Medium-range ordering in glass structures as a background of photosensitivity in silica fibers

Using picosecond stimulated Raman scattering (SRS) in silica-based fibers as a spectroscopic tool, some correlations between medium-range ordering and photosensitivity in silica glasses were experimentally established. Apart from the fundamental Raman-active vibrations attributed to the silica tetrahedra and dopant groups and their overtones, the intense Raman combination bands were observed. Based on the overtone band sequences, the vibrational anharmonicity constants were calculated and analyzed, including the ones corresponding to the 570 cm−1 band. This band is forbidden in Raman scattering in silica crude selection rules are used. However, the transition is activated owing to the tetrahedron distortions by outer shells of the first and second peripheral atoms in a silica network. It is the intensity of the 570 cm−1 Stokes band that grows enormously in the SRS spectra of germanium-doped silica fibers as a result of photoinduced changes in the glass structure resulting in second harmonic generation (SHG). The experimental data are interpreted using a modern glass theory. Practical applications of picosecond SRS spectroscopy for glass microstructural studies are discussed.