Residual amplitude modulation cancellation in passive Resonant Fiber Optic Gyroscope

In a resonator fiber optic gyroscope (RFOG), a residual amplitude modulation (RAM) may appear with the light phase modulation used to generate the error signals, which are necessary for locking the frequency of the laser on the optical cavity resonance and measure the angular velocity. The RAM causes an unstable bias on the resonance frequency measurement and thus limits the gyro performances. A well-known method to suppress the RAM was introduced by Wong and Hall in 1985. The intensity of the modulated beam is monitored, before entering the resonator, to generate a continuous voltage controlling the natural birefringence of the modulator crystal. We show that this technique seems to be not adapted to setup with an imperfect polarizer (having a limited extinction ratio) at the output of phase modulator followed by component exhibiting polarization dependent losses. To counter this limit, we propose a new strategy and we illustrate its effectiveness with two types of phase modulator to probe a cavity resonance in transmission and reflection.