Photoacoustic microscopy using four-wave mixing in a large mode-area fiber

Many applications in photoacoustic microscopy (PAM) require laser pulses at multiple wavelengths. High repetition rate multi-wavelength lasers have been demonstrated using nonlinear fiber optics, which produce a wide wavelength range but tend to suffer from low pulse energy (e.g. 100 nJ). This paper presents our preliminary investigation into multimode fibers as a path to achieve higher output laser pulse energy for multi-spectral PAM. Laser pulses from a 532 nm pulsed Nd:YAG laser are coupled into a large mode area (LMA) fiber that supports six spatial modes at 532 nm. Phase-matched four-wave mixing (FWM) between fiber modes produces a broad continuum extending past 800 nm. The pulse energy at 550, 650, and 750 nm spectral bands is 0.84 uJ, 1.31 uJ, and 0.73 uJ, respectively. Multi-spectral imaging experiments successfully differentiate vessel phantoms containing Allura Red, methylene blue, and IR-820 dyes. These results show the potential of using intermodal FWM in a few-mode multimode fiber to achieve uJ-level pulse energies for multi-spectral PAM applications.