The precise determination of residual stress profiles in optical fibers allows drawing conclusions about radiation-induced density-changes of the fibers doped core glass. The corresponding index changes partially contribute to the phenomenon of photosensitivity, i.e. the modification of the refractive index of a glass through the irradiation with an appropriate laser source. In this book, a highly sensitive polariscope with sub-micron resolution is presented. The captured birefringence data is interpreted to be basically strain and not stress-induced, which allows the proper discrimination between an inelastic, stress-free strain contribution and a strain part essentially accompanied by elastic stress. The polariscope is used to determine irradiation-induced stress changes for different optical fibers and irradiating lasers, ranging from femtosecond to cw sources. The radiation-induced core stress changes are correlated with corresponding index changes. Thus, challenging conclusions about the impact of core compaction to the overall index change are identified.