Fig. 1. Principle of OFDR. (a)Basic configuration of OFDR; (b) generation of beat frequency signal^[4]

Fig. 2. Beat signal diagram of nonlinear optical frequency tuning^[4]

Fig. 3. Configuration of small-scale fiber-tapering device^[11]

Fig. 4. Spectral bandwidth comparison under 1 m grating length. (a) FBG spectrum; (b) ROUGH spectrum^[3]

Fig. 5. Experiments on sensing micro distributed strain or strain resolution under the highest spatial resolution of 3 mm^[16]

Fig. 6. Schematic diagram of the OFDR system^[19]

Fig. 7. Experimental result. (a) Beat spectrum without nonlinearity correction; (b) beat spectrum with time-scale factor correction^[19]

Fig. 8. PA-OFDR experimental setup^[25]

Fig. 9. Schematic illustration of the spectrum for the frequency sweep with high-order sidebands of external modulation^[33]

Fig. 10. Four-wave mixing process. (a) Broadening the frequency sweep using first stage FWM; (b) using second stage FWM ^[33]

Fig. 11. OFDR for aircraft bulkhead monitoring^[40]

Fig. 12. OFDR for PCB strain monitoring^[41]

Fig. 13. Layout diagram of sensor fiber on transformer core^[45]

Fig. 14. Magnetic field measurement using FBG pasted on Terfenol-D rod^[52]

Fig. 15. Schematic of the custom setup built to glue three fibers into a fiber triplet^[55]