Tm,Ho:YAG Laser Output Power Limitation: The Role of Up-Conversion Loss and Dopant Concentration

The unfavorable up-conversion loss, which is a crucial factor in limiting the high output power of Tm,Ho:YAG laser and efficient operation, is strongly related to dopant concentration. The dopant concentrations of Tm3+ and Ho3+ generally range from 4% to 6% and 0.3% to 1.5%, respectively. However, this relatively high concentration has some disadvantages. High c(Tm3+) ensures high absorption efficiency and efficient cross-relaxation, but massive excited ions accumulated in the upper level significantly enhance the up-conversion processes, which is linearly dependent on c(Tm3+), generating excessive heat by nonradiative decay. This issue is particularly important when pumping is intense, and the thermally induced optical distortions will compromise the laser performance. We use the notations c(Tm3+) and c(Ho3+) to represent the dopant concentration of Tm3+ and Ho3+, respectively.