High spectral energy density all-fiber nanosecond pulsed 1.7 μm light source for photoacoustic microscopy

Seongjin Bak; Sang Min Park; Yuon Song; Jeesu Kim; Tae Won Nam; Dong-Wook Han; Chang-Seok Kim; Soon-Woo Cho; Brett E Bouma; Hwidon Lee

doi:10.1016/j.pacs.2025.100744

High spectral energy density all-fiber nanosecond pulsed 1.7 μm light source for photoacoustic microscopy

Photoacoustics. 2025 Jun 16:44:100744. doi: 10.1016/j.pacs.2025.100744. eCollection 2025 Aug.

Authors

Seongjin Bak¹, Sang Min Park¹, Yuon Song², Jeesu Kim^{1

2}, Tae Won Nam³, Dong-Wook Han², Chang-Seok Kim^{1

2}, Soon-Woo Cho^{1

4}, Brett E Bouma⁵, Hwidon Lee^{1

2}

Affiliations

¹ Engineering Research Center for Color-Modulated Extra-Sensory Perception Technology, Pusan National University, Busan 46241, Republic of Korea.
² Department of Cogno-Mechatronics Engineering, Pusan National University, Busan 46241, Republic of Korea.
³ Department of Materials and Science and Engineering, Pusan National University, Busan 46241, Republic of Korea.
⁴ Department of Biomedical Engineering, Duke University, Durham, NC 27708, USA.
⁵ Wellman Center for Photomedicine, Harvard Medical School and Massachusetts General Hospital, Boston, MA 02114, USA.

Abstract

We present a high spectral energy density all-fiber nanosecond pulsed 1.7 μm light source specifically designed for photoacoustic microscopy (PAM). The system targets the 1st overtone absorption of C-H bonds near 1720 nm within the near-infrared-III (NIR-III) window, where lipids exhibit strong optical absorption, and tissues benefit from reduced scattering and high permissible fluence. To achieve narrow-linewidth, high pulse energy, and high pulse repetition rate (PRR), we developed a master oscillator fiber amplifier architecture based on stimulated Raman scattering. A 1589.80 nm Raman pump and a custom-built narrow-linewidth Raman seed laser were employed to generate spectrally pure 1719.44 nm pulses (∼0.10 nm linewidth). The proposed light source delivers nanosecond pulses (∼5 ns) with high pulse energy (≥2.2 μJ) and tunable PRRs up to 300 kHz, resulting in a spectral energy density of approximately 22 μJ/nm-significantly higher than that of conventional 1.7 μm light sources. Performance of the NIR-PAM system was validated through resolution testing with a 1951 USAF target, demonstrating a spatial resolution of approximately 4.14 μm and an axial resolution of approximately 85.5 μm. Phantom imaging of CH₂-rich polymer films and ex vivo lipid-rich biological tissues confirmed the system's high spatial fidelity and strong contrast for lipid-specific structures. This compact, stable, and spectrally refined light source with high spectral energy density can offer an effective solution for high-resolution, label-free molecular imaging and represents a promising platform for clinical photoacoustic imaging applications involving lipid detection and metabolic disease diagnostics.

Keywords: 1.7 µm light source; All-fiber nanosecond pulsed light source; High spectral energy density; Narrow-linewidth; Photoacoustic microscopy (PAM); Stimulated Raman scattering (SRS).