Judgment and Compensation of Deviation of the Optical Interferometric Sensor's Operating Point From the Interferometer Quadrature Point

Optical interferometric sensors have been widely used for highly sensitive detection of various physical quantities, and most of them operate in simple intensity interrogation mode. The deviation between the sensor's operating point and its quadrature point (Q-point) can cause signal distortion and sensitivity reduction. In this work, a new method for judging and compensating the above-mentioned deviation is proposed for improving sensor performance. Judgment of the degree of deviation is performed using the amplitude of the second harmonic (SH) component derived from the sensor's output signal, and the deviation compensation is achieved through wavelength tuning. The wavelength tuning direction is determined by the relationship between the two phases of fundamental frequency (FF) and SH components, which eliminates the ambiguity of the sign of wavelength change and expedites the tuning speed. The method was fully verified by simulation and experiment based on a Fabry-Perot interferometer (FPI) based fiber-optic microphone with a tunable laser. Experimental results show that the FF and SH amplitudes oppositely changes with changing the ambient temperature after fixing the laser wavelength and fast tuning of the laser wavelength minimizes the SH amplitude close to zero and simultaneously maximizes the FF amplitude, consequently maintaining the operating point of the microphone at its Q-point. Moreover, the proposed method shows high precision and strong robustness to the laser power fluctuation. According to the criterion that the total harmonic distortion allowed is less than 3%, the phase deviation between the microphone's operating point and its Q-point should be smaller than 0.004°, which can be easily satisfied by the method proposed in this work.