An optical modulator comprising a spacing-controllable etalon having at least one sprung micro- mirror suspended above a substrate. At least one electrically insulating stop is provided between the micro-mirror and the substrate to avoid short-circuit when the micro-mirror is drawn towards the substrate by an applied voltage. An optical detector detects the time of arrival of a first laser pulse. A control circuit predicts from this an arrival time of the next incident laser pulse and, responsive to a control signal, either retains the micro-mirror in its pulled-down state held against the insulating stops or releases the micro-mirror at a time predicted to maximise or minimise the light transmitted through the modulator. After a time interval calculated to permit a predetermined number of mechanical oscillations, the micro-mirror is pulled back down onto the stops.