Transient absorption in pumped Yb fibers opens a path to photodarkening

We investigated the strong absorption change appearing in ytterbium (Yb)-doped silica simultaneously with Yb3+ excitation and vanishing immediately after pump power switch-off. The transient absorption peaks below 300 nm for all the investigated host glass compositions. It extends at least to 500 nm for samples with aluminum codoping or without any codoping and is therefore supposed to constitute the first step needed to enable photodarkening (PD). Our observations indicate that defect complexes including one Yb3+ ion can be activated stepwise. Firstly, their original absorption spectrum is shifted by the energy of the Yb3+ excitation. Secondly, this transient absorption opens the path to the generation of PD color centers by pump photons or energy transfer from neighboring excited Yb3+ ions. In samples codoped with phosphorous only, we found transient absorption of comparable strength but limited to the wavelength range below 350 nm. Thus, the second step of activation seems to be inhibited, which might be an explanation for the consistently observed low PD effects in this fiber type.