OPTICAL FIBRE TERMINATION

Abstract: A termination for an optical fibre (4) comprises a planar support block which has an aperture there-through for receiving and accurately locating an optical fibre end (4). It is constructed out of a plurality of platelets (1), the aperture being provided by aligned, laser-drilled holes (2) which are formed in each platelet (1). The holes (2), which are characteristically tapered, all taper in the same direction. They can therefore act as guides during insertion of a fibre end (4). Alternative arrangements are described. The termination is for use wherever an optical fibre must be accurately located relative to another component. It finds particular application as a support for a rear-entry photodiode (3), as described.

Description: OPTICAL FIBRE TERMINATION The present invention relates to optical fibre terminations, and finds particular application in substrates for opto-electronic transducers. It is often necessary that the end of an optical fibre be mounted so that it is aligned along a particular axis. For instance, if light is to be transferred between an optical fibre and another component such as a photodiode, the light transfer efficiency may be maximised by alignment of the optical fibre end along a particular axis in relation to the other component. It is known to mount the end portion of an optical fibre in a termination comprising a block of ceramic material having an aperture therethrough. The wall of the aperture at least substantially aligns the fibre end portion along a known axis in the block of material. Before being placed in the aperture, the fibre end portion is stripped down to the outermost surface of the glass cladding layer or layers. There are two reasons for this. Firstly, the fibre end portion can be aligned with much greater accuracy and, secondly, it can be much more effectively sealed at the aperture if the seal can be made directly to the glass cladding. This second feature is particularly useful where the aperture provides a path for the fibre end portion into an otherwise hermetically sealed optical package. Because the aperture must be suitable to align a stripped fibre end, its diameter must be only slightly greater than the diameter of a stripped fibre. With «§* fibres commonly having diameters of about 125μm or 150 μm, significant manufacturing difficulties present themselves: it is not at all easy to construct an aperture accurately to a predetermined diameter when working at such small dimensions. Two methods of constructing such an aperture are known. In a first method, the aperture is bored through a block of material in a single operation while in a second method, grooves are made in each of two half blocks of material then brought into register to form the aperture about the fibre end portion. The first method suffers not only from the difficulty of making such a fine aperture but also from a risk of damage to the stripped fibre during its insertion into the bored, aperture. A fibre end without its protective coverings is extremely sensitive to damage which can cause considerable light loss from the fibre in use. To compound the difficulty, the ceramic materials which can be most suitable for use in this context, for other reasons, tend to be extremely abrasive as well as being diffcult to machine. The second method largely avoids the possibility of damage to the stripped fibre end but still suffers from disadvantages. The accurate formation of two grooves of suitable dimensions and their being brought into register still present difficulties. Additionally, if the termination is to be used to couple an optical fibre to a photodiode, then problems arise in holding the two half blocks together. Two alternatives are to use epoxy resin or solder between the two half blocks. However, a particularly convenient arrangement is to mount the photodiode directly on the termination so that it overlies one end of the aperture. This means that it is difficult to avoid creating an interface between the resin or the solder and the photodiode. If epoxy resin is used to hold the half blocks together, contact between it and the photodiode can affect the material of the photodiode and consequently damage its action. If solder is used instead of epoxy resin, it can cause electrical difficulties such as earthing of the photodiode. It is an object of the present invention to provide an improved optical fibre termination in which an end portion of an optical fibre may be mounted and accurately located, and a method of making the termination. According to a first aspect of the present invention, there is provided a method of manufacturing an optical fibre termination, for receiving and locating an end portion of an optical fibre, which comprises the steps of: (i) preparing a plural ty of platelets of material, each platelet having a laser-drilled hole therethrough; and (ii) assembling the platelets into a layered structure, the holes being positioned so as to contribute to a common alignment aperture through the termination, and having nominally identical minimum diameters. Methods according to the present invention allow the great dimensional accuracy with which laser-drilled holes can be constructed, even at sizes comparable to stripped optical fibre diameters, to be taken advantage of in locating a fibre end portion. Conveniently step (ii) above may be carried out by inserting a common locating p n through the holes and arranging the platelets in relation to the locating pin. According to a second aspect of the present invention there is provided an optical fibre termination, for receiving and locating an end portion of an optical fibre, comprising a plurality of platelets mounted in a fixed relationship to one another, each platelet havi ng a laser-drilled hole therethrough, which holes have nominally identical minimum diameters and are positioned so as to contribute to a common alignment aperture through the termination, the alignment aperture providing location means adapted to receive, and to locate with respect to the termination, an optical fibre end portion. To use a termination according to the present invention, an optical fibre end portion is inserted into the alignment aperture from one end. Laser-drilled holes are characteristically tapered. Preferably therefore, all the tapered holes open towards the end of the aperture via which a fibre end portion will be inserted. The taper of each hole .is then particularly suited to provide guidance during insertion of the fibre end portion. Embodiments of the invention find particular application as supports for opto-electronic components such as the so-called substrate entry photodiode. Discussion of these photodiodes may be found in an article by Lee et al. entitled "small area InGaAs/InP p-i-n photodiodes, Electronics Letters 198016 (4) pages 155-6. A photodiode of this type comprises a small block which has a sensitive area on one face. In use, an optical fibre must be coupled to the sensitive area. To achieve that coupling, the photodiode is mounted with its face in contact with a support, on aperture being provided in the support through which the fibre can be optically coupled to the sensitive area. A termination according to the present invention, for use as a support for an opto-electronic component, comprises a layered block of material, the platelets providing at least some of the layers. The alignment aperture opens into a face of the block which provides a site for mounting the opto-electronic component. Supports for substrate entry photodiodes according to first and second embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures 1n which: Figure 1 shows a cross section of a support according to a first embodiment of the Invention, in use; and Figure 2 shows a cross-section of a support according to a second embodiment of the invention, not in use. Referring to Figure 1, the support according to a o first embodiment of the invention comprises three contiguous, platelets 1. Each platelet 1 is provided with a laser-drilled hole 2, all the holes 2 being aligned to provide an alignment aperture through the support. An optical fibre end 4 is mounted in the aperture so provided s and a substrate entry photodiode 3 is mounted on the support, overlying the aperture. The fibre end 4 is optically coupled to the photodiode 3 via its sensitive area. The platelets 1 of the support consist of 96°/o 0 alumina and are each 635um thick. Because the holes 2 are laser-drilled, each one is conical. This is due to the fact that during a single drilling operation the rise in temperature produced by the beam in the material of the platelets 1 has more time to propogate through the material, away from the beam, where the beam is in close proximity to it for the longest. Hence the beam has the greates effect, in terms of material removed, at the face of the platelet 1 where the beam first impinges. As material at increasing depth through the platelet 1 is considered, the total time for which the beam will be in close proximity to it decreases since the beam takes time to penetrate through the platelet. Hence a decreasing amount of material will be removed and the laser-drilled hole 2 produced will be generally conical. The smallest diameter of each hole 2 1s 152μm and it Is this dimension at the small end of each hole 2 which acts to locate the optical fibre end 41n the support. The largest diameter is 254um. A first face 5 of the support provides a mounting site for the photodiode 3 such that the photodiode 3 lies over the smaller end of an hole 2'. The optical fibre lies in abutment or near abutment with the photodiode 3 to provide the optical coupling therewith. 10 Although lying loose through the aperture provided by the holes 2, the fibre end 4 is attached to a second face 6 of the support which is remote from the photodiode 3. In order to lend added support to the fibre end 4 at the attachment point, and to minimise damage to the fibre by 15: bending, a coaxial support tube 9 is provided. The tube 9 is a brass tube, being 8mm long and of internal and external diameters 0.43mm and 0.61mm respectively. The tube 9 abuts the second face 6 of the support, around the margin of the hole 2" opening into that face. Both the 2σ fibre end 4 and the tube 9 are attached to the support by means of solder 8 which provides a seal between the outer surface of the fibre end 4, the margin of the relevant aperture 2", and the internal surface of the tube 9. The fibre end 4 is thus unattached within the 25. alignment aperture but located in position by the combined effect of the minimum diameters of the holes 2. These minimum diameters provide location of the fibre end 4 at three positions spaced along the fibre end 4 at intervals of 635μm. 3σ Electrical connections 12 are made to the photodiode 3 by means of contact pads 11 on the first face 5 of the support. In order to manufacture the support described above, firstly the three platelets 1 are prepared. Each platelet 1 has a hole 2 laser-drilled individually therethrough. The manner 1n which the holes 2 may be laser-drilled will be known to a person skilled in the art and is not therefore further described. In order to provide suitable surfaces for the soldering of the fibre end 4 and the tube 9 to the completed support, the fibre end 4, the internal surface of the tube 9, and the surface of one of the platelets 1 which is to become the second face 6 of the support are all metallised in the region where the solder 8 is to provide a seal. Metallisation on the surface of the platelet 1 is in the form of an annulus which extends around the opening of its respective hole 2". A length of about 0.5mm of the fibre end 4, adjacent its end face, is left unmetallised in order to keep stray capacitance levels low in use of the completed support. In order to facilitate the soldering of the photodiode 3 to the completed support, the surface of a second of the platelets 1, which is to become the first face 5 of the support, is also metallised. Metallisation is applied to that part of this second platelet 1 which will provide the mounting site for the photodiode 3. The contact pads 11 are conveniently deposited in the same operation as the metallisation. To assemble the layers 1 into the layered structure of the support, they are stacked together and fired with a thick film film dielectric ink between adjacent faces. The ink, once fired, maintains the stacked structure. In order to ensure that the holes 2 of the platelets 1 are aligned as stacking takes place, a piece of wire is inserted into the holes 2 and only withdrawn when the platelets 1 and ink have been fired. To mount the fibre end 41n the alignment aperture provided by the holes 2, it is fed in from the second face 65 of the support. In this way, It 1s guided into each hole 2 by its tapered nature. Before the fibre end 4 is mounted 1n the hole, the support tube 91s threaded onto it and held to one side so that 1t can later be brought into Its supporting position. When the fibre end 4 is in position in the aperture, it is fixed there by means of an annulus of solder. The solder is applied to encircle the metallised fibre end 4 and to seal it to the annulus of metallisation on the first face 5 of the support. The tube 9 is then moved into position abutting the support and the solder is reheated so that it flows against the internal, metallised surface of the tube 9, sealing it also to the annulus of metallisation on the support. To mount the photodiode 3, it is placed on the mounting site provided by the first face 5 of the support with the fibre end 4 being previously fixed in place in the aperture. Light is passed along the fibre end 4 so that the degree of optical coupling between it and the photodiode 3 can be monitored, so allowing the photodiode 3" to be manoeuvred until maximum coupling is achieved. The photodiode 3 is then soldered into the position where such coupling has been achieved. It may not be necessary that the third platelet 1 be provided. In feeding the fibre end 4 into the aperture, little guidance may be required at the third of the holes 2 since the fibre end 4 will be aligned by the first two. Consequently, However, there are constraints other than that of accurate alignment which make it preferable that there should be three platelets 1 or even more. One of these constraints is the stray capacitance present in the arrangement, 1n use. It is preferable to maintain such stray capacitance at as low a level as practicable, during use, because of its deleterious effect on the function of the photodiode 3. One factor which can affect the stray capacitance is the proximity of the solder 8 to the photodiode 3 and its electrical contacts 11. By using at least three platelets 1 in the support, the solder 8 can be substantially retained at a distance of at least two platelets' thickness from the photodiode 3 and contacts 11. Using a support as described above, with reference to Figure 1, wherein the layers 1 are constructed out of 96°/o alumina, stray capacitance levels of about 0.06 pF have been observed, excluding the photodiode capacitance. In a variation, the alumina was replaced by steatite, stray capacitance levels of about 0.03 pF then being observed. Referring to Figure 2, an alternative construction uses only two platelets 1 having a laser-drilled hole 2 therethrough, and a third, intervening layer 12 having a comparatively large hole 13 therethrough, made by any convenient means. This construction offers slightly lower stray capacitances, 0.025 pF having been observed using platelets and a layer constructed out of steatite, but is slightly more complicated to manufacture. A support such as that described above, by offering a hermetically sealed mounting for the optical fibre end 4, is suitable for being incorporated into the wall of an optical device package. The accuracy with which an optical fibre end can be located along a preselected axis in a termination according to the present invention will be determined in part by the relationship between the size of the laser-drilled holes and the outer diameter of the fibre end, and in part by the degree of alignment of the holes. The accuracy In locating an optical fibre end that Is required, however, is determined by the spatial tolerance of the optical coupling that 1s to be made with the fibre end. Typically the core diameter of a monomode optical fibre is 8μm. If the fibre 1s to be coupled to a photodiode having a sensitive area of for instance 40μm diameter, the accuracy required is likely to be relatively low. If, however, it is to be coupled to a photodiode having a smaller sensitive area, or to a different type of component with a relatively small coupling area, then clearly the accuracy required will be higher. Although the support for rear-entry photodiodes described above comprises platelets which are 635μm thick, the- thickness of the platelets may clearly be varied. The depth of the platelet, and the material from which it is constructed, will both have an effect on the dimensions of the laser-drilled hole in that platelet. Therefore the nature of the. hole required may at least partially determine the depth of the platelets used and the material they consist of. Optical fibres for different purposes can vary considerably in outer diameter, typical examples including fibres of outer diameters 250μm and 125 μ m. Hence the minimum diameter of a relevant laser-drilled hole in one case may be required to be of up to and including 300μm diameter, and in another case may be required to be of up to and including 160μm.

Claims: - CLAIMS - 1 A method of manufacturing an optical fibre termination, for receiving and locating an end portion of an optical fibre, which comprises the steps of: (1) preparing a plurality of platelets of material, each platelet having a laser-drilled hole therethrough; and (11) assembling the platelets Into a layered structure, the holes being positioned so as to contribute to a common alignment aperture through the termination, and having nominally identical minimum diameters. 2 A method according to Claim 1 wherein step (ii) is carried out by means of inserting a common locating pin through the laser-drilled holes and arranging the platelets in relation to the locating pin. 3 An optical fibre termination, for receiving and locating an end portion of an optical fibre, comprising a plurality of platelets mounted in a fixed relationship to one another, each platelet having a laser-drilled hole therethrough, which holes have nominally identical minimu diameters and are positioned so as to contribute to a common alignment aperture through the termination, the alignment aperture providing location means adapted to receive, and to locate with respect to the termination, an optical fibre end portion. 4 A termination according to Claim 3 wherein the laser-drilled holes taper in a common direction. 5 A termination, according to either one of Claims 3 or 4, wherein the termination comprises three platelets, the platelets being contiguous with one another. 6 A termination, according to either one of Claims 3 or 4, wherein the termination comprises two platelets, the platelets being spaced apart. 7 A termination, according to Claim 6, wherein the two platelets are spaced apart by means of an intervening layer, the intervening layer having a hole therethrough adapted to allow the reception and location of an optical fibre end portion by the alignment aperture. 8 A termination according to any one of the preceding Claims 3 to 7 wherein the platelets are constructed out of a ceramic material. 9 A termination according to any one of the preceding Claims 3 to 8 wherein each platelet is less than err equal to 650μm thick. 10 A termination according to any one of the preceding Claims wherein the minimum diameter of each laser-drilled hole is less than or equal to 300μm. 11 A termination according to Claim 10 wherein said minimum diameter is less than or equal to 160μm. 12 A termination according to any one of the preceding Claims 3 to 11 wherein a face of the termination provides a mounting site for an opto-electronic transducer, the site being such that an opto-electronic transducer mounted on the site is capable of being optically coupled to an optical fibre end portion received and located in the alignment aperture. 13 A termination according to any one of Claims 3 to 12, having an optical fibre end portion received and located therein, the optical fibre end portion being secured and hermetically sealed to the termination by means of an annulus of fixing material which extends between the optical fibre end portion and a face of the termination. 14 A termination according to Claim 13 as dependent on Claim 12 wherein the fixing material is solder and the face of the termination to which the fixing material extends Is parallel and opposed to the face which provides a mounting site. 15 A termination substantially as described herein¬ before with reference to the accompanying Figures. 16 An optical assembly comprising a termination according to Claim 12, an optical fibre end portion received in the alignment aperture, being secured to the termination, and an opto-electronic transducer mounted on the mounting site, the transducer being optically coupled to the optical fibre. 17 An optical assembly substantially as described hereinbefore with reference to the accompanying Figures.

Inventors: CONNELL IAN MURRAY, MACDONALD BRIAN MICHAEL, STANGHAN CHRISTOPHER JOHN, CONNELL, IAN, MURRAY, MACDONALD, BRIAN, MICHAEL, STANGHAN, CHRISTOPHER, JOHN
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: G02B6
IPC codes: G02B6