  We have argued that a high-purity Yb-doped silica glass can potentially be
cooled via anti-Stokes fluorescence optical refrigeration. This conclusion is
reached by showing, using reasonable assumptions for the host material
properties, that the non-radiative decay rate of Yb ions can be made
substantially smaller than the radiative decay rate. Therefore, an internal
quantum efficiency of near unity can be obtained. Using spectral measurements
of the fluorescence emission from a Yb-doped silica optical fiber at different
temperatures, we estimate the minimum achievable temperature in Yb-doped silica
glass for different values of internal quantum efficiency.
