Detection of Large Dynamic Range Acoustic Signal Using OPD-Based Fiber-Optic Interferometric Demodulation With SNR Enhancement

The measurement of acoustic signals at high frequencies with a large dynamic range remains a challenge for phase-demodulation-based fiber acoustic sensing systems due to the phase wrapping phenomenon. Here, an optical path difference (OPD)-assisted demodulation method aiming at breaking the limitations of phase demodulation is reported. By interrogating an imbalanced Michelson interferometer (IMI) with a frequency-modulated pulse light, a time-resolved interference spectrum is obtained and used for OPD demodulation. Theoretical analysis reveals that the variation amplitude of demodulated OPD signal not only is linearly dependent on the amplitude of the acoustic signal applied to the IMI, but also experiences a gain that is directly proportional to the signal frequency. Consequently, compensation of this gain introduces a noise suppression that favors high-frequency signals, resulting in a higher signal-to-noise ratio (SNR) at high frequencies. The method's capability for detecting both single-frequency and multi-frequency acoustic signals is demonstrated by simulations and experiments, and the results show that the upper-frequency limit of the proposed scheme is at least 40 times the limit of the phase demodulation method for an acoustic signal with an amplitude of 1.135 μϵ. The proposed method can be easily extended to distributed acoustic sensing systems and is of great potential to break the limitation of a restricted dynamic range encountered in traditional phase demodulation methods.