Mechanism of Radiation Induced Loss in Optical Fibers and Development of Radiation-Resistant Fibers

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Various optical absorption bands are induced in <gamma> -ray irradiated pure-silica-glass. The cause of seven of these absorption bands, existing in the UV to near IR regions, were not understood. Using ESR and optical absorption experiments, we show that four of these bands can be explained by factors such as impurity content (Cl, OH groups), and nonstochiometry between Si and O in the glass. In the course of the investigation, we found that a fiber with good radiation resistance characteristics is not necessarily a high OH content fiber, as was previously believed.In a number of fibers, the growth of radiation induced defects is different even though the impurity content of the fiber is basically the same. This rather unusual phenonena is caused by stoichiometric imbalance of the ppm order between oxygen and silicon. In oxygen excessive glass, peroxy radicals ( Si-O-O・ ) are formed from either oxygen molecules or other excess oxygen such as peroxy linkages. In oxygen deficient glass on the other hand, the 245nm absorption (caused by Si-Si Bonds ) is observed. Molecular orbital calculations show that in order for the Si-Si bond to be the cause of the 245nm absorption, the distance between the two Si atoms must be 3.08 A. This distance is very near the distance between Si and Si in a normal Si-O-Si bond, which is 3.10 A. These results indicate that the structure of silica glass used as the core in optical fibers is far more orderly than previously believed.