Spectroscopic photoacoustic microscopy in the 1064–1300 nm range using a pulsed multi-color source based on stimulated Raman scattering

Photoacoustic microscopy (PAM) provides excellent image contrast based on optical absorption. Lipidrich tissue, such as atherosclerotic plaques and myelinated nerve fibers, exhibits an optical absorption peak near 1210 nm. Unfortunately, pulsed lasers operating in this wavelength range use expensive optical parametric oscillator (OPO) systems. We demonstrate a simple approach to convert an inexpensive 1064 nm pulsed laser into a multi-wavelength source suitable for spectroscopic PAM of lipid-rich tissue. Wavelength conversion is achieved by using stimulated Raman scattering (SRS) in an optical fiber. A sufficiently intense laser pulse nonlinearly interacts with the internal vibrations of the glass molecular structure to produce a series of down-shifted frequency components (Stokes lines). We use a Q-switched Nd:YAG microchip laser producing 0.6 ns duration pulses at 1064 nm with 8 uJ of energy at a 7.4 kHz repetition rate. The laser pulses are coupled into a 20 meter long single-mode fiber. The multi-color fiber output goes through a band pass filter, where the selected wavelength is sent to a photoacoustic microscopy system employing optical focusing. Pulse energies above 200 nJ are produced within spectral bands at 1064, 1100, 1175, 1225, 1275, and 1325 nm. Imaging experiments with phantoms containing butter clearly show the expected absorption peak near 1210 nm. A major advantage of our technique is the simple arrangement to convert a single-wavelength laser into a multi-color source for spectroscopic PAM. We believe this multi-color technique is a promising method to achieve high-speed spectroscopic photoacoustic microscopy of lipid-rich tissue.