Non-destructive testing of subsurface damage for early indication of laser-induced damage threshold in fused silica

Abstract

Subsurface damage (SSD) in optical components is almost unavoidably caused by mechanical forces involved during grinding and polishing and can be a limiting factor, particularly for applications that require high laser powers. In this contribution, non-destructive characterization techniques are evaluated with respect to their capability to determine SSD in fused silica. For this, differently polished surfaces with different SSD levels have been prepared. An initial destructive analysis using etching in hydrofluoric acid in combination with white light interferometry revealed a high amount of SSD in one of the sample types compared to a very low amount of SSD in a second one. It is shown that nondestructive absorption as well as scattering measurements are sensitive towards SSD related differences in the samples. Finally, laser-induced damage tests proved a significant impact of SSD on the laser stability by determining a reduced damage threshold of 31 ± 3 J/cm² for the sample with high amount of SSD compared to 45 ± 5 J/cm² for the high-quality polished sample.