WO2010128327A1

WO2010128327A1 - Optical collimator and light collection assembly having a concave reflecting surface

Info

Publication number: WO2010128327A1
Application number: PCT/GB2010/050739
Authority: WO
Inventors: Boris Povazay
Original assignee: University College Cardiff Consultants Ltd
Priority date: 2009-05-08
Filing date: 2010-05-06
Publication date: 2010-11-11
Also published as: GB0907985D0

Abstract

Optical collimators, light collectors and focussing devices using reflective optics. A monolithic construction may be used.

Description

OPTICAL COLLIMATOR AND LIGHT COLLECTION ASSEMBLY HAVING A CONCAVE REFLECTING SURFACE

FIELD OF THE INVENTION

The present invention relates to an optical collimator assembly and a light collection assembly.

BACKGROUND OF THE INVENTION

In existing optical fibre collimators, there is no direct physical contact between the optical fibre emission facet and the input optical face of the collimator. This makes it difficult to reliably reposition an optical fibre within the collimator. This is particularly problematic when the optical fibre emission facet is angled (e.g. FC/APC optical fibre facets which are angled at up to 8° to avoid back reflections) as both the distance between the fibre facet and the input optical face of the collimator and their respective rotational orientations must be precisely controlled. Therefore, when the optical fibre is re-polished and/or replaced within the collimator, an incorrect positioning of the fibre facet with respect to the collimator often results.

Common collimators also use primarily refractive optics (e.g. singlet, doublet, GRIN lenses) which can only physically be designed for limited wavelength ranges. Furthermore, surface reflections which commonly arise from transmittive optics are undesirable.

SUMMARY OF THE INVENTION

The invention is defined in the claims to which reference should now be made.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, certain embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figures la-c show various perspective views of a monolithic block; Figures 2a-d show the monolithic block of Figures la-c in use as an optical collimator where the collimator's interactions with an input light beam are illustrated;

Figures 3a-d show the monolithic block of Figures la-c in use as an optical collimator where the path of an input light beam is illustrated both as collimated and as undisturbed by the collimator;

Figures 4a-c are top elevation views of the monolithic block of Figures 1-3, again in use as an optical collimator;

Figures 5a-c are cross sectional views taken along lines A-A, B-B and C-C as indicated in Figures 4a-c respectively;

Figures 6a-c and 7a-c are side and back elevation views of the monolithic block of Figures 1-5;

Figures 8a-e show various perspective views of a monolithic block similar to that of Figures 1-7 but with a mount for receiving an optical fibre;

Figure 9a shows an optical patch cord terminated with a standard FC/PC connector;

Figure 9b shows the FC/PC connector secured to the mount of the block of Figures 8a-e;

Figure 10a shows an optical patch cord terminated with a standard FC/APC connector;

Figure 10b illustrates a first method of achieving parallel physical contact between an angled optical fibre emission facet and the monolithic block of Figures 8a-e;

Figure 11 illustrates a second method of achieving parallel physical contact between an angled optical fibre emission facet and the monolithic block of Figures 8a-e;

Figures 12a-b show the monolithic block of Figures 8a-e with external reflector(s) for repositioning the collimated beam;

Figure 13 shows the monolithic block in use as a light collector; and Figure 14 shows two monolithic blocks being used in series to pass light from one optical fibre to another.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Figures la-c show various perspective views of a transparent monolithic block 1 which has two flat opposing side faces 2, 3 and a flat top face 4 extending between them. The flat top face 4 also extends between a flat front face 5, which extends orthogonally between the side faces 2, 3, and an off-axis internal parabolic reflector 6. The parabolic reflector 6 extends between the side faces 2, 3 and also between the aft edge of the top face 4 and the lower edge of the front face 5. The internal parabolic reflector 6 conforms to a three dimensional parabolic shape - that is, it appears concave when viewed in both side elevation (see Figure 6a) and in top elevation (see Figure 4a). The parabolic reflector 6 has a focal point 8 which is located on the top face 4 of the block 1. The block 1 is preferably made from a moulded glass material such as fused silica, but other materials such as moulded plastics (or indeed any other optically transparent material) may also be used.

As shown schematically in Figures 2a-d and Figures 3a-d, the monolithic block 1 can be used as an optical collimator. Light is emitted towards the parabolic reflector 6 from a light source 10, such as a laser diode or optical fibre, whose emission facet is positioned at the focal point 8 on the top face 4 of the block 1 and faces the parabolic reflector 6. As shown in Figures 3a-d, the light emitted by the light source 10 diverges to form a light cone 14, a small portion of which (i.e. light not supported by the internal reflection angle) may be lost through the reflector 6 of the block 1. The rest of (or all of) the light is reflected off the parabolic reflector 6 by total internal reflection. As the light is emitted from its focal point 8, it is always reflected in the same direction by the parabolic reflector 6 - i.e. towards the front face 5. Thus, the reflected light forms a substantially cylindrical collimated beam, illustrated as a cylinder 12, which is emitted through the front face 5 of the block 1. As the reflector 6 conforms to a three dimensional parabolic shape, the light is collimated in two dimensions. That is, all of the light rays which are reflected by the parabolic reflector 6 are collimated into the cylindrical beam 12, regardless of the direction in which they diverge from the light source 10 into the light cone 14. To decrease the amount of light lost through the reflector 6 in cases where internal reflection reflects insufficient light (or to protect the surface of the reflector), the external surface of the reflector may be coated with a reflective material such as aluminium.

Figures 4a-c are top elevation views of the monolithic block 1. Figures 5a-c are cross sectional views taken along lines A-A, B-B and C-C as indicated in Figures 4a-c respectively. Figures 6a-c and 7a-c are side and back elevation views of the monolithic block 1 respectively. Figures 4a, 5a, 6a, 7a show in schematic the light cone 14 emitted by the light source 10 and the collimated light beam 12 which is formed by the parabolic reflector 6. Figures 4b, 5b, 6b, 7b show schematically the portion of the light cone 14 which is reflected by the parabolic reflector 6 and the internal portion of the collimated light beam 12. The light beams are omitted in Figures 4c, 5c, 6c, 7c.

Figures 8a-e show various perspective views of a monolithic block Ia which is similar to the monolithic block 1 described above with reference to Figures 1-7. The same reference numerals will be used for identical features. Again, the monolithic block Ia can be used as an optical collimator. In this case, the monolithic block Ia has a mount

20a for receiving an optical fibre 22 (see Figures 9a-b) which carries light from, for example, a laser diode or an LED (not shown). The mount 20a is securely attached to the flat top face 4 of the monolithic block Ia with glue or a UV cured epoxy to ensure that, when the optical fibre is inserted into the mount, the position of its emission facet coincides with the focal point 8 of the parabolic reflector 6. It is noted that the focal point 8 does not have to be precisely located on the top face 4 of the monolithic block.

Indeed, particularly when the optical fibre 22 emits high optical powers, it can be beneficial to locate the focal point 8 either slightly above or below the top face 4 to prevent the destruction of the optical fibre facet in the event that dirt or dust particles foul the facet or the top face during use.

Referring to Figure 9a, the optical fibre 22 is housed within a patch cord terminated with a standard FC/PC optical connector 26a. The FC/PC connector 26a comprises a cylindrical ferrule 28 which has a central bore for housing a distal end of the optical fibre. The central bore extends through the front face 28a of the ferrule 28 such that the emission facet of the optical fibre 22 is flush with the front face 28a of the ferrule 28. The FC/PC connector has substantially hollow concentric inner and outer cylindrical casing elements 29, 30 and a screw cap 31 which is slidably attached to the outer surface of the outer casing element 30. The ferrule 28 protrudes from the centre of the inner casing element 29. The outer surface of the inner cylindrical casing element 29 has a nipple 32 which is used for positioning the FC/PC connector in the mount 20. Referring back to Figures 8a-e, the mount 20a has substantially hollow concentric inner and outer cylindrical receptacles 33, 34. The outer cylindrical receptacle has a threaded outer surface 35 and a notch 36. To secure the FC/PC connector to the mount, the nipple 32 is inserted into the notch 36 in the outer receptacle 34 and the ferrule 28 is inserted into the inner receptacle 33. Meanwhile, the inner casing element 29 is slotted between the inner and outer receptacles 33, 34 and the screw cap 31 is slid over the outer casing element 30 towards the ferrule 28 and screwed onto the threaded outer surface of the outer receptacle 34.

To guarantee that they are precisely aligned, the FC/PC connector and the mount 20a are sized to ensure that the emission facet of the optical fibre (and consequently the front face 28a of the ferrule 28) and the flat top face 4 of the block Ia are brought into direct parallel physical contact when the screw cap 31 is fully screwed onto the mount 20a (see Figure 9b). This allows the optical fibre patch cord to be replaced simply by disconnecting the FC/PC connector from, and reconnecting it to, the mount 20a. No further active alignment procedures are required. Consequently, the resultant collimated beam is highly reproducible, even if the optical fibre patch cord is replaced.

Referring now to Figure 10a, the optical fibre patch cord may alternatively be terminated with an FC/ APC connector 26b. The FC/ APC connector 26b is similar to the FC/PC connector 26a described above and the same reference numerals will be used for identical features. The FC/ APC connector is identical to the FC/PC connector with the exception that both the front face 28b of the ferrule 28 and the emission facet 22b of the optical fibre are angled at approximately 8° to the principle optical axis of the optical fibre to minimise back reflections. To guarantee the alignment between the emission facet 22b of the optical fibre and the top face 4 of the monolithic block, they are again brought into direct parallel physical contact. In this case, the angled surface of the optical fibre emission facet 22b must also be accounted for. One method of doing this is illustrated in Figure 10b. Here, the upper surface 40 of the mount 20b is inclined with respect to the flat top face 4 such that the optical fibre 22 is inserted into the mount at an angle of 8° to a line D perpendicular to the flat top face 4 of the block Ib. This allows a parallel physical contact to be achieved between the flat top face 4 of the block Ib and the angled emission facet 22b of the optical fibre 22. Note that, in this case, the shape of the parabolic reflector 6 must be adapted to account for the angled input of the light from the emission facet 22b of the optical fibre in order to achieve a centrally symmetric collimated beam. Note also that, as before, the emission facet 22b of the optical fibre 22 is positioned at the focal point of the parabolic reflector 6 and the central (chief) ray is taken care of accordingly by the tilt of the reflector 6.

Clearly, to guarantee a parallel physical contact between the emission facet 22b of the optical fibre 22 and the top face 4 of the block Ib, the FC/APC connector must be inserted into the mount with a predefined rotational orientation which matches the incline of the mount 20b. To ensure that the FC/APC connector has the required rotational orientation, the fibre-carrying ferrule of the FC/APC connector is set in place by the nipple 32 on the outer surface of the inner cylindrical casing element 29 which, as above, fits into the corresponding notch (not shown) on the mount 20b. The screw cap 31 can be screwed onto the mount 20b fully only if the nipple 32 is aligned with the notch - i.e. only if the optical fibre connector has the required rotational orientation. This again minimises the requirement for active alignment of the ferrule with the collimator and ensures that the resultant collimated beam is highly reproducible even when the optical fibre is replaced.

Another method of accounting for an angled optical fibre emission facet is illustrated in Figure 11 which shows a monolithic block Ic which is similar to the monolithic block 1 described above with reference to Figures 1-7. The same reference numerals will be used for identical features. Here, the top face 4c of the monolithic block Ic is inclined from the front face 5 down towards the parabolic reflector 6 at an angle of 8° to a line E (see Figure 11) which is perpendicular to the front face 5. To account for the incline on the top face 4c of the monolithic block Ic, the lower surface 42 of the mount 20c is also inclined correspondingly. If the optical fibre connector is inserted into the mount 20c with the required rotational orientation, a parallel physical contact is achieved between the emission facet 22b of the optical fibre and the inclined top face 4c of the monolithic block Ic. As before, the rotational orientation at which the optical fibre connector is inserted into the mount 20c is set by a nipple (not shown) which fits into a notch (not shown) on the mount 20c. This again minimises the requirement for active alignment of the ferrule. As before, it is again noted that the end facet 22b of the optical fibre 22 is positioned at the focal point of the parabolic reflector 6. It is noted that any other optical fibre connector could be employed, in which case the mount and, if necessary, the monolithic block would be adapted for the particular type of connector.

To alter the position of the collimated beam 12 and/or the direction in which the collimated beam 12 propagates, one (see Figure 12a) or more (see Figure 12b) reflector(s) may be positioned outside the monolithic block 1. In Figure 12a, one reflector 50 is positioned proximate the front face 5 of the block 1 to redirect the collimated beam 12. In Figure 12b, two further reflectors 51, 52 are positioned to further alter the position of the collimated beam 12.

In the embodiments shown in Figures 8-14, the monolithic blocks la-Id are formed from a material (such as fused silica) with a refractive index which matches that of the core of the optical fibre 22 or for which the refractive index difference is compensated by the shape of the reflector 6. This minimises the build up of unwanted optical reflections which can otherwise arise at the interface between the fibre and the top face of the collimator block. It is noted, however, that this matching of materials is unnecessary if the emission facet of the optical fibre is a standard fibre end as there are no chromatic effects at the beam waist (the angle of incidence is zero). The front face 5 of the monolithic block may be tilted, or otherwise altered, again to minimise the build up of unwanted optical back reflections. Alternatively, an anti-reflection coating may be applied to the front face 5.

As well as being suitable for use as an optical collimator, the monolithic blocks 1-lc, may also be used for light collection. This is illustrated in Figure 13 which shows a monolithic block Id which is similar to monolithic blocks Ia-Ic described above. The same reference numerals will be used for identical features. The monolithic block Id has an optical fibre mount 2Od attached to its top face 4d. In this case, a collimated light beam 12d incident on its front face 5d is focussed by the parabolic reflector 6 onto its focal point 8 on the top face 4d. As above, the position at which the mount 2Od holds the optical fibre facet 22d (substantially) coincides with the focal point 8. Therefore, the collimated light beam 12d will be focussed onto the optical fibre facet 22d (which thus becomes a collection facet rather than an emission facet), thereby maximising the quantity of light collected by the optical fibre.

It is noted that, in this case, the optical fibre/optical fibre mount may be replaced with a detector, such as a photodiode to convert the optical signal to an electrical signal.

Other than the convenience it provides in terms of minimising active alignment, one of the main benefits of the collimator/light collection assemblies described above is that the monolithic block is largely wavelength independent. However, it is noted that if more than one wavelength is to be collimated/collected simultaneously and unwanted back reflections are suppressed by tilting input/output surfaces, an optical wedge made from the same/similar material to the monolithic block may be included at either the top face or the front face of the block to compensate for any unwanted chromatic effects.

Another major benefit of this design is that the monolithic block may be manufactured at low cost using moulding techniques. The monolithic block is also highly mechanically and temperature stable.

Divergent light emitted by a source can be regarded as diverging from a focal point and convergent light emitted by a source can be regarded as converging towards a focal point. In the case where the source is an optical fibre emission facet that to an approximation is a point source, the focal point is coincident with the emission facet. In the foregoing embodiments, the focal point of the optical fibre emission facet is made substantially co-incident with the focal point of the parabolic reflector, meaning that the emission facet is substantially coincident with the focal point of the parabolic reflector. In other embodiments however, a convergent (or divergent) light source with an emission surface displaced from its focal point can be used. In such a case, the focal point of the source, rather than its emission surface, is made substantially coincident with the parabolic reflector's focal point.

As well as being used to collimate/collect light, similar monolithic block assemblies to those described above may be used for passing light from a transmitting optical fibre with a first numerical aperture/core size into a collecting optical fibre with a second numerical aperture/core size. For example, but not exclusively, such a monolithic block assembly may be used to transmit light from a single mode fibre terminated in an FC/PC connector to a single mode fibre terminated in an FC/ APC connector. This is illustrated in Figure 14. In this case, a first monolithic block 60 is positioned with its front face 66 in parallel physical contact with the front face 67 of a second monolithic block 61. The emission facet 62 of the transmitting optical fibre 63, which is terminated in an FC/PC connector, is positioned at the focal point of the parabolic reflector 64 of the first monolithic block 60. Light 65 emitted from the transmitting optical fibre 63 is reflected (and collimated) through the front face 66 of the first block 60 and into the front face 67 of the second block 61. This collimated light 68 is focussed by the reflector 69 of the second block 61 towards its focal point on the top face of the second block 61. The collecting optical fibre 70, which is terminated with an FC/APC connector, is held in position by the second block's optical fibre mount 71 to collect the focussed light 72 at the focal point of the reflector 69. This allows the light to be passed from one optical fibre to another without incurring significant losses. Note that, in a similar way to the embodiment described above with respect to Figure 11, the inner surface of the mount 71 and the lower face 73 (in the view of Figure 14) of the block 61 are tilted to ensure parallel physical contact between the face 73 of the block and the ferrule of the FC/APC connector. As an alternative to this method, it may be preferable to use only the first single monolithic block 60 but to substitute the parabolic reflector 6 with an ellipsoidal reflector. As an ellipsoid has two focal points, an ellipsoidal reflector would be able to both collimate the light diverging from the optical fibre held by the FC/PC connector and also focus the light into the collecting optical fibre held by the FC/APC connector. In this case, the collecting FC/APC connector would be positioned on the front face 66 of the monolithic block. As well as the fibre coupled designs described above, the monolithic blocks 1-ld may also be used for modular optics where the optical elements are optically contacted with each other (i.e. to convert, focus, diffract, refract or passively modulate or alter the beam).

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. An optical collimator assembly comprising: an optical collimator having a concave reflective surface with a focal point; and a mount adapted to receive a light source having an emission surface having a focal point; wherein the mount is arranged to receive a light source with the emission surface facing the reflective surface and with the emission surface's focal point substantially coincident with the reflective surface's focal point.

2. The optical collimator assembly of claim 1 wherein the collimator is monolithic.

3. The optical collimator assembly of claim 1 or 2 wherein the mount is arranged to direct divergent light from the emission surface of a received light source into said collimator for internal reflection from the reflective surface to collimate light from the emission surface.

4. The optical collimator assembly according to any one of claims 1, 2 or 3 wherein the mount is arranged to bring the emission surface of a received light source into parallel physical contact with an external surface of the collimator.

5. The optical collimator assembly according to claim 4 wherein the external surface of the collimator has an inclined surface with respect to a principle optical axis of a received light source.

6. The optical collimator assembly of any preceding claim wherein the mount is fixedly coupled to an external surface of the collimator.

7. The optical collimator assembly of any preceding claim further comprising an optical wedge.

8. A light collection assembly comprising: a light focussing element having a concave reflective surface with a focal point; and a mount adapted to receive a light collector having a light collection surface having a focal point; wherein the mount is arranged to receive a light collector with its collection surface facing the reflective surface and with the collection surface's focal point substantially coincident with the reflective surface's focal point.

9. The light collection assembly of claim 8 or 9 wherein the light focussing element is monolithic.

10. The light collection assembly of claim 9 wherein the mount is arranged to collect light in the collection surface of a received light collector, the light having been focussed by internal reflection from the reflective surface of said focussing element.

11. The light collection assembly of any of claims 8 - 10 wherein the mount is arranged to bring the collection surface into parallel physical contact with an external surface of the light focussing element.

12. The light collection assembly according to claim 11 wherein the external surface of the light focussing element has an inclined surface with respect to a principle optical axis of a received light collector.

13. The light collection assembly of any of claims 8-12 wherein the mount is fixedly coupled to the light focussing element.

14. The light collection assembly according to any of claims 8-13 wherein the mount is adapted to receive a transducer for converting the collected light into an electrical signal.

15. The light collection assembly of any of claims 8-14 further comprising an optical wedge.

16. The assembly according to any preceding claim wherein light is reflected by the reflective surface by total internal reflection.

17. The assembly according to any preceding claim wherein the reflective surface has a reflective coating.

18. The assembly of any preceding claim wherein the mount is arranged to receive an optical fibre.

19. The assembly according to any preceding claim wherein the reflective surface conforms to a three dimensional parabolic shape.

20. The assembly according to any of claims 1- 18 wherein the reflective surface conforms to a three dimensional elliptical shape.