In this contribution the influence of fabrication technique (solution doping, gas-phase doping) and the choice of suitablematerial systems (Al, P, Yb:SiO_2 and Al, F, Yb:SiO_2) for high power fiber laser materials on their optical properties isanalyzed. The materials under analysis contain low amounts of codopants (Yb < 0.15 mol%, other <1.2 mol%). The effectson refractive index, attenuation, absorption and emission cross section as well as on photodarkening are addressed. Themain part concerns with the analysis of photodarkening, in fact the evolution of individual defect centers are spectrally andtemporally investigated by means of 2D curve fitting. It is suggested that this spectro-temporal fitting procedure can leadto new insights in the development of photodarkening on a level of the defects themselves.
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