Fig. 1. OFDR technology development process and milestones

Fig. 2. Basic structure and principle schematic of OFDR

Fig. 3. Classification and influence of noise in OFDR system

Fig. 4. Influence of the laser source phase noise on the PSF^[66]. (a)(b) Influence of frequency tuning nonlinearity; (c) PSF under the influence of stochastic phase noise (linewidth is 200 kHz, tuning rate is 5 THz/s)

Fig. 5. Compensation results at each position when the measurement distance is 100 m^[79]

Fig. 6. Flowchart of PPNE-deskew-filter algorithm^[56]

Fig. 7. Compensation results of 8 km measurement distance^[56]. (a) Comparison of the results by optimized deskew filter and PPNE deskew filter; (b) comparison of spatial resolution

Fig. 8. Transfer process of parameter under test based on RBS sensing

Fig. 9. Mapping diagram of RBS in spatial domain, spectral domain and phase domain

Fig. 10. Relationship diagram of OFDR sensing method performance

Fig. 11. Validation of extremal relationships for strain accuracy measurements. (a) Signal-to-noise ratio versus strain accuracy; (b) sweep range versus strain accuracy; (c) strain spatial resolution versus strain accuracy

Fig. 12. Demodulation process based on RBS spectral^[103]

Fig. 13. Differential relative phase demodulation process based on RBS phase^[107]

Fig. 14. OFDR sensing technology development thread

Fig. 15. Strain measuring results by SSM-OFDR^[65]. (a) Results before and after spectral splicing; (b) close-up view; (c) large strain measuring

Fig. 16. Schematic illustrations of range ambiguity and spectral mismatch caused by strain^[101,114]. (a) Principle of range ambiguity; (b) principle of spectral mismatch

Fig. 17. Algorithmic workflow^[101,114]. (a) Local inter-correlation; (b) position compensation

Fig. 18. Workflow of the BM3D-SAPCA algorithm^[119]

Fig. 19. Demodulation principles of SV-OFDR

Fig. 20. SV-OFDR distributed strain measuring results. (a) Large strain demodulation results; (b)(c) vibration signal demodulation results

Fig. 21. Typical OFDR instruments. (a) Luna OBR-4600; (b) Mega-Sense Co. LGA50; (c) GDUT OFDS

Fig. 22. Ultra-long distance optical fiber and link measuring application^[55]. (a) Fiber link loss of OFDR controlled by optical phase locking; (b)(c) measurement accuracies of optical fiber loss at 10 km and 100 km

Fig. 23. Ultra-high precision optical chip measuring applications. (a)(b) Fabricated LiDAR chip and measured refection spectrum, where ∆R=10 µm/ng^[129]; (c)(d) Y-waveguide and distributed polarization crosstalk measurement results^[42]

Fig. 24. Typical application of distributed acoustic sensing^[138]. (a) Observed acoustic signal in long-term deep sea floor observatory; (b)measured seismic profiles of the surface wave

Fig. 25. 3 km fiber coil strain measurement results. (a) Strain results under different temperature excitations; (b) interlayer strain; (c) interturn strain