Photoacoustic microscopy using four-wave mixing in a multimode fiber

Many applications in photoacoustic microscopy (PAM) use contrast agents that preferentially absorb light at wavelengths other than 532 nm. This usually requires the use of expensive dye or optical parametric oscillator (OPO) lasers. We have previously demonstrated a cost-effective alternative using an inexpensive 532 nm laser and a photonic crystal fiber, where multiple wavelengths are produced by stimulated Raman scattering (SRS). In this paper, we demonstrate a new source using four-wave mixing (FWM) in SMF-28e, an industry standard telecommunications fiber. Our system is based on a Q-switched Nd:YAG laser producing 2 ns duration pulses at 532 nm with 15 uJ of energy at a 20 kHz repetition rate. The laser pulses are coupled into 100 meters of SMF-28e, a step-index fiber that is designed to be single-mode at 1310 and 1550 nm but supports a few optical modes at 532 nm. The fiber output spectrum contains both discrete lines below 625 nm and a broad continuum from 625 to 900 nm. This continuum is most likely due to phase matched FWM between multiple fiber modes, particularly the LP01, LP11, and LP02 modes. The fiber output goes through a dielectric filter, where the selected spectral window is sent to a PAM system employing optical focusing and laser scanning. PAM imaging of a USAF target produced an image resolution of 7 μm using 800 nm light. Imaging experiments are performed on a dye phantom, where one tube is filled with diluted red ink and the other with 200 μM of IR-820 dye. Dielectric filters were used to select either a 750-900 nm (1.3 uJ pulse energy) or 532-700 nm (1.0 uJ pulse energy) spectral window. The resulting images easily differentiate between the two absorbers. The advantages of this multi-wavelength optical source are the very simple apparatus, the flexibility in choosing the desired spectral range, and the use of a very inexpensive optical fiber.