Abstract

The formation of photorefractive Bragg gratings in fibers has recently attracted attention. To achieve high reflectivity, high-Ge fibers are usually considered to be better than telecommunication fibers. However, Limberger et al. reported that a reflectivity of 94% and an index change of 1.3×10⁻³ were obtained in a standard telecommunication fiber with a Ge concentration of 3%. ¹ It is also noted that Archambault et al. reported type II gratings with a reflectivity of 99.8% and an index change of 6×10⁻³ in a high-Ge fiber.² Therefore the relation between index change and Ge concentration is still unclear, and the mechanism of index changes is under discussion. These reported index changes are calculated values from reflectivities of gratings, and thus a directly measured value is rare. Hand et al. measured index changes at 266 nm using interferometry. ³ In this paper, we describe a measurement on refractive index changes induced by a 248-nm KrF laser. We used a telecommunication fiber and a 50wt%- GeO₂ fiber. Analysis with the Frantz-Nodvik equation is performed, and absorption cross sections of the related defects (Ge-Si and Ge²⁺) are calculated.

© 1995 Optical Society of America