Modeling and experimental comparison of the plasma jet induced etch front for subsurface damage determination

The performance of optical systems can be compromised by subsurface damage (SSD) caused by mechanical processing. This study presents a destructive method of determining SSD depth in fused silica surfaces using atmospheric plasma jet etching (PJE). A simplified mathematical model describes how the etching front evolves under the assumption of isotropic etching. By comparing simulated surfaces with topographies determined experimentally after each etching step, an isotropy factor (IF) is calculated to identify deviations from isotropy. Areas with an IF greater than one exhibit anisotropic etching behavior, indicating the presence of SSD. The method was validated using defined Vickers indentations, scratches and conventionally polished samples. The maximum SSD depth correlates with both the maximum IF value and the slope of the cavity volume versus etching depth. For the samples examined, SSD depths ranged from 6.37 µm to 52.47 µm. The results agreed well with OCT measurements (deviation < 10%). The developed approach not only enables the quantitative determination of SSD depth, but also the three-dimensional reconstruction of crack morphology. Combining computer-aided modelling with experimental comparison provides a robust method for characterizing the quality of optical components.