Fig. 1. Typical structure of OFDR system

Fig. 2. Demodulation process of OFDR system. (a) Demodulation steps; (b) schematic diagram of signal processing

Fig. 3. TLS sweep frequency and spatial resolution^［7］. (a) Without nonlinear sweep frequency; (b) with nonlinear sweep frequency

Fig. 4. Structure of long-distance OFDR system based on OPLL^[14]

Fig. 5. Dynamic frequency noise of OFDR system using OPLL^[14]

Fig. 6. Schematic diagram of sweep laser generation controlled by optically locked phase^[15]

Fig. 7. Hardware compensation method^[16]. (a) Schematic diagram of beat signal and sampling point under nonlinear frequency sweep; (b) schematic diagram of beat signal generation

Fig. 8. Zero-crossing detection OFDR^[21]. (a) Auxiliary interferometer signal zero-crossing detection circuit scheme; (b) timing analysis of each node of the circuit

Fig. 9. NUFFT signal processing steps^[25]

Fig. 10. Schematic diagram of PNC-OFDR system using GCP for phase compensation ^[26]

Fig. 11. Rayleigh intensity curve of PNC-OFDR system before and after phase noise compensation and spatial resolution at 40 km^[27]

Fig. 12. Signal processing diagram of declining filter method^[29]

Fig. 13. TGD-OFDR system experimental configuration^[33]

Fig. 14. PDIR algorithm flow^[37]

Fig. 15. Typical OFDR and DSB-PNC OFDR^[46]. (A) Typical OFDR; (B) DSB-PNC OFDR

Fig. 16. OFDR system with bidirectional decision algorithm^[51]

Fig. 17. OFDR system based on I/Q detection^[52]

Fig. 18. FUT strain distribution solved by recursive demodulation compensation method^[58]

Fig. 19. CCSA process^[61]

Fig. 20. OFDR system based on moving time gating method^[63]

Fig. 21. Signal processing flow of moving time gating method^[63]

Fig. 22. Vibration sensing system combined with TGD-OFDR and MZI^[64]

Fig. 23. Schematic diagram of sensor prototype^[67]

Fig. 24. γ radiation measurement using OFDR^[69]. (a) Experimental setup; (b) experimental result