Resonant diffraction mechanism, nonreciprocity, and lock-in in the ring-laser gyroscope

Diffraction is shown to play a dramatic role in the frequency characteristics of ring-laser gyroscopes. It is proved theoretically and experimentally that a small misalignment of the cavity leads to a nonreciprocity of the resonant diffraction mechanism. The resonant diffraction losses of the two counterpropagating modes being then different, the asymmetries of their output-power versus cavity-frequency profiles are different. This leads to the existence of a frequency bias between the two modes. Diffraction is also shown to play a fundamental role in the lock-in mechanism. It is indeed experimentally demonstrated that the asymmetric evolution of the lock-in threshold with the laser frequency is governed by resonant diffraction mechanisms.