A sensor is described which allows the detection of rotation with respect to an inertial frame. The sensor is based on the Sagnac effect, which is measured with a fibre-optic ring interferometer. The fibre gyro is characterized by the following features: a phase modulation scheme to obtain a high sensitivity; a single-mode filter to achieve low drift; depolarized light in order to get a constant scale factor; light with short coherence length to avoid noise due to backscattering.