Direct evidence of two types of UV-induced glass changes in silicate-basedoptical fibers

Two spatially distinct types of UV-induced refractive index changes in phos phosilicate glass fibers have been differentiated by the authors. The index change close to the core-cladding interface is less stable than the index change in the bulk fiber core. It is shown that by using presensitization t echniques, in which the use of hydrogen is minimized so as to optimize the index changes possible within the core center, gratings of better stability than those prepared in fully hydrogen-loaded optical fiber are obtained.