Experimental investigations of spectroscopy and anti-Stokes fluorescence cooling in Yb-doped silicate fibers

Several silicate fibers doped with 0.5-6 wt.% Yb were evaluated experimentally for optical cooling, namely three nanoparticle-doped fibers (LaF₃, BaF₂, and YbF₃) and three fibers with network modifiers (borophosphosilicate, fluorosilicate, and aluminosilicate). Their performance was compared to that of a commercial Yb-doped silica fiber. Simulations were first carried out to investigate the influence of absorptive loss and concentration quenching on the relationship between temperature change and pump power. This analysis provided a method for inferring the values of these two quantities from the measured dependence of the temperature change on pump power. For fair comparison of the cooling performance of different fibers, we show that the temperature change exhibited by the fibers must be compared at the same pump power absorbed per unit length. Although none of the fibers exhibited negative temperature changes, this metric was used to show that nanoparticles and network modifiers effectively reduce heating and increase the cooling efficiency. The borophosphosilicate and BaF2 nanoparticle fibers performed best, exhibiting 92.7% and 93.9% less heating than the silica fiber. Based on this result, we propose a borophosphosilicate fiber design with a lower Yb concentration and a larger core that is predicted to produce cooling at atmospheric pressure by -12 mK for 100 mW of 1020-nm pump.