A gyroscope that detects rotation by using a ring laser, i.e. a laser consisting of two beams of light with the same polarization but travelling in opposite directions in a ring (loop). It detects rotation by making use of the Sagnac effect. A related device is the fibre optic gyroscope, which is also based on the Sagnac effect but uses an external laser in which the two counter-moving beams are injected into a coiled optical fibre.