Phase modulation of a ring-laser gyro - Part II: Experimental results

Experimental results are presented for the multimode ring-laser gyro with intracavity phase modulation. By proper choice of modulator frequency, the oppositely directed traveling waves (ODTW) exist as either pulse trains or FM signals. Conditions have been achieved, depending upon the time at which pulses arrive at the modulator, in which lock in between ODTW does not occur at low rotation rates. This result is not adequately explained, but is clearly useful for rotation sensing. In this condition, rotation sensitivity is noise limited and shows an improvement of two orders of magnitude over the same system without modulation. When lock in does occur, it is accompanied by a hysteresis effect that is explained in terms of an optical damage effect in the KDP modulator crystal. In the FM region of operation, the beat frequency between ODTW contains an offset that drifts with time and so this region of operation is not as useful for rotation sensing as is the pulse-train region.