A fiber optic rotation sensor, employing the Sagnac effect comprising all fiber optic components positioned along a continuous, uninterrupted strand of fiber optic material. The rotation sensor includes a detection system utilizing a modulator or modulators for phase modulating at first and second harmonic frequencies light waves which counter-propagate through a loop formed in the fiber optic strand. Each modulator is operated at a specific frequency to eliminate amplitude modulation in the detected optical output signal.A phase sensitive detector generates a feedback error signal proportional to the magnitude of the first harmonic in the output optical signal. The feedback error signal controls a modulator which controls the amplitude of the second harmonic driving signal for the second harmonic phase modulator such that the first harmonic component in the output signal from the rotation sensor is cancelled or held within a small range of amplitudes.