Abstract
An ytterbium doped silica optical fiber with a core diameter of $900\;\unicode{x00B5}{\rm m}$ has been cooled by 18.4 K below ambient temperature by pumping with 20 W of 1035 nm light in vacuum. In air, cooling by 3.6 K below ambient was observed with the same 20 W pump. The temperatures were measured with a thermal imaging camera and differential luminescence thermometry. The cooling efficiency is calculated to be $1.2 \pm 0.1\%$. The core of the fiber was codoped with ${\rm{A}}{{\rm{l}}^{3 +}}$ for an Al to Yb ratio of 6:1, to allow for a larger Yb concentration and enhanced laser cooling.
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