RESUMO
We report on a bulk thulium laser operating on the 3H4 â 3H5 transition with pure upconversion pumping at 1064â nm by an ytterbium fiber laser (addressing the 3F4 â 3F2,3 excited-state absorption (ESA) transition of Tm3+ ions) generating 433â mW at 2291â nm with a slope efficiency of 7.4% / 33.2% vs. the incident / absorbed pump power, respectively, and linear laser polarization representing the highest output power ever extracted from any bulk 2.3â µm thulium laser with upconversion pumping. As a gain material, a Tm3+-doped potassium lutetium double tungstate crystal is employed. The polarized ESA spectra of this material in the near-infrared are measured by the pump-probe method. The possible benefits of dual-wavelength pumping at 0.79 and 1.06â µm are also explored, indicating a positive effect of co-pumping at 0.79â µm on reducing the threshold pump power for upconversion pumping.