Generation of watt-class, sub-50 fs pulses through nonlinear spectral broadening within a thulium-doped fiber amplifier.

We demonstrate direct generation of sub-50 fs pulses from a thulium-doped fiber amplifier. Broad spectra are obtained by exploiting nonlinear effects within the amplifier fiber itself. High fractional inversion densities of thulium ions achieved by a core-pumping scheme helped to extend spectra into the shorter wavelength region around 1.7 µm. Pulses with a duration of 48 fs are obtained at an average power of 2.5 W directly after the amplifier fiber, i.e., without using a compressor.