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 changes^9,10 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.

© 1995 Optical Society of America