Fig. 1. Multipath effects

Fig. 2. Phase demodulation scheme in Φ-OTDR. (a) Interferometer structure; (b) double pulse structure; (c) digital domain coherent demodulation; (d) coherent demodulation based on optical hybrid

Fig. 3. Phase shift double pulse method for eliminating interference fading^[64]

Fig. 4. Pulse phase shift for eliminating interference fading^[66]

Fig. 5. Temporally sequenced multi-frequency ^[71]

Fig. 6. Optical frequency comb for eliminating interference fading^[43]

Fig. 7. Rotated-vector-sum method for eliminating interference fading^[51]

Fig. 8. Positive and negative frequency band for eliminating interference fading^[82]

Fig. 9. Continuous chirped-wave phase-sensitive OTDR with interference fading elimination^[83]

Fig. 10. Multi-carrier non-linear frequency modulation for eliminating interference fading^[84]

Fig. 11. In the case of multi-degree-of-freedom superimposition, relationship between superimposition number M and reconstructed demodulation signal gain and fluctuation^[52]. (a) Reconstructed demodulation signal gain; (b) fluctuation

Fig. 12. Linearized pulse-coding phase-sensitive OTDR^[44]

Fig. 13. Spectrum extraction and remix method for eliminating interference fading^[93]

Fig. 14. Non-matched filter method for fading immunity Φ-OTDR^[100]

Fig. 15. SPEA in frequency domain for fading immunity Φ-OTDR^[101]

Fig. 16. Dechirp operation and SPEA in time domain for fading immunity Φ-OTDR^[102]