Abstract: In a closed loop type fiber optic gyro the number of waves of a ramp voltage for providing a phase difference between right-handed light and left-handed light which pass through an optical fiber coil is counted, the ramp voltage is converted into a digital value, and the digital value is used as a fraction of the calculated number of waves of the ramp voltage, thereby improving the accuracy of the azimuth angle being measured.

Abstract: A modulated phase difference is detected between two rays of light phase modulated by a phase modulator, having propagated through an optical fiber coil in opposite directions and interfering with each other. A ramp voltage controlled in polarity and frequency in accordance with the detected phase difference is generated. The ramp voltage is applied to the phase modulator to control it so that a maximum phase shift of light thereby is 2.pi. k rad (where k=.+-.1, .+-.2, . . . ). To correct the temperature characteristic of the phase modulator, a peak value control circuit creates a reference voltage corrected by a voltage detected by a temperature sensor and controls the peak value of the ramp voltage by the reference voltage, thereby ensuring that the maximum phase shift of light in the phase modulator is always 2.pi. k rad regardless of temperature changes.

Abstract: In a fiber optic gyro in which a biasing phase modulator and a ramp phase modulator are provided at the one end and the other end of an optical fiber coil, a biasing voltage and a ramp voltage are applied to the phase modulators to induce a phase difference between two rays of light which propagate through the optical fiber coil in opposite directions, the interference light is detected by a photodetector, the phase difference component is obtained from the photodetector output by a synchronous detector, and the synchronous detector output is used to control the polarity and the frequency of the ramp voltage, an external signal is added to the synchronous detector, output to fluctuate the frequency of the ramp voltage, or a component of an image frequency spaced apart from the frequency of the biasing voltage by a value twice the intermediate frequency is removed by a subtractor from the synchronous detector output, thereby improving the scale factor of the fiber optic gyro.