Abstract
The name "Photosensitivity" is associated with the process by which the refractive index of certain types of silica glasses is permanently modified by exposure to short wavelength optical radiation [1]. The process was discovered in germano-silicate optical fibers exposed to intense blue light (λ=488 nm) from an Argon ion laser [2] but further experiments revealed that the photoinduced index change grows as the square of the laser intensity, indicating a two-photon process[3]. It was eventually found that the photosensitivity was due to an absorption band with a maximum near 242 nm [4], similar to the absorption observed in oxygen deficient germania (i.e. GeO sites in GeO2 )[5]. With direct excitation from intense UV light in the absorption band, the refractive index change in standard optical fibers reaches 10-4 and the process becomes sufficiently efficient for writing Bragg gratings in fibers by side exposure [4],[6]. These advances have led to the developement of several important applications [7]. Finally, apart from using special fibers with enhanced photosensitivity (from higher germanium concentration, co-dopants, or processing in reducing atmospheres), two methods have been proposed to increase the photosensitivity of standard fibers: flame brushing [8] and low temperature hydrogen (H2) loading [9]. With these techniques, refractive index changes in the 10-3 -10-2 range have been reached, extending the variety of useful fiber devices and allowing other applications to become practical [10-12].