporting optical fibre cable according to an embodiment ofthe present invention; Figure 2 shows a second embodiment of a non-self-supporting optical fibre cable according to the present invention; Figure3is a schematic drawing of a core partofthe cable of Figure 1 and is used to explain the design ofthe cable, and Figure4shows schematically partofthe manufacturing apparatus for manufacturing the cable shown in Figure 1.

Referring to Figure 1 a non-electrically-conductive slotted core of homogeneous material in the form of a C-section profile 1 is made from glass-fibre reinforced plastics by a Pultrusion or similar process; it could be made from some otherfibre-reinforced composite which is non-metallic and which acts as a cable strength member and armourand is resilient with a modulus of at least 40000 N/mm2. For example the fibres could be an aramid fibre (such as Kevlar-RTM) or carbon fibres. The resin is a polyester-based material. A modulus for the glass-fibre-reinforced material would be at least 40000 N/mm2 and ofthe order of 45000 N/mm2 and this can be achieved using E-grade glass. Howeverthe higherthe modulus, the better, and moduli of 70000 are attainable using T-grade glass (Japan).

Aslot2 runs straight along the profile 1 and is arranged to be on the outside ofthe profile as it is bentaround a capstan 9to obtain excess fibre, explained in more detail in Figure 4. The slot 2 accepts optical fibres 3, housed in a loose tube 3A. Around the outside of the C-section profile is a composite plastics sheath comprising a longitudinal tape 13, a woven yarn wrapping 15 and a extruded outer sheath 14. This maintains the optical fibres 3 within the slot 2. There may, optionally, be a filler member 5 (shown in broken line)which closesthe slot and prevents the optical fibres 3 from slopping around inside the slot 2, and in that variation, it would not be necessary to have the tape 13.

In manufacturing the cable it is importantthatthefinished cable has an excess-length offibre intheslot2.

Referring to Figures 3 and 4this is achieved, in the preferred embodiment of the method, by running the profile 1 from a storage reel 7 which has a brake 8 which can be applied to brake rotation ofthe reel 7, over a capstan 9 and on to the slot 2 to ensure that the slot is maintained directed radially outwardly with respect two the capstan 9, although it is found that the profile has a natural tendency to offer this slot outwardly when bent. The diameter ofthe capstan 9 is close to or equal to the designed minimum bend diameterofthe cable. Asthe profile proceeds towards the capstan 9, tubed optical fibres 3, 3A are fed in from a storage reel 12 and the composite sheath is formed by applying a longitudinal polyethelyne tape such as 13 from a reel 13A,with overlapping edges.Overthe longitudinal tape 13 is applied a helically-wound binder 15 of polyester material, art station 16. The partially-sheathed profile 1 is then passed around the capstan 9 whose diameter is close to the minimum designed bending diameterofthe cable, and would lie in the range 0.6m foran 8mm diameter profile core to 1.5m for a 12mm core. The profile would have uptofourturns around the capstan andthe purpose of the capstan is to induce an excess length of fibre in the cable.Then a low density polyethelyne sheath 14which is UV stable is extruded overthe helically wound binder at station 14A and amalgamateswith the longitudinal tape 13 to prevent slippage of the composite sheath relative to the profile 1 for examplewhen it is clamped.

The distance between tubed fibre payoff 12 and capstan 9 is kept short e.g. 1 to 2 metres, although it could be longer provided the excess fibre can be "drawn" by the capstan without feedback around the capstan occurring.

The excess length of thetubed optical fibre within the constructed cable is achived due to the circumferential differences ofthe optical fibres 3A in the slot 2 ofthe profile 1. Excess fibre is additionally achieved by shrinking thetube 3A around the optical fibres 3 when thefibres 3 are laid up inthetube3A in an earlier process, by careful cooling and drying ofthe tube 3A after extrusion around the fibres 3. Thus as shown in Figure3, provided the neutral axis 1 A ofthe profile 1 is beneath the bottom ofthe slot 2 then an excess oftubed optical fibres 3Awill be achieved and become effective when the cable is straightened out in use.

The profile when it straightens out after leaving the capstan 9 shortens at the upper slot side and lengthens on its lower sidle remote from the slot. Itwill, in effect, probably rotate or bend about its centre of mass and the element that has been placed in the slotwill therefor become slack having acquired excess length over the slot.

This will occur provided the "excess" cannot work backwards along the capstan. This requires a reasonably high degree offriction, ora large contact area. This may be able to be provided by a 360' turn around the capstan 9, i.e. 1 turn, but 2,3 or even 4turns may be found necessary to prevent feedbackfrom the straight portion between capstan 9 and take-up drum 10 effectively reducing the excess fibre. The take-up drum 10 will be largerthanthe capstan and similar in size to the reel 7, e.g. 1.2m to 1 .7m diameter. The degree of excess element in the slot will be determined by the distance of the bottom ofthe slot from the neutral axis ofthe profile.The largerthis distance then the large the proportional excess ofelementwhich is achived.

lftheprofile bends around a radius r (Figure 3) and the element around a radius r+a+b,wherea isthe distance of the base ofthe slotfrom the neutral axis of the profile, and b is the distance ofthe axis of rotation of the element from the base ofthe slot, then the path length difference is 2n(r+a+b) rr2r ~ 6 +b 27rr r lfr is the minimum bend radius ofthe profile e.g. 300 mm and a+b is 1 mm,then the excessslack = 1/300 = 0.0033 = 0.33% This would more than doublethe maximum tension the cable could endure.

The excess optical fibre is held in by the binding and tape indicated by reference numerals 13, 15 and 14.

There are two suitable sizes for the core 1: 8mm diameterforthe profile member, suitable for pylon spans of 1100 to 1400 feet in regions where there is no possibility of ice, e.g. Sudan or India; and 12mm forcountries where ice has to be taken into account, e.g. UK. The maximum diameter envisaged is 14mm. For the 8mm size the slotwill be4mm deep and 3mm wide, butwould optimally be 2.5mm wide. Forthe 12mm profile,theslot depth will be between 4and 6 mm andthewidth between 2.5mm and3.2mm.

There will befour or sixfibres 3 (although there could be as many as ten) and these are pre-housed inthe plastics tube 3A. This has been found to produce a reliable excess of fibre when installed in the slot in the profile. An excess of about 0.2 to 0.25% can be achieved in thetubed fibre by controlled shrinkage ofthetube during extrusion and cooling of the tube around the fibres. However this in itself is unlikely to be sufficient and further excess is achieved by the cabling technique already described which provides an additional excess of about 0.5%, making a total of 0.7 - 0.75%. A better excess may be achievable, allowing 1.00% strain on the finished cable without straining the fibres beyond 0.25%, which is a universally adopted standard.

Thus the excess length oftube compared to cable is greaterthan the excess length offibre compared to tube when the cable is straight and untensioned.

The profile has a preferred plane of bend 1 B (Figure 3) which coincides with the central longitudinal plane of symmetry ofthe slot 2. It may be preferableto modifythetips 1C ofthe profile, which undergo the greatest strain around the capstan 9, by incorporating fibres having greater ultimate strain atthose extremities than for the remainderofthe profile. This is possible using the Pultrusion process and ensures that failure ofthetip fibres does not occur around the capstan 9.

Referring to Figure 2, there is shown an alternative design intended to be supported from a supportwire.

Here a C-section profile 21 of less than 8mm and about3 or4mm diameter is made of the same material as profile 1 of Figure 1, and has a narrow slot 22 containing several loose acrylate-coated fibres 23. Acomposite sheath is applied in the same way as in Figure 1 and like reference numerals represent like parts. The cable is slung from a support member 24 by a sling 25. Since the cable will suffer less stress than the embodiment of Figure 1, a large excess of fibre is not required and the fibres 23 can be fed into the slot 22 under notension whilethe member 21 is under sometension, to thus provide a slight excess length offibre in thefinished cable.

Hence the bottom ofthe slot 22 will lie at or below the neutral axis of member21.

The slot 22 is very narrow, much narrower than in Figure 1.

In both the embodiments of Figures 1 and 2 the pultruded profile 1 or 21 provides the sole longitudinal strength member of the cable per se and the solid armour and crush-resistant member of the cable, in a single integrally-formed element. This provides significant economy of production and high speed production.

Another advantage of this design of cable is the ease with which the fibres can be accessed by simply cutting through the composite sheath above the slot, and withdrawing thefibresthrough the side ofthe slot. Thus there is provided a "weak" line for gaining access to thefibres and it is proposed to incorporate a rip cord (16 in Figure 1) either in the slot or between the binding 15 and the longitudinal tape 13to enable access to thefibres by pulling the rip cord. Such a rip cord could be similarly applied to Figure 2.

Yet another advantage is that glass reinforced plastics does not "creep" in tension, unlike steel and other materials.

The embodiments of Figure 1 would have a permissible tensile load of 36kN (1 2mm dia. version) and 20kN (8mm dia. version). The thermal expansion coefficientof glass reinforced plastics would match very closely thatforthe optical fibres and would be about 0.7 x 1 0-6 per "C. The permissible span allowing a 12mm ice radial and a 55mph wind would be 1100 to 1400ftat- 5.6'C.With an optical safetyfactorof 1.3 and a mechanical safetyfactorof 2. At O"C the maximum sag with the same ice radial would be about33ft.

Claims (11)

1. An optical fibre cable comprising an elongate core member of non-electrically -conductive material defining at least one longitudinally-extending slot, said core member having a modulus of at least40000 N/mm2 and forming the sole tensile strength member ofthe cable, there being at least one optical fibre located in said slot, and a sheath surrounding the core member, said core member also forming the sole crush-resistantarmouring around the optical fibre.

2. An optical fibre cable as claimed in claim 1,wherein the core member comprises a pultruded profile.

3. Acable as claimed in claim 1 or claim 2,whereinthe core member is made ofglass-reinforced-plastics material.

4. A cable as claimed in claim 1, claim 2 orclaim 3, wherein the cable is a self-supporting aerial cable, and comprises a tubular member within said slot and containing said fibres, there being an excess length of fibres within said tube and an excess length oftube within said cable.

5. A cable as claimed in claim 4, wherein the excess length oftubewithin the cable isgreaterthanthe excess length offibre within the tube, where the cable isstraightand under negligibletension.

6. Acable as claimed in claim 1,2 or3, and designed as an aerial cableforsupportfrom a supportingwire, said slot containing loose optical fibres and the core member having a diameter of less than eight millimetres.

7. A cable substantially as herein before described with reference to and as illustrated in the accompanying drawings.

8. A method of making a fibre optic cable comprising providing an elongate core member of non-electrically-conducting material defining at least one longitudinally-extending slot, feeding at least one optical fibre into the slot, and sheathing the core member with a sheath to close the slot, wherein the core member is made of a non-metallic material having an elastic modulus of at least 40000 N/mm2.

9. A method of making a cable as claimed in claim 8wherein the core member is bent around a capstanto provide excess fibre in the cable when straightened.

10. Amethod as claimed in claim 9, wherein the fibres are housed in atube andthere is excessfibrewithin the tube priorto introducing the tubed fibres into the slot.

11. A method of making a cable substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.