Photodarkening in Tb(3+)-doped phosphosilicate and germanosilicate optical fibers

The presence of Tb(3+) is found to sensitize greatly phosphosilicate and germanosilicate optical fibers to photodarkening when they are exposed to 488-nm light. The darkening is a three-photon process and may involve the photoionization of Tb(3+) to Tb(4+). This sensitivity raises the possibility of side writing refractive-index gratings into silica-based optical fibers with 488-nm light.