"Mixed" Tm:Ca(Gd,Lu)AlO₄ - a novel crystal for tunable and mode-locked 2 µm lasers

We report on the crystal growth, spectroscopy characterization and first laser operation of a new tetragonal disordered "mixed" calcium aluminate crystal, Tm:Ca(Gd,Lu)AlO₄. The introduction of Lu³⁺ leads to an additional inhomogeneous broadening of Tm³⁺ absorption and emission spectra compared to the well-known Tm:CaGdAlO₄. The maximum stimulated-emission cross-section for the ³F₄ → ³H₆ Tm³⁺ transition is 0.91 × 10^-20 cm² at 1813 nm for σ-polarization, and the emission bandwidth is more than 200 nm. A continuous-wave diode-pumped Tm:Ca(Gd,Lu)AlO₄ laser generates 1.82 W at 1945 nm with a slope efficiency of 29%. Under Ti:Sapphire laser pumping, a continuous tuning of the laser wavelength from 1836 to 2083 nm (tuning range: 247 nm) is demonstrated. The Tm:Ca(Gd,Lu)AlO₄ crystal is promising for tunable/femtosecond lasers at ~2 μm.