In vivo three- and four-photon fluorescence microscopy using a 1.8 $\mu$m femtosecond fiber laser system

Multiphoton microscopy has enabled us to image cellular dynamics in vivo. However, the excitation wavelength for imaging with commercially available lasers is mostly limited between 650-1040 nm. Here we develop a femtosecond fiber laser system that produces $\sim$150 fs pulses at 1.8 $\mu$m. Our system starts from an erbium-doped silica fiber laser, and its wavelength is converted to 1.8 $\mu$m using a Raman shift fiber. The 1.8 $\mu$m pulses are amplified with a two-stage Tm:ZBLAN fiber amplifier. The final pulse energy is $\sim$1 $\mu$J, sufficient for in vivo imaging.