Defect centers in a pure-silica-core borosilicate-clad optical fiber: ESR studies

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Abstract
An electron‐spin‐resonance (ESR) study has been carried out on γ‐ray‐induced color centers in a pure‐SiO2‐core B2O3⋅3SiO2‐clad prototype optical fiber and in bulk glasses of the core and cladding compositions. Defects in the core material included the familiar siliconE′ center and other centers tentatively attributed to oxygen‐associated structures. Approximately 102 times more centers per unit mass and unit γ‐ray dose were created in the claddingmaterial than in the core glass; these comprised just three defect types: the boron‐oxygen hole center (BOHC), the siliconE′ center, and a boron analog of the latter, the boronE′ center. These same defects were also observed in the irradiated prototype fiber in concentrations comparable to those attained in the bulk glass of the cladding composition. The unexpectedly high radiation sensitivity of the fiber, therefore, would appear to be due to the efficient production of (intrinsic) boron‐related defects in the cladding. Considerable insight has been gained into the electronic structures of the BOHC and the boronE′ center, by means of careful computer simulations of their ESR line shapes in B2O3⋅SiO2glasses, as functions of both frequency and boron isotopic enrichment.