Fig. 1. Backward Rayleigh scattering caused by scattering medium in optical fiber^[3]

Fig. 2. OFDR system and its principles based on Michelson interferometer. (a) Schematic setup of OFDR system; (b) beat frequency signals induced by frequency-swept laser source^[16]

Fig. 3. Research on improvement of OFDR performances by optoelectronic measurements and signal processing methods. (a) Mach-Zehnder interferometer for frequency-sweeping nonlinear calibration^[17]; (b) bandwidth segmentation phase compensation algorithm improving sweeping rate^[23]; (c) time gate control algorithm for software nonlinear compensation^[25]; (d) improving spatial resolution with position deviation compensation algorithm^[29]

Fig. 4. Applications of OFDR in temperature monitoring. (a) High temperature sensing using gold-coated optical fiber^[27]; (b) monitoring temperature distribution along superconducting degaussing cables^[32]

Fig. 5. Applications of OFDR in civil and energy infrastructures. (a) Strain monitoring for wind turbine blades under large wind load^[35]; (b) integrated fiber sensors for bridge reinforcement testing^[38]; (c) high-resolution embedded OFDR sensors for concrete structure testing^[40]

Fig. 6. Differential mode delay measurement. (a) Differential mode delay measurement using OFDR analysis method^[47]; (b) spatial-spectral near-far field imaging method for mode identification^[50]; (c) adaptive optics applied to multimode fiber for mode excitation^[54]; (d) phase reference technology for mode delay measurement^[55]

Fig. 7. High-order mode group identification using coherent polarized OFDR in six-mode fiber.^{(a) Linearly polarized mode group and its degenerate modes in six-mode fiber; (b) mode separation of six-mode fiber in beat frequency domain[57]}

Fig. 8. Mode characterization in fluorine-trench two-mode fiber. (a) Fundamental mode LP₀₁ and high-order mode LP₁₁ in dual-mode fiber; (b) mode identification with hybrid mode right in the middle^[58]

Fig. 9. Different coherent birefringence measurement methods and their results. (a) Birefringence measurement by Faraday effect^[64]; (b) birefringence measurement by polarization graded receiver^[4]; (c) birefringence distributed measurement of ribbon fibers using polarization-sensitive OFDR^[66]; (d) birefringence measurement by new distributed polarization analysis system^[68]

Fig. 10. Measurement and characterization of birefringence in dual-hole microstructured optical fibers. (a) Two polarized modes in fiber; (b) separation of polarized modes in beat frequency domain for dual-hole microstructured optical fiber^[69]

Fig. 11. Measurement and regulation of birefringence in dual-hole microstructured optical fiber. (a) Two polarized modes and their phase/group dispersion curves in optical fiber; (b) exchange of fast and slow axes at zeros in group birefringence with temperature increment^[70]