Up-Conversion Loss in Tm,Ho:YAG Lasers: Impact on Output Power and Efficiency

The unfavorable up-conversion loss is a crucial factor that limits the high output power and efficient operation of Tm,Ho:YAG laser. This loss is strongly related to dopant concentration, with 'Tm3+' and 'Ho3+' dopant concentrations ranging from 4% to 6% and 0.3% to 1.5%, respectively. However, the relatively high concentration of these dopants has a few disadvantages. To represent the dopant concentration of 'Tm3+' and 'Ho3+', we use the notations 'c(Tm3+)' and 'c(Ho3+)'. Although high 'c(Tm3+)' ensures high absorption efficiency and efficient cross-relaxation, it also leads to the accumulation of massive excited ions in the upper level, which significantly enhances the up-conversion processes that generate excessive heat by nonradiative decay. This issue is particularly important when pumping is intense, and the thermally induced optical distortions can compromise the laser performance. Therefore, the concentration of dopants needs to be carefully controlled to balance the trade-off between absorption efficiency and up-conversion loss.