We study the upconversion properties of Ta<sub>2</sub>O<sub>5</sub> and Nb<sub>2</sub>O<sub>5</sub> based TeO<sub>2</sub> glasses doped with 0.5 mol% Er<sub>2</sub>O<sub>3</sub> after excitation into the <sup>4</sup>I<sub>1½</sub> level using 980 nm radiation. Green and red emission is observed originating from the (<sup>2</sup>H<sub>1½</sub>, <sup>4</sup>S<sub>3/2</sub>) and <sup>4</sup>F<sub>9/2</sub> states, respectively. Excited state absorption and energy trasnfer are discussed as possible mechanisms for the upconversion.
Lanthanide doped tellurite glasses containing heavy metals were prepared by the melt/quench technique. The absorption spectra in the visible and in the near infrared were measured and the behavior of the electric dipole oscillator strengths of the lanthanide 4f→4f transitions with the amount of modifier oxide in the glass matrix are discussed. The luminescence spectra in the visible and in the near infrared of the dopants were measured. The stimulated emission cross sections were reported and compared with the ones obtained for oxide glasses of relevant technological applications.