Efficient Tm:LiYF₄ Lasers at ${\sim}2.3~\mu$ m: Effect of Energy-Transfer Upconversion

Abstract

The <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> → <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> transition of Thulium ions (Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ), which features laser emission at ~2.3 μm is studied in details. We revise the conditions for efficient laser operation using a rate-equation model accounting for the ground-state bleaching, cross-relaxation and energy-transfer upconversion (ETU). We show that ETU has a crucial role in reaching more than unity pump quantum efficiency (QE) for ~2.3 μm Tm lasers based on highly-doped crystals. A Ti:Sapphire pumped quasicontinuous-wave 3.5 at.% Tm:LiYF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> laser generated 0.73 W at 2306 nm with a record-high slope efficiency of 47.3% (versus the absorbed pump power, for double-pass pumping) featuring a QE of 1.27. Diode-pumping of this crystal yielded a peak output power of 2 W. The first 2.3 μm Tm waveguide laser is also reported based on Tm:LiYF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> epitaxial layers with even higher doping of 6.2 at.% generating 0.23 W with a slope efficiency of 19.8%. The spectroscopic properties of Tm:LiYF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> relevant for the ~2.3 μm laser operation are revised as well.