Fig. 1. (a) The calculated deformations of the fiber surrounded by water in response to plane ultrasound waves. Left: TR₂₁ mode at 22 MHz. Right: TR₂₂ mode at 39 MHz. Arrows: local displacement. (b) Analogous model of a spring-mass oscillator. (c) Schematic of the phase cancelation effect. (d) Frequency responses with different source-to-fiber distances.

Fig. 2. (a,d) Schematics of a straight sensor and curved (bending radius R=30 mm) sensor subjected to a point ultrasound source. The dashed curves show the acoustic phase variation along the fiber. (b–c) Calculated spatial sensitivities of a straight sensor in the x-z and x-y planes. (e–f) Calculated spatial sensitivities of a curved sensor in the x-z and y-z planes. Figure reproduced with permission from ref.²⁹, Optical Society of America.

Fig. 3. (a) Schematic of ultrasound detection by using a fiber-laser sensor. (b) Unperturbed and ultrasonically perturbed beat signal output from the sensor. Figure reproduced with permission from ref.³⁰, under a Creative Commons Attribution 4.0 International License.

Fig. 4. (a) Temporal and (b) frequency response of the sensor. (c) Measured NEP as a function of input power at the photodetector. (d) Stability test result by continuously measuring the photoacoustic signals from two discrete absorbers over 30 min. Figure reproduced with permission from ref.³¹, Optical Society of America.

Fig. 5. (a) Schematic of a fiber-based PAM apparatus. (b) Schematic (left) and result (right) of the field-of-view measurement. (c) PAM result of a mouse ear that contains branches of blood vessels and capillaries. (d) Three consecutive snapshots showing the blood flow in the mouse ear. (e) Hemoglobin concentration image of a mouse brain obtained with a 532 nm pulse laser. (f) Oxygen saturation images enabled by a dual-wavelength laser source^{33, 34}.

Fig. 6. (a) The optical signal duplication system. The ultrasonically modulated laser light is duplicated into 8 copies. (b) Demodulated ultrasound signals of the 8 copies. (c) B-scan signals before and after signal averaging, showing a 3.5 dB reduction in the background noise. (d) In vivo PAM results before and after signal averaging. The visibility has been effectively enhanced by the signal averaging method. Figure reproduced with permission from ref.³⁵, Optical Society of America.

Fig. 7. (a) Schematic of a rotary-scanning PACT with the fiber laser sensor. Photoacoustic images of (b) a zebrafish and (c) a mouse brain. SB: swim bladder; SC: spinal cord; AF: anal fin; CV: cortical vessels; SSS: superior sagittal sinus; ICV: inferior cerebral vein. Figure reproduced with permission from ref.²⁹, Optical Society of America.

Fig. 8. (a) Photograph of the holder for adjusting the bending curvature of the sensor. (b) Schematic of the multidepth PACT. (c) Photoacoustic images of human hairs at different depths in the x-z plane. (d) In vivo photoacoustic image of the abdominal subcutaneous vasculature of a rat. (e) Photograph of the rat with the fur removed before imaging. The red dashed box in (e) corresponds to the green box in (d). The yellow dashed line in (e) corresponds to the white dashed line in (d). Figure reproduced with permission from ref.³⁶, AIP Publishing.

Table 1. Comparison of piezoelectric and optical ultrasound transducers.