Laser-induced stress changes in optical fibers

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. Photosensitivity is widely exploited for the fabrication of fiber Bragg gratings which are in-fiber filter devices whose spectral response can be designed with great versatility. Fiber Bragg gratings find broad application nowadays in the field of telecommunication as well as in sensing applications. The polarimetric assessment of the integrated transverse fiber birefringence serves as a means to gather information about both stress and strain profiles of the fiber. Birefringence is introduced in the optical fiber due to the photo-elastic effect. In this work, a highly sensitive polariscope with sub-micron resolution has been developed. The captured birefringence data has been interpreted to be basically strain and not stress-induced, which allowed the proper discrimination between an inelastic, stress-free strain contribution and a strain part essentially accompanied by elastic stress. In earlier interpretations of the polarimetric data, all birefringence had been attributed to be purely stress-induced, which excluded the existence of inelastic strain and even led to contradictory results in some cases. The reinterpretation in terms of strain, however, results in a consistent picture of both stress and strain-induced birefringence. The polariscope has been used in the following to determine radiation-induced stress changes for different optical fibers and irradiation sources. Femtosecond-laser induced core stress changes in SMF-28® standard telecommunication fibers have been measured tomographically and correlated with corresponding index changes. Furthermore, stress has been correlated to the refractive index change in highly germanium-doped fibers irradiated with cw-irradiation at 244 nm. The annealing behavior of stress, strain and index in this fiber has been studied. Approximately the same linear correlation between stress and index change was found for all germanium-doped fibers under investigation. In addition, stress changes were measured for phosphorus- and nitrogen-doped fibers drawn at different tensions before and after irradiation with an ArF excimer-laser. The impact of the drawing force on fiber stress and inelastic strain anisotropy has been determined as well as radiation-induced stress modifications. The observed stress increase has been compared to those obtained for germanium-doped fibers. Atomic Force Microscopy of cleaved and subsequently etched fiber end-faces is a powerful means to gather topographic information on a nanometer scale due to differential etching of the doped core and the undoped silica cladding. Within this work, the technique has been used to acquire information about etch rate changes in fibers drawn from the same nitrogen-doped preform with different drawing conditions. Furthermore, the impact of UV-irradiation on the etch rate has been determined for phosphorus- as well as nitrogen-doped fibers. Comparison of the results, with stress measurements allowed the attribution of the etch rate changes to a modification of the color center population.