An acousto-optic later 30 for shifting the frequency of an optical signal comprises a block 32 formed of a dielectric material having an acoustic transducer 34 mounted to a first surface 35 for launching acoustic waves in the block. End face 36, 38 of the block are oriented relative to the first surface such that an optical beam of a selected frequency incident upon one of the end face at a selected angle refracts into the block and is diffracted by the acoustic waves to form an output beam from the block that is collinear with the incident beam and shifted in frequency therefrom. The dielectric block preferably is formed of arsenic trisulphide. The end faces are preferably angled relative to the transducer so that the optical beam incident upon the acousto-optic modulator preferably is parallel to the transducer. The modulator is used in fibre optic ring interferometers which relay on the sagnac effect and are employed as rotation sensors. <IMAGE>
