Abstract: A ring laser comprises a closed optical path defined by two mirrors and a partially-reflecting surface, and one arm of the path includes a laser active medium which produces two counter-rotating beams. The partially-reflecting surface diverts part of each beam by way of an optical energy reflecting device so that the two diverted beams intersect in a plane to produce an interference pattern. An optical grating of similar period to the pattern is located adjacent to and parallel to the plane of the interference pattern, and a detector is located so as to detect movement of the resulting Moire fringe pattern.