Abstract

In 1978, K. O. Hill et al. [1] reported the first observation of photosensitivity in a germanosilicate fiber. Interest in understanding and optimizing this phenomenon was further heightened when Meltz et al. demonstrated that an external exposure method could form a grating for use in telecommunication windows [2]. Three mechanisms have been suggested in order to explain the refractive index changes induced in the fiber core. The first is based on a colour - center model [3]. The bleaching of the UV absorption band at 242nm leads through Kramers - Kronig relationship to a refractive index changes in the visible and infrared part of the spectrum. The second is based on UV induced stress relief of the fiber core [4]. The last model is based on structural modifications which induced compaction in the glass matrix [5]. Such UV induced densification has been already reported in preform plates as a change of volume leading to corrugations at the sample surface [6]. These experiments were performed on bulk sample. Therefore, it is of interest to confirm if compaction is photoinduced or not in photosensitive fibers. The aim of this work is to make microscopic investigations of the fiber core at the Bragg grating place in order to look if the UV exposure changes the structural or chemical properties of the material. The characterization of the grating structure photowritten in the optical fiber was realized by Atomic Force Microscopy on etched fibers.

© 1995 Optical Society of America