Annealing of pre-darkened ytterbium doped silica — Kinetic model

The photodarkening (PD) loss appears as time dependent broadband loss with peak at visible wavelengths and a long tail extending to near infrared wavelengths. The origin of PD is still under debate but in [1] it is proposed to be due to a “1½” bond color center (CC). The mechanism for CC absorption is by photon electric field excitation to the ionic three electron bond (TEB) of =Si<;O
₂
:· Yb inducing forces in opposite directions to the TEB and Yb that finds resonance in σ- to σ*-bond excitation of the σ-bond in parallel to the TEB. Here the “1½” bond CC is for the first time shown to describe the annealing behavior of pre-darkened fibers [2-4].