Emerging trends in laser-cooling of Yb-doped silicate fibers

The recent development of Yb-doped silica fibers that can be optically cooled via anti-Stokes fluorescence is fostering a new generation of fiber lasers and amplifiers that can run without external coolers. Thanks to their greatly reduced thermal loads, these devices are expected to exhibit superior properties in terms of improved power and frequency stability, and reduced thermal lensing and transverse-mode instability. This paper presents a detailed comparative study of the measured cooling properties of eight fibers with record-high Yb quenching concentrations in aluminosilicate and aluminophosphosilicate compositions. Analysis of the experimental cooling data provides valuable quantitative information about the quenching rate and background absorption of each fiber, the two parameters (besides Yb concentration) that influence the cooling performance most strongly. This study shows that aluminophosphosilicates perform better than aluminosilicates, and that very high Yb concentrations (up to 6 wt.%) can be achieved with very low quenching when the glass is co-doped with sufficient phosphorus (∼12 wt.%) and aluminum (1-7 wt.%). With the best aluminophosphosilicate composition, it is now possible to cool a fiber with a small core by as much as -250 mK at atmospheric pressure. Cooling is no longer limited by quenching, but by background absorption. Future work will focus on identifying the impurities in precursor materials responsible for this background absorption and reducing their content to lower this loss from its current 15-dB/km record value.