Optical-resolution photoacoustic microscopy based on the overdriving of a CW laser diode

Abstract

Although the feasibility of the photoacoustic microscopy (PAM) based on the overdriving of a continuous wave (CW) laser diode was demonstrated already in biomedical photoacoustics [1], it was limited to an acoustic-resolution PAM because generated pulsed laser beams were delivered by a 400-μm core diameter multi-mode optical fiber, thereby resulting in a poor lateral resolution. The reason that such a large diameter optical fiber was employed was to minimize energy loss during the delivery of the generated laser pulses to the scanning head. In this study, we developed an optical-resolution (OR) PAM system that works based on the overdriving of a 450 nm laser diode (LD), which was originally manufactured to provide a 7 W average power at a CW operation mode. Most importantly, the system was implemented to have a light guiding section implemented using a 25-μm core optical fiber, which was the core element that enabled OR-PAM in conjunction of a GRIN lens. Achieved pulse width and energy after a light delivery by the optical fiber were ~20 ns and ~300 nJ, respectively, at 1 kHz repetition rate. Through our first imaging test using an US Air Force resolution target, we could see that the produced energy was enough to acquire photoacoustic signals at a SNR of ~20 dB even after the light delivery by the optical fiber.