FIBRE TERMINATION

Abstract: An optical fibre termination which reduces the effect of reflections from the transfer face (12). The transfer face is skewed to the axis of the termination so that reflections are not propagated along the external path. The transfer face is also skewed to the core direction so that refraction makes the external path parallel to, and preferably co-incident with, the axis. To assemble a termination, the fibre is prepared by removing the end portion of its plastics sheath to leave about 1 cm of bare glass filament exposed. If the other end of the fibre is not available (because the fibre is too long) the housing is placed on the fibre as a preliminary even though is not used at the start of that assembly. The fibre (10) is clamped with its bare end (40) horizontal and the nose-piece (30) (which is too light to bend the fibre) is placed on the bare end (40). Because the centre of mass is towards the rear the nose-piece tilts to its angle of repose. Thus the critical skew defined above is established and, at this time a thermosetting or other filler material is introduced into the nose-piece so that the configuration is finalised even when the fibre is removed from its horizontal alignment. The length and diameter of the control tube (31) determine the angle which a fibre, located diagonally in the control tube (31), makes with the axis of the control tube (31).

Description: FIBRE TERMINATION This invention relates to fibre terminations and especially to the termination of an optical fibre comprising a filament surrounded by a protective sheath, e.g. of a plastics or rubbery material. The filament usually has a glass core for the propagation of optical signals and a glass cladding to confine the signals to the core. At its ends an optical fibre is usually associated with external devices for optical intercommunication. It has been observed that reflections from the transfer face (i.e. the face of the fibre through which optical signals must pass for linkage with an external device) often cause operating difficulties. These difficulties are exacerbated if there are also reflections from the external device. Reflections to a signal generator can result in oscillation whereas reflections to a receiver can cause noise. It is an object of tins invention to reduce the effect of such reflections. This invention is based upon the principle that skewing the transfer face directs reflections received on the external path of the termination away from said external path whereby the reflections do not reach an external device located on said path. In so far as the reflections do not reach the external device, they do not affect its operation. However, skewing the transfer face causes refraction so that the external path assumes an arbitrary orientation which makes it difficult to locate the external device for coupling with the fibre. Therefore this invention also skews the core of the fibre to the direction of the external path and this angle of skew is chosen so that diffraction restores die external path to a predetermined direction, e.g. parallel to the geometric axis of the termination. Preferably the core ends on the geometric axis so that the external path is co-incident with the axis. It is particularly suitable for the angle of skew to be in the range 1° to 20° e.g. 8° to 10 0 . Conveniently, the termination includes a nose-piece which holds the end of the filament at ti e skew angle. The nose- piece is located in the housing of the termination. The nose- piece facilitates the assembly of the termination. In a preferred embodiment, the nose- piece has a counterbalance which cases which causes it to tilt and adopt its angle of repose on the horizontal filament. As will be explained in greater detail below, the dimensions of the nose-piece determine the angle of repose so that the desired angle of skew is produced. After the fixing the nose-piece, e.g. by using a filler material such as a thermosetting plastics, it is convenient to cut or grind the end to establish the angle of the transfer face so as to cause the amount of refractive bending needed to orient the external path as described above. The skewing of the core and the transfer face in accordance with the invention results in a termination which has the desirable features, e.g. the external path is parallel to and preferably co-incident with a desired axis, of a conventional termination but reflections are directed off the external path so that the effect on external devices is minimised. The invention will now be described by way of example with reference to the accompanying drawings in which: - Figure 1 is a diagram illustrating important orientations of the invention; Figure 2 illustrates in diagrammatic form further orientations at the end of a termination; Figure 3 is a longitudinal cross section through a nose-piece; Figures 4A and 4B illustrate important dimensions of the nose-piece; Figures 5 A and 5B is a longitudinal cross section showing a nose-piece fixed to an optical fibre; and Figure 6 is a longitudinal cross section through an assembled termination in accordance with the invention. Figure 1 illustrates the end portion of an optical fibre 10 which has a path region 11 and a transfer face 12. In the receive mode, optical signals exit the fibre 10 via the transfer face 12; in the transmit mode optical signals enter the fibre 10 via the transfer face 12. In a simple configuration (not shown) the transfer face 12 is normal to the direction of the core at the end of the fibre but this simple configuration has important disadvantages. An optical signal generator would need to be placed at 17 so that light focused on the end of the fibre would be acquired for transmission. However, not all the light would be acquired; some would be reflected. Since, in the simple case, the face 12 is normal to the path of the light, the reflected light returns to the signal generator and it disturbs the operation thereof. For example, in the case of a laser the reflection is equivalent to unwanted resonance cavity. Similar considerations apply in the case of the receive mode. In order to reduce the effect of reflections, and in accordance with the invention, the transfer face is skewed in that it is inclined to the external path at an angle which directs the reflections away from the device 17. Figure 1 is drawn in the plane which includes the core 11 and the normal (14,15) to the transfer face 12. ' is the angle between the internal normal 15 and the core 11 (the core being small enough to be considered as a line). Simple geometry shows that I is also the angle between the extension 13 and the external normal 14. Because of refraction, the external path 16 is inclined to the external normal at an angle R and, by the definition of refractive index:- sin R = n sin I where n is the refractive index of the core 11 relative to air. (Many fibres have cores of Geθ2/SiO 2 and for these fibres n is close to 1.47). Therefore. R is greater than I, and the external path 16 is above the external normal 13 as shown. Figure 1 shows that these values are capable of defining the important structural and optical orientations at the end of the fibre. Because the transfer face 12 is skewed as shown, reflections cause less interference than in the simple case described above. Figure 2 illustrates, in diagrammatic form, the end of a termination which utilises the configuration of Figure 1 but still retains the external path 16 in a convenient direction, e.g.parallel to and preferably co-incident with, the axis 21 of the termination. As shown in Figure 2, the fibre 10 is enclosed in a tubular housing 20 but it is skewed so that the core 11 is inclined to the axis 21 at an angle R-I. In the plane of Figure 2, the transfer face 12 makes an angle (90-1) to the wall of tubular housing. Preferably (R-I) is in the range 1° to 20°, especially 8° to 10°. In all important aspects, except reflection, the termination according to the invention behaves exactly as a conventional, non-skewed termination. Thus, the external path 16 is located on the axis of the termination for optimal coupling and even close examination may fail to reveal the skew which constitutes the feature of the invention. Nevertheless, the skew directs reflections so that their operational effect is substantially reduced. A method of assembling a preferred embodiment of the invention will now be described. This preferred embodiment includes a nose-piece which will be described with reference to Figure 3 The nose-piece, generally indicated by the numeral 30, has a control tube 31 at its front and a counterbalance 32 at its rear so that the centre of mass is towards the rear. The external diameter of the counterbalance 32 provides a sliding fit into the tubular housing 20. The length and diameter of the control tube 31 determine the angle which a fibre, located diagonally in the control tube 31, makes with the axis of the control tube 31. This feature will be explained in more detail with reference to Figures 4A and 4B. Figure 4A is a cross section through the control tube 31 but only those aspects defining the dimensions are shown. In cross section the tube is rectangle having a length L and a diameter D. The fibre located diagonally in.the tube has a thickness T. It is desired to arrange that the angle of the fibre is (R-I) as defined above. The thickness of the fibre is known and the length L can be selected to a convenient value, e.g.4mm. This enables the diameter to be calculated as :- D = [L sin(R-I) + T]/[cos(R-I)] Figure 4B is a re-drawing of Figure 4A but without the dimensions and with centre-lines added. In addition, the location of the transfer face 12 is shown and it should be noted that it passes through the intersection of the two axes. Figure 4B relates to the preferred embodiment in which the external path is co-incident with the axis 21. The orientations are important and Figure 4B implies that the axis 21, the core 11 and the normal to die transfer face 12 are co-planar. It is convenient to produce the configuration shown in Figure 4B by removing material to the right of the intended face 12 using a grinding wheel with its axis of rotation normal to the face. This means that it is necessary to mark the plane of the core 11 and the axis 21 which can be done with an index mark on the nose-piece 30. e.g. by an external flat parallel to said plane. To assemble a teπnination, the fibre is prepared by removing the end portion of its plastics sheath to leave about 1cm of bare glass filament exposed. If the other end of the fibre is not available (because the fibre is too long) the housing is placed on the fibre as a preliminary even though is not used at the start of that assembly. As shown in Figure 5, the fibre 10 is clamped with its bare end 40 horizontal and the nose-piece 30 (which is too light to bend the fibre) is placed on the bare end 40. Because the centre of mass is towards the rear the nose-piece tilts to its angle of repose which means that the horizontal fibre is located as mentioned with respect to Figures 3 , 4A and 4B . Thus die critical skew defined above is established and, at tins time a d ermosetting or other filler material is introduced into the nose-piece so that die configuration is finalised even when die fibre is removed from its horizontal alignment. As stated with respect to Figure 4B, the nose-piece 30 is indexed so that its orientation can be ascertained and it is presented to a grinding wheel at the correct angle to produce the transfer face 13. The control tube and die fibre located therein are ground away so that half the length is removed. That is the grinding is continued to the point where the core crosses the axis of the nose-piece so that d e overall result is that the core terminates on said axis and, although the core is skewed to the axis the transfer face is skewed to the core at the compensating angle so that refraction will make the external path co-incident witii the axis, as also shown in Figure 5B. Finally, as shown in Figure 5, the housing 20 (already on die fibre) is moved over the noise-piece 30. This makes the axis of die nose-piece coincide with the axis of the housing which completes the assembly of the termination. The fibre is sufficiently flexible for it to bend enough so tiiat, although it is skewed at the end, it becomes parallel with the termination axis before it reaches the rear end. It will be apparent that the simple technique described assembles a termination with die complicated configuration specified above giving die advantage of reducing the effect of reflections.

Claims: CLAIMS 1. A fibre termination comprising a tubular housing for protecting die end portion of an optical fibre, said termination providing an external optical path orientated in a predetermined direction, said fibre ending in a transfer face which provides optical connection for optical signals to pass between the core of the fibre and its external path: wherein the direction of the core at the transfer face is skewed to both:- (a) the external path, and (b) the normal to the transfer face, the angles of skew in both (a) and (b) being matched so that refraction at the transfer face causes die external path to be oriented in said predetermined direction. 2. A fibre termination according to claim 1, wherein the predetermined direction is parallel to the geometric axis of the termination. 3. A fibre termination according to claim 2, wherein the core of the fibre ends on die geometric axis whereby d e external path is substantially co-incident with die extension of die geometric axis. 4. A fibre termination according to any one of claim 1, claim 2 or claim 3, wherein die optical fibre comprises a filament inside a protective sheath, the end portion of said sheatii being removed to expose said filament; wherein the termination also comprises a nose-piece attached to the exposed portion of the filament, said nose- piece holding said exposed portion at die skew angle specified in (a). 5. A fibre termination according to any one of die preceding claims, wherein die end face of die fibre is skewed at die angle specified in (b). 6. A fibre termination according to any one of d e preceding claims, wherein the angle specified in (a) is witiiin the range 1° to 20°. 7. A fibre termination according to claim 6, wherein die angle is within the range 8° to 10° . 8. A method of making a fibre termination according to any one of the preceding claims, which method comprises:- ( A) removing the protective sheath from the end portion of die fibre to expose the end portion of the filament and locating said exposed portion horizontally, (B) locating a nose-piece on the exposed filament, said nose-piece including a control tube and a counterbalance such that the nose-piece adopts an angle of repose which is determined by die length and diameter of die control tube, said length and diameter being selected such that the angle between die filament and axis of the nose-piece is the angle defined in (a), (C) fixing die position of die nose-piece by introducing filler material into the nose-piece, (D) removing die end portion of the nose-piece and die filament contained therein to establish the transfer face at the angle specified an (b). 9. A method according to claim 8, wherein die filler material used in step (C) is a thermosetting plastics. 10. A method of making a fibre termination according to either claim 8 or claim 9, wherein step (D) is achieved by grinding away material to produce the transfer face. 11. A method of making a fibre termination according to any one of claims 8 - 10, which method also comprises locating and fixing the nose-piece in the housing.

Inventors: MUSK ROBERT WILLIAM, MUSK, ROBERT, WILLIAM
Applicants: BT & D TECHNOLOGIES LTD, MUSK ROBERT WILLIAM, BT&D TECHNOLOGIES LIMITED, MUSK, ROBERT, WILLIAM
Application year: 1993
Main classification: G02B6
IPC codes: G02B6