Self-focusing of tightly focused laser beams

Laser-induced damage (LID) thresholds of pure fused silica have been measured with single-temporal-mode nanosecond pulses at 1.064 μm. The laser beam was focused by spherical lens or axicon into 2.7 μm diameter spots with identical cross-sectional distribution of the central lobe irradiances. The measurements have shown about a 30% higher threshold value for the beam focused by the axicon, which is inherently not subjected to self-focusing. This result indicates the strong influence of self-focusing on standard LID threshold measurements performed even with tightly focused beams.