  A ytterbium doped silica optical fiber has been cooled by 18.4K below ambient
temperature by pumping with 20W of 1035nm light in vacuum. In air, cooling by
3.6K below ambient was observed with the same 20W pump. The temperatures were
measured with a thermal imaging camera and differential luminescence
thermometry. The cooling efficiency is calculated to be 1.2+-0.1%. The core of
the fiber was codoped with Al3+ for an Al to Yb ratio of 6:1, to allow for a
larger Yb concentration and enhanced laser cooling.
