Fig. 1. Optical fiber ultrasonic sensor based on transmission loss. (a) Bending loss^[25]; (b) cantilever coupling^[26-27]; (c) reflection coupling^[28]

Fig. 2. Optical fiber coupler-based ultrasonic sensor^[29]. (a) Optical fiber coupler-based ultrasonic sensing structure; (b) time-domain response of the ultrasonic sensor

Fig. 3. Optical fiber interferometry-based ultrasonic sensors. (a) Optical fiber Mach-Zehnder interferometer; (b) optical fiber Michelson interferometer; (c) optical fiber Sagnac interferometer; (d) optical fiber Fabry-Perot interferometer

Fig. 4. Some highly sensitive optical fiber ultrasonic sensors. (a) Optical fiber MI based on a diaphragm^[40]; (b) optical fiber MZI based on spiral silicon waveguide^[41]; (c) multimode interference-based fiber-optic ultrasonic sensor ^[43]

Fig. 5. Optical fiber SI ultrasonic sensors. (a) Noncontactive optical fiber SI ultrasonic sensor^[49]; (b) optical fiber SI ultrasonic sensing array^[50]

Fig. 6. FPI ultrasonic sensors with different cavities. (a) FPI ultrasonic sensor with reflective mirrors^[57]; (b) FPI ultrasonic sensor with an air cavity^[59]; (c) optical fiber tip-based FPI ultrasonic sensors with polymer cavity^[60]; (d) optical fiber tip-based FPI ultrasonic sensors with polymer plano-concave cavity^[61]

Fig. 7. FPI ultrasonic sensors with different diaphragms. (a) Schematic diagram of the diaphragm-based FPI ultrasonic sensors; (b) two-photon 3D printing optical fiber tip-based FPI ultrasonic sensor with polymer diaphragm^[64]; (c) two-photon 3D printing optical fiber tip-based FPI acoustic sensor with microspring-based diaphragm^[65]; (d) single-photon 3D printing optical fiber tip-based FPI acoustic sensor with polymer spirally-suspended cavity^[66]

Fig. 8. Comparison of sensitivity of optical fiber FPI ultrasonic sensors with different diaphragm materials^[68]

Fig. 9. Optimum OPD versus the spectral width of the FBG sensor for unbalanced interferometric method^[14]

Fig. 10. Two-photon 3D printing phase-shifted Bragg grating waveguide ultrasonic sensors^[20]. (a) SEM of phase-shifted Bragg grating waveguide; (b) optical transmission spectrum of the sensor; (c) time response signal detected by the sensor

Fig. 11. Optical fiber tip-based SPR ultrasonic sensor^[86-89]. (a) Schematic of the proposed sensor; (b) SEM image of the SPR sensor; (c) picture of the fabricated sensor; (d) time response signal detected by the sensor

Fig. 12. Fabricated optical fiber integrated-waveguide micro-ring resonator-based ultrasonic sensor^[21]. (a) SEM image of the sensor; (b) simulated electric field distribution of the guided mode; (c) light profile of the micro-ring resonator sensor at a resonant wavelength; (d) detected time response signal

Fig. 13. 3D printing optical fiber partial discharge acoustic emission sensor^[109]. (a) Schematic diagram of the sensing head; (b) demodulated signals of the proposed sensing head and optical fiber ring; (c) power spectral densities of the signals detected by the proposed sensing head and optical fiber ring; (d) schematic diagram of the sensor head layout; (e) demodulated signals of the sensor#1, sensor#2 and sensor#3

Fig. 14. Dynamic strain response of the FBG sensor and the PZT transducer^[14]. (a) Time response of the sensors; (b) time frequency response of the FBG sensor; (c) time frequency response of the PZT transducer

Fig. 15. Illustration of possible research directions for optical fiber ultrasonic sensing technologies