Reduction of photodarkening in Yb/Al-doped fiber lasers

A strong charge-transfer band at UV-wavelengths is found to play a major role for the observed induced optical losses (photodarkening) in ytterbium doped high-power fiber lasers. This is correlated to the valence stability of the ytterbium ion in the silicate glass matrix, which we believe is the origin of photodarkening. We have performed UV-irradiation experiments on ytterbium-doped preform samples and accelerated photodarkening experiments on Yb-doped fibers, by using 915 nm high power diodes. Our results show that photodarkening can be reduced, to low levels, either by preparing the preform glass in a reducing atmosphere or by hydrogen loading the fiber in a pressure chamber at room temperature.