Filamentary Tracks Formed in Transparent Optical Glass by Laser Beam Self-Focusing. I. Experimental Investigation

Abstract

The permanent damage in optical glass investigated here is characterized by filamentary tracks of fine fractures. These tracks are a few micrometers in diameter, straight to within 0.7 μm, up to 9 cm long, and collinear with the incident laser beam. There may also be damage stars (regions of fracture gross compared with the track diameter). These damage stars are usually near the upstream ends of the tracks. Track formation is characterized by a flash of side-scattered white light from the track, laser light side-scattered from the damage stars, a marked increase in the exit divergence angle of the laser beam, and a weak back-scattered pulse of laser light. The back-scattered pulse preserves the polarization of the incident beam, is of shorter duration than the incident laser pulse, and has a frequency shift corresponding to Brillouin scattering from a free compressional sound wave in the glass. Track formation is accompanied by a detectable cylindrical sound wave. The track formation threshold is repeatable at different locations in the glass sample. Both the power threshold and the energy-density threshold are rapidly varying functions of the incident beam radius at focus. The threshold power is as low as 10 kW for a ruby-laser beam sharply focused in dense flint glass, and more than 2 MW for an unfocused beam in fused silica.