Mode locking in the ring laser gyroscope: Reduced threshold for two cavity modes

Abstract

A ring laser gyroscope is a device which employs a ring laser to measure rotation. A ring laser supports two beams propagating in opposite directions around the ring resonator. When the gyroscope is rotated, the frequencies of the two beams split by an amount proportional to the rate of rotation: the device works by measuring this frequency splitting. The main problem of ring laser gyroscope design is the frequency synchronisation - lock-in - of the two beams at low rates of rotation. Lock-in arises from weak mutual coupling caused by backscattering at the mirrors and results in a dead band around zero. One of the possible solutions to this problem is a two-mode ring laser in which two modes oscillate simultaneously and interact to reduce the dead band. The present work reviews the theory of lock-in and offers a theoretical basis for this approach, as well as providing experimental evidence to support it.