Lasers angular rate sensor

A ring laser which includes a laser active medium, two or more optical energy reflecting devices and a partially-reflecting device arranged around a closed path such that laser energy generated in the active medium circulates as two counter-rotating beams around the closed path, the partially-reflecting device being arranged to divert a small part of the energy in each beam out of the closed path, an optical energy reflecting element located so as to direct the diverted parts of both of the counter-rotating beams of energy towards one another to produce an interference pattern in a plane outside the closed path, and of such form that the diverted parts of the counter-rotating beams of energy intersect with one another in the same sense as that in which the counter-rotating beams strike the partially-reflecting device, an optical grating located in a plane adjacent to and parallel to that of the interference pattern and comprising a series of parallel lines spaced from one another at a pitch equal to or slightly different from that of the interference pattern, and a detector arranged to detect the presence and movement of any Moire fringe pattern set up by the relative movement of the interference pattern and the optical grating.