Radiation damage of germanium-doped silica glasses: Spectral simplification by photo- and thermal bleaching, spectral identification and microwave saturation characteristics

Abstract

Germanium-doped silica glasses are important materials used in the fabrication of preforms from which optical waveguides are drawn. The preforms consist of an outer cladding of silica and an inner core of germanium-doped silica. These separate portions have been subjected to varying types of ionizing radiation and the centers generated have been studied by electron-spin resonance spectroscopy. Core samples show the generation of a single type of germanium E′ center in the silica matrix which is surrounded by three next-nearest-neighbor germanium atoms and is specified as Ge(3) after xenon lamp irradiation at room temperature. If γ irradiation is used, several germanium E′ centers with varying numbers of next-nearest-neighbor germanium atoms ranging from 0 to 3 are generated and are designated as Ge (n) centers, where n=0, 1, 2, and 3. The Ge(1) and Ge(2) centers disappear after heat treatment at a temperature of ∼500 K leaving only the Ge(3) center as the most stable of these species. The intensity of the Ge(3) centers after γ irradiation and heat treatment increases in intensity with the γ-irradiation dose to at least ∼0.5 Mrad. Photobleaching by xenon lamp irradiation of γ-irradiated core samples also decreases the intensity of the Ge(1) and Ge(2) centers. Gamma irradiation of cladding samples generates two different types of silicon E′ centers, E′α1 and Eα2. At a lower radiation dose one sees a mixture of E′α1 and Eα2, but at a higher radiation dose, E′α2 dominates. Microwave power saturation data at different temperatures on both the core and the cladding samples are used to further differentiate and characterize these various species. It is found that a relaxation mechanism involving modulation of the g anisotropy is probable.