Photoacoustic microscopy with a pulsed multi-color source based on stimulated Raman scattering

Photoacoustic microscopy (PAM) provides excellent image contrast based on optical absorption. A very common pulsed optical source is a frequency-doubled Q-switched Nd:YAG laser. However, the fixed 532 nm output is not suitable for spectroscopic PAM. We demonstrate a simple approach to increase the number of wavelengths 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). The number of spectral peaks depends on the fiber length, peak intensity, and polarization of the propagating laser pulse. We use a Q-switched Nd:YAG microchip laser producing 0.6 ns duration pulses at 1064 nm with 10 μJ of energy at a 7.5 kHz repetition rate. A 10 mm long frequency-doubling KTP crystal produces 2 μJ pulses at 532 nm. After passing through a half-wave plate for polarization control, the 532 nm laser pulses are coupled into a 6 meter long polarization-maintaining single-mode silica 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. The individual pulse energy is over 80 nJ at four wavelengths. Imaging experiments on scattering phantoms clearly distinguish tubes filled with red and blue inks. A major advantage of our technique is the simple arrangement to convert a single-wavelength laser into a multi-color source for spectroscopic PAM. The discrete number of Stokes lines does not allow arbitrary wavelength selection, but the wavelength spacing is sufficiently close for practical applications (e.g. oxygenation measurements). Straightforward improvements to the system can achieve pulse energies over 100 nJ, which is sufficient for in vivo applications. We believe this multi-color technique can significantly benefit spectroscopic photoacoustic microscopy.