This dissertation describes a new digital signal processing method for the closed-loop fiber optical gyroscope. A fiber optical gyroscope is an interferometric instrument, which measures the rotation rate utilizing the Sagnac effect. The output signal is cosinusoidal form of phase difference. Thus, it has the properties of 2π ambiguity, a narrow dynamic range, and low sensitivity. To solve the problems, the closed loop signal processing is generally used. In the closed loop approach, the detector output of gyroscope is used as error signal for the feedback loop. The feedback phase shift compensates for the Sagnac phase shift induced by the rotation.
In proposed method, it merges the bias and feedback phase modulation signals in one phase modulation signal and its frequency is $1/τ_4$. It has 4 states ($τ_1$, $τ_2$, $τ_3$, $τ_4$) in one period (4τ). And its values in the first ($τ_1$) and the third ($τ_3$) are varied with rotation rate. Every period, 4τ, the rotation rate is calculated. And the phase difference between Sagnac and feedback phase shift is updated for compensation. As a result, the sensing speed in this method is faster than that in the conventional serrodyne method. The value of the modulated output signal is converted directly into digital data at each one-fourth period ($τ_1$, $τ_2$, $τ_3$, $τ_4$). Then, the value of the third ($τ_3$) is subtracted digitally from that of the first ($τ_1$). Such digital demodulation is intrinsically free of any source of electronic long-term drift, such as the input offset in the AD converter. And we have used a digital integrator to reduce noises, such as thermal noise, shot noise, and so on. The use of a digital integrator yields the same noise reduction as low-pass analog filtering.
