MOUNTING OPTICAL WAVEGUIDES

Abstract: A pair of optical fibres (13) is mounted in a support tube (1) by heating the tube at two positions using separate coils (11, 12) while the fibres are supported by a jig (8). Glass sealant is introduced through a hole (5) in the tube (1) and the heating step is terminated to allow the sealant to harden and fix the optical fibres in the tube.

Description: MOUNTING OPTICAL WAVEGUIDES The invention relates to methods for mounting optical waveguides, such as optical fibres, in a support tube. Many optical devices are fabricated in a substrate, such as lithium niobate. These devices typically comprise an optical waveguide whose transmission characteristics can be varied by applying electrical or acoustic pulses to the substrate. In order to make use of such a device, it has to be coupled to other optical waveguides and in the past this has been achieved by mounting an optical waveguide, such as an optical fibre, in a metal tube and then welding the tube and fibre to the substrate in alignment with the waveguide in the substrate. This is described for example in JP-A-5758369. More recently, optical devices have been developed in which at least two waveguides are fabricated in the device, the ends of the waveguides terminating relatively close together. An example of such a device is a directional coupler in which the waveguides are 250 microns apart. Since typical optical fibres have a diameter of 125 microns it is very difficult to use the conventional technique described above to connect optical fibres separately to the ends of each waveguide. A more practical alternative to connecting optical fibres singly to each waveguide is to pre-assemble the fibres into an array, and then to align and fix the whole array as a unit. This imposes strict tolerances on the position of each fibre in the array. In the case of lithium niobate devices, the core-to-core separation of the fibres in the array must be maintained to a tolerance of ±lμm or better to avoid excessive optical coupling losses. EP-A-0274222 describes a method in which a single optical waveguide is mounted in a metal tube. A glass bead is inserted in the tube which is subsequently heated by applying heat through the metal tube. The glass bead then melts and on cooling rehardens securing the optical fibre within the tube. This method cannot be readily adapted to solve the problem outlined above since on melting the glass bead tends to alter the set spacing of the fibres. However, in order to ensure low loss coupling to and from the fibres it is important to maintain the set spacing between the fibres within the tube. According to a first aspect of the present invention, a method of mounting a plurality of optical fibre waveguides in a support tube comprises the steps of inserting the fibres into the tube, locating their free ends in a jig capable of holding them at their desired separation, applying a sealant to be drawn under the influence of capillary action along the fibres and the tube, and causing the sealant to harden thereby to provide a seal fixing the fibres in the tube. Where the fibres are silica glass fibres, the sealant is preferably glass which flows and can be drawn along the tube and fibres at a temperature above its melting point but low enough to avoid damaging the optical fibres, an appropriate temperature profile along the tube being established by heating an extended region of the tube. Conveniently, the fibres are temporarily secured in their desired configuration before insertion into the tube. At least one of the fibres may carry a spacer element to assist maintaining correct fibre spacing before the sealant has hardened. The tube may be heated for the temperature at a first position to be above the melting point of the sealant and to be near to but below the melting point at a second position. According to a second aspect of the present invention, a method of mounting an optical waveguide in a support tube comprises heating an extended region of the tube with a waveguide inserted therein; causing sealant to be drawn along the tube; and terminating the heating step to allow the sealant to harden and fix the optical waveguide in the tube, wherein the temperatures during the heating step are selected to be such that they do not damage the optical waveguide. The drawing of sealant along the tube, caused by capillary action, requires that the tube is heated for a much longer time than in the conventional method. If this were achieved by applying heat to a single position, the temperature required would be so high, due to the significant heat loss into the remainder of the tube, that the waveguide, in the case of an optical fibre, would become brittle, or be damaged in some other way. However, by heating an extended region of the the tube, for example by applying heat at two spaced positions, heat loss is considerably reduced thus reducing the overall time required and therefore the total heat input. The temperature will be selected so as to be- high enough to melt the sealant but be low enough to prevent damage to the optical waveguide. The invention is particularly suitable for mounting two or more optical waveguides in the support tube. Thus, in accordance with a third aspect of the present invention, there is provided a . method of mounting two or more optical waveguides in a support tube in which an accurately controlled separation between the waveguides is established by using a jig to maintain the waveguide positions while a sealant is allowed to harden and fix the waveguides in position. The mounting of two or more optical waveguides within the same tube is very simple and straightforward a the sealant is drawn into the tube by capillary action rather than having to be provided in situ in the form of a glass bead, for example. The tube may be heated at a first position to a temperature above the melting point of the sealant, and at a second position to a temperature below its melting point. In accordance with a third aspect of the present invention, a method of mounting an optical waveguide in a support tube comprises heating the tube with a waveguide inserted therein at two positions spaced along the tube; causing sealant to be drawn along the tube; and terminating the heating step to allow the sealant to harden and fix the optical waveguide in the tube, wherein the temperatures at the two positions are selected such that the amount of heat supplied to the tube during the heating step does not damage the optical waveguide. Preferably, the sealant is introduced through an aperture in the tube positioned between the two heating positions. Typically, the sealant will comprise glass particularly where a hermetic seal is to be achieved between the waveguides and the device to which they are to be mounted. However, other forms of sealant such as an epoxy could be used in other applications. In some cases, where more than one optical waveguide is to be mounted in the tube, there is a risk that they could twist together within the tube and/or they could diverge as they emerge from the tube. To overcome this problem, the optical waveguides may be joined together at one or more points adjacent their ends. 5 The support tube will typically be of approximately circular cross section or have at least circular aperture; however, where appropriate, the tube may be of different shape, have a different cross section, or both. For instance, even though the 1 0 aperture may be oval or otherwise elongate in cross section to accommodate several fibres arranged in one plane, the tube will often be externally of circular cross section for greater ease of mounting in device housings. 1 5 An example of a method of mounting optical fibres in a support tube according to the invention will now be descried with reference to the accompanying drawings, in which:- Figure 1 is a plan of the support tube; 20 Figure 2 is a perspective view of the mounting apparatus; Figure 3 is a plan of the support tube with the heating elements; ' 25 Figure 4 is a longitudinal section of the apparatus shown in Figure 3, and Figure 5 is a schematic cross section of a modified 30 embodiment of the support tube of Figure 1. Referring first to Figures 1 and 5, the illustrated support tube 1 is made from stainless steel. The support tube 1 has a wide diameter bore portion 2 and a narrow diameter bore 3 5 portion 3 connected by a tapering portion 4. The internal diameter of the portion 3 is drilled out to 0.4 mm over a length of about 3 mm to accommodate two optical fibres 18, 19, with the required separation. Two holes 5, 6 are drilled in the side of the tube 1 in the tapering portion 4 and the wide diameter bore portion 2 respectively. The hole 5 is to allow access for glass sealant to fix the aligned fibres, and the hole 6 is to allow access for adhesive to protect the uncoated fibres to the rear of the glass seal. Prior to use, the tube 1 should be cleaned in trichloroethylene, then placed in an ultrasonic bath of soap solution, rinsed in deionised water and finally cleaned in methanol. The tube 1 is crimped onto a length of PTFE sleeving 7 as shown at 16. Referring now also to Figure 2, the tube is then loaded into an assembly jig 19. The principal components of the jig 19 are an aluminium mould 8 with impressions of two fibre ends 9, 10 to position the fibres to be fixed accurately with their cores 250 microns apart; and two platinum heater coils 11, 12. The heater coils 11, 12 are provided around the narrow portion 3 and the end of the wider portion 2 adjacent the tapering portion 4 of the tube 1 respectively. The aluminium mould 8 has been manufactured by impressing optical fibres of the same external diameter as the fibre ends 9 and 10 into the mould metal, in a manner similar to that described in GB-A-2046466 in relation to the manufacture of optical fibre joints. At this stage, the tube 1 is preheated for two to three minutes at about 500° C to remove any residual fluid and to help form an oxide layer onto which the glass will wet. In one case, this temperature was obtained with currents of 2 amps and 3.4 amps on the coils 11, 12 respectively. In order to learn the temperature at the critical point within the narrow portion 4 of the tube, a sub-assembly tube with a thermocouple mounted inside it is loaded into the tube and the currents are adjusted accordingly. At temperatures in excess of 600° C the fibre becomes brittle so that care has to be taken to avoid too much heat. The two optical fibres 18, 19 to be mounted have about 50 mm of primary coating 13 along their length removed. The bare fibres are cleaned with isopropyl alcohol, cleaved 10 mm from the coating and inserted into a jig where the fibres are made to lay parallel to one another with the cleaved ends in the same plane. As indicated schematically in Figure 5 at 17, the fibres are fixed together (Norland UV Curing Type 81) at two points, about 30 mm and 200 mm from the cleaved ends, to keep the fibres 18, 19 from twisting as they are inserted into the sleeving and to keep the ends in the same plane. The joined fibres 18, 19 are then loaded into the tube 1 through the sleeving 7 and located into the grooves 9, 10 under a glass clamp 14 such that when the stainless steel tube 1 has expanded with the heat required to melt the glass, there is about 600 microns of fibre protruding from the tube. The tube 1 should, when heated, be at least 200 microns from the aluminium mould 8. The coils 11, 12 are then energised to heat the tube 1 to about 500° C as before and with the aid of an instrument manipulator (not shown) a glass rod is offered up to the hole 5. The heat of the tube 1 in this region causes glass at the end of the rod to melt and this melted glass is then drawn into the hole 5 by capillary action. When the glass ceases to be drawn into the hole, the current in the coil 12 is increased to 3.8 amps to encourage the flow along the inside of the tube 1 and the fibres 18, 19. The current should be reduced again to 3.4 amps as soon as the glass begins to be drawn in or else the temperature will cause the glass to flow around the hole as well as into it. When the glass will not be drawn even at 3.8 amps on the coil 12, its current is lowered to 3.2 amps and that on the coil 11 is increased to 2.6-2.8 amps, causing the glass to be drawn to the end of the tube. When a meniscus is observed around the fibre ends as shown in Figure 5a, this indicates that the glass sealant has passed throughout the length of the narrow portion 3 of the tube whereupon the coils 11, 12 should be cooled down slowly (0.4 amps/minute) until the coil 11 is at 1.0 amps and the coil 12 is at 2.0 amps, when both currents can be switched off. Referring now specifically to Figure 5, a modification of the method just described consists in slipping a spacer in the form of a sleeve 15 over one of the fibres. The sleeve 15 is chosen to have a wall diameter corresponding to the required gap between the outsides of the fibres. Thus, if a separation of 250μm is desired between the fibre cores, and the fibres are 125μm in diameter, the sleeve wall will need to be also 125μm in thickness. The effect of the sleeve 15 is to reduce the tendency of the fibre ends to splay apart after the sealant has set and the fibre ends are removed from the mould 8. After carefully removing the assembly from the jig, a few drops of Epotek 301 are drawn into the hole 6 to protect the uncoated fibres behind the glass seal. The adhesive is cured for 60 minutes at about 65°C. This finished assembly can then be connected to an optical device such as a directional coupler by offering up the exposed ends of the fibres to the device and then welding the tube to the device in a conventional manner.

Claims: CLAIMS 1. A method of mounting a plurality of optical fibre waveguides in a support tube, the method comprising the steps of inserting the fibres into the tube, locating their free ends in a jig capable of holding them in their desired separation, applying a sealant to be drawn under the influence of capillary action along the fibres and the tube, and causing the sealant to harden thereby to provide a seal fixing the fibres in the tube. 2. A method as claimed in claim 1 wherein the sealant is glass drawn along the tube and fibres at a temperature above its melting point but low enough to avoid damaging the optical fibres, an appropriate temperature profile along the tube being established by heating an extended region of the tube. 3. A method as claimed in any preceding claim wherein the fibres are temporarily secured in their desired configuration before insertion into the tube. 4. A method as claimed in any preceding claim wherein at least one of the fibres carries a spacer element to assist maintaining correct fibre spacing before the sealant has hardened. 5. A method as claimed in any one of claims 2 to 4 wherein the tube is heated at two or more spaced positions and wherein the temperature at one of those positions is above the melting point of the sealant and at another one below the melting point. 6. A method of mounting an optical waveguide in a support tube comprises heating an extended region of the tube with a waveguide inserted therein; causing sealant to be drawn along the tube; and terminating the heating step to allow the sealant to harden and fix the optical waveguide in the tube, wherein the temperatures during the heating step are selected to be such that they do not damage the optical waveguide. 7. A method of mounting two or more optical waveguides in a support tube in which an accurately controlled separation between the waveguides is established by using a jig to maintain the waveguide positions while a sealant is allowed to harden and fix the waveguides, in position. 8. A method according to claim 7, wherein the sealant is introduced into the tube by heating an extended region of the tube with waveguides inserted therein; causing sealant to be drawn along the tube; and terminating the heating step to allow the sealant to harden and fix the optical waveguides in the tube, wherein the temperatures during the heating step are selected such that they do not damage the optical waveguides. 9. A method according to claim 7 or claim 8, wherein the waveguides are joined together at least at one point along their length prior to the mounting process. 10. A method according to claim 6 or claim 8, or claim 9 when dependent on claim 8, wherein the sealant is introduced through an aperture in the tube situated between the two heating positions. 1 1. A method according to any of claims 6 to 10, wherein the sealant comprises glass. 12. A method according to any of claims 6 to 11, wherein the or each optical waveguide comprises an optical fibre. 13. A method as claimed in any one of claims 6 to 12, wherein the tube is heated at a first position to a temperature above the melting point of the sealant, and at a second position to a temperature below the melting point. 14. A method of manufacturing a waveguide device having a plurality of waveguides connected to respective optical fibres, the method comprising mounting a plurality of fibres in a support tube according to any one of the preceding claims, and mounting the support tube such that the fibres are in aligned relationship with the respective waveguides. 15. An assembly comprising a support tube and a plurality of optical fibres sealingly fixed therein, wherein the sealant has been drawn along the fibres and the tube under the influence of capillary action. 16. An assembly as claimed in claim 15 wherein the sealant is glass. 17. An optical waveguide device comprising a plurality of waveguides having optical fibres connected thereto, wherein the fibres are mounted in an assembly according to claim 15 or 16.

Inventors: HARMON ROBERT ARTHUR, PRESTON KEITH ROBERT, ROSE PETER JOHN, HARMON, ROBERT, ARTHUR, PRESTON, KEITH, ROBERT, ROSE, PETER, JOHN
Applicants: BT & D TECHNOLOGIES LTD, BT&D TECHNOLOGIES LIMITED
Application year: 1989
Main classification: G02B6
IPC codes: G02B6