Efficient continuous-wave Tm,Ho:CaF₂ laser at 2.1µm

We report on the first, to the best of our knowledge, continuous-wave laser operation of a Tm³⁺,Ho³⁺-codoped calcium fluoride crystal at ∼2.1 µm. Tm,Ho:CaF₂ crystals were grown by the Bridgman method, and their spectroscopic properties were studied. The stimulated-emission cross section for the ⁵I₇ → ⁵I₈ Ho³⁺ transition is 0.72 × 10^-20 cm² at 2025 nm, and the thermal equilibrium decay time is 11.0 ms. A 3 at. % Tm, 0.3 at. % Ho:CaF₂ laser generated 737 mW at 2062-2088 nm with a slope efficiency of 28.0% and a laser threshold of 133 mW. Continuous wavelength tuning between 1985 and 2114 nm (tuning range: 129 nm) was demonstrated. The Tm,Ho:CaF₂ crystals are promising for ultrashort pulse generation at ∼2 µm.