Experiment of laser induced damage threshold for different sub-surface cracks on fused silica

The micro sub-surface cracks of fused silica are observed by AFM and optical microscope. Due to the morphology characters, the cracks are sorted into three types: Boussinesq-point-force crack(BPFC), Hertzian-conical scratch(HCS) and plastic indent(PI). The laser induced damage thresholds of the cracks are measured. The damage mechanism is also discussed. The results show that the damage threshold of sharp tip BPFC is less than 2.0 J/cm2, that of HCS get to 2.6 J/cm2, and that of PI with large shape change is 2.8 J/cm2. The laser induced damage threshold of PI with small change is parallel with that of perfect surface. It is obvious that BPFC and HCS with depth more than 1 μm are the main factors to decrease the laser induced damage threshold of fused silica.