Multispectral photoacoustic microscopy using a photonic crystal fiber supercontinuum source

Photoacoustic microscopy (PAM) provides excellent image contrast based on optical absorption. Microchip lasers are attractive optical sources for PAM, as they are compact and provide nanosecond pulse durations at several kHz repetition rates. However, spectroscopic imaging is not possible with microchip lasers due to their fixed wavelength output. We are investigating multispectral PAM with a supercontinuum source based on a photonic crystal fiber (PCF) pumped with a microchip laser. 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 tunable band-pass filter before being focused into the object. Photoacoustic detection is performed with a 25 MHz spherically focused f/2 transducer. En-face imaging experiments were performed on ink phantoms. Images are acquired at seven different wavelengths from 575 to 875 nm. A simple discriminant analysis of the multispectral photoacoustic data produces images that clearly distinguish the different absorbing regions of the sample. These preliminary results suggest the potential of the supercontinuum PCF source for multispectral PAM.