Fig. 1. (a) and (b) The basic theory of the new HOPE. The desired optical pulses are generated in an all-fiber filterless OPO. It includes an HNLF, which provides the parametric gain, a feedback fiber, and a short TDF for gain compensation. The cavity is driven by an optical pulsed pump. The two insets, respectively, represent the gain transformations of points a and b in the OPO, in which the dashed lines show that the indicated signal energy decreases and disappears. WDM, wavelength-division-multiplexer; HNLF, highly nonlinear fiber; TDF, thulium-doped fiber; and OC, optical coupler.

Fig. 2. (a) and (b) Schematic representation of HOPE system and PA microscope. EML, electro-absorption modulated laser; WDM, wavelength-division-multiplexer; CIR, circulator; LT, light trap; PC, polarization controller; HNLF, highly nonlinear fiber; TDF, thulium-doped fiber; OC, optical coupler; VDL, variable delay line; PM coil, 1-km polarization-maintaining fiber core; TDFA, thulium-doped fiber amplifier; Col, collimator; $L1$, $f=75\mathrm{mm}$ lens; $L2$, $f=150\mathrm{mm}$ lens; M, mirror; Obj, microscope objective lens; and UST, ultrasound transducer.

Fig. 3. Characterization of the new HOPE’s output. (a) Output spectrum of the new HOPE. (b) Time trace of the new HOPE’s output. (c) Zoomed-in single pulse of (b). (d) Pulse-to-pulse widths of the new HOPE’s output at the start and after 1 h. (e) Pulse intensity histogram. (f) The relationship between the output power of the new HOPE and the pump power of TDFA.

Fig. 4. (a) PA signal of water obtained at 1930 nm and its frequency spectrum with Gaussian fitting. (b) FFT spectrum of PA signal and its Gaussian fitting result.

Fig. 5. (a) PA signal of water (red) and butter (black) obtained at 1930 nm. (b) Zoomed-in signal of butter in panel (a). (c) Photograph of the sample and the imaging region. The black dashed box encloses the area imaged in the experiment, in which the adipose tissue lies within the region bounded by the yellow dashed line (scale bar: 2 mm). (d) PAI of the water content at 1930 nm (scale bar: 1 mm; image acquisition time: $\sim 40\min$). The region bound by the green dotted line denotes adipose tissue.

Fig. 6. (a) Photograph of the sample and indication of the illuminating direction. (b) 3D PA image of muscle surface with image acquisition time: $\sim 40\min$. (c) B-mode image of (b). (Scale bar: 1 mm.)

Fig. 7. (a) Photograph of the zebrafish embryos, where region $R1$ is the yolk sac and $R2$ is the tail. (b) The water PAI of the zebrafish embryos with image acquisition time: $\sim 30\min$. (c) The water PAI of the zebrafish embryos after 24 h. (d) The water PAI of the zebrafish embryos after 48 h with image acquisition time: $\sim 30\min$. (Scale bar: $500\mu \mathrm{m}$.)

Fig. 8. (a) Orthotopic mouse liver tumor xenograft sample in which the green dashed line box represents the imaging region (scale bar: 2 mm, region $R1$: tumor tissue; and region $R2$: healthy tissue). (b) PA water image of liver cancer sample with image acquisition time: $\sim 30\min$. (Scale bar: 1 mm.)

Fig. 9. PAI of water distribution in the ear with normalized intensities with image acquisition time: $\sim 40\min$. (a) The water PAI for a mouse normal ear. (b) The water PAI for the ear of a mouse with edema, where the presence of cartilage is marked $R1$ and $R2$. (c) PA amplitude values corresponding to the axial yellow lines passing through approximately the same structures in the left and right ears in (a) and (b), respectively. (d) and (e) Depth-encoded maximum amplitude projection images corresponding to (a) and (b), respectively. (Scale bar: 1 mm.)

Fig. 10. (a) PAI of water content acquired at 1930 nm with image acquisition time: $\sim 40\min$. (b) PAI of lipid acquired at 1750 nm with image acquisition time: $\sim 55\min$. (c) Photograph of the sample, where the yellow dashed line box represents the imaging region. (d) The merged image of (a) and (b). (e) The subtraction image of (a) and (b). (f) and (g) The amplitude profile plots of lines I and II in panel (d). (red, PAI for water; green, PAI for lipid). (Scale bar: 1 mm.)