Frequency noise of laser gyros

Abstract

Laser gyros are powerful tools used to test the predictions of the general theory of relativity. The precision of a measurement of the rotation rate with a laser gyro is limited by the frequency noise of the beat between two counterpropagating modes of a ring laser. The frequency noise of a single mode of a laser is limited by quantum mechanical constraints because it is related to the maximum precision with which the phase of a coherent state can be measured. If two modes are not correlated, the variance of the fluctuations of the difference of their frequencies is the sum of the variance of the frequency noise of the two modes. If the two modes are correlated, this result does not hold any longer. In this paper, we show that a laser gyro has mechanisms capable of dynamically locking the two modes together without forcing them to the same frequency. The lock of modes decouples the noise of the beat note from the frequency noise of the individual modes, thus allowing the realization of sub-shot noise laser gyros.