GB2283105A - Forming optical fibre bundle terminations in a metal tube - Google Patents

Abstract

Method of forming optical fibre bundle terminations wherein a fibre optic bundle is inserted into a metal outer sleeve, and the bundle is heated preferably by electrical resistance heating of the metal tube. Once the fibre core within the tube is heated to reach a softening temperature, the metal tube is crimped at 16 so as to define a waisted region within which the fibre bundle is close packed and fused. The tube is subsequently cut to define the fibre optic bundle terminations 24, 26. <IMAGE>

Description

Improvements Relating to Optical Cables This invention relates to optical cables and in particular is concerned with the provision of effective optical cable ends or terminations.

Optical cables are used extensively as a means of light delivery or collection but to perform efficiently in this regard they need to have efficiently formed ends where the light is emitted or collected.

The cables are formed of optical fibres and either single optical fibres or fibre bundles may be used for these cables.

The use of fibres as a means of light deivery or collection, is much preferred, as they can provide a relatively large optical diameter combined with mechanical flexibility which cannot be achieved with single large diameter fibres. Fibre bundle optical cables have wide application, ranging from miniature industrial sensing to large scale illumination schemes delivering high volumes of light.

An essential element of any system is the method of terminating the fibre optic cable ends. Conventional methods of termination rely on the use of thermosetting materials such as epoxy resins to bond the numerous individual fibres together in a tight packed bundle. After curing to a hard composite, the ends are sawn, ground and polished to optical quality. There are several problems with this technique such as limited operating temperature (typically 100 - 2000C max), restrictive manual process with little mechanisation potential, slow curing cycle for epoxies, and the inbuilt packing fraction loss of a composite structure.

Existing patents such as UK 1556046 have suggested heat fused fibres as a means of increasing the operating temperature of fibre bundle terminations for industrial sensing applications. The methods described have the disadvantage of requiring specialised precision components and may also require a long process time, not dissimilar to the conventional epoxy processes. The proposed method below is both rapid and uses low cost components to achieve a heat fused termination.

The present invention is concerned with a method providing optical cable ends which at least in its preferred form obviates or mitigates the disadvantages of existing methods.

According to the present invention there is provided a method for forming an optical cable termination wherein an assembly is formed comprising an optical fibre core located so as to extend axially through an outer tube, electrical current is caused to flow through the tube to heat same and to heat the core in the tube to the softening temperature of the fibre core, the tube is forcibly constricted to crimp it onto the hot core to deform same and the assembly is cooled, the material of the fibre core and tube being selected so that upon cooling the outer tube remains a compression fit on the optical fibre core, and the assembly is cut transversely to define an optical cable termination.

The optical fibre core preferably is defined by an optical fibre bundle so that the fibres deform to a close packed structure before the cooling step.

The said step of crimping may be carried out by multijaw crimping.

The core preferably is of glass fibres, the metal and glass being chosen to have such expansion co-efficients such that upon cooling the assembly the metal tube remains a compression fit on the fused fibre bundle.

The cooling preferably will be forced cooling, but it is to be mentioned that the invention extends to the case where the cooling is natural cooling.

The resulting assembly may be such that the tube after the crimping operation has a central inner assembled region with ends flaring up to the original diameter. When the assembly is cut, preferably the cutting is at the ends of the central narrow portion so that said flared portions define the optical cable terminations. The optical faces of the terminations may be ground, polished and/or otherwise treated to finish same.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein: Fig. 1 shows schematically an assembly ready to be worked by the method of the present invention; Fig. 2 is a view similar to Fig. 1 but showing how the assembly is heated; Figs. 3, 4 and 5 show in sequence operational steps involved in the method; Fig. 6 shows a practical embodiment of the invention; and Fig. 7 shows a fibre-optic conductor terminated by the method illustrated in Fig. 6.

Referring to the drawings, Fig. 1 shows an assembly comprising a fibre core 10 made up of a bundle of optical fibres. The diameter of this bundle in the Fig. 1 assembly is slightly less than that of an outer metallic tube 12 through which the bundle passes axially as shown.

The requirement is that the assembly should be used to provide at least one termination for a fibre optic cable, and in fact as will be explained the method illustrated can be utilised for providing two fibre optic cable terminations.

One might assume therefore that the fibre optic bundle extends to opposite sides of the tube 12 to any required extent defining the length of fibre optic cables.

In order to form the fibre bundle terminations, the assembly shown in Fig. 1 is heated and this is illustrated diagrammatically in Fig. 2 in that the circle 14 indicates the hot zone or zone of heating when heating is effected.

It is preferred that the heating be effected by electrical resistance heating of the metal tube 12. Typically, 100 amps may be conducted through the tube between two clamped contacts. This arrangement allows heating of the zone 14 to take place very quickly e.g. in the order of 1 minute and by electric resistance heating control of the temperature of the hot zone can readily be effected.

The heating takes place until the fibres reach their softening temperature and at this stage, as shown in Fig. 3, the metal tube 12 is mechanically constricted, suitably by multijaw crimping, onto the hot fibres so as to deform same as shown in Fig. 3 in that a region 16 wherein the tube 12 is crimped to a smaller diameter is defined. The crimping is such that the section 16 is of relatively constant cross section although smaller than that of the original tube 12, and the ends of the section 16 are connected to flared portions 18, 20 which extend up to the original diameter.

When the crimping has been completed, the assembly is cooled, and this cooling may be by natural cooling for example by leaving the assembly to cool through heat loss to atmosphere, or preferably forced cooling is used. The cooling step results in that the metal tube as crimped remains as a compression fit on the fused fibre bundle, it being mentioned that when the crimping takes place the fibres of the bundle deform to form a close packed structure, and indeed may fuse into a relatively homogenous mass. To ensure that the tube remains crimped as shown in Fig. 3, it is preferable that the expansion co-efficients of the material of the fibres e.g.

glass and the material of the tube 12, are suitably related.

When the crimping has been completed and the assembly has been cooled, the assembly is cut transversely by means for example of a suitable saw so that a centre section 22 as shown in Fig. 4 is extracted leaving two terminations 24 and 26 defined by the flared portions 18 and 20 of the crimped tube 12. The faces 28 and 30 of the terminations which are defined by the fused and crimped optical fibres may be ground and polished to finish same. It is these faces 28 and 30 which form the light emitters or collectors. The fibre optic terminations can be considered as the faces 30 and the adjacent tube portions 24 and 26 which form ferrules holding the fibres of the fibre bundles together.

The embodiment described in relation to the drawing has a number of advantages as follows.

Firstly, it is not necessary to use any thermosetting material to bond the numerous individual fibres together and consequently there is no need to effect any curing of any resin.

Secondly, there are no problems concerning limitation of the operating temperature.

Thirdly, the method lends itself to mechanisation.

Fourthly, it is believed that the arrangement described will not suffer from any inbuilt packing fraction loss as there is no composite structure.

Fig. 6 illustrates a practical embodiment of the invention wherein an annealed stainless steel tube 32 having an annular flange 34 located near its end and containing a core packed with optical fibres running axially along its length is being treated. The metal tube 32 has a threaded end 36 which is engaged in a copper terminal block 38. A short distance from the end of the tube, and between the terminal block 38 and the flange 34 is positioned a clamp 40 consisting of brass blocks held together by means of a pinch bolt around the metal tube 32. Both the terminal block 38 and the clamp 40 are held in effective electrical contact with the metal tube 32, and an electrical circuit 42 is connected between the terminal block 38 and the clamp 40. The electrical circuit 42 includes wire connections 44, transformer 46 and an ameter 48.The transformer converts mains current, that is to say 250 volts, down to 1 volt which is generated across its terminals connected into the electrical circuit 42. The metal tube 32 contains approximately 300 fibres each being of 50 microns in diameter.

In order to heat the metal tube and fuse the fibres therein, the transformer is switched on for a short period, suitably 30 seconds or 1 minute, during which time a current of approximately 100 amps flows in a circuit 42 and along the portion of the metal tube 32 situated between the clamp 40 and the terminal block 38. Most of the voltage drop in the circuit occurs across this portion of the metal tube, which rapidly heats to a temperature of several hundred degrees, sufficient to soften the optical fibres contained within that portion of the tube 32. Once the tube and its contents are sufficiently heated, a multijaw crimp is used to constrict the diameter of the tube and to rearrange the optical fibres therein to a close-packed relation.

It is preferred that the period over which heating by electrical resistance is effected is kept to a minimum, since once the temperatures of the clamp 40 and the terminal block 38 are elevated by conductance of heat from the metal tube 32 and also by resistance effects, the resistance of these connector parts is increased excessively and this results in a wasteful usage of power within the circuit 42.

Once the metal tube is allowed to cool, the constricted portion formed by crimping adjacent the end of the metal tube 32 is cut and polished and otherwise treated so as to form a good optical termination, resulting in the optical conductor shown in Fig. 7. The optical conductor includes a fused fibre end which can withstand high working temperatures such as 3500C and high pressures such as 600 psi, and at the other end there are formed two fibre optic bundles. The conductor shown in ideally suited to the task of testing turbines within a jet engine, the length and metal tubing 32 being deformable to allow the fused fibre end 50 to be inserted within the tip of the turbine, the flange serving as an abuttment for the correct positioning of the fibre bundle end 50 adjacent the turbine tips. At the other end of the sensor, light is projected along one fibre bundle tail 52, the light emmerging at the fused fibre bundle end 50 and the light collected after reflection from the turbine tip is monitored at the receiving fibre bundle tail 54.

It will be appreciated that the present invention lends itself to various modifications and the substitution of various equivalents of the features described in the specific embodiment will be appreciated by a man skilled in the art to fall within the scope of the invention.

Claims (8)

1. A method for forming an optical cable termination wherein an assembly is formed comprising an optical fibre bundle core located so as to extend axially through a metal tube, electrical current is caused to flow through the metal tube to heat same and to heat the core in the tube to the softening temperature of the fibre core the tube is forcibly constricted onto the hot core to deform same and the assembly is cooled, the material of the fibre core and tube being selected so that upon cooling the outer tube remains a compression fit on the optical fibre core, and the assembly is cut transversely to define an optical fibre bundle termination.

2. A method according to claim 1 or 2 wherein the step of crimping is carried out by multi-jaw crimping.

3. A method according to claim 1 or 2 wherein the cooling is forced.

4. A method according to any previous claim, wherein the resulting assembly is such that the tube after the crimping operation has a central inner assembled region with ends flaring up to the original diameter.

5. A method according to claim 4 wherein the assembly is cut at the end of the central narrow portion so that one of said flaired portions defines the optical cable termination.

6. A method according to any previous claim wherein the optical face of the termination is ground, polished and/or otherwise treated to finish same.

7. A method according to any previous claim wherein the assembly is cut to define two optical cable terminations.

8. A method of forming an optical cable termination substantially as hereinbefore described with reference to the accompanying diagrams.