A relative movement sensor for measuring movement of an object (15) and the sensor relative to each other, the sensor comprising a plurality of lasers (3,5) for generating respective measuring beams and illuminating the object (15) therewith, and a plurality of respective photodiodes (4,6) for measuring changes in operation of each laser cavity caused by reflected measuring beam radiation re-entering the respective laser cavity and the optical wave in the laser cavity. The plurality of lasers (3,5) are operated in a time-multiplexed manner such that only a subset (i.e. one or more) of the lasers is operated at any one time, the remaining lasers being idle. This reduces peak power dissipation and prevents erroneous interference caused by measuring beam radiation from one laser entering the cavity of another laser.