Study of multi-fractal spectrum of sol-gel hydrophobic anti-reflective SiO2 coating after laser-conditioning

Abstract

Hydrophobic AR SiO2 coating was deposited on the K9 substrate with sol-gel dipping technique，and laser-conditioning was done in “R on 1" mode with Nd:YAG laser (1064nm，7.5ns). Laser-induced damage thresholds (LIDT) were measured by “1 on 1" mode，and fore-and-aft changes of the surface morphology and fractal structure of the coating caused by laser conditioning were investigated with atomic force microscope (AFM) and multi-fractal spectrum (MFS), respectively. The results showed that after laser-conditioning the LIDT of the coating increased obviously，the mean-square roughness decreaseds lightly and the surface of the coating became more smooth，and that width of MFS shrinked and the uniformity of distribution of fractal framework was improved. It means that the micro-structure of the surface of the coating become, so regular that the coating could resist more powerful laser irradiation. Further more，MFS is an effective method to research mechanism of high power laser irradiation on optical coating because we can acquire more information about the change of the surface structure of the coating.