Energy-transfer processes in Yb : Tm-doped KY3F10, LiYF4, and BaY2F8 single crystals for laser operation at 1.5 and 2.3 mu m

Energy-transfer processes have been quantitatively studied in various Tm:Yb -doped fluoride crystals. A comparison between the three host crystals whic h have been examined (KY3F10, LiYF4, and BaY2F8) shows clearly that the eff iciency of the Yb-->Tm energy transfers is larger in KY3F10 than in LiYF4 o r BaY2F8. The dependence of the energy-transfer parameters upon the codopan t concentrations has been experimentally measured and compared with the res ults calculated on the basis of migration-assisted energy-transfer models. Using these energy-transfer parameters and a rate equation model, we have p erformed a theoretical calculation of the laser thresholds for the H-3(4)-- >F-3(4) and H-3(4)-->H-3(5) laser transitions of the Tm ion around 1.5 and 2.3 mu m, respectively. Laser experiments performed at 1.5 mu m in Yb:Tm:Li YF4 then led to laser threshold values in good agreement with those derived theoretically. Based on these results, optimized values for the Yb and Tm dopant concentrations for typical values of laser cavity and pump modes wer e finally derived to minimize the threshold pump powers for the laser trans itions around 1.5 and 2.3 mu m.