Blue and green up-conversion in (Yb3+, Tb3+) co-doped fluorophosphate glasses

Up-conversion processes are studied in (Yb 3+ , Tb 3+ ) co-doped fluorophosphate glasses. These glasses contain a large concentration of terbium fluoride, up to 36 mol%, and still luminesc. Samples are excited at 980 nm, within the Yb 3+ absorption band. For `low' Yb 3+ concentrations, i.e., below 16 mol%, the typical green luminescence of terbium is observed. With increasing Yb 3+ concentration, the green luminescence vanishes and a blue emission appears. Both emissions are due to a cooperative effect between two Yb 3+ ions.