Fig. 1. DAS system structure diagram

Fig. 2. Flow chart of FFT vibration signal extraction

Fig. 3. Optical fiber directional phase unwinding diagram after phase difference of vibration points

Fig. 4. Unwinding diagram of phase difference in time direction and time-distance diagram of phase difference after unwinding of time direction. (a) Phase-time diagram with time direction unwinding; (b) diagram of phase difference with time-distance variation

Fig. 5. Phase-time diagrams of vibration signals extracted by three demodulation methods. (a) FFT demodulation; (b) IQ demodulation; (c) Hilbert transform demodulation

Fig. 6. Spectra of extracted signals by three demodulation methods. (a) FFT demodulation; (b) IQ demodulation; (c) Hilbert transform demodulation

Fig. 7. Power spectra of demodulation signals in three demodulation modes. (a) FFT demodulation; (b) IQ demodulation; (c) Hilbert transform demodulation

Fig. 8. Phase-time diagrams of intermediate- and high-frequency signals in FFT demodulation mode. (a) Intermediate-frequency signal of 2000 Hz; (b) high-frequency signal of 5000 Hz

Fig. 9. Signal spectra under FFT demodulation mode. (a) Intermediate-frequency signal of 2000 Hz; (b) high-frequency signal of 5000 Hz