Low-Frequency Optical Fiber Fabry-Perot Acoustic Sensor Based on All-Phase Cross-Correlation Demodulation

Abstract

A high-sensitivity low-frequency fiber-optic Fabry-Perot (F-P) acoustic sensing scheme was proposed based on optical cross-correlation (OCC). The finite element method (FEM) is used to analyze the OCC optical path distribution. The demodulation of the F-P cavity length is fulfilled by measuring the initial phase of the OCC signal. An improved phase demodulation method combined with all-phase Fast Fourier Transform (FFT) and Buneman frequency estimation is applied in sound pressure detection. The discrete wavelet transform and Hilbert transform are used for denoising and de-envelope the OCC signal. The possibility of phase jump is reduced by tracking 511 pixels. Acoustic experiments prove that the scheme has a high linear responsivity in the frequency range of 2–250 Hz. At the frequency of 100 Hz, the sound pressure sensitivity and calibration values are 0.683 rad/Pa and 137.8 nm/rad. The minimum detectable pressure (MDP) is measured to be 24.1 μPa/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1/2</sup> , which is over ten times lower than peak demodulation. The scheme provides a low-cost and high-sensitivity acoustic demodulation system that is suitable for low-frequency acoustic detection such as hydrophone measurement, seismic detection, and photoacoustic spectroscopy.