Broadly tunable continuous-wave Tm,Ho:SrF₂ and Tm,Ho:BaF₂ lasers

We report on the first, to our knowledge, room-temperature continuous-wave laser operation of Tm³⁺,Ho³⁺-codoped barium and strontium fluoride crystals at ∼2.1 µm. The 3 at.% Tm, 0.5 at.% Ho:BaF₂ laser generated 160 mW at 2073 nm with a slope efficiency of 31.0% and a laser threshold of 43 mW. The continuous wavelength tuning of this laser from 2010 to 2090 nm (tuning range, 80 nm) was demonstrated. The spectroscopic properties of Tm,Ho:SrF₂ and Tm,Ho:BaF₂ crystals were also determined, showing enhanced Tm³⁺ → Ho³⁺ energy transfer in rare-earth clusters. For Ho³⁺ ions in BaF₂, the stimulated-emission cross section is 0.40 × 10^-20 cm² at 2044 nm, and the thermal equilibrium luminescence lifetime is as long as 14.1 ms.