Optical modulation

1. A modulator for modulating an optical signal, the modulator comprising a spacing-controllable etalon comprising at least one sprung micro-mirror suspended over a substrate, at least one insulating stop located between the micro-mirror and the substrate and further comprising a control circuit arranged, in operation, to apply a voltage between the micro-mirror and substrate whereby to displace the micro-mirror from its equilibrium position towards the substrate into a snap-down state in which the sprung micro-mirror is retained in contact with the at least one insulating stop and in which, responsive to a control signal, the control circuit is arranged to release or not release the micro-mirror at successive times calculated to maximise or minimise transmission through the etalon of incident light in successive pulses received from a remote laser source, whereby to modulate the transmitted pulses.

2. A modulator according to claim 1 in which the electrical control circuit is arranged to reduce the voltage applied between micro-mirror and substrate whereby to release the micro-mirror.

3. A modulator according to claim 1 in which the control circuit is arranged to release the micro-mirror responsive to a time of arrival at the modulator of a laser pulse.

4. A modulator according to claim 1 in which the control circuit is further arranged to re-apply the voltage after a pre-determined time responsive to the time of release of the micro-mirror.

5. A modulator according to claim 1 in which the micro-mirror is located in a volume of low atmospheric pressure, preferably substantially a vacuum.

6. A modulator according to claim 1 further comprising a retro-reflector arranged to retro-reflect light transmitted through the etalon back through the etalon.

7. A modulator for modulating an optical signal the modulator comprising a plurality of a spacing-controllable etalons each comprising at least one sprung micro-mirror suspended over a substrate, at least one insulating stop located between the micro-mirror and the substrate, and further comprising a control circuit arranged, in operation, to apply a voltage between respective micro-mirrors and the substrate whereby to displace the micro-mirrors from their equilibrium positions towards the substrate into a snap-down state in which the sprung micro-mirror is retained in contact with the at least one insulating stop and in which, responsive to a control signal, the control circuit is arranged to release or not release the micro-mirror at successive times calculated to maximise or minimise transmission through the etalon of incident light in successive pulses received from a remote laser source, whereby to modulate the transmitted pulses.

8. A modulator according to claim 7 in which the plurality of spacing-controllable etalons are arranged in an array.

9. A modulator according to claim 7 in which the plurality of spacing-controllable etalons are partitioned into distinct areas each of which is separately controllable whereby, in operation, to impress a spatial pattern on light transmitted through the modulator.

10. A modulator according to claim 9 in which the spatial pattern is characterised by at least one of phase and intensity of transmitted light.

11. A spatial light modulator comprising a modulator according to claim 1 .

12. A modulator according to claim 7 comprising an array wherein groups of etalons are controlled separately so as to produce a spatial pattern on the transmitted beam, causing the array to behave as a spatial light modulator.

13. A modulator according to claim 7 further comprising a retro-reflector arranged to retro-reflect light transmitted through the etalon back through the etalon.

14. A modulator according to claim 12 in which the retro-reflector comprises at least one retro-reflective element and in which a plurality of spacing-controllable etalons is located in front of at least one such retro-reflective element.

15. A system for optical communications comprising a modulator according to claim 1 .

16. A system according to claim 15 in which the optical signal is a continuous wave signal.

17. A method of modulating light, the method comprising:

receiving incident light at a surface of a spacing-controllable etalon comprising at least one sprung micro-mirror suspended over a substrate and at least one insulating stop located between the micro-mirror and the substrate, and

applying a voltage between the micro-mirror and substrate whereby to displace the micro-mirror from its equilibrium position towards the substrate into a snap-down state in which the sprung micro-mirror is retained in contact with the at least one insulating stop and in which, responsive to a control signal, the control circuit is arranged to release or not release the micro-mirror at successive times calculated to maximise or minimise transmission through the etalon of incident light in successive pulses received from a remote laser source, whereby to modulate the transmitted pulses.

18. A method according to claim 17 in which the incident light is in the infrared spectrum (1 μm-15 μm wavelengths).

19. A modulator according to claim 1 in which communication is achieved by switching or not switching the micro-mirrors, and a detector detects the presence or absence of modulation.

20. A modulator according to claim 1 in which communication is achieved by switching the micro-mirrors at different times to achieve logic one or logic zero, and a detector detects the presence or absence of modulation.

21. A modulator according to claim 1 in which the at least one insulating stop is dimensioned such that the snap-down state corresponds to a low optical transmission state of the etalon with respect to the optical signal.