OPTICAL FIBRE TERMINATING DEVICE AND A METHOD OF TERMINATING OPTICAL FIBRES

Abstract: An optical fibre terminating device and a method of terminating optical fibres are shown. Fibres are secured in a receptacle (12) which is formed by two parts (13,14) joined together to form a precise "V" shaped groove having an internal angle of sixty degrees. The groove is arranged to receive a close packed array of small diameter juxtaposed fibres (11) which are permanently fixed within the groove.

Description: The present invention relates to an optical fibre terminating device and a method of terminating optical fibres in which the ends of a plurality of fibres are permanently secured in a receptacle. The advantages of optical fibres, over copper conductors, for signal transmission are well known and include, for example, broad bandwidth and immunity to inductive pick up. However a particular problem with optical fibres is that of termination to facilitate splicing and switching etc. Unfortunately developments which have improved transmission qualities have resulted in fibres having a small outer diameter with an even smaller transmission core. Terminating monomode fibres having a small outer diameter (in the range of two hundred micro-meters down to ten micro-meters) is therefore particularly difficult. Optical fibres are preferably terminated in groups or bundles rather than individually. This not only facilitates mechanical handling of the fibres but also allows additional functions to be provided, for example signal switching and multiplexing. A method of terminating small diameter fibres is shown in British Patent No 1524874 of Chinnock. Figure 1 of this patent shows a receptacle in the form of a holder 10 in two Sections 11 and 12. Each section has a series of parallel grooves having an internal angle of ninety degrees. A fibre is placed in each of the grooves of Section 11 and then covered by a co-operating groove in Section 12. The fibres are then permanently secured by an epoxy adhesive. In the above known terminating device the relative position of the fibres can be maintained within a very accurate tolerance range. However a disadvantage of the device is that it only provides a linear band of fibres and not a multi layer array, the latter being desirable in many applications. A multi layer array could be fabricated by securing a plurality of holders together or alternatively, intermediate holder sections may be provided with grooves on both sides. However both of these techniques result in an array in which the distance between fibres is relatively very large. Closely packed arrays may be produced for large diameter multimode fibres by known techniques. An example of grouping large diameter fibres is shown by Tomlinson in United States Patent 4208094. However it must be realised that handling large diameter fibres is relatively easy and represents a totally different branch of the optical fibre art. It is therefore possible to glue seven large diameter fibres together to produce an array consisting solely of fibres and glue. This approach would not be delicate. Before being glued, the seven fibres of Tomlinson are supported in a "V" shaped groove with three dummy fibres. The groove is formed in a support block and techniques are known providing a groove which is accurate enough for large diameter fibres. However such a groove could not be machined to the required tolerances for small diameter fibres. JP58205106 discloses a similar technique to that taught by Tomlinson for providing a circular array of fibres by insertion of a multiplicity of fibres in a "V" groove, supporting the usable fibres using dummy fibres. In the aforementioned Japanese patent the technique is used for fibres having diameters of several hundreds of microns. Accordingly, any slight inaccuracy in machining the base of the "V" groove for a mololithic block does not substantially affect the alignment of the circular array. However, if the method and terminating device proposed in JP 58205106 is used with smaller diameter fibres (ie fibres having a diameter of less than 200 microns) a minute inaccuracy in the base of the "V" groove may result in a gross inaccuracy in the alignment of the fibres. It is therefore an object of the present invention to provide an improved terminating device of the kind disclosed in JP58205106 or US4208094 and a method of manufacturing and using such a device which alleviates the alignment problems which may be caused by inaccuracies in the manufacture of the device. According to a first aspect of the invention there is provided an optical fibre terminating device in which, in use, the ends of a plurality of small diameter fibres are permanently secured in a receptacle having a "V" shaped groove with an internal angle of sixty degrees, the "V" shaped groove being arranged to receive a close packed array of small diameter juxtaposed fibres which, in use, are permanently fixed within the groove; characterised in that the receptacle comprises first and second parts having respective first and second planar surface portions, and respective first and second further planar surface portions which make with their associated first and second planar surface portions two respective external angles therewith having a total of four hundred and twenty degrees, the parts being joined together such that they are in contact with each other over said respective further planar surface portions whereby said first and second planar surface portions form the "V" groove with an internal angle of sixty degrees, and a line of intersection between a said planar surface portion and its associated further planar surface portion forms the base line of the "V" groove. An advantage of the first aspect is that a multi layer array is provided wherein the fibres are secured within the device while being accurately displaced relative to one another. Preferably each of said first and second parts has its planar surface portion and its further planar surface portion intersecting along said base line, and the external angle between its planar surface portions is two hundred and ten degrees. An advantage of this preferred embodiment is that the two parts may be made symmetrical and thus may be cut from a common length of material. Preferably the device includes a wedge for contacting the row of fibres furthermost from the common line. This embodiment has the advantage of enclosing the fibres totally within the material of the receptacle. Preferably the receptacle is fabricated in silica facilitating bonding between the receptacle and the fibres. In a preferred embodiment the fibres form an array having a triangular cross section, the axis of a central fibre lies on the inter-section of the three lines bisecting the angles of the triangular array, and the receptacle is arranged to rotate about the axis of the central fibre. An advantage of this preferred embodiment is that it is possible to switch from one fibre of the array to another similar fibre by rotating the device. It should be noted that six fibres surround the inner most or central fibre wherein an array fabricated according to the Chinnock teaching would only provide four fibres surrounding an inner most fibre. In a preferred embodiment the receptacle is substantially cylindrical and the central fibre is positioned along the central axis of the cylindrical receptacle. An advantage of this preferred arrangement is that the whole device may be accommodated in a mounting similar, or identical, to a mounting used for single large fibres. Furthermore rotation of the array about the central fibre merely involves rotation about the central axis of the cylindrical receptacle. A small diameter fibre may be defined as having a diameter in the range of ten micrometers to two hundred micrometers, however, preferably the "V" shaped groove is arranged to receive fibres having an outer diameter in the range of forty micrometers to one hundred and seventy micrometers. The device is therefore advantageously arranged to accommodate monomode fibres. According to a second aspect of the present invention there is provided a method of forming an optical fibre terminating device according to Claim 1, characterised by the steps of providing a receptacle comprising first and second parts having respective first and second planar surface portions and respective first and second further planar surface portions which make two respective external angles therewith having a total of four hundred and twenty degrees, at least one of said external angles being at a line of intersection between its associated planar surface portions bringing the further planar surface portion of said first part into contact with the further planar surface portion of said second part, and joining said first and second parts together such that said first and second planar surface portions form a precise sixty degree "V" groove in the receptacle having a base line formed by said line of intersection bending small diameter fibres and introducing them to the receptacle with a force acting generally towards the base line of the "V" groove thereby forming a close packed array of juxtaposed fibres received in said groove, and permanently securing said received array of juxtaposed fibres in the receptacle. Preferably the method is further characterised by bending a first fibre and applying force in the direction of the base line of the groove so that the fibre lies along the groove, bending two further fibres and applying force in the direction of the groove so that the further fibres are juxtaposed with the first, and introducing further fibres in the same manner to form an optical fibre array having a triangular cross-section. An alternative preferred method is further characterised by grouping the fibres in a "V" shaped guide to form a triangular array, inclining the guide at an angle to the receptacle so as to bend the fibres and forcing the fibres in juxtaposition into the receptacle. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional side view of an optical fibre terminating device according to an embodiment of this invention; Figure 2 is a view showing on method of forming such device; and Figure 3 is a view showing an alternative method of formation. Referring to Figure 1, this shows a multi-layer optical fibre termination, viewed in section taken normal to the axes of optical fibres 11. There are seven layers of fibres 11 in a triangular array. The fibres are held in a receptacle 12, the body of which is formed in two parts, 13 and 14, of fused silica. The parts 13 and 14 are alike; each has a respective plane surface 15, 16 (constituting first and second planar surface portions of the invention) inclined to a respective plane surface 17, 18 (constituting first and second further planar surface portions of the invention), and the parts are joined by fusing surfaces 17, 18 together, so that the four surfaces 15, 16, 17, 18 meet along an edge indicated by 19 in the Figure. This line is the base line of the "V" groove formed by surfaces 15, 16 which are inclined at sixty degrees to one another. By forming the body in two parts the two surfaces 15, 16 can be made to meet very precisely at edge 19 so that the base of the "V" groove is a sharp sixty degree angle. This ensures that the lowermost fibre fits into the bottom of the groove as shown and all the fibres of the array can be juxtaposed. A rounded base to the groove can lead to the bottom fibre being incorrectly positioned, and there will then be a gap between the two fibres in the second row, and gaps between other fibres in the array. The fibres are held together by adhesive and a silica wedge 20 is disposed on the top row of fibres 11. The receptacle 12 may be held in a standard connector normally used for one fibre, or for a single row of fibres. One fibre, preferably the central fibre 11', is aligned in the connector in a standard manner. Depending on the application, for example for circular switching, the connector may be rotated about the axis of this fibre. The fibres 11 are monomode fibres having an outer diameter of one hundred and twenty-five micrometers when stripped of their protective coating. Larger or smaller diameter fibres may be used if desired, for example, fibres having a diameter of fifty micrometers could be located in the receptacle 12. The fibres used would normally have diameters of less than two hundred micrometers and typically be within the range of forty micrometers to one hundred and seventy micrometers, or ten micrometers to one hundred and seventy micrometers. One method of forming the device of Figure 1 is illustrated in Figure 2. The receptacle 12 is clamped with the "V" groove lying generally horizontal. An inclined surface 21 is supported at an angle of about twenty degrees to the horizontal a short distance from the rear end of the receptacle. The fibres for the array are chemically stripped of their protective coating to expose the fibre cladding. A first fibre is introduced to the "V" groove so that its front end portion lies in the groove and an intermediate part rests on surface 21. The fibre is thus bent and there will be a force on the front portion acting towards the base line of the groove. The fibre is then pushed forwards so that its front end extends past the front edge of the "V" groove, and this end is clamped so that the downward force on the fibres is increased and the fibre is held tight against the base of the groove. The hold on this fibre is maintained while two further fibres are brought down on top of it, by advancing them along the surface 21, until they also extend beyond the front end of the "V" groove. These fibres are then clamped so they press on the first fibre, which is released. The procedure is repeated, with the next row of three fibres being introduced to the "V" groove, clamped and the two fibres below released. Further rows of four, five, six and seven fibres are added. The top row of seven fibres is held clamped until wedge 20 has been fitted into the top of the groove and clamped. A high viscosity expoxy adhesive, such as rapid Araldite (Trade Hark), is applied at the rear of the groove. A very low viscosity expoxy resin such as Perma Bond E27 (Trade Mark) is then introduced to the front of the "V" groove. A hot air gun is used to force the resin to flow along the groove between the fibres, and also cures the resin. Flow of resin past the rear end of the groove is prevented by the high viscosity adhesive. The device thus formed can now be introduced to a standard connector, such as Amphenol SMA, ITT Canon or Weco connectors, and the central fibre 11' of the array aligned by standard procedures. After alignment the termination is cemented in the connector using a standard expoxy adhesive. The front surfaces of the fibres are machined so that they are normal to the axis of fibre 11, and then lapped and polished to give a transmission surface. The device has rows of fibres lying with their axes accurately parallel to one another with each fibre contacting adjacent fibres, walls of the "V" groove or the wedge 20. The central axes of adjacent fibres are therefore equally spaced so that the device can be used satisfactorily for a variety of applications. These include linear and circular switches, wavelength multiplexors and demultiplexors using gratings (which may be switchable), integrated optical matrix switches, DCG holographic reflection filters, Fabry-Perot multipass cavity devices, and multifibre connectors. An alternative to the above method is now described with reference to Figure 3. In this embodiment, a guide 31 in the form of a large "V" is provided. All of the fibres to be introduced into the "V" groove of the receptacle 12 (twenty eight fibres) are placed in guide 31, where the outer coating of the fibres is chemically stripped. Guide 31 is then inclined and clamped so that it is a short distance from receptacle 12. The fibres are forced forwards so they enter the "V" groove of the receptacle. Forward movement continues until the ends of the fibres pass beyond the edge of the "V" groove. The fibres, properly juxtaposed, are clamped at the front and rear of the receptacle. Adhesive is applied as described for the first method and the device is aligned in a connector. It will be observed that in the second method, no wedge (20) is used. The above methods may be used for forming devices having larger arrays than the one illustrated, for example fifty five fibre arrays with fibres having a diameter of fifty micrometers or one hundred and twenty five micrometers.

Claims: An optical fibre terminating device in which, in use, the ends of a plurality of small diameter fibres (11) are permanently secured in a receptacle (12) having a "V" shaped groove (15-19-16) with an internal angle of sixty degrees, the "V" shaped groove being arranged to receive a close packed array of small diameter juxtaposed fibres (11) which, in use, are permanently fixed within the groove; characterised in that the receptacle comprises first and second parts (13, 14) having respective first and second planar surface portions (15, 16), and respective first and second further planar surface portions (17, 18) which make with their associated first and second planar surface portions (15, 16) two respective external angles having a total of four hundred and twenty degrees, the parts (13, 14) being joined together such that they are in contact with each other over said respective further planar surface portions (17, 18), whereby said first and second planar surface portions (15, 16) form the "V" groove with an internal angle of sixty degrees, and a line of intersection (19) between a said planar surface portion (15, 16) and its associated further planar surface portion (17, 18) forms the base line (19) of the "V" groove. An optical fibre terminating device according to Claim 1 characterised in that each of said first and second parts (13, 14) has its planar surface portion (15, 16) and its further planar surface portion (17, 18) intersecting along said base line (19), and the external angle between its planar surface portions (15 and 17, 16 and 18) is two hundred and ten degrees. An optical fibre terminating device according to Claim 1 characterised in that it further includes a wedge (20) for contacting the row of fibres furthermost from the base line (19). An optical fibre terminating device according to Claim 1 or Claim 2 characterised in that the receptacle (12) is fabricated from silica. An optical fibre terminating device according to any of Claims 1 to 3 characterised in that the fibres (11) form an array having a triangular cross section, the axis of a central fibre (11') lies on the cross section of the three lines bisecting the angles of the triangular array, and the receptacle is arranged to rotate about the axis of the central fibre. An optical fibre terminating device according to Claim 4 characterised in that the receptacle (12) is substantially cylindrical and the central fibre (11') is positioned along the central axis of the cylindrical receptacle. An optical fibre terminating device according to Claim 1, characterised in that the groove (15, 19, 16) is arranged to receive fibres having an external diameter within the range of forty micrometers to one hundred and seventy micrometers. A method of forming an optical fibre terminating device according to Claim 1, characterised by the steps of providing a receptacle (12) comprising first and second parts (13, 14) having respective first and second planar surface portions (15, 16) and respective first and second further planar surface portions (17, 18) which make two respective external angles therewith having a total of four hundred and twenty degrees, at least one of said external angles being at a line of intersection (19) between its associated planar surface portions (15 and 17, 16 and 18); bringing the further planar surface portion (17) of said first part (13) into contact with the further planar surface portion (18) of said second part (14), and joining said first and second parts (13, 14) together such that said first and second planar surface portions (15, 16) form a precise sixty degree "V" groove in the receptacle having a base line (19) formed by said line of intersection (19); bending small diameter fibres and introducing them to the receptacle with a force acting generally towards the base line (19) of the "V" groove thereby forming a close packed array of juxtaposed fibres received in said groove, and permanently securing said received array of juxtaposed fibres in the receptacle (12). A method of forming a terminating device according to Claim 8 characterised by bending a first fibre (11) and applying force in the direction of the base line (19) of the groove so that the fibre lies along the groove, bending two further fibres and applying force in the direction of the groove so that the further fibres are juxtaposed with the first, and introducing further fibres in the same manner to form an optical fibre array having a triangular cross-section. A method of forming a terminating device according to Claim 8 characterised by grouping the fibres in a "V" shaped guide (31) to form a triangular array, inclining the guide at an angle to the receptacle so as to bend the fibres and forcing the fibres in juxtaposition into the receptacle. A method of forming a terminating device according to any of Claims 8 to 10 characterised by positioning a wedge (20) across the row of fibres furthermost from the base line (19).

Inventors: NICHOLLS, SIMON THOMAS
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: G02B6
IPC codes: G02B6