Low-cost and compact second harmonic laser for photoacoustic imaging

Photoacoustic imaging (PAI) combines the high contrast of optical imaging with the high resolution of ultrasound imaging, which has garnered significant attention in applications such as vascular imaging and liver function reserve (LFR) assessment. The realization of low-cost and miniaturized light sources is a crucial development direction for the future application and commercialization of PAI. Traditional high-power pulsed lasers (hundreds of millijoules range) are bulky and expensive, although light-emitting diodes (LEDs) are lower in cost and more compact, their output energy is relatively weak (microjoules range), and their pulse width is wide (tens of nanoseconds range), making them unsuitable for high-resolution PAI of clinical application. This paper proposes a low-cost, compact PAI light source based on a miniature laser commonly used in laser communication and laser radar systems, combined with a second-harmonic generation (SHG) crystal. The primary diode pumped solid-state laser (DPSS) has the following specifications: dimensions of 110 × 40 × 24 mm³, weight of 220 g, operating wavelength of 1535 nm, single-pulse energy of 2.7 mJ (pulse stability is ±5%), pulse width of 10 ns, repetition rate of 5 Hz, and spot diameter of 0.8 mm (divergence angle is 4%). To meet the requirements of PAI, the nonlinear effect of a KTiOPO₄ second-harmonic crystal is utilized, resulting in an output wavelength of 767.26 nm, pulse width of 10 ns, and single-pulse energy of 0.68 mJ. To verify this light source, we performed PAI of blood vessels in the human finger and dorsal hand, as well as PAI detection analysis with different concentrations of Indocyanine green (ICG). The results demonstrated that the proposed low-cost, compact second-harmonic laser has the potential for vascular imaging and LFR evaluation. The proposed method for the excitation source offers a reliable approach for the development of low-cost, compact PAI and contributes to the advancement of miniaturized, portable PAI systems.