Optical alignment system

Abstract

There is herein described an alignment system for aligning an optical transmitter and optical receiver for transmission of an optical beam therebetween over free space, the alignment system comprising a plurality of sub-receivers and an adjustment system, where the plurality of sub-receivers are arranged to receive the optical beam from the optical receiver and are spaced apart in a direction perpendicular to the longitudinal axis of the optical beam, where the plurality of sub-receivers are adapted to provide an indication of a characteristic of the optical beam received at the plurality of sub-receivers to the adjustment system, where the adjustment system is adapted to use the indication to determine an adjustment to the relative orientations of the optical receiver and the optical transmitter and where the adjustment system is adapted to output the determined adjustment.

Description

[0001] Optical Alignment System

[0002] Numerous applications require the communication of optical signals over free space, e.g. satellites, drones and shipping. For such communications to be successful it is necessary that the transmitter and receiver are very accurately aligned, and remain so for the duration of the session. This is challenging. It is more challenging still if one or both the transmitter and receiver are moving.

[0003] Fig 1 shows a known apparatus for optical communication over free space. In particular, there is a transmitter 50 and a receiver 51 . At the transmitter 50, an optical signal is transmitted from a laser 1 into a transmission telescope 2. The transmission telescope 2 outputs the signal over free space to a receiver telescope 4 (which has a reverse orientation to the transmission telescope). The signal transmitted by transmission telescope 2 diverges in flight so as to define a cone-shaped beam 7. The receiver telescope 4 focusses the beam 7 onto the proximal end of an optical fibre 6. The optical fibre 6 leads to a photodetector 8. Alignment of transmitter 50 and receiver 51 is achieved by minute adjustment of the orientation of the telescopes 2, 4. In particular, alignment of transmission telescope 2 is achieved by adjustment of the orientation of platform 3. Alignment of transmission telescope 4 is achieved by adjustment of the orientation of platform 5. This adjustment process is challenging.

[0004] It would be desirable to overcome and / or substantially mitigate some or all of the above- mentioned and / or other disadvantages of the prior art.

[0005] According to a first aspect of the invention there is provided an alignment system for aligning an optical transmitter and optical receiver for transmission of an optical beam therebetween over free space, the alignment system comprising: a plurality of sub-receivers; and an adjustment system, where the plurality of sub-receivers are arranged to receive the optical beam from the optical receiver and are spaced apart in a direction perpendicular to a longitudinal axis of the optical beam; where the plurality of sub-receivers are adapted to provide an indication of a characteristic of the optical beam received at the plurality of sub-receivers to the adjustment system; where the adjustment system is adapted to use the indication to determine an adjustment to the relative orientations of the optical receiver and the optical transmitter and / or to a focus level of the optical beam; and where the adjustment system is adapted to output the determined adjustment.

[0006] The characteristic of the optical beam may be the intensity of the optical beam. The plurality of sub-receivers may each comprise an aperture for receiving the optical beam. The apertures of the plurality of sub-receivers may all lie in the same plane, which may be the plane that is perpendicular to the longitudinal axis of the optical beam. Each one of the plurality of sub-receivers may comprise an optical fibre core. The plurality of subreceivers may each comprise an optical fibre core within the same multi-fibre core. The multi-fibre core may be longer than 10cm and may be longer than 50 cm. The multi-fibre core may comprise a plurality of fibre cores extending along the length of the multi-fibre core. The multi-fibre core may comprise one core which is centrally-disposed within the multi-fibre core and a plurality, preferably six, further cores which are circumferentially- disposed around the centrally-disposed core.

[0007] Each one of the plurality of sub-receivers may further comprise a photo detector to which the optical fibre core is optically connected. The plurality of photo detectors may each be electrically connected to the adjustment system. The adjustment system may comprise an adjustment control unit. The adjustment control unit may be further electrically connected to an adjustable transmitter support and / or an adjustable receiver support. The adjustment control unit may be adapted to transmit the determined adjustment to the adjustable transmitter support and / or the adjustable receiver support. The adjustment control unit may be further adapted to instruct the adjustable transmitter support and / or an adjustable receiver support to make the determined adjustment. The adjustable transmitter support may comprise a platform upon which the optical transmitter is mounted, where an angle at which the platform is oriented is adjustable. The angle at which the platform is oriented may be adjustable in three dimensions. The adjustable receiver support may comprise a platform upon which a telescope portion of the optical receiver is mounted, where an angle at which the platform is oriented is adjustable. The angle at which the platform is oriented may be adjustable in three dimensions. The alignment system may be located on a vehicle, such as a satellite, a ship or an aircraft. Alternatively the alignment system may be located at a ground station.

[0008] According to a further aspect of the invention there is provided a method of aligning an optical transmitter and optical receiver for transmission of an optical beam therebetween over free space, the method comprising:

[0009] Providing a plurality of sub-receivers, the plurality of sub-receivers being arranged to receive the optical beam from the optical receiver, where the plurality of sub-receivers are spaced apart in a direction perpendicular to a longitudinal axis of the optical beam; providing an indication of a characteristic of the optical beam received at each of the plurality of sub-receivers; using the provided indication to determine an adjustment to the relative orientations of the optical receiver and the optical transmitter and / or to a focus level of the optical beam, outputting the determined adjustment.

[0010] The characteristic of the optical beam may be the intensity of the optical beam. The determined adjustment may be an adjustment that causes the optical transmitter to be more closely aligned with the optical receiver. The method of the invention may be repeated until the optical transmitter and optical receiver have been aligned to within a pre-determined tolerance. The method of the invention may be repeated until a predetermined illumination pattern is received at the plurality of sub-receivers. The predetermined illumination pattern may comprise illumination of only the centrally-disposed sub-receiver with the optical beam. Alternatively, pre-determined illumination pattern may comprise a different pattern of illumination.

[0011] An embodiment of the invention will now be described in detail, for illustration only, with reference to the appended drawings, in which:

[0012] Fig 1 is a schematic view of an apparatus according to the prior art;

[0013] Fig 2 is a schematic view of an apparatus according to the invention;

[0014] Fig 3 is a schematic view of the end face of a multicore fibre receiving a first sub-optimal illumination pattern; Fig 4 is a schematic view of the end face of a multicore fibre receiving a second sub- optimal illumination pattern;

[0015] Fig 5 is a schematic view of the end face of a multicore fibre receiving a desired illumination pattern;

[0016] Fig 6 is a flow chart showing the steps of a method in accordance with embodiments of the invention.

[0017] Fig 2 shows an apparatus in accordance with the invention. In particular, there is a transmitter 250 and a receiver 251. At the transmitter 250, an optical signal is transmitted from a laser 21 into a transmission telescope 2. The transmission telescope

[0018] 22 outputs the signal over free space to a receiver telescope 24 (which has a reverse orientation to the transmission telescope 22). The signal transmitted by transmission telescope 22 diverges in flight so as to define a cone-shaped beam 27. A difference between the invention and the prior art is that, in the invention, the receiver telescope 24 focusses the beam 27 onto the proximal face of a multi-core optical fibre 26, rather than a single-core optical fibre. Multi-core fibre 26 is a cable containing multiple optical fibre cores that extend in parallel. Each core leads to a respective photodetector 28. Having multiple cores assists in the alignment process as, if the beam received at the multi-core fibre 26 from the receiver telescope 24 is not perfectly aligned with the multicore fibre 26, the different cores within the multi-core fibre 26 will be illuminated to different extents. This difference can be detected at the detectors 28.

[0019] The transmission telescope 22 is supported on adjustable platform 23. Receiver telescope 24 is supported on adjustable platform 25. The orientation of the platforms

[0020] 23 and 25 (and hence the orientation of transmission 22 and receiver 24 telescopes) can be adjusted. This has the effect of changing the pattern of light arriving at the multi-core fibre. The orientation of the platforms 23 and 25 are adjusted until a desired illumination pattern is detected by the detectors 28. In a typical desired illumination pattern the central core is the most intensely illuminated core. A feedback loop is provided for this purpose. This process is controlled by control module 29 which connects electronically to platforms 23 and 25. Figs 3, 4 and 5 are examples of light patterns striking the proximal face of the multicore fibre. In Fig 3 the left-most two cores are maximally-illuminated and so are shown as white. The central three cores are partly illuminated and so are shown with crosshatching. The right-most two cores are not illuminated and so are shown in black. This pattern is sub-optimal as the light is impacting the left side of the face of the multi-core fibre, rather than the centre.

[0021] In Fig 4 the central core is maximally-illuminated and so is shown in white. The six cores surrounding the central core are partly illuminated and so are shown with crosshatching. This pattern is also sub-optimal as the light is spread over all the cores rather than being focussed on the central core.

[0022] In Fig 5 the central core is maximally-illuminated and so is shown in white. The six cores surrounding the central core are not illuminated and so are shown in black. This is the desired illumination pattern as the light is focussed on the central core.

[0023] Figure 6 is a flow chart showing a method according to the invention.

[0024] At step s1, laser 21 at transmitter 250 transmits light into transmission telescope 22.

[0025] At step s2, transmission telescope 22 outputs a defocussed beam 27.

[0026] At step s3, at receiver 251 , the defocussed beam 27 enters receiver telescope 24 which outputs a focussed beam onto the proximal face of multicore fibre 26.

[0027] At step s4, each component core of multicore fibre 26 carries light arriving at it to one of the photodiodes 28.

[0028] At step s5, each photodiode 28 provides an output signal to alignment control unit 29 that is proportional to the amount of light received at the core to which it is connected.

[0029] At step s6 alignment control unit 29 uses the output signals to compare the measured pattern of relative light powers at the photodiodes 28 to the desired pattern of relative light powers at the photodiodes 28. If the measured pattern identical to the desired pattern, the process stops as the desired illumination pattern has been achieved. If the measured pattern is not identical to the desired pattern, alignment control unit 29 calculates the adjustments to the orientation pf platforms 23 and 25 that are required to bring the illumination pattern closer to the desired pattern. The alignment control unit outputs control signals to the transmitter 23 and receiver 25 support units, causing them to make the calculated adjustments to their orientations. The process then returns to step s1 and the process is repeated until the measured pattern is determined to be identical to the desired pattern at step s6.

Claims

Claims1. An alignment system for aligning an optical transmitter and optical receiver for transmission of an optical beam therebetween over free space, the alignment system comprising: a plurality of sub-receivers; and an adjustment system, where the plurality of sub-receivers are arranged to receive the optical beam from the optical receiver and are spaced apart in a direction perpendicular to a longitudinal axis of the optical beam; where the plurality of sub-receivers are adapted to provide an indication of a characteristic of the optical beam received at the plurality of sub-receivers to the adjustment system; where the adjustment system is adapted to use the indication to determine an adjustment to the relative orientations of the optical receiver and the optical transmitter and / or to a focus level of the optical beam; and where the adjustment system is adapted to output the determined adjustment.

2. An alignment system as claimed in claim 1 , wherein the plurality of sub-receivers each comprise an aperture for receiving the optical beam and the apertures all lie in a plane perpendicular to the longitudinal axis of the optical beam.

3. An alignment system as claimed in claim 1 or claim 2, wherein each one of the plurality of sub-receivers comprises an optical fibre core.

4. An alignment system as claimed in any preceding claim in which the plurality of subreceivers each comprise an optical fibre core within a single multi-fibre core.

5. An alignment system as claimed in claim 4, wherein the multi-fibre core may comprise one core which is centrally-disposed within the multi-fibre core and a plurality, preferably six, further cores which are circumferentially-disposed around the centrally-disposed core.

6. An alignment system as claimed in any of claims 3 to 5, wherein each one of the plurality of sub-receivers further comprises a photo detector to which the optical fibre core is optically connected.

7. An alignment system as claimed in any preceding claim, wherein the adjustment system is further electrically connected to an adjustable transmitter support comprising a platform upon which the optical transmitter is mounted, where an angle at which the platform is oriented is adjustable.

8. An alignment system as claimed in any preceding claim, wherein the adjustment system is further electrically connected to an adjustable receiver support comprising a platform upon which a telescope portion of the optical receiver is mounted, where an angle at which the platform is oriented is adjustable.

9. An alignment system as claimed in any preceding claim where the characteristic of the optical beam is the intensity of the optical beam.

10. A method of aligning an optical transmitter and optical receiver for transmission of an optical beam therebetween over free space, the method comprising:Providing a plurality of sub-receivers, the plurality of sub-receivers being arranged to receive the optical beam from the optical receiver, where the plurality of sub-receivers are spaced apart in a direction perpendicular to a longitudinal axis of the optical beam; providing an indication of a characteristic of the optical beam received at each of the plurality of sub-receivers; using the provided indication to determine an adjustment to the relative orientations of the optical receiver and the optical transmitter and / or to a focus level of the optical beam, outputting the determined adjustment.

11. A method according to claim 10, wherein the method is repeated until the optical transmitter and optical receiver have been aligned to within a pre-determined tolerance.

12. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to perform the method according to claim 10 or claim13. A computer-readable medium comprising the computer program of claim 12.

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