Frequency-locking Technology And FPGA Design In Resonator Optic Gyro

Resonator optic gyroscope is a new high-precision angular rotation sensor based on Sagnac effect. The optic ring resonator, as the key rotation sensing element, is implemented with a short sensing fiber coil or an integrated optics. It has the unique advantages for miniaturization and integration. In the resonator optic gyroscope, the angular rotation velocity is measured through the detection of the resonant frequency difference between the clockwise and the counter-clockwise lightwaves counter-propagating in the resonator. There are many kinds of reciprocity noises in the loop as the actual optical devices such as laser source, optic ring resonator, etc subject to environmental factors such as temperature and stress fluctuations, while the resonant frequency difference caused by the Sagnac effect is non-reciprocity and extremely weak. Therefore, the laser frequency must be locked to one of the resonant frequency of the resonator through the modulation and demodulation methods. A high-precision frequency locking method affects the gyro sensitivity directly in the practical gyro system. On the one hand it is essential to establish a good frequency locking loop in order to reduce the impact of reciprocity noise. The detection sensitivity of the frequency locking loop finally depends on the accuracy of the signal processing circuits. Under the above background, a digital signal processing circuit including the digital LIA and the frequency servo loop based on the PI controller is designed and implemented for the resonator optic gyroscope with a 7.9 cm length optical waveguide ring resonator. All the processing circuit is implemented with a single FPGA. Specifically, this paper carried out following research works:By analyzing the characteristics and the transfer patterns of the reciprocity noises in the frequency locking loop, it was found the laser, the optical ring resonator, the photodetector, and the unit gain of the demodulation circuits could be looked as one model. A variable gain K is used to describe the transfer characteristic of this equivalent mathematical model. Thus a simplified loop model for the frequency locking loop is established. A proportional integral controller is introduced into the frequency servo loop instead of the traditional proportional controller. The step response of the steady-state error is eliminated by the PI controller servo loop. The influence of reciprocity noises is effectively reduced. Furthermore, the capturing and tracking features of the loop are analyzed and the loop parameters are optimized to improve the sensitivity of the frequency locking loop.The detection accuracy of the analog lock-in amplifier is limited by the inherent 1/f noise and the temperature drift noise of the analog devices, especially by the noise of the multipliers. A digital lock-in amplifier based on the CORIDC algorithm is presented and fabricated in this paper. The digital processing circuits include the function of LIA and the PI controller. It is implemented with s single FPGA to avoid the 1/f noise and the temperature drift noise of the analog devices. Measurements show that the minimum detectable signal is 45 nV, corresponding to a peak-peak rotation sensitivity of 0.03°/s. The frequency servo loop accuracy is about 0.02°/s, and the step response time is 60 ms.Based on the above analysis and works on the electronic circuits, the whole measurements on the gyro system based on the phase modulation spectroscopy techniques are carried out. An actual rotation signal is observed successfully. Then the gyro output at the rest state is tested for 200 second duration with the integration time of 10 second. The bias error is estimated to -12°/s. A bias stability of 0.1°/s is demonstrated in a waveguide ring resonator with the ring length of 7.9 cm.