UV- induced stress changes in optical fibers

Abstract

Photosensitivity in optical fibers has first been observed by Hill et al. in 1978.1 Recently, the application of Bragg gratings has considerably been increased due to the introduction of side exposure techniques.2 Index changes of 10-3 have been achieved even in standard telecommunication fibers.3 Reliable fabrication of Bragg gratings depends on the detailed knowledge of the underlying mechanisms of photo-induced index changes. The basis of all proposed mechanisms is the ionization of a GeO deficiency center that exhibits an absorption bands in the UV at 240 nm. A model based on the change of color centers that include 3 or 4 absorption bands,4,5 a stress relaxation model,6-8 and structural changes9,10 have been proposed as origin of photosensitivity. In the color center model the index changes can be calculated from absorption changes using the Kramers-Kronig relationship. Including 4 absorption bands, an index change of 2 × 10-4 has been calculated.5 Higher index changes have been explained using a model based on thermoelastic stress relaxation where the photoelastic effect leads to the index change.6-8 The largest achievable stress and photoelastic index changes would then be determined by the initial thermoelastic stress. Even higher values may be explained by glass densification.9’10 Recently, the core stress in fibers with different Ge concentrations before and after UV irradiation with different doses has been measured.11,12 Such stress measurements allow directly to test the stress relief model and to correlate stress changes with refractive index changes. Here, we report on extended measurements performed on 3 fibers with Ge concentrations of 9%, 12% and 18% and propose a new model to explain the results.