Fig. 1. Schematic diagram of heterodyne Φ-OTDR system

Fig. 2. Flexible multi-frequency modulation scheme

Fig. 3. Experimental setup of high fidelity Φ-OTDR system

Fig. 4. Multi-frequency beat signals. (a) Time domain signal; (b) frequency domain signal; (c)-(e) time domain beat signals of 40, 90, and 150 MHz after band-pass filtering

Fig. 5. Demodulation results of 40, 90, and 150 MHz beat signals. (a) Cumulative phase information; (b) differential phase information; (c) intensity information

Fig. 6. Demodulation and reconstruction of vibration signals. (a) Normalized intensity information of 40, 90, and 150 MHz beat signals; (b) vibration signal reconstruction with 90 MHz beat signal

Fig. 7. High-fidelity phase information of reconstructed PZT simulated vibration signal

Fig. 8. Normalized intensity cumulative distribution and frequency multiplexing results of 40, 90, and 150 MHz beat signals

Fig. 9. A π phase shift is introduced into half of 40 MHz beat signal spectrum

Fig. 10. Intensity distribution of beat signal before and after introducing π phase shift. (a) 150 MHz; (b) 40, 90, and 150 MHz

Fig. 11. Normalized intensity cumulative distribution of 40, 90, 150 MHz beat signals and new beat signals

Fig. 12. Differential phase information. (a) Distribution of time-spatial domain; (b) distribution of vibration position