The role of Nd3+ in the population inversion between 1G4 and 3H6 states of Tm3+ in Yb:Nd:Tm:YLF crystal

In this paper, we present the energy transfer rates of YLF:Yb:Tm:Nd system, identifying the most important processes that lead to the thulium blue upconversion emission under excitation around 792 nm. The calculation of the rate equations system based on numerical method using Runge-Kutta method was employed for analyzing the population inversion between 1G4 and 3H4 states of Tm3+. Population inversion was achieved only in Yb:Nd:Tm doped YLF crystal, which exhibits a pumping rate threshold of 620 s-1. The Nd3+ absorption near 792 nm is very important because the excitation is rapidly transferred to Yb3+, that also interacts with Tm3+.