The photosensitivity of various cerium-doped fibers has been experimentally investigated for both Excimer- and Femtosecond-laser illumination. The results of single-pulse, few-pulse and multi-pulse inscription of fiber-Bragg-gratings with both laser systems and the thermal aging of those gratings demonstrated the restrictions of the conventional color center model. To explain the short-term stability of single-pulse gratings against long-term stability of multi-pulse gratings, an extension into a new two-step-model was deduced.