High quality-factor Kerr-lens mode-locked Tm:Sc2O3 single crystal laser with anomalous spectral broadening

We report on a high quality-factor Kerr-lens mode-locked Tm:Sc2O3 laser operating around 2.1 μm. Using a simple four mirror Z-shaped cavity consisting of an output coupler (OC) of 0.5% transmittance, two folding mirrors, and a chirped mirror, pulses as short as 72 fs with an average power of 130 mW were obtained at a center wavelength of 2108 nm. By replacing the 0.5% OC with a ∼0.3% one, we observed anomalous broadening of the spectrum into the range between ∼2250 and 2350 nm where Tm:Sc2O3 shows no gain.