The photodarkening resistivity of ytterbium doped alumosilicate fibers can be remarkably improved by cerium codoping. Here we report on systematical investigations of the influence of cerium on an optimized fiber design. Fibers with different ytterbium, aluminium and cerium contents have been prepared both under reducing and oxydizing conditions and characterized concerning refractive index, absorption and emission from UV to NIR. Typical spectral features in the UV and visible range have been analysed with respect to the ratio of Ce3+ / Ce4+. Photodarkening tests have been accomplished in order to correlate the power stability with the Ce content and valency state.