Development of a Photoacoustic Microscopy System

Photoacoustic microscopy (PAM), as an imaging modality, has shown promising results in imaging angiogenesis and cutaneous malignancies like melanoma, revealing systemic diseases including diabetes, hypertension, coronary artery, tracing drug efficiency and assessment of therapy, monitoring healing processes such as wound cicatrization, brain imaging and mapping. Clinically, PAM has been emerging as a diagnostic tool. Laser parameters (particularly, pulse duration, pulse energy, pulse repetition frequency (PRF), and pulse-to-pulse stability) affect signal amplitude and quality, data acquisition speed and indirectly, spatial resolution. Lasers used in PAM are typically Q-switched lasers, low-power laser diodes, and recently, fiber lasers. The key laser parameters cannot be adjusted independently of each other, whereas microvasculature and cellular imaging, spectroscopic measurements, e.g., have dfferent requirements. We report an integrated fiber laser system producing nanosecond pulses, covering from 600 nm to 1300 nm, developed specifically for photoacoustic excitation. The system comprises of Yb-doped fiber oscillator and amplifier, an acousto-optic modulator (AOM) and photonic-crystal fiber to generate supercontinuum. Complete control over the pulse train, including generation of non-uniform pulse trains, is achieved via the AOM through custom-developed field-programmable gate-array (FPGA) electronics. The entire system is fiber-integrated; guided-beam-propagation renders it misalignment free and largely immune to mechanical perturbations. The laser is unique in thatall the important parameters are adjustable: pulse duration (1-3 ns), energy (up to 10 µJ), repetition rate (50 kHz - 3 MHz). Different photocoustic imaging probes can be excited with the ultrabroad spectrum, real-time imaging can be performed thanks to its high PRF, and the pulse energy can be adjusted by means of FPGA circuit.