  Ytterbium (Yb) doped fiber lasers are known to be affected by the creation of
color centers during lasing (socalled photodarkening (PD)). In a previous work,
this defect creation was investigated from a spectroscopic point of view,
showing the presence of traces (ppb) of thulium (Tm) in the Yb doped fiber. It
was shown that Tm has a strong impact on the defect creation process involved
in PD. In this paper, we compare the results from the literature with our Tm
hypothesis, without finding any contradiction. Moreover, this hypothesis can be
an explanation for the discrepancies in the literature.
