| 摘要 |
A photodetector generates an electric signal by receiving a light signal which is obtained by coupling light streams, propagated through an optical fiber sensing loop in clock-wise and counter clock-wise directions and modulated in a phase modulator. The electric signal is detected in a synchronous detection circuit to generate a fundamental wave component of the phase modulation signal, a second harmonic wave component, a quadruple harmonic wave component, and an orthogonal component which is obtained in accordance with a phase shift of the quadruple harmonic wave component by one quarter period. In the first phase adjustment, a phase relation of the fundamental wave component, the second harmonic wave component, and the orthogonal component is maintained in a predetermined phase relation, while a relation between the phase of the components and a phase of the phase modulation signal is adjusted. At this time, a level of the orthogonal component is controlled to be zero. In the second phase adjustment, the phase of the phase modulation signal is adjusted to make a level of the second harmonic wave component zero. Subsequently, the phase of the phase modulation signal is shifted in a positive output direction by one-eighth wavelength of the second modulation signal. Then, the phase of the phase modulation signal is adjusted to make the level of the quadruple harmonic wave component zero, and the phase of the phase modulation signal is shifted in the positive output direction by the one-eighth wavelength.
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