Robust fiber-optic microphone with modified dual-wavelength demodulation algorithm for low-frequency sound detection

A fiber-optic Fabry-Perot (FP) microphone with a prestressed nickel diaphragm was prepared for low-frequency environmental noise detection. By using a modified dual-wavelength demodulation (MDWD) algorithm proposed in this work, the time-domain displacement of the microphone diaphragm is determined without the need to know the initial length of the FP cavity. The MDWD algorithm enables the microphone to operate at non-quadrature points and extends the microphone response beyond the linear region of the FP interferometer. The simulation results show that the time-domain displacement of the microphone diaphragm determined by the MDWD algorithm is accurate and the measurement error is less than 3.3 % in the case of the diaphragm displacement below 300 nm. The mechanical response of the fiber-optic FP microphone to 100 Hz sound waves at different pressures was measured using the MDWD algorithm and compared with that obtained by the broadband interferometric demodulation method. The two experimental results are in good agreement with each other, verifying the reliability of the MDWD algorithm. The mechanical sensitivity of the prepared microphone was measured to be 49.41 nm/Pa over the dynamic range of 0.14–3.16 Pa using the MDWD algorithm. Finally, the field detection of subway noise was performed using the MDWD-based fiber-optic FP microphone. The work demonstrated that the MDWD algorithm can significantly improve the performance of fiber-optic FP microphones for low-frequency sound detection.