Highly efficient dual-wavelength mid-infrared CW Laser in diode end-pumped Er:SrF₂ single crystals

The spectral properties and laser performance of Er:SrF₂ single crystals were investigated and compared with Er:CaF₂. Er:SrF₂ crystals have larger absorption cross-sections at the pumping wavelength, larger mid-infrared stimulated emission cross-sections and much longer fluorescence lifetimes of the upper laser level (Er³⁺:⁴I_11/2 level) than those of Er:CaF₂ crystals. Dual-wavelength continuous-wave (CW) lasers around 2.8 μm were demonstrated in both 4at.% and 10at.% Er:SrF₂ single crystals under 972 nm laser diode (LD) end pumping. The laser wavelengths are 2789.3 nm and 2791.8 nm in the former, and 2786.4 nm and 2790.7 nm in the latter, respectively. The best laser performance has been demonstrated in lightly doped 4at.% Er:SrF₂ with a low threshold of 0.100 W, a high slope efficiency of 22.0%, an maximum output power of 0.483 W.