Tm:CALGO lasers at 2.32 µm: cascade lasing and upconversion pumping.

We report on the first laser operation of a disordered Tm:CaGdAlO crystal on the H → H transition. Under direct pumping at 0.79 µm, it generates 264 mW at 2.32 µm with a slope efficiency of 13.9% and 22.5% vs. incident and absorbed pump power, respectively, and a linear polarization (σ). Two strategies to overcome the bottleneck effect of the metastable F Tm state leading to the ground-state bleaching are exploited: cascade lasing on the H → H and F → H transitions and dual-wavelength pumping at 0.79 and 1.05 µm combining the direct and upconversion pumping schemes. The cascade Tm-laser generates a maximum output power of 585 mW at 1.77 µm (F → H) and 2.32 µm (H → H) with a higher slope efficiency of 28.3% and a lower laser threshold of 1.43 W, out of which 332 mW are achieved at 2.32 µm. Under dual-wavelength pumping, further power scaling to 357 mW at at 2.32 µm is observed at the expense of increased laser threshold. To support the upconversion pumping experiment, excited-state absorption spectra of Tm ions for the F → F and F → H transitions are measured for polarized light. Tm ions in CaGdAlO exhibit broadband emission at 2.3 - 2.5 µm making this crystal promising for ultrashort pulse generation.