Electron spin resonance (ESR) and optical absorption of optical fibers irradiated by Ύ and treated by hydrogen gas were observed. A hyperfine structure (hfs) in ESR spectrum not previously reported was found. It is induced by hydrogen treatment only when peroxy radicals are observed. The hfs is explainable by assuming that peroxy radicals form a hydrogen bond with a hydrogen molecule. The 1.52 μm band appears in the same fibers as does the hfs, and diminishes at 90° C in a similar manner to the hfs. This band is thought to be caused by a hydrogen molecule that forms a hydrogen bond with a peroxy radical.
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