Abstract

We report on highly efficient and power-scalable laser operation in a thulium-doped high-phonon-energy crystal [monoclinic double tungstate, ${\rm KLu}{({{\rm WO}_4})_2}$] on the ${^3{{\rm H}}_4} \to {^3{{\rm H}}_5}\;{{\rm Tm}^{3 +}}$ transition giving rise to the short-wave infrared emission at ${\sim}{2.3}\;\unicode{x00B5}{\rm m}$. A 3 at. % Tm-doped crystal generated a maximum continuous-wave output power of 1.12 W at ${\sim}{2.22}$ and 2.29 µm with a record-high slope efficiency of 69.2% (versus the absorbed pump power), a slightly multimode beam (${\rm M}_{x,y}^2 = {2.2}$ and 2.6), and a linear laser polarization. The ${\sim}{2.3}\;\unicode{x00B5}{\rm m}$ laser outperformed the one operating on the conventional ${^3{{\rm F}}_4} \to {^3{{\rm H}}_6}$ transition (at ${\sim}{1.95}\;\unicode{x00B5}{\rm m}$). The effect of the Tm concentration on the ${\sim}{2.3}\;\unicode{x00B5}{\rm m}$ laser performance indicates a gradually increasing pump quantum efficiency for the ${^3{{\rm H}}_4}$ upper laser level with the Tm doping. For the 3 at. % Tm-doped crystal, it reached ${1.8} {\pm} {0.1}$ (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 ${^3{{\rm H}}_4} \to {^3{{\rm H}}_5}$ and ${^3{{\rm F}}_4} \to {^3{{\rm H}}_6}$ 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 µm lasers.

© 2021 Optical Society of America

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