Optical fiber microphones based on twice envelope demodulation algorithm

Abstract

In order to improve the detection performance of a sound signal with the phase-sensitive optical time domain reflectometer, a demodulation algorithm based on twice envelope extraction is reported for the distributed sensing of sound signal. After the introduction of the sound sensing principle with the optical fiber microphone, a sensitivity enhancement structure which is composed by a piece of austenitic sheet is chosen for the detection of sound signal generated by the loudspeaker. The complete algorithm flow chart of twice envelope demodulation algorithm is then introduced. The first envelope extraction of the time-domain signal is acquired by the wavelet decomposition method in order to search the smooth region in the curve of time-domain signal. After the threshold comparison, the second envelope extraction of the smooth segment is realized by the envelope estimation algorithm, which could be used for the elimination of the noise factor. A high speed acquisition system based on FPGA is also proposed which could achieve signal acquisition through high speed AD acquisition module. The experimental results verify the effectiveness of the demodulation method. It can detect and locate the sound signal in the frequency range of 300˜1 kHz at a distance of 1000 m with the position error within 20 m. This demodulation method provides a new solution for the detection of sound signal by the phase-sensitive optical time-domain reflectometer.