9.1 INTRODUCTION

Photosensitivity is another amazing feature of most specialty optical fibers (SOFs). The photosensitivity phenomenon is different from photo-darkening and radiation-darkening, which induces excess losses. There is then no added background losses due to the fiber Bragg gratings (FBGs) inscribed in the fiber s core besides filtered wavelengths.

Photosensitivity of a medium is defined as its capacity to have its refractive index permanently changed by a modification of its physical or chemical properties through light exposition. Photosensitivity is a complex phenomenon because of the diversity of both parameters and effects that are observed. Fiber composition, fabrication process, operation wavelength, and even light source are all different parameters that can have a significant influence on the photosensitive properties of a fiber. Effect of interest in this chapter is the photosensitivity of Ge-doped silica fiber for which the core refractive index can be permanently modified by an ultraviolet (UV) irradiation [1].

In 1978, photosensitivity was first observed by Hill et al. [1, 2] at the Communication Research Centre (CRC) in Canada. To improve the performance of his tunable fiber Raman laser, Ken Hill ordered two spools of high numerical aperture (HNA) single-mode SOF from Bell Northern Research (BNR). These fibers turned out to be inadequate for use in the fiber Raman laser but accidentally led to the discovery of the photosensitivity. The experiment consisted of injecting light from a single-frequency Argon laser (514 nm) into the core of a doped silica...