Tm:CALGO lasers at 2.32 µm: cascade lasing and upconversion pumping
Hippolyte Dupont, Pavel Loiko, Aleksey Tyazhev, Luidgi Giordano, Zhongben Pan, Hongwei Chu, Dechun Li, Bruno Viana, Ammar Hideur, Lauren Guillemot, Alain Braud, Patrice Camy, Patrick Georges, Frédéric Druon
Abstract
We report on the first laser operation of a disordered Tm:CaGdAlO 4 crystal on the 3 H 4 → 3 H 5 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 3 F 4 Tm 3+ state leading to the ground-state bleaching are exploited: cascade lasing on the 3 H 4 → 3 H 5 and 3 F 4 → 3 H 6 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 ( 3 F 4 → 3 H 6 ) and 2.32 µm ( 3 H 4 → 3 H 5 ) 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 3+ ions for the 3 F 4 → 3 F 2,3 and 3 F 4 → 3 H 4 transitions are measured for polarized light. Tm 3+ ions in CaGdAlO 4 exhibit broadband emission at 2.3 - 2.5 µm making this crystal promising for ultrashort pulse generation.