Broadly tunable continuous-wave Tm:CALYO laser operating on the ³H₄→³H₅ transition

We report broadly tunable diode-pumped laser operation of a disordered Tm:CALYO aluminate crystal on the ³H₄ → ³H₅ transition. By applying an intracavity birefringent filter, continuous tuning ranges of 258 nm (2254-2512 nm) and 225 nm (2235-2460 nm) were achieved for the π- and σ-polarized eigen polarization states of this uniaxial crystal, respectively. In the free-running regime, the Tm:CALYO laser delivered a maximum continuous-wave output power of 1.10 W at 2.3 µm with a slope efficiency of 6.4%. The polarized spectroscopic properties of Tm³⁺ ions in CALYO were systematically investigated to explain the observed laser behavior. The stimulated-emission cross section for π-polarized light reaches 0.23 × 10^-20 cm² at 2353 nm, corresponding to an emission bandwidth exceeding 250 nm, while the intrinsic luminescence lifetime of the ³H₄ upper laser level is about 400 µs. Attention was devoted to crystal annealing conditions, which were found to play a critical role in optimizing laser performance and represent a pathway for further improvement. Owing to its exceptionally broad emission bandwidth and good thermal properties, Tm:CALYO emerges as a promising gain material for femtosecond pulse generation at 2.3-2.4 µm.