Highly Efficient 2.3 Mu M Thulium Lasers Based On A High-Phonon-Energy Crystal: Evidence Of Vibronic-Assisted Emissions

We report on highly efficient and power-scalable laser operation in a thulium-doped high-phonon-energy crystal monoclinic double tungstate, KLu(WO4)(2)] on the H-3(4) -> H-3(5) Tm3+ transition giving rise to the short-wave infrared emission at similar to 2.3 mu m. A 3 at. % Tm-doped crystal generated a maximum continuous-wave output power of 1.12 W at similar to 2.22 and 2.29 mu m with a record-high slope efficiency of 69.2% (versus the absorbed pump power ), a slightly multimode beam (M-x,y(2) = 2.2 and 2.6), and a linear laser polarization. The similar to 2.3 mu m laser outperformed the one operating on the conventional F-3(4) -> H- 3(6) transition (at similar to 1.95 mu m). The effect of the Tm concentration on the similar to 2.3 mu m laser performance indicates a gradually increasing pump quantum efficiency for the H-3(4) upper laser level with the Tm doping. For the 3 at. % Tm-doped crystal, it reached 1.8 +/- 0.1 (almost two-for-one pump process), which is attributed to efficient energy-transfer upconversion. We discuss the physical nature of the laser emissions occurring at intermediate wavelengths between the electronic H-3(4) -> H-3(5) and F-3(4) -> H-3(6) transitions and highlight the role of electron-phonon coupling (vibronic processes) in the appearance of such laser lines. This allowed us to better understand the near- and mid-infrared emission from thulium ions, which can be used in broadly tunable and fs mode-locked 2-2.3 mu m lasers. (C) 2021 Optical Society of America