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In situ observation of the Yb2+ 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.