Abstract
In this work we present experimental evidence that the valence instability of the ytterbium ion play a key role for the observed photodarkening mechanism in Yb-doped fiber lasers. Luminescence and excitation spectroscopy performed on UV irradiated Yb/Al doped silica glass preforms and near-infrared diode pumped photodarkened fibers show a concentration increase of Yb(2+) ions. A concentration decrease in Yb(3+) could also be observed for the UV irradiated preform. The findings contribute to an increased understanding of the kinetic processes related to photodarkening in Yb-doped high power fiber lasers.
Highlights
Near infrared (NIR) diode laser induced optical losses in ytterbium (Yb) doped fiber lasers has been under extensive investigation during the last decade
In this work we present experimental evidence that the valence instability of the ytterbium ion play a key role for the observed photodarkening mechanism in Yb-doped fiber lasers
Luminescence and excitation spectroscopy performed on UV irradiated Yb/Al doped silica glass preforms and near-infrared diode pumped photodarkened fibers show a concentration increase of Yb2+ ions
Summary
Near infrared (NIR) diode laser induced optical losses (photodarkening) in ytterbium (Yb) doped fiber lasers has been under extensive investigation during the last decade. The first model implies that the valence instability of the Yb-ion is the origin of PD where a valence state change is believed to occur through a charge transfer (CT) process [6, 7]. This will result in pair generation of divalent Yb ions together with bound hole centers, which most likely can be assigned to aluminum oxygen hole centers (Al-OHCs) [8]. Conclusive experimental evidence have so far not been presented for the presence of divalent ytterbium in photodarkened near infrared (NIR) diode pumped Yb/Al doped fibers. As will be shown the results are in favor for the first model above and we are able to present experimental evidence for generation of divalent ytterbium in the PD process of Yb-doped fiber lasers
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