Closed-loop locking of an optical frequency comb to a large ring laser

We report on the frequency locking of a 16 m2 ring laser to a single tooth of an optical frequency comb referenced to a hydrogen maser, obtaining a frequency stability of 1 kHz over several days. In common mode operation, where the counterpropagating laser beams run on the same longitudinal mode index, a sensitivity to rotation of 3×10(-9) relative to Earth's rotation is obtained. To test a proposal to bypass time-varying backscatter-induced readout errors in large ring laser gyroscopes, we have operated the laser on adjacent longitudinal cavity modes. The Sagnac frequency due to Earth's rotation obtained in this fashion was strongly influenced by atmospheric pressure changes because the counterpropagating beams within the cavity are affected differently by geometric cavity fluctuations.