Abstract: 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).

Abstract: 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. Related apparatus and signals are also provided.

Abstract: A method of synchronising an optical transmitter and modulator comprising transmitting a calibration sequence of pulses from the transmitter to the modulator; varying a control timing drawn from the set comprising the transmit interval timings and the modulator transmission interval timings; monitoring the resulting pulses transmitted by the modulator and selecting a preferred control timing; controlling the optical transmitter and/or modulator responsive to the selected control timing. Corresponding apparatus, systems, signals and programs for computers to implement or control the arrangement are also provided.