We conclude that there are two photochemical reactions of germanosilicate glass with hydrogen: one with Ge defects and another with regular Ge-O bonds. Bleaching of germanium oxygen deficient centers forms paramagnetic H(II) and E' centers in hydrogen-loaded glass, but this process plays a minor role in the index change mechanism. By using light at 300 nm in hydrogen-loaded glass one can avoid the formation of these unstable products yet still create a large and thermally stable refractive-index change (Δn～10~(-2)).
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