Upconversion emissions in Yb–Tm-doped tellurite glasses excited at 976 nm

Intense Tm blue upconversion emission has been observed in Tm–Yb codoped oxyfluoride tellurite glass under excitation with a diode laser at 976 nm. Three emission bands centered at 475, 650 and 796 nm corresponding to the transitions G→ H, G→ H and F→ H, respectively, simultaneously occur. The dependence of upconversion intensities on Tm ions concentration and excitation power are investigated. For fixed YbO concentrations of 5.0 mol%, the maximum upconversion intensity was obtained with TmO concentration of about 0.1 mol%. The blue upconversion luminescence lifetimes of the Tm transitions G→ H are measured. The results are evaluated by the possible upconversion mechanisms.