(54) IMPROVEMENTS IN OR RELATING TODEVICES FOR JOINING OPTICAL FIBRES(71) We, THE GENERAL ELEcTRIc COMPANY LiMrTFD, of 1 Stanhope Gate, LondonW1A 1EH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:This invention relates to devices for use in joining one or more pairs of vitreous optical fibres together in end-to-end relationship. The invention is also concerned with a method of forming an optical joint, using the device described.
It is known to form a butt joint between the ends of two optical fibres by placing the end portions of the fibres in a groove, of suitable dimensions for accommodating the fibres without permitting them lateral movement, in a support such as a metal plate or block, for example of brass or other copper alloy or of an aluminium alloy, and fixing the fibres in position in the groove, with their ends butted together, by means of an adhesive material of refractive index substantially matching that of the material of the fibres, for example an epoxy resin or a suitable optical cement, some of the adhesive material usually being inserted between the butted fibre ends. A multi-fibre joint is formed in a similar manner, using a plurality of grooves, one for each matching pair of fibres. An optical fibre joint of this form can initially have very low attenuation losses, for example less than 1 dB. However, in use of an optical fibre cable incorporating such ajoint, a problem can arise as a result of a difference between the coefficient of thermal expansion of the metal support and that of the vitreous material of the fibres: thus if the joint is subjected to a marked increase in temperature, due to a change in the weather conditions or any other cause, the resulting expansion of the metal joint support tends to pull the fibre ends apart, possibly causing the adhesive to fracture, any resulting separation of the fibre ends producing an increase in the attenuation loss at the joint.
It is an object of the present invention to provide an improved device for use in forming an optical fibre joint of the kind described above, whereby the aforesaid problem can be overcome.
According to the invention, in a device for joining one or more pairs of vitreous optical fibres in end-toend relationship which consists of a rigid support member having one or more grooves formed in a surface thereof for receiving butting end portions of said pair or pairs of fibres together with an adhesive material, each groove being of such shape and dimensions that it will accommodate a butted pair of fibres, if desired with a protective resin coating on the fibres, without permitting lateral movement of the fibres, the said support member is wholly or mainly formed of a material having a low coefficient of thermal expansion which either is substantially equal to that of the vitreous material of which the fibres are composed, or is greater than that of the said fibre material to an extent such that the ratio of the thermal expansion coefficient of the support member material to that of the fibre material is not greater than 2.5 l at 20 "C, and the said groove or grooves is or are formed in a surface layer of the said support member by machining by mechanical means.
Since the thermal expansion coefficient of the support member material is substantially equal to that of the optical fibre material or is greater than that of the fibre material only to the extent specified above, any differential expansion of the fibres and the support member material taking place on heating of the joint will not be sufficient to cause damage to the adhesive employed in forming the joint.
Ideally the fibre jointing support member would consist wholly of a plate or block of material having a thermal expansion characteristic substantially equal to that of the material of the fibres, for example a silica plate or block would be used for joining silica fibres.
However, silica is difficult to machine accurately for the formation of the necessarily small grooves, requiring a special tool which may not be readily available. Therefore one form of fibre jointing support member in accordance with the invention consists of a plate or block of material which has a thermal expansion coefficient substantially equal to that of the material of the fibres to be joined, but which is relatively difficult to machine by mechanical means, with a thin layer of more readily machinable material attached to one surface of said plate or block, the groove or grooves for receiving the fibres being formed in the said layer by machining. It will accordingly be understood that the above statement that the support member may consist 'mainly' of a low expansion material as aforesaid means that the said member consists of such material except for a said thin surface layer only of sufficient thickness to enable the groove or grooves to be formed therein by machining. Since such a surface layer need only be of a thickness slightly greater than the depth of the groove or grooves, the thermal expansion coefficient of the material forming said layer is immaterial, for the expansion of the support member as a whole will be controlled by that of the underlying plate or block of low expansion material.
A second form of fibre jointing support member in accordance with the invention consists wholly of a plate or block formed of a metal or alloy which has a thermal expansion coefficient substantially equal to or not more than 2.5 times greater than that of the fibre material, and which can readily be machined with sufficient accuracy to form a groove or grooves of the required dimensions in one surface of the plate or block. This form of support member is preferred since, although the metal or alloy will usually have a thermal expansion coefficient somewhat higher (within the specified limit) than that of the fibre material, the possible disadvantage of the resulting differential expansion is outweighed by the advantage of good machinability, which eliminates the requirement for the provision of an additional surface layer to facilitate the machining of the grooves, thus simplifying the manufacture of the support member.
Each of the fibre-receiving grooves is preferably V-shaped, and of such dimensions, in relation to the diameter of the fibres to be joined, that the sides of the V will make tangential contact with the fibre lying in the groove, and that the fibres will stand slightly proud of the surface of the support member on each side of the groove.
In use of a fibre jointing support member in accordance with the invention, a suitable adhesive material as aforesaid may if desired be applied around and between the fibre ends, for attaching the butted fibre ends together as well as fixing the fibres into the groove. Alternatively, according to a feature of the invention, adhesive material is placed only in or near both ends of the groove, so that the fibre portions lying in the groove are fixed into the groove at or near their outer ends, that is to say at points remote from the butted fibre ends. In this case, the butted fibre ends may be surrounded by a drop of liquid, which may be non-adhesive, but which has a refractive index matching that of the fibre material, to ensure continuity of the optical path between the fibres. The presence of such a liquid around the fibre ends allows separation of the fibre ends to occur as a result of slight differential expansion between the fibres and the support member, without increasing the optical loss at the joint.
Some specific forms of optical fibre jointing support member in accordance with the invention will be described in the following examples with reference to the diagrammatic drawing accompanying the Provisional Specification, in which two forms of support member are shown in perspective in Figures 1 and 2 respectively.
Each of the support members shown in the drawing is designed for use in forming ajoint between two lengths of optical cable incorporating several, for example seven, optical fibres each consisting of a graded index or step index silica fibre waveguide 120 microns in diameter with a protective coating of polyurethane resin, giving an overall diameter of 150 microns for the coated fibre; the coating is preferably formed in several layers and contains fillers consisting of carbon powder in the inner layers and titania powder in the outer layers, the outermost layer also containing colouring material for colour coding of the individual fibres.
Example 1The fibre jointing support member shown inFigure 1 consists of a rectangular plate 1, 25 mm by 50 mm in area and 3 mm thick.
formed of an alloy consisting of 63.8% iron, 36.0% nickel, and 0.2% carbon, by weight. This alloy has a thermal expansion coefficient of approximately 1 x 10-6 at 20 OC, which although higher than that of silica, namely 0.4 x 10-6 at 20 OC, will not result in differential expansion to a sufficient extent to damage the joint. Seven V-grooves 2, each 100 microns in depth, with an appropriate angle of the V for accommodating the resin-coated fibres described above, are formed in the upper surface of the plate by precision machining.
Example 2The fibre jointing support member shown inFigure 2 consists of a vitreous silica plate 3, of the same dimensions as the alloy plate described in Example 1, with a layer 4 of a thermosetting resin, for example an epoxy resin of the type sold under the Registered TradeMark 'Araldite', adherent to the upper surface of the silica plate, the resin layer being 200 microns thick. Seven suitably angled Vgrooves 5, 100 microns in depth, are formed in the resin layer by precision machining.
A specific method of forming a multi-fibre joint, using either of the jointing plates described in the above examples, will now be described, also by way of example.
The ends of both sets of fibres are first prepared in known manner, by scoring and tensioning the fibre to break off an end portion in such a manner as to give an optically flat end surface orthogonal to the fibre axis. A protective coating of filled polyurethane resin, as described above, can be retained on the silica fibres for the end preparation and jointing procedures. The two sets of fibre end portions are then inserted into the grooves in the jointing plate so that the prepared ends abut, both sets of fibres being arranged in the same order so that each fibre in one set is matched with the corresponding fibre in the second set (as indicated by the colours of the fibre coatings). A suitable liquid adhesive material, for example an epoxy resin, is applied over the top of the jointing plate in the regions of both outer ends of all the grooves, so that the adhesive penetrates into the grooves around the fibres only in these end regions, and a drop of a suitable liquid having a refractive index matching that of the fibres, for example glycerol, is applied around and between the pair of butted fibre ends in each groove. Alternatively, an epoxy resin of refractive index matching that of the fibres may be applied over the whole of the jointing plate so as to surround the butted fibre ends as well as the fibre portions on the end regions of the grooves. Before the adhesive has set a cover plate (not shown in the drawing), preferably formed of glass, is placed over the jointing plate and is pressed down to ensure that the fibres are well seated down in the grooves and adhered to the groove walls at and near the ends thereof.
We have found that optical fibre joints formed in this manner, by means of either of the jointing plates described with reference to the drawing, exhibit no increase in attenuation loss when heated over the temperature range from -40 "C to +60 OC WHAT WE CLAIM IS:- 1. A device for joining one or more pairs of vitreous optical fibres in end-toend relationship which consists of a rigid support member having one or more grooves formed in a surface thereof for receiving butting end portions of said pair or pairs of fibres together with an adhesive material, each groove being of such shape and dimensions that it will accommodate a butted pair of fibres, if desired with a protective resin coating on the fibres, without permitting lateral movement of the fibres, wherein the said support member is wholly or mainly (as hereinbefore defined) formed of a material having a low coefficient of thermal expansion which either is substantially equal to that of the vitreous material of which the fibres are composed, or is greater than that of the said fibre material to an extent such that the ratio of the thermal expansion coefficient of the support member material to that of the fibre material is not greater than 2.5 l at 20 OC, and the said groove or grooves is or are formed in a surface layer of the said support member by machining by mechanical means.
2. A device according to claim 1, wherein said support member consists of a plate or block of material which has a thermal expansion coefficient substantially equal to that of material of the fibres to be joined, with a layer of material, which is more readily machinable than the material of said plate or block, attached to one surface of said plate or block, in which layer the said groove or grooves is or are formed by machining, the said layer being only of sufficient thickness to enable the groove or grooves to be so formed therein.
3. A device according to claim 1, wherein the said support membezconsists wholly of a plate or block formed of a metal or alloy which has a thermal expansion coefficient substantially equal to or not more than 2.5 times greater than that of the material of the fibres to be joined, the said groove or grooves being formed in one surface of the said plate or block by machining.
4. A device according to claim 1, 2 or 3, wherein each of the said grooves is V-shaped and is of such dimensions, in relation to the diameter of the fibres to be joined, that the sides of the V will make tangential contact with the fibres lying in the groove, and that the fibres will stand slightly proud of the surface of the support member on each side of the groove.
5. A device according to claim 1, substantially as shown in Figure 1 or Figure 2 of the drawing accompanying the Provisional Specification, and as described in the foregoingExample 1 or Example 2 with reference to saidFigure 1 or Figure 2 respectively.
6. A method of joining a pair of optical fibres in end-to-end relationship, by means of a device according to any preceding claim, wherein adhesive material is placed only in or near both ends of a said groove, so that the said butting end portions of a pair of fibres lying in the groove are fixed into the groove at points remote from the butted fibre ends, and the butted fibre ends are surrounded by a nonadhesive liquid having a refractive index matching that of the fibre material.
7. A method of forming a multi-fibre joint, using a device according to claim 5, and carried out substantially as hereinbefore described by way of example.
**WARNING** end of DESC field may overlap start of CLMS **.