Abstract: A method and apparatus are disclosed for closed-loop phase modulation, intensity demodulation, control, and estimation of the non-reciprocal phase shift induced in a rotation sensing interferometer with a reciprocal phase modulator near one end of a closed light path. A bias modulation signal and a feedback modulation signal are generated, and the two signals are summed to form a summed signal which is applied to the phase modulator. The step transition frequency of the bias modulation signal is chosen so as not to fall on even harmonics of the proper frequency f.sub.p =1/2.tau..sub.o. The feedback modulation signal can consist of any periodic amplitude-modulated square wave with spectral frequency components at harmonics of the bias modulation step transition frequency. The spectral power of the feedback modulation signal must be distributed across frequencies other than even harmonics of the proper frequency f.sub.p.