Excited-state absorption and upconversion pumping of Tm-doped potassium lutetium double tungstate.

We report on a bulk thulium laser operating on the H → H transition with pure upconversion pumping at 1064 nm by an ytterbium fiber laser (addressing the F → F excited-state absorption (ESA) transition of Tm ions) generating 433 mW at 2291 nm with a slope efficiency of 7.4% / 33.2% vs.