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

The 3 H 4 → 3 H 5 transition of Thulium ions (Tm 3+ ), 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 4 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 4 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 4 relevant for the ~2.3 μm laser operation are revised as well.