GB2158607A - Fiber optic coupler - Google Patents

Abstract

For use particularly in a local splice monitor, a fiber optic coupler for coupling light into and out of an optical waveguide (31) without breaking the waveguide has a transparent body (12) through which a passage extends. The passage accommodates a waveguide and has an angled section (38) with the apex of the angle sufficiently sharp that light propagating along the waveguide (31) is emitted as a beam at the angled section (38). The light is detected by a photodetector (42). Alternatively, light from a light emitting source (29) is injected into the waveguide at the fiber angle (38). In another embodiment, the fibre is pressed by a transparent block so as to extend around part of the circumference of a cylindrical rod (see Figure 9 - not shown). <IMAGE>

Description

SPECIFICATION Fiber optic coupler This invention relates to a fiber optic coupler particularly for minimizing light loss when splicing monomode fibers.

In the fabrication and installation of optical cable spans, lengths of optical waveguide are joined together end-to-end using for example a fusion splice. An example of splicing equipment and an operating procedure for using that equipment are described in United Stages Patent No. 4,274,707 (Pacey). Particularly for single mode waveguide in which light propagates substantially within a small core of the order of 10 microns in diameter, the cores of the waveguides which are to be spliced must be accurately aligned so that light from the core of one waveguide passes into the core of the other waveguide.

One technique for optimizing waveguide end alignment at a splice site is to inject light into the remote end of the waveguide upstream of the site and monitor the light output level at the remote end of the waveguide downstream of the splice site. The two fiber ends at the splice site are then manipulated until light transmission between the two fibers is maximized. The technique requires three people at locations remote from one another and is therefore costly and laborious to set up.

Alternative local monitoring techniques have been proposed for example, Campbell et al, "Fiber Optic Connection System", International Wire and Cable Proceedings, 1982. In this technique, light is injected locally into the waveguide just upstream of the splice site and its level is monitored at a location just downstream of the splice site. In one known specific implementation of this latter technique, waveguides at the injection and detection sites are bent around convex formers. Over the curved fiber length at the detection site, light in the fiber core passes firstly into the cladding, then into the jacket, and finally is emitted from the jacketed fiber, the light emission being distributed along the curved fiber length.Any lightwhich passes from the core of the upstream waveguide into the cladding of the downstream waveguide is quickly stripped out of the waveguide so that only the light transferred at the core region reaches the detection site. A photodetector positioned within the plane of the curved waveguide length detects a portion of the light. A similar arrangement is used upstream of the splice site for injecting light into the waveguide. However at the upstream site the light detector is replaced by a light emitting source.

Although this arrangement is satisfactory for light injection into, and light emission from, a multimode fiber having a core of about 50 microns, it is unsuitable for monomode fiber having a core of less than 10 microns in diameter since the smaller core presents a much smaller target at the injection site so the light entering the downstream waveguide is substantially less. Also at the injection site, whereas the waveguide must be curved in order to accept light from outside the waveguide, light is in fact continuously lost from any curved part of the waveguide immediately downstream of the actual injection site.

These disadvantages are overcome by using a fiber optic device according to one aspect of the invention comprising a transparent body and a passage extending through the body, the passage having an angled section therein, the apex of the angle being sufficiently sharp that light propagating along a waveguide retained within the passage is emitted as a beam at the angle.

The body can have two complementary parts, the passage comprising a locating groove extending along the surface of one of the parts. The groove is preferably V-section and of a depth less than the waveguide diameter whereby the waveguide is securely fixed between the two parts when the parts are brought together. The other body part can have a pair of notches for initially positioning the waveguide in the coupler. The device can include a lockable biasing mechanism to press the one part of the light transparent body down against the other part to clamp the fiber in the groove.

For launching light the device can have a light emitting source positioned to launch light into one end of a pigtail fiber, the other end of the pigtail fiber being positioned to direct light towards a lens. Light from the source is focussed by the lens at the fiber angle. The lens is preferably mounted within the body so as to be very close to the fiber angle, and to focus light precisely at the fiber angle in the center of the fiber core.

For detecting light, a photodetector mounted within or on a surface of the body can intercept a beam of light directed from the fiber angle.

Particularly for coupling light into and out of a waveguide loop the coupler can have two active devices, one device for emitting light into, or detecting light from, the fiber angle in a first direction and the other active device for emitting light into, or detecting light from, the fiber angle in another direction.

The body is preferably made of Plexiglass (Trade Mark). An indexing matching material can be used at the fiber angle to reduce reflection losses. Preferably the coupler is retained within a light-tight enclosure. To ensure that the fiber accurately adapts the desired curvature at the apex of the fiber angled section, there is proposed according to another aspect of the invention, a fiber optic coupler comprising a cylindrical rod, means for mounting the fiber to extend around a part of the circumference of the rod, a transparent body located on a side of the fiber remote from the rod, spring means for pressing the fiber between the rod and the transparent means to bias the fiber to adopt the radius of curvature of the rod and to form an intimate contact between the transparent body and the curved part of the fiber, and a light input or output device positioned to direct light at or receive light from the curved fiber part through the transparent body.

The rod, preferably made of metal, can adhere in a groove formed in a first block. Similarly the transparent body can form part of a second block, the two blocks having complementary surfaces and being mounted relative to one another inside a box fixed with a lid said spring means can be located between the lid and the first block, this arrangement permitting insertion of a fiber between the blocks with automatic applications of the spring bias to the fiber on closing the lid.

A groove can extend around at least said part of the rod circumference for location of the fiber therein. Aligned slots can be formed in one or both of the blocks whereby when the fiber is stretched taut between a pair of the slots, it is aligned with said groove in the rod. The transparent body can be refractive index matched to the fiber outer coating, the outer coating being resilient to ensure the establishment of said intimate contact between the fiber and the transparent body. The transparent body can have a durable surface facing the fiber to prevent scratching, the durable surface provided by a glass upper layer overlying a mass of transparent epoxy, the light input or output device projecting into said transparent epoxy mass. The light input device can be a laser and for a light ouput unit, the device can be a PIN or avalanche photodiode.Alternatively the input or output device can be combined with a fiber optic pigtail for guiding light to or from a detector or source respectively located outside the coupler blocks.

When the blocks are located in a position clamping the fiber therebetween, the complementary surfaces thereof should be spaced from one another whereby the contact location is limited to the fiber contacts with the first and second blocks.

Preferably control means are provided to the pressure means whereby to insure that when splicing two fibers together the light output from the output unit is a maximum for a fixed input intensity and splice site loss.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a longitudinal sectional view of a device for coupling light into a waveguide; Figure 2 is a cross-sectional view on the line ll-ll of Figure 1; Figure 3 is a longitudinal sectional view of a device for coupling light out of a waveguide; Figure 4 is a longitudinal sectional view of a device combining the functions of the Figures 1 and 3 couplers; Figure 5 shows a longitudinal sectional view of a universal device which can be adapted for coupling or detecting light in two directions; and Figure 6 is a perspective view of an arrangement of the Figure 1 and Figure 3 devices used in conjunction with fusion splicing equipment; Figure 7 is a perspective view from the front of a packaging arrangement for a coupler according to the invention; Figure 8 shows a perspective view of a coupler according to the invention; Figure 9 is a cross-section through the device of Figure 8; and Figure 10 is a sectional view on the line X-X of Figure 9.

Referring to Figure 1 in detail a coupler 8 has a Plexiglass block having two parts 10, 12. The two parts have complementary angled surfaces 14, 16 respectively. Extending along the surface 14 of the part 10 is a V-section groove 18 as shown in Figure 2. The groove 18 has a depth of 100 microns and an included angle of 60'. The two parts 10, 12 have planar surface sections meeting at an angle of 146 . At the angle region, the apex of the angle in both of the parts 10 and 12 has a radius of curvature of 125 microns. The lower part 12 has notches 19 which are 300 microns in width and which are vertically aligned with the groove 18.

Extending into the lower block 12 is a bore 22 of 1/8 inch diameter which terminates in a further bore 24 of 1/16 inch diameter. A graded refractive index rod 26 which functions as a converging lens is lodged in the smaller bore and a multimode pigtail fiber 28 with protective jacketing is secured by adhesive within a ferrule which is itself mounted by an epoxy within the wider bore 22. The end of the pigtail fiber projects from the ferrule and is aligned with the axis of the graded refractive index rod 26. The end surface of the rod 26 remote from the fiber adheres using a UV cured adhesive which is refractive index matched to the block 12 to a surface 25 produced by forming a groove within the block part 12, polishing the groove site 25 and filling in the groove except where it is aligned with the rod 26.The other end of the fiber is secured in a position in which it receives the output of a Ga A1As semiconductor laser 29 having an output wavelength of 0.84 microns. In plan view the block is rectangular and has two rods 30 which extend down through the top part 10 of the block and are anchored within the lower part 12. A plate 32 is mounted horizontally at the top of the two rods and a locking pin 34 extends down through the plate 32 and has a lower end engaged within a centrally disposed hole in the top of the block part 10. A compression spring 36 extends between the undersurface of the plate 32 and the top surface of the block part 10 to press the two parts of the block together.To release the two parts from one another, the pin is twisted to a position allowing it to be withdrawn upwardly and partially through the plate 32 and is then twisted to lock the pin 34 in that raised position. The top part of the block can then be reciprocated along the vertical rods 30.

In use an optical fiber 31 is positioned so as to extend between notches 19 and the top body part 10 is slid down the rods 30, and locked against the body part 12 using the resilient pin 34. Because the notches 19 are aligned with groove 18, the fiber automatically locates within the groove as the two body parts come together. The jacketed fiber is 250 microns in diameter and the V-groove is 100 microns in depth so that the fiber projects beyond the groove 18 by about the thickness of the fiber jacketing and is secured against the lower block part 12. A relatively sharply angled region 38 is developed in the fiber, the radius of curvature of the fiber at the fiber angle being of the order of 125 microns.The lens 26 is positioned so as to focus light from the pigtail fiber 28 directly at the fiber angle 38 which is as sharp an angle as possible commensurate with limiting short term stress on the fiber 31 below that which might result in fracture. If the bend in the fiber has a relatively large radius of curvature, then light injected at one point of the fiber is, to some extend, scattered out of the fiber in the curved portion downstream of where the light enters. By ensuring that the injected light is properly focussed at the fiber angle 38 and by ensuring that the apex of the fiber angle 38 is sharp, light loss downstream of the light entry point is minimized. A low wavelength laser is used since it has a greater V number; i.e. the number of modes which can be launched into a fiber is proportional to the inverse of the laser wavelength.In addition, the local launch device is intended to be used with a photocell and one of the more sensitive types of device is the silicon photodetector.

Referring to Figure 3, the detection coupler 37 is similar in many respects to the Figure 1 coupler except that a single 1!2 inch bore 40 is formed in the block part 12 in the plane of the groove 18.

Within the bore is mounted a photocell 42 having a light sensitive surface facing towards the fiber angle 38. Leads 44 from the photodetector are taken through a protective rubber sleeve 4fi which is secured to an end face of the block. The photocell is for example a silicon photodetector/preamplifier combination available from Silicon Detector Corporation under the product number SD-100-41-11-231.

In use, light propagating through the fiber from the left as shown in Figure 3 is emitted from the fiber angle 38 and is directed as a narrow beam 48 towards the photosensitive face of the photocell 42.

Typically the light detected is 25 to 35 dB down on the light level on the upstream side of the coupler.

The detector output level depends for example on whether the plastic coating at the fiber angle 38 is of irregular thickness. Also it is common practice to colour code fibers by colouring the plastic jacket material. Some colouring materials may be more absorbent than others.

Using a low frequency synchronous detection method with a gallium aluminum arsenide source and a highly sensitive silicon detector, a signal level of 75 dBM can be detected.

Referring to Figure 4, there is shown a combination of the devices of Figures 1 and 3 which is adapted for use at a transmission and receiving station located on a fiber optic loop. The device has a detector 40 for monitoring light emitted at the fiber angle from an upstream fiber part 50 and an emitter 26 located to direct light into the fiber angle to propagate through a downstream fiber part 52. Several transmitting and receiving devices can be located around the fiber loop. Timing circuitry can be used in the transmit mode to ensure that data is placed on the loop from a particular station only during an appropriate time slot and in the receive mode to determine from what station particular data originates.To avoid light injected at one station being coupled directly to the detector at that station an inhibiting circuit can be used to inhibit operation of a detector circuit while light is being injected into the loop from its associated light emitting device. With this arrangement, stations can be added and taken away from the loop at any position and at any time. Every part of the fiber is a potential station and is specifically made into a transmit/receive station by clamping that part of the fiber within a device such as that shown in Figure 4. Compensation may be required for the light lost from the loop at each receive station.

Figure 5 shows a universal coupler using the principles of the Figures 1 and 3 couplers. Identical bores 54 within the block part 12 extend away from the fiber angle 38, each of the bores having a flange 56 for engaging a corresponding annular recess 58 in the outside of active device plugs 60,62.

An emitter plug 60 houses a graded refractive index lens (not shown) coupled via a pigtail fiber 64 to a light emitting source 66. A detector plug 62 houses a photodetector having an electrical output on leads 68.

The choice of fiber angle 38 is important since it is a compromise between minimizing beam size and minimizing fiber stress. The fiber included angle of 146 , the angle between the fiber 31 and the axis of the launch or detection device of 24.5 , and the apex radius of 125 microns recited previously are specific values which depend on the relative refractive index of the fiber, the jacket material and the coupler block. They depend also on the ability of the fiber to withstand bending stresses. For other types of fiber, the fiber included angle, the angle between the fiber span 31 and the electrooptic device, and the apex radius will be different.

When using the launch and detection couplers in conjunction with a splicing equipment as shown in Figure 6, the couplers are fixed on either side of the splicing equipment. An upstream fiber 74 is led through the light input or injection coupler into a splicing zone 78 where the end of the fiber is clamped. The downstream fiber 76 is led through a light output or monitoring coupler 37 and its end portion is clamped at the splicing zone 78. In the splicing zone one of the fiber ends can be moved incrementally in x, y and z directions using a micromanipulator unit.

In use, light is launched from the laser into the pigtail fiber housed within shield 88 and at a graded refractive index rod (not shown) is focussed at the core in the fiber angle at the upstream side of the splice site. In turn, light is monitored by a silicon photodetector within the downstream detector unit 37 and directed to a level detection circuit. In a preferred embodiment of the invention, the detector output is used directly to control the movement of the micromanipulator until the light transmission through the splice is maximized. To ensure accuracy of monitoring, the output level of the laser is stabilized and detector sensitivity is related to the laser output level.

For use with fibers having a high index jacket such as acrylate, the fiber angle is chosen to direct a beam of light into and out of the fiber core. Because acrylate has a high refractive index, any light which enters the outer part of the cladding is quickly stripped from the fiber by the acrylate.

Consequently, the only light received at the detector is that which propagates within the core until emitted at the fiber angle.

With a low refractive index jacket material such as silicone, the situation is somewhat different since light can propagate for quite long distances within the cladding region of the fiber. In this situation, the only useful coupler is that used to detect downstream light. To be properly used in a splice loss measuring operation, light must exist only in the core of the upstream fiber and is preferably launched at a remote location upstream of the fiber so that any light launched into the cladding region has been attenuated by the time the splice site is reached. The light detector coupler can then be used to detect that portion of the upstream core light which is launched into the cladding of the downstream fiber.This is made possible by the light in the downstream fiber cladding emerging from the fiber angle at a different angle from that at which core light emerges from the fiber angle.

In fact the light detector can be positioned to detect either one of the core and cladding light from the splice point but it is easier to examine for a minimum in the cladding light than for a maximum in the core light. Fiber alignment at the splice point corresponds to a minimum in the detected cladding light.

Referring in detail to the perspective view of Figure 7 there is shown a packaging arrangement for a coupler according to the invention. The arrangement has upper black plastic, and lower transparent, Plexiglass block parts 10 and 12, the block part 12 being fixed within a box 84 and the part 10 being fixed to a lid 86 for the box. Sides of the box 84 have grooves 90 which are aligned with the notches 19 in the lower part 12. A shield 88 provides electromagnetic screening for the silicon detector or physical protection for a coiled fiber pigtail used in the launch example. The lid 86 is hinged to the box 84 and when the top part 10 is brought against the bottom part 12 provides a light- tight enclosure for the portion of fiber within the box 84.To ensure accurage alignment of the top part 10 in the bottom part 12 the two parts are initially aligned using a pair of rods (not shown) which extend through bores within the top part into corresponding bores in the bottom part 12.

Located in this position, the lid is brought down against the adhesive covered rear surface of the top part 10 and the two rods are then removed from the package. This provides an automatic alignment mechanism for the two transparent block parts. A latch 92 having an operating plunger 94 is used to render the box light tight. The box is dimension so that when closed, the lid presses down on the fiber sufficiently to distort the acrylate jacketing without unduly stressing the fiber itself.

The layer of adhesive between the lide 86 and the block part 10 ensures that the desired pressure on the fiber is not exceeded.

Referring to Figures 8 and 9, the unit shown has an upper metal block part 110 and a lower metal block part 112, the block part 112 being fixed within a box 114 and the part 110 being fixed to a lid 116. A compression spring 150 extends between the block 110 and the lid 116. Extending into the sides of the box are slots 118 which are aligned with corresponding slots 120 within the lower block 112.

As shown in sectional view in Figure 9, the two blocks 110, 112 have generally complementary angled surfaces 122, 124. Extending along the apex and protruding from the angled part of the top block 110 is a cylindrical metal rod 126 which adheres in a cylindrical groove 128 along the apex. A groove 130 (Figure 10) having a depth of 100 microns and an included angle of 60 extends around the rod 126 in alignment with the slots 120.

The lower block has angle surfaces meeting at 146 with a light input or output device 140 aligned to a line projecting from the apex of the surfaces at an angle of 7.5 below the horizontal and 24.3 below the top surface of a glass plate 134. The rod 126 has a radius of curvature of 2.8 millimeters.

The metal lower block 112 has a column 132 of transparent epoxy and a top glass plate 134 flush with the angle face. Extending into the lower block is a bore 136 of 1/4 inch diameter which terminates in a further bore 138 of 1116 inch diameter. A graded refractive index rod 140 which functions as a converging lens is lodged in the smaller bore 136 and a multimode pigtail fiber 142 with protective jacketing is secured by adhesive within a ferrule 144 which is itself mounted by an epoxy within the wider bore 136. The end of the pigtail fiber projects from the ferrule 144 is aligned with the axis of the lens 140. The end of the lens remote from the fiber projects into the epoxy mass 132.The other end of the fiber 142 is secured in a position in which it receives the output of a GaAslGaA1As semiconductor laser 146 (not shown) having an output wavelength of 0.84 microns.

In use an optical fiber 148 having a resilient jacket 149 is positioned so as to extend between the slots 120 whereby it is aligned to the groove 130. The lid 116 is then biassing the fiber 148 by means of the spring 150 between the top block 110 and the bottom block 112. The arrangement provides a light-tight enclosure for the portion of the fiber 148 within the box 114. The pressure set on the fiber is determined by the stiffness of spring 150 and the depth of two cylindrical holes 151, 152 in the lid and the upper block 110. The force should not exceed 1 kg otherwise damage to fiber coating 149 occurs. A minimum force of 500 grams is required in order to ensure intimate contact of resilient fiber coating 149 with top glass plate 134.

At this pressure, the rod 26 presses the fiber 148 against the planar faces of the lower block immediately adjacent to the angle. This has two effects.

Firstly the fiber is caused to adopt the curvature of the rod outer circumference where it contacts the rod at groove 130, and secondly, the resilient coating 149 on the fiber is deformed slightly so as to effect an intimate contact between the fiber and the glass plate 134. The epoxy 136, the glass of plate 134, and the fiber coating material 149 are index matched to one another so that there is mini mal light lost from reflection. The glass plate is 1 millimeter thick and the epoxy is available under the tradename EPO + TEK 301-2, being an opitically clear epoxy whose index is close to that of the glass and the UV curabie acrylate fiber coating.

At the rod 26 a relatively sharply angled region is developed in the fiber, the radius of curvature of the fiber being that of the rod. The lens 140 is positioned so as to focus light from the pigtail fiber 142 directly at the fiber angle which is as sharp an angle as possible commensurate with limiting short term stress on the fiber 148 below that which might result in fracture. If the bend in the fiber had a relatively large radius of curvature, then light injected at one point of the fiber would to some extent, be scattered out of the fiber in the curved portion downstream of where the light enters.By ensuring: (i) that the injected light is properly focussed at the fiber angle; (ii) the apex of the fiber angle is sharp, and (iii) the axis angles of 140 and 142 are 7 5G below horizontal (i.e. 24.55 below the angle of the glass plate surface) light loss downstream of the light entry point is minimized. A low wavelength laser is used since the number of modes which can be launched into a fiber is proportional to the inverse of laser wavelength. In addition, the local launch device is intended for use with a photocell using particularly sensitive silicon photodetectors which have a peak response in the low wavelength regime.

In a corresponding detector unit (not shown) the laser and the lens 140 are replaced by a photocell which can be sited either within the coupler or at the remote end of a pigtail fiber such as fiber 142.

In the operation of the Figure 8 unit the fiber retained between the two blocks accurately adopts the desired curvature at the apex of the angle. The light input device in the case of a local launch unit functions to focus light at a point which assumes a close contact between the fiber and the protruding angle. If the fiber does not in fact contact the block over the critical part of the protruding angle then light launched into the fiber core is not maximized.

Similarly detection of maximum light by the local detection unit assumes close contact between the fiber and the protruding angle. Aiso, if the fiber is clamped in a position such that the angle through which the fiber is bent adjacent to the protruding angle is greater than 146 then the fiber is subjected to more stress than it need be.

Claims (30)

1. A fiber optic coupler comprising a transparent body and a passage extending through the body, the coupler characterized by said passage having an angled section (38) therein, the apex of the angle being sufficiently sharp that light propagating along a waveguide (31) retained within the passage is emitted as a beam at the angle (38).

2. A coupler as claimed in claim 1 further characterized in that the body has two complementary parts (10, 12) the parts having opposed surfaces (22, 24), the passage being a groove (18) extending along one of the opposed surfaces.

3. A coupler as claimed in claim 2 further characterized in that the groove (18) has a V-section.

4. A coupler as claimed in claim 2 further characterized in that one of the body parts (10) is hinged relative to the other body part (12).

5. A coupler as claimed in claim 2 further characterized by adjustment means (32, 34, 36) for moving one of the body parts relative to the other body part from a position in which the opposed surfaces are spaced apart to a position in which the opposed surfaces are abutting.

6. A coupler as claimed in claim 1 further characterized in that one of the body parts (12) houses a means (42) for detecting light, said means (42) located to intercept a beam emitted from a fiber angled part (38) located at said angled section.

7. A coupler as claimed in claim 6 further characterized in that the detection means (42) is a photodetector mounted within a bore (40) one of the body parts (12).

8. A coupler as claimed in claim 1 further characterized in that a means for emitting light (29, 26) is located to direct light at an angled part of the fiber (31) located at the angled section (38) of the passage.

9. A coupler as claimed in claim 8 further characterized in that the light emitting means comprises a graded refractive index lens (26), the lens coupled to a pigtail fiber (28), a remote end of the pigtail fiber (28) being coupled to a semiconductor laser (29).

10. A coupler as claimed in claim 1 further characterized in that the transparent body (12) has a first bore (54) thererin located to intercept a beam emitted from the angled part of the fiber after propagation in a first direction along the fiber (50, 52) and a second bore (54) located to intercept light emitted from the fiber at said angled part following propagation of light in a reverse direction in the fiber (50, 52).

11. A coupler as claimed in claim 10 further characterized in that the bores (54) have first engagement means (56) for engaging with a second corresponding engagement means (58) associated with an active device plug unit (60, 62) whereby to retain the plug unit (60, 62) in one or other of the bores (54).

12. A coupler as claimed in claim 11 further characterized in that the active device plug (60) is adpated to emit light.

13. A coupler as claimed in claim 11 further characterized in that the active device plug (62) houses a light detection means.

14. A coupler as claimed in claim 1 further characterized in that at the angled section (38), the fiber (31) has a radius of curvature at the apex of the fiber angle in the range 50 microns to 250 microns.

15. A coupler as claimed in claim 1 further characterized in that the coupler elements are confined within a light-tight enclosure (84, 86).

16. A coupler as claimed in claim 7 further characterized in that a photodetector circuit is located within an electromagnetic shield (88) mounted adjacent the body.

17. A coupler as claimed in claim 9 further characterized in that the laser (29) and the pigtail fiber (28) are located within a protective housing (88) mounted adjacent to the body.

18. A coupler as claimed in claim 1 further characterized in that one of the body parts (12) houses a means (42) for detecting light, said means located to intercept a beam emitted from a fiber angled part located at said angled section (28), and in which a means (26, 29) for emitting light is located to direct light at an angled part of the fiber located at the angled section (38) of the passage.

19. A fiber optic coupler comprising a first body, means for mounting a fiber having a core, a cladding and a resilient coating to extend around a part of the first body, a second transparent body located on a side of the fiber remote from the first body, means for pressing the fiber between the first body and the second transparent body characterized in that said first body (126) is a cylindrical rod and the fiber mounting is such that the fiber (148) extends around a part of the circumference of the rod 126, whereby the fiber (148) adopts the radius of curvature of the rod and the coating (149) intimately contacts the transparent body (134), and a light input or output device (140) positioned to direct light at or receive light from the fiber core at the curved fiber part, said light passing through the transparent body (134).

20. A coupler as claimed in claim 19 further characterized in that the rod (126) is a metal rod.

21. A fiber optic coupler as claimed in claim 19 further characterized in that the rod (126) adheres in a groove within a plastic block (110).

22. A coupler as claimed in claim 19 further characterized in that a fiber locating groove (130) extends around at least a part of the circumference of the rod (126) in a plane perpendicular to the axis of the rod (126).

23. A coupler as claimed in claim 19 further characterized by means for setting the pressure on the fiber in a region of said rod whereby to deform the resilient coating (149) of the fiber into intimate contact with the transparent body (134).

24. A coupler as claimed in claim 19 further characterized in that the transparent body (134) has a durable glass surface to contact the fiber (148).

25. A coupler as claimed in claim 24 further characterized in that the glass surface forms one surface of a glass plate (134), the plate overlying a mass (132) of transparent epoxy refractive index matched to said glass plate (134) and wherein the light input or output device (140) projects into the epoxy mass.

26. A coupler as claimed in claim 19 further characterized in that the light input device is a laser having a focussing lens (140) to focus emitted light at the core of the fiber at said curved fiber part.

27. A coupler as claimed in claim 19 further characterized in that the rod (126) is fixed within a first angled block (110) and the transparent body (134) if fixed within a second angled block (112), a protruding angle in the first block (110) projecting into and substantially complementary to a recessed angle in the second block (112), the blocks so mounted that contact between the blocks is lim- ited to a zone immediately adjacent the rod (126).

28. A coupler as claimed in claim 27 further characterized in that the blocks (110,112) are housed within a light tight housing (114, 116).

29. A coupler as claimed in claim 19 further characterized by means for setting the contact pressure between the rod (126), the fiber (148) and the transparent body (134).

30. A coupler as claimed in claim 19 further characterized in that the rod (126) has a diameter of about 3 millimeters.