Abstract: An optical gyroscope includes an optical fibre ring or coil (2) around which light may propagate in clockwise and counter-clockwise directions, means (1, 3, 4, 5, 6) for inputting a beam of light in either direction into the coil (2), a phase modulator (7) for generating and applying a phase modulation at a modulation frequency between the clockwise and counter-clockwise beams, a detector (8) for detecting the combined clockwise and counter-clockwise beams and processing means for monitoring the intensity of the output of the detector means (8) at a plurality of integral multiples of the modulation frequency to determine the rate applied to the gyroscope.