Excited-state absorption and upconversion pumping of Tm³⁺-doped potassium lutetium double tungstate

We report on a bulk thulium laser operating on the ³H₄ → ³H₅ transition with pure upconversion pumping at 1064 nm by an ytterbium fiber laser (addressing the ³F₄ → ³F_2,3 excited-state absorption (ESA) transition of Tm³⁺ ions) generating 433 mW at 2291 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 2.3 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 1.06 µm are also explored, indicating a positive effect of co-pumping at 0.79 µm on reducing the threshold pump power for upconversion pumping.