An improved device and demodulation method for fiber-optic distributed acoustic sensor based on homodyne detection

An improved fiber-optic distributed acoustic sensor (DAS) using a LiNbO₃ straight through waveguide electro-optic phase modulator and a novel phase demodulation method based on homodyne detection is proposed in this paper. The principles of the phase-sensitive optical time domain reflectometer (Φ-OTDR) system and the multi-point disturbances phase demodulation algorithm are described in detail. The system structure can solve the problems of large size, difficult integration, low modulation frequency and unstable performance of piezoelectric transducer (PZT), which exists in the traditional homodyne detection system based on Michelson interferometer (MI). Experiments show that it can realize the location and identification of multi-point disturbances, and the acoustic signal can be demodulated accurately. The detectable acoustic frequency depends no longer on the phase modulation frequency. The signal-to-noise ratio (SNR) is above 20 dB, the noise level (NL) is less than 4 × 10⁻³ rad/√Hz, and the spatial resolution is about 10 m at 5 km sensing fiber. Compared with the heterodyne detection system and traditional homodyne detection system, the system has the advantages of lower cost, higher detection frequency, more stable performance and easier to integrate, which can be applied in some specific fields, such as oil and gas exploration, pipeline detection, border security and so on.