Non-linear optical investigations of glass structure

Abstract

Non-linear spectroscopy, including stimulated Raman scattering (SRS), has been proposed for the experimental verification of modern color center models in silica glasses. The selection rules in spontaneous and stimulated Raman processes were identical; however, the shape and enormous intensity of SRS lines in silica optical fibers (OFs) are determined by the threshold nature of SRS. The observed SRS spectra in OFs consisted of a number of narrow lines, due to silica network vibrations, to dopant and ‘microdefects’ also and more than 20 overtones and combination vibrations. The anharmonicity constant, χ, and frequencies of harmonic vibrations are calculated using the multiwavelength SRS in OFs for the first time. It is shown that the minimal anharmonicity is characteristic for deformation vibrations of silica tetrahedrons with Δv = 440 cm −1. The χ values for the ‘microdefect’ vibrations in silicate and doped phosphate groups were greater by ∼ 2 and ∼ 8 times, respectively, than the χ values for 440 cm −1 silica vibrations. The different interactions between ‘silica’ or ‘phosphate’ microdefects and the bath of silica vibrations are established. It is emphasized that the SRS possesses the simple and universal character for express-analytical investigations of type, concentrations and spatial distributions of dopant and microdefects in any transparent glass media, which could be manufactured as OFs with optical losses no greater than ∼ 100 dB/km. The future application of SRS in OFs as an experimental tool for investigation of the changes in glass structure under mechanical stresses, irradiation and thermal annealing are indicated.