Excitation scheme for the upper energy levels in a Tm:Yb:BaY_2F_8 laser crystal

Energy-transfer processes (including upconversion) are quantitatively studied in a $\mathrm{Tm}:\mathrm{Yb}:{\mathrm{BaY}}_2{\mathrm{F}}_8$ laser crystal. Several energy-transfer, cross-relaxation, and upconversion parameters are determined. We prove that at excitation of the crystal via Yb ions, the Tm levels ${ }^3{F}_4,$ ${ }^3{H}_4,$ ${ }^3{F}_{3,2},$ ${ }^1{G}_4,$ and ${ }^3{P}_0$ are populated via Yb–Tm interaction, and the level ${ }^1{D}_2$ is likely populated owing to ${ }^3{F}_3 \mathrm{Tm}{–}^3{F}_3 \mathrm{Tm}$ upconversion. The possibility of resonant excited-state absorption at 706 nm and resonant energy-transfer upconversion populating the level ${ }^1{D}_2$ are discussed. The possibility of avalanche pumping of $\mathrm{Tm}:\mathrm{Yb}:{\mathrm{BaY}}_2{\mathrm{F}}_8$ at 706 nm is shown.