Fig. 1. Configuration of optical frequency domain reflectometry

Fig. 2. The beat principle of optical frequency domain reflectometry. (a) Linear sweep frequency of local and probe signals; (b) corresponding beat signals at each delay

Fig. 3. Flow chart of wavelength demodulation method

Fig. 4. Flow chart of phase demodulation method

Fig. 5. The influence of sweep nonlinear noise. (a) Nonlinear sweep curve; (b) frequency of beat signal

Fig. 6. Classification of methods for suppressing noise of sweep nonlinearity

Fig. 7. Configuration of external clock sampling method

Fig. 8. Simplified optical frequency domain reflectometry based on single interferometer^[39]

Fig. 9. Sensing results of optical frequency domain reflectometry based on single interferometer^[39]. (a) Measured 100 m distance domain signal; (b) cross correlation coefficient within 170 m range

Fig. 10. Principle of external modulation optical sweeping

Fig. 11. Ultra-linear sweep light^[49]. (a) Sweep frequency spectrum; (b) instantaneous optical frequency

Fig. 12. Measured 2 km distance domain signal^[49]

Fig. 13. Coherent fading phenomenon. (a) Distance domain signal; (b) enlarged view

Fig. 14. Smoothing and filtering of phase

Fig. 15. Principle of pulse internal frequency division

Fig. 16. Fabrication of weak scattering point array using femtosecond laser^[65]. (a) Processing schematic diagram; (b) distance domain signal of weak scattering point array

Fig. 17. Phase distribution of single-mode fiber segment and fiber segment with weak scattered point array demodulated by phase method^[65]

Fig. 18. Fabrication of weak scattering point array using femtosecond laser in multi-core parallel connection for 3D shape sensing^[84]. (a) Distance domain signal of scattering points; (b) 3D shape reconstruction results

Fig. 19. Fiber optic 3D shape sensing instrument

Fig. 20. Schematic diagrams of spectral spatial position mismatch^[16]. (a) Spectral position without strain; (b) spectral position at large strain

Fig. 21. Large strain sensing results obtained using different optical fibers. (a) Scattering enhanced fiber^[16]; (b) weak reflection point array fiber^[65]

Fig. 22. High temperature sensing result^[37]

Fig. 23. High temperature sensing results obtained using different post-processing fibers^[107]. (a) Weak grating array fiber; (b) weak micro-cavity array^[108]

Fig. 24. Large bending fiber

Fig. 25. Fiber corroded by hydrofluoric acid^[119]

Fig. 26. Corroded multi-core optical fiber