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).