Hydrogen treatment of optical fiber waveguides with pure silica cores and boron, fluorine co-duped silica claddings was found to effectively reduce the y-ray induced loss increases in visible wavelength region. Germaniuni doped core fibers free from metal impurities bad good resistivity against y-ray. The induced-losses were pointed out to be dependent on the dose rate, a similar behavior tu that of pure silica core fibers. Metal impurities such as phosphorus or alkalies added to germanium doped silica core were found to significantly change the glass structure to produce the precursors of defect center related to the y-ray induced loss, and that increased sensitivity to y-irradiation.

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