Efficient holmium-doped fluoride fiber laser emitting 2.1 μm and blue upconversion fluorescence upon excitation at 2 μm

We demonstrate a highly efficient and high power Ho³⁺-doped fluoride glass fiber laser that is resonantly pumped with a Tm ³⁺-doped silicate glass fiber laser operating at 2.051 μm. The laser operates at 2080 nm and generated 6.66 W at a slope efficiency of 72%. We observe strong visible upconversion fluorescence centered at a variety of wavelengths including 491 nm which results from three sequential energy transfer upconversion processes; the fluorescence to pump energy ratio for this emission is one the largest reported to date.