The 630-nm loss band recently identified in unclad fibers made from low-OH-content vitreous silica is shown to be introduced in the fiber-drawing process. The band has an absorptive component as well as resonance fluorescence whose peak intensity amounts to about 16% of the total losses. A red fluorescence was observed when the same fibers were illuminated with the 325-nm line of a HeCd laser. The loss band could be eliminated by annealing for 3 h at 700 °C. Spontaneous annealing at room temperature was also observed in some cases. It is proposed that the 630-nm band is associated with drawing-induced defects of the silica network.
P. Kaiser, "Drawing-induced coloration in vitreous silica fibers," J. Opt. Soc. Am. 64, 475-481 (1974)