High-efficiency ∼2 µm CW laser operation of LD-pumped Tm³⁺:CaF₂ single-crystal fibers

The Tm:CaF₂ single-crystal fibers were successfully grown by modified temperature gradient technique method. The J-O intensity parameters, spontaneous radiative transition rates, radiative lifetimes and fluorescence branching ratios of Tm³⁺ were calculated with Judd-Ofelt theory. A systematic study of the fluorescence characteristics has been carried out. Simulated emission cross-sections of the ³F₄ → ³H₆ transition were calculated to be 6.68×10^-21 cm^-2 and 4.65×10^-21cm^-2 for crystal doped with 3 at.% and 4 at.% Tm³⁺. The 64.4% slope efficiency with output power of 2.23W was achieved in 3 at.% Tm:CaF₂ single-crystal fiber. The slope efficiency decreased to 44.5% and maximum output power decreased to 1.65W in 4 at.% Tm:CaF₂ single-crystal fiber. Obtained results show that Tm-doped CaF₂ single-crystal fibers are promising materials for IR laser action generation.