The output characteristics of Mode-Locked Fiber Laser Gyroscope(ML-FLaG) is analyzed by a simple theoretical model. Output pulse shape is simulated using the model and is compared with experimental results with a good agreement. A new normalization scheme using DC component is suggested in order to suppress the dependence on total optical power in lock-in detection method.
In ML-FLaG using erbium-doped fiber(EDF) as a gain medium, the mode-locked pulse train is unstable due to gain competition between pulses. This problem can be suppressed by detuning the phase modulation frequency from the longitudinal mode spacing of the resonator. In this case, the output pulse shape becomes asymmetric. Moreover, the peak powers of two neighboring pulses become different from each other when ML-FLaG is rotating. These observations have not been fully understood before. To explain these phenomena, output pulse shape is simulated by a simple theoretical model. Both gain recovery time and gain saturation time in EDF are much longer than the time interval between pulses. This means it can be assumed that gain modulation in EDF due to intensity modulation of incident light(pulse) is neglected and the gain is saturated by average power of the pulses. The simulation result in pulse shapes agreed well with the experimental results, and the related output characteristics can be easily understood.
As a signal processing method for ML-FLaG, lock-in detection has advantages that the output is linearly proportional to the rotation rate and the error caused by the fluctuation in the relative intensity of the two consecutive pulses can be suppressed. However, the output depends on the total optical power, which leads to significant error. A method is suggested in which the output is normalized by the DC component to suppress the error. It is experimentally demonstrated that this method can reduce the error by factor of 10 for the rotation rate up to 200 deg/sec(1.1 rad).
