Linearity analysis of fiber-optic gyroscope

Fundamental principles of closed-loop fiber-optic gyroscopes are described. Based on the closed-loop condition, simulated by computer, the linearity of scale factor is analyzed. The result shows, for a given gated time, it is possible to find the best amplitude ratio between the first and the second harmonics of feedback signal so that the optimum linearity of scale factor for fiber-optic gyroscope can be achieved. The scale factor of practical fiber-optic gyroscope should have good linearity in a wide dynamic range. Two techniques are used for this purpose, electrical signal processing and phase nulling. Limited dynamic range of fiber-optic gyroscopes is caused by the nonlinear relation between output signal amplitude and phase difference in the interferometer. The fundamental principle of electrical signal processing is to recover the original phase information in sensing loop. Pseudoheterdyne detection technique is to transfer Sagnac phase shift caused by gyroscope rotation into a phase of low frequency signal, so the dynamic range is extended. Phase nulling is to induce artificially a compensating phase whose amplitude is equal to Sagnac phase shift, but in opposite direction. As such, the gyroscope is operated at zero phase shift and gyroscope dynamic range is limited only by the feedback control component. Gated phase modulation approach can realize closed-loop operation of the gyroscope. The key component to this technique is a phase modulator giving two functions of biasing and nulling. For normal cylindrical piezoelectric phase modulator, it is difficult to have flat frequency response. To improve linearity of scale factor, one must control relative amplitudes and phases of different harmonics of nulling signal. The necessary nulling signal in closed-loop operation of fiber-optic gyroscope can be simulated by a microcomputer. Simulated result shows that the scale factor of fiber- optic gyroscope depends on the ratio of the fundamental and the second harmonics and the duty cycle of gating signal. For a given gating time a reasonable amplitude ratio for optimum linearity of scale factor of gyroscope can be found. The schematic diagram of fiber-optic gyroscope is shown in fig 1, where fb is the bias modulation frequency. Output signal of a pulse generator is applied to a low-pass filter and relative phase of different harmonics is adjusted to get a composite signal shown in fig 2, including the fundamental and the second harmonics. This signal used to give a compensating phase is added to the modulator to realize closed-loop operation. The output signal of the fiber-optic gyroscope is applied to PSD via a preamplifier and a bandpass filter. The error signal from PSD, whose output includes the information of rotation rate and direction is used to control the amplitude and the polarity of nulling signal. PSD output signal is represented as: