Resonantly pumped tunable Tm,X:CaF₂ lasers: effect of buffer ions (X = Y, La, Gd, and Lu)

We report on the eye-safe laser performance of Tm³⁺-doped calcium fluoride crystals, modified with optically inactive "buffer" cations (Y³⁺, Lu³⁺, Gd³⁺, and La³⁺), under in-band diode-pumping at 1.68 µm. In the free-running regime, the 1.5 at.% Tm, 4 at.% Y:CaF₂ laser operates with a high slope efficiency of 47% with respect to absorbed pump power. By employing an MgF₂ birefringent plate, the emission wavelength of this laser is continuously tuned over 242 nm (1820-2062 nm), highlighting its potential for broadly tunable and ultrafast oscillators. The spectroscopic properties of Tm³⁺ ions in CaF₂ crystals with and without buffer ions are also compared.