Fig. 1. Phase optical time domain reflector based on coherent detection.

Fig. 2. Process of signal extraction in coherent Φ-OTDR.

Fig. 3. Pathways to suppress the effect of noise.

Fig. 4. Comparison of demodulated signals in the time domain is shown in (a) for both the 12-bit and single-bit signals. Additionally, (b) displays the comparison of PSD between the 12-bit and single-bit signals^[63].

Fig. 5. Principle of the twice-differential method^[65].

Fig. 6. Sinusoidal waveform with a 0.1 Hz frequency by the twice-differential method^[65].

Fig. 7. Schematic diagram of the experimental setup of coherent Φ-OTDR^[73].

Fig. 8. Indicative result of phase extraction in coherent Φ-OTDR^[73].

Fig. 9. Indicative result of suppressing non-fading noise. (a), (b) Phase changes extracted from the amplitude signal. (c), (d) Phase changes while the outlier (fading) has been removed. (e) Trace of one position of the fiber for wavelet denoising. (f) Trace of one pulse for data fitting^[76].

Fig. 10. Frequency of the sinusoidal perturbation signal is 500 Hz. (a) Original data, (b) wavelet denoising, (c) EMD-soft denoising, and (d) EMD-PCC denoising^[79].

Fig. 11. Schematic diagram of the hollow cylinder structure and optical fiber sensitization^[83].

Fig. 12. Coherent Φ-OTDR embedded with an ultra-weak FBG^[85].

Fig. 13. Demodulation result of external perturbation. (a) Results of the single pulse, (b)  Results of the single pulse after averaging 4 times, and (c) Results of 128-bit Golay coding^[91].

Fig. 14. 2D-SenseNet network with integrated demodulation and noise reduction^[101].