Method of providing duplex optical communications and optical modulator therefor
Methods of providing duplex free-space optical, communication comprising receiving a time-shift keying (TSK) encoded signal and selectively re-modulating—and optionally retro-reflecting —received TSK pulses so as to transmit an on-off keying (OOK) signal wherein modulation is achieved by operating a micro-opto-electronic mechanical system (MOEMS) device having a oscillation period, the difference in timing between logic 1 and logic 0 pulses of the TSK encoded signal being such that each pulse arrives at a time within a single MOEMS device oscillation period chosen to ensure high or low transmissivity through the MOEMS device independent of incident TSK encoded signal pulse value (0 or 1).
1. A method of providing duplex optical communication comprising receiving a time-shift keying encoded signal and selectively modulating received pulses so as to transmit an on-off signal wherein the step of selectively modulating comprises operating a MOEMS device having an oscillation period (t c ) and the differences in timing between logic 0 and logic 1 of the time-shift keying encoded signal are such that arrival time of each pulse within a single MOEMS device oscillation is chosen, responsive to angle of incidence of the time-shift keying encoded signal and to start time of the oscillation to correspond to high or low transmissivity through the MOEMS device independent of time-shift keying encoded signal pulse value.
2. A method according to claim 1 in which the modulator selectively reflects received pulses.
3. A method according to claim 1 in which the modulator selectively retro-reflects received pulses.
4. An optical signal modulator comprising a selectively transmissive element arranged, in operation, to selectively modulate successive pulses of an incident time-shift keying encoded signal whereby to provide an on-off encoded signal wherein the step of selectively modulating comprises operating a MOEMS device having an oscillation period (t c ) and the differences in timing between logic 0 and logic 1 of the time-shift keying encoded signal are such that arrival time of each pulse within a single MOEMS device oscillation is chosen, responsive to angle of incidence of the time-shift keying encoded signal and to start time of the oscillation to correspond to high or low transmissivity through the MOEMS device independent of time-shift keying encoded signal pulse value.
5. An optical modulator according to claim 4 further comprising a reflector arranged to reflect selectively modulated successive pulses.
6. An optical modulator according to claim 5 in which the reflector is a retro-reflector.
7. A method according to claim 1 wherein the MOEMS device comprises a micro-mirror having the oscillation period.
8. A method according to claim 1 wherein the MOEMS device comprises a Fabry-Perot etalon.
9. A method according to claim 1 wherein differences in timing between logic 0 and logic 1 pulses are equal to an oscillation period of a micro-mirror used in the step of selectively modulating.
10. A method according to claim 1 wherein the step of selectively modulating comprises oscillating a micro-mirror such that the first half (Q 1 ) of single oscillations is interrogated by logic pulses representing a first logic value and the second half (Q 2 ) of single oscillations is interrogated by logic pulses representing a second logic value.
11. A modulator according to claim 4 in which the modulator selectively reflects received pulses.
12. A modulator according to claim 4 in which the modulator selectively retro-reflects received pulses.
13. A modulator according to claim 4 in which the MOEMS device ( 11 ) comprises a micro-mirror ( 10 ) having the oscillation period.
14. A modulator according to claim 4 in which the MOEMS device comprises a Fabry-Perot etalon.