GB2017392A - Phase-locked loop laser gyroscope - Google Patents

Abstract

A laser gyroscope has a phased-locked loop circuit 5, Fig. 1, coupled to a frequency difference output from a laser gyroscope cavity 110. Waves of four frequencies propagate around the gyroscope cavity, each characterised by one of the two different senses of circular polarization and one of the two different directions of propagation. Output signals DELTA f1 and DELTA f2 are formed each having a frequency equal to the difference between the frequencies of a corresponding wave pair. The frequency difference signal DELTA f1 for one wave pair is phase-locked to an external reference source 118, Fig. 3. A signal applied to a Faraday rotator bias coil 114 provides the frequency shifting of the wave pairs necessary to achieve a constant frequency difference DELTA f1. The amplitude of the signal applied to the Faraday bias coil can be employed as a precise measure Vout of the rate of rotation. Alternatively, a second, external phase-locked loop, Fig. 4, includes a voltage controlled oscillator 138 followed by a frequency divider 135 whereby the oscillator frequency divided by N is phase locked to DELTA f2. The output of the voltage controlled oscillator provides an output signal Vout which has a frequency in proportion to but many times higher than the frequencies of the wave pair difference signals. in a further alternative not controlling the frequency difference in the laser, the control signal for the voltage controlled oscillator 138 provides the measure of rate of rotation. Other embodiments employ phase- locked loops only of the external type. <IMAGE>