We report on the observation of upconversion blue fluorescence pumped
in the near-infrared of Tm3+ and Pr3+ doped BIGaZYbTZr glasses. Fluore
scence dynamics is interpreted in terms of a cooperative energy transf
er in which two excited Yb3+ ions transfer simultaneously their energy
to either TM3+(1G4) or Pr3+(P-3(0)) ions.