Self-phase-modulated femtosecond laser source at 1603 nm and its application to deep-brain 3-photon microscopy in vivo.

3-photon microscopy (3PM) excited at the 1700 nm window enables deep-tissue imaging in vivo, especially in brain. PC rod soliton source has previously been exclusively used as the excitation source, which is rather costly and difficult to align. Here we demonstrate a novel nonlinear optical technique to build femtosecond laser source at the 1700 nm window, based on self-phase modulation (SPM) in a short span of large-mode-area fiber. The spectral broadening experienced by the pump pulse leads to the generation of a red-shifted sidelobe at 1603 nm. After spectral filtering, this sidelobe corresponds to 170-fs, 167-nJ pulses at 1603 nm. Using this SPM source, we further demonstrate deep-brain 3 PM to a depth of 1500 μm below the mouse brain surface in vivo. Our SPM femtosecond laser source thus provides a cost effective and easy-to-align alternative excitation source to the PC rod soliton source.