Temperature-dependent fluorescence study of ytterbium-doped borophosphosilicate fiber for optical cooling applications

Borophosphosilicate (BPS) optical fibers have been developed to mitigate nonlinear optical phenomena in high power fiber lasers. The spectroscopic properties of Yb 3+ in such a BPS glass host are notably different than in aluminosilicates (AS), which are more conventional hosts for active optical fibers. The beneficial spectroscopy of Yb-doped BPS fibers has also permitted their use in solid-state optical refrigeration at or near ambient temperature. Spurred by this, temperature-dependent fluorescence spectroscopy is carried out herein on a cooling grade Yb-doped BPS fiber fabricated using modified chemical vapor deposition (MCVD). The Stark levels of the Yb 3+ manifolds are identified and confirmed to be much closer than in aluminosilicates, enabling thermal occupation of higher lying sub-levels to persist at temperatures well into the cryogenic range. The temperature dependence of the mean fluorescence wavelength degrades the cooling efficiency faster than in aluminosilicates, having a slope of −41pm/K, about a 30%–40% increase in magnitude relative to the AS system. Using a combination of measured and estimated material parameters, the cooling efficiency is modeled at low temperatures. The results suggest that the BPS glass is a better candidate for low temperature, bulk solid-state optical refrigeration compared to aluminosilicates.