Investigation of the photodarkening of Yb-Li co-doped silica fiber

In this work, we systematically studied the mitigation of photodarkening (PD) in ytterbium-doped silica fiber co-doped with lithium (Li). Adding a proper concentration of Li+ ions to the core glass composition, the PD-induced excess loss can be reduced by 25%. The results showed that the effect on numerical aperture and laser efficiency was very small. Compared with the sodium (Na) co-doped Yb/Al silica fiber at the same concentration, the mitigation of the PD effect through Li+ and Na+ ions co-doping was analyzed and discussed. Furthermore, a hypothesis was proposed for the mechanism of alkali metal Li and Na co-doping to mitigate the PD effect. More effective core composition and doping concentration were proposed to provide guidance of high output power gain fiber preparation.