Fig. 1. Research progress and application of ultrasound transducer technology based on optical fiber

Fig. 2. Working mechanism of ultrasound transducer. (a) Schematic of ultrasound emission; (b) schematic of ultrasound detection

Fig. 3. Metallic material fiber optic ultrasound transmitters. (a) Schematic of the objective gold nanopores on the fiber end face^[5]; (b) fiber-optic ultrasound transmitter coated with gold nanoparticles/PDMS composite material^[9]; (c) schematic of excitation angle control and (d) diagram of the sound field distribution based on fiber optic ultrasound transmitter array^[11]

Fig. 4. Nonmetallic material fiber optic ultrasound transmitters. (a)-(c) SEM images of fiber optic ultrasound transmitter coated with carbon nanofiber^[19]; (d) SEM image of a optical fiber end coated with MWCNT^[18]; (e) SEM image of fiber optic ultrasound transmitter coated with MWCNT/PDMS composite^[18]; (f)(g) top view and side view of fiber optic ultrasound transmitter based on CIS-QDs/PDMS composite material^[20]; (h) absorption spectrum of the CIS-QDs/PDMS based fiber optic ultrasound transmitter^[20]

Fig. 5. Intensity modulated fiber optic ultrasound sensors. (a) Intensity modulated fiber optic ultrasound sensor based on fiber coupler; (b) fiber optic ultrasonic sensor based on reflectivity of fiber end face

Fig. 6. Phase modulated fiber optic ultrasound sensors. (a) Fiber optic ultrasound sensor based on MZI; (b) fiber optic ultrasound sensor based on MI; (c) fiber optic ultrasound sensor based on FPI; (d) fiber optic ultrasound sensor based on SI

Fig. 7. Wavelength modulated fiber optic ultrasound sensors. (a) Fiber optic ultrasound sensor based on fiber grating^[68]; (b) fiber optic ultrasound sensor based on fiber laser^[70]；(c) fiber optic ultrasound sensor based on micro-ring resonator^[37]; (d) fiber optic ultrasound sensor based on SPR^[71]

Fig. 8. Separated fiber optic ultrasound transducers. (a) Fiber optic ultrasound probe composed of fiber optic ultrasound transmitter and a fiber optic FPI ultrasound sensor^[86]; (b) fiber optic ultrasound probe composed of fiber optic ultrasound transmitter and fiber laser type ultrasound sensor^[87]

Fig. 9. Integrated fiber optic ultrasound transducers. (a) Schematic of ultrasound detection based on an integrated fiber optic ultrasound transducer^[88]; (b) an integrated fiber optic ultrasound transducer based on selective absorption composite film^[90]

Fig. 10. Photoacoustic tomography applications of fiber optic ultrasound transducer technology^[97]. (a) Photoacoustic tomography system based on an FPI type fiber optic ultrasound sensor; (b) photoacoustic tomography system based on a fiber laser ultrasound sensor

Fig. 11. Photoacoustic microscopy imaging applications of fiber optic ultrasound transducer technology. (a) Fast photoacoustic microscopy imaging system based on a fiber FPI ultrasound sensor^[101]; (b) schematic of rapid microscopy imaging system and hemodynamics based on a fiber laser ultrasound sensor and blood flow images^[100]

Fig. 12. Endoscopic imaging applications of fiber optic ultrasound transducer technology. (a) Forward endoscopy imaging system based on a fiber optic ultrasound sensor array^[106]; (b) lateral endoscopy imaging system based on a fiber optic ultrasound sensor^[107]

Fig. 13. Ultrasound imaging applications of fiber optic ultrasound transducer technology. (a) 3D pulse-echo ultrasound image of ex vivo porcine aorta^[62]; (b) lateral ultrasound imaging system based on a fiber optic ultrasound transmitter and a fiber laser ultrasound sensor, and cross-section ultrasound image of a pig trachea^[87]; (c) freehand ultrasound imaging probe and application of human vascular imaging based on all-fiber ultrasound transducer technology^[112]

Fig. 14. Nondestructive testing applications of fiber optic ultrasound detecting technology. (a) Damage detection system of carbon fiber composite laminates using piezoelectric ultrasound transmitter and FBG ultrasound sensors^[115]; (b) a flower-shaped FBG ultrasound sensor structure for sound source location^[118]; (c) a nondestructive testing method based on the combination of a FBG ultrasound sensor and an MZI ultrasound sensor^[123]; (d) schematic of acoustic emission detection of structural damage based on phase-shifted grating ultrasound sensor^[121]; (e) Lamb wave signals measured by FBG ultrasound sensor and identification results^[121]

Fig. 15. Applications of fiber optic ultrasound detection technology in partial discharge. (a) Schematic of discharge detection based on Sagnac fiber interferometer^[127]; (b) discharge detection system based on fiber FPI^[128]; (c) discharge detection system based on distributed FBG ultrasound sensors^[129]; (d) discharge detection system based on fiber MI^[130]; (e) partial discharge detection system based on φ-OTDR and the measured discharge signal position^[132]

Table 1. Comparison among fiber optic ultrasound transmitters

Table 2. Comparison among fiber-optic ultrasound sensors