A series of glass samples the tungsten–tellurite system TeO2-WO3-Bi2O3-(4-x) La2O3-xEr2O3, x = 0; 0.4; 0.5; 0.7; 1.2; 2; 4 mol%, CEr 0 - 15 × 1020 cm?3 were synthesized from high-purity oxides in an oxygen flow inside a specialized sealed reactor. In all series, extremely low content hydroxyl groups was achieved (~n 1016 cm?3, more than orders magnitude lower concentration erbium ions), which g...