A quantitative description of optical refrigeration in Yb3+-doped ZBLAN glass in the presence of transition-metal and
OH impurities is presented. The model includes the competition of radiative processes with energy migration, energy
transfer to transition-metal ions, and multiphonon relaxation. The cooling efficiency is sensitive to the presence of both
3d metal ions with absorption in the near infrared and high-frequency vibrational impurities such as OH. The calculation
establishes maximum impurity concentrations for different operating temperatures and finds Cu2+, Fe2+, Co2+, Ni2+, and
OH to be the most problematic species. Cu2+ in particular has to be reduced to <2 ppb, and Fe2+, Co2+, Ni2+, and OH have
to be reduced to 10-100 ppb for a practical ZBLAN:Yb3+ optical cryocooler to operate at 100-150 K.
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