Wavelength agile photoacoustic microscopy with a pulsed supercontinuum source

We are developing a multi wavelength photoacoustic microscopy system based on a pulsed optical source with high repetition rate (i.e. several kHz) and rapid wavelength tunability. The key feature is the generation of an ultrabroadband spectrum by propagating laser pulses from a microchip laser through several meters of photonic crystal fiber (PCF). The air-silica honeycomb-like microstructure of these fibers provides extremely tight optical confinement and favorable dispersion properties to dramatically enhance nonlinear optical propagation. The Q-switched Nd:YAG microchip laser produces 0.6 ns duration pulses at 1064 nm with 8 uJ of energy at a 6.6 kHz repetition rate. These pulses are sent through 7 meters of PCF with a 5 um diameter core and a zero dispersion wavelength of 1040 nm. The supercontinuum is sent through a prism-based monochromator that can rapidly select the desired wavelength. The selected wavelength is sent to a photoacoustic microscopy system employing optical focusing. Detection is performed with a 25 MHz spherically focused f/2 transducer. Ten different wavelengths (570 to 930 nm) are acquired in less than 500 ms for each image pixel. For each wavelength, approximately 200 signals are averaged together before storage. The multiwavelength images were processed with a simple approach based on linear discriminant analysis. Multiwavelength imaging is tested on phantoms with different color inks. A major advantage of our tunable source is the rapid access to widely separated wavelengths. The small pulse energy (i.e. 30 nJ) limits this source to photoacoustic microscopy systems employing optical focusing. Nevertheless, we believe this rapidly tunable laser can significantly benefit spectroscopic photoacoustic microscopy.