Ultrafast thulium fiber laser operating at 1750 nm

Passively mode-locked femtosecond fiber lasers operating in the telecom band around 1.5 μm are well discovered since many years. Thulium- and Holmium-based hght sources in 2 μm region have greatly gained interest in recent years. Because of the absence of optical transitions in fibers doped with rare-earth ions, the gap between these two spectral regions, namely laser sources at wavelengths around 1750 nm, have hardly been investigated, even though this wavelength region has some interesting applications, such as multi-photon fluorescence imaging [1] or medical applications [2].