Fig. 1. The typical OTDR configuration

Fig. 2. The typical COTDR configuration

Fig. 3. OFDR configuration

Fig. 4. Locating principle of OFDR system

Fig. 5. TGD-OFDR configuration

Fig. 6. Principle of vibration measurement via RBS phase method

Fig. 7. The DAS demodulation scheme based on unbalanced Mach-Zehnder interferometer. (a) Phase demodulation using 3×3 coupler^[30]; (b) phase demodulation using phase-generated carrier technology^[31]

Fig. 8. Suppression of coherence fading. (a) Dual probe pulses with π-phase shift^[47]; (b) inner-pulse frequency-division in TGD-OFDR^[47]; (c)‒(e) schematic of rotated-vector-sum method^[41]

Fig. 9. Principle of frequency demodulated method in DAS

Fig. 10. Distributed sensing using Rayleigh frequency pattern and the cross correlation algorithm^[53]

Fig. 11. Demodulation using RBS curve matching^[58]

Fig. 12. Longitudinal shift of the RBS trace when temperature changes are applied to the fiber^[60]

Fig. 13. Chirped pulse and short-time Fourier transform of corresponding pulse^[62]

Fig. 14. Walk-away VSP images from conventional geophone data and DAS^[65]

Fig. 15. Standard architecture of a DAS and pattern recognition system^[68]