High quantum fluorescence yield of Er 3+ at 1.5 μm in an Yb 3+, Ce 3+-codoped CaF 2 crystal

For the first time, a quaternary doping system of Er ³⁺, Yb ³⁺, Ce ³⁺, Na ⁺:CaF ₂ single crystal was demonstrated to have high fluorescence yield in the eye-safe 1.5 μm region under 980 nm laser diode pumping, with relatively broad and flat gain curves. A simplified model was established to illustrate the effect of Ce ³⁺ on the branching ratio for the Er ³⁺⁴I _11/2 → ⁴I _13/2 transition. With 0.2-at.% Er ³⁺ and 2.0-at.% Ce ³⁺ in the quaternary-doped CaF ₂ crystal, the branching ratio was estimated to be improved more than 40 times by the deactivating effect of Ce ³⁺ on the Er ³⁺⁴I _11/2 level. The quaternary-doped CaF ₂ system shows great potential to achieve high laser performance in the 1.5 μm region.