Yb3+-doped fiber is one of the most promising hosts for high-power fiber lasers . However, in the late 1990s,
photodarkening effect, i.e., the creation of color centers induced by light, was observed and reported . Like in other
rare-earth-doped materials, a broad visible and near infrared absorption band appears during laser operation, which
strongly lowers the laser efficiency. We have shown how thulium impurities, present at the parts-per-billion weights
(ppb) level in the raw doping material, can induce UV emission which in turn creates defects responsible for
photodarkening in ytterbium-doped continuous-wave fiber lasers . This new vision is not in contradiction with others
but it gives for the first time a possible explanation for UV defect creation by an infrared beam.