Periodic structures in photo-thermo-refractive glasses induced by homogeneous UV irradiation

Samples of photo-thermo-refractive glasses were exposed to a homogeneous collimated beam of a He-Cd laser at 325 nm. A refractive index change is observed in the exposed glass upon thermodevelopment due to photoinduced crystallization. A periodic structure was observed inside this exposed area by optical microscopy. This structure is a system of parallel continuous aligned filaments having widths of ten's of microns oriented along the direction of light propagation. Variation of incident light polarization direction does not effect the resulting structure.