Observation of anti-Stokes-fluorescence cooling in a ZBLAN fiber with a Yb-doped cladding

This paper reports the first experimental observation of anti-Stokes cooling of fibers in which both the core and the cladding doped with Yb3+ to increase to number of Yb ions contributing to cooling and induce greater refrigeration. Two ZBLAN fibers were designed, fabricated by Le Verre Fluore, and evaluated experimentally. Two cladding profiles were tested, both with asymmetric boundaries to induce greater mode mixing, and therefore better pump filling of the fiber and greater cooling. Temperature measurements showed that the fiber with a double-D cladding did not perform as well (it cooled to –78 mK for 240 mW of input pump power at 1025.5 nm) largely due to limited mode mixing. The octagonal cladding profile of the second fiber produced greater cooling, down to –1.3 K with 3 W. Fitting experimental results to a model showed good agreement with theory, and confirmed the high critical quenching concentration (Nc = 3.2x1027 Yb/m3), low absorptive background loss (40 dB/km), and good filling ratio (~38%) achieved in this second fiber. This study establishes that with straightforward improvement in mode filling, a cladding-pumped ZBLAN fiber can readily be cooled to ~10 K below room temperature at atmospheric pressure with only ~15 W of pump power.