Effect of temperature on surface error and laser damage threshold for self-healing BK7 glass

Cracks caused during the lapping and polishing process can decrease the laser-induced damage threshold (LIDT) of the BK7 glass optical elements, which would shorten the lifetime and limit the output power of the high-energy laser system. When BK7 glass is heated under appropriate conditions, the surface cracks can exhibit a self-healing phenomenon. In this paper, based on thermodynamics and viscous fluid mechanics theory, the mechanisms of crack self-healing are explained. The heat-healing experiment was carried out, and the effect of water was analyzed. The multi-spatial-frequency analysis was used to investigate the effect of temperature on surface error for self-healing BK7 glass, and the lapped BK7 glass specimens before and after heat healing were detected by an interferometer and atomic force microscopy. The low-spatial-frequency error was analyzed by peak to valley and root mean square, the mid-spatial-frequency error was analyzed by power spectral density, and the high-spatialfrequency error was analyzed by surface roughness. The results showed that the optimal heating temperature for BK7 was 450 degrees C, and when the heating temperature was higher than the glass transition temperature (555 degrees C), the surface quality decreased a lot. The laser damage test was performed, and the specimen heated at 450 degrees C showed an improvement in LIDT. (C) 2018 Optical Society of America