Fig. 1. Schematic setup of 1725-nm HOPE; inset, visualization of the HOPE dual gain scheme. DFB-LD, distributed feedback laser diode; AM, amplitude modulator; PC, polarization controller; LNA, optical low-noise amplifier; WDM, wavelength-division multiplexer; CIR, circulator; LT, light trap; HNLF, highly nonlinear fiber; TDF, thulium-doped fiber; SMF, single-mode fiber coil; VDL, variable delay line; OC, output coupler; FL, fiber laser; ISO, optical isolator; BPF, bandpass filter.

Fig. 2. (a) 400-nm emission spectrum of 1725-nm HOPE showing residual pump and suppressed noise floor. (b) Emission spectrum of 1725-nm HOPE with a center wavelength of 1725 nm and a 3-dB bandwidth of 1.4 nm. (c) 3000 pulses from the HOPE; inset, pulse train showing a PRR of ∼194  kHz. (d) Output power versus pump power of the second-stage TDFA with the first-stage pump power fixed at 600 mW.

Fig. 3. (a) Standard test conducted to verify the depth and concentration resolution capabilities of the 1725-nm-HOPE-based PAM system by submerging a prism-shaped block of agar gel in olive oil. (b) Results of depth imaging with linear fit. (c) Normalized PA signals at three different points of the agar prism.

Fig. 4. (a) i, photo of healthy liver sample with normal steatosis, ii, 2D PAM image, iii and iv, 3D PAM images of healthy sample. (b) i, photo of fatty liver sample with severe steatosis, ii, 2D PAM image, iii and iv, 3D PAM images of fatty liver sample. Scale bars: 1 mm.

Fig. 5. Segmentation results of photoacoustic images of liver samples using Otsu’s thresholding method. (a) Healthy liver sample with three pairs of regions of interest (ROIs): H1 (yellow, H1-liver and H1-lipid), H2 (cyan, H2-liver and H2-lipid), and H3 (green, H3-liver and H3-lipid). (b) Fatty liver sample with three ROI pairs: F1 (yellow, F1-liver and F1-lipid), F2 (cyan, F2-liver and F2-lipid), and F3 (green, F3-liver and F3-lipid). (c) Segmentation outcomes from the three respectively color-coded ROI pairs in the healthy liver image. (d) Segmentation outcomes from the three respectively color-coded ROI pairs in the fatty liver image. (e), (f) Segmentation outcomes from the averages of the three threshold pairs for the healthy and the fatty liver samples, respectively. Colors within segmented images: black for the water background, gray for the liver region, and white for the lipid-rich steatotic lesions. Scale bars: 1 mm.

Table 1. Quantification of Steatosis Level from the PAM Image of Healthy Liver Sample

Table 2. Quantification of Steatosis Level from the PAM Image of Fatty Liver Sample