Abstract: A sensor for measuring inertial rotation is provided which includes a passive ring, a means for generating two beams of monochromatic electromagnetic radiation, means for passing the beams into the passive ring to form two countertravelling beams and means adapted to lock the beams entering the passive ring to the cavity resonant frequencies of the countertravelling beams. The amount and direction of inertial rotation is determined by determining the frequency of the beams entering the passive ring effected by the locking means and the frequency shift of the countertravelling beams resulting from the inertial rotation of the passive ring.