In situ observation of the Yb^2+ emission in the radiodarkening process of Yb-doped optical preform

This Letter relates the clear evidence of Yb²⁺ formation under 2.5 MeV electron irradiation in optical fiber preforms showing a darkening of the core. We thus detected by in situ photoluminescence measurements the green emission of divalent Yb²⁺ under the 355 nm excitation. Moreover, we showed the existence of two types of Yb²⁺ ion species with different stabilities. We demonstrated that the radiodarkening mechanism is based on a pair association of Yb²⁺ with aluminum oxygen hole center point defects.