Subsurface micro-cracks will be generated during the grinding and polishing processes of optical components. Microcracks have a modulation effect on the laser, thereby reducing the laser damage threshold. The FDTD method is used to simulate the light intensity distribution modulated by micro-crack. By comparing the simulation results of radial crack, parabolic crack and elliptic crack, the modulation mechanism of micro-crack is revealed. The results show that for the crack with the same width and depth, light intensity enhancement factor (LIEF) modulated by radial crack on the rear surface and parabolic crack on the front surface is the largest; LIEF modulated by elliptical crack on the rear surface and radial crack on the front surface is the smallest. In addition, when the crack width-depth ratio is the same, the larger the depth, the higher the LIEF. As the width-depth ratio increases, the LIEF value increases firstly, then decreases, and finally approaches a stable value.
Subsurface defects and contaminations will be generated during the grinding and polishing processes of optical components. Combined modulation is one of the important factors for the laser-induced damages of fused silica. In this paper, by using 2D finite-difference time-domain method, the light intensity distribution modulated by both radial crack and contaminant is studied on front/rear surface, respectively. The results show that the light intensity distribution is significantly affected by the aspect ratio of radial crack and the relative position between radial crack and contaminant. The simulations of the combined modulation on rear surface show that larger LIEFs are generated at certain relative positions compared with those in the single modulation of radial crack or contaminant. Meanwhile, with the increase of distance, the LIEFs are wave-like up and down fluctuations, and gradually tend to stable values. When there is no total internal reflection, the LIEF in contaminant on the crack wall rises significantly with increase of distance, the maximum LIEF occurs when the contaminant is near the intersecting line between radial crack and rear surface. The simulation of the combined modulation on front surface show that the variation of LIEFs in global domain are not very prominent.
Chemical etching is usually utilized to improve the laser damage performance of optical glass by mitigating microcracks, while it inevitably produces some reaction products (RPs). In this paper, two K9 glasses with good quality and two K9 glasses with micro-cracks are etched, statically or dynamically (high-frequency ultrasonic agitation). The morphologies of cracks and RPs are characterized, and the laser-induced damage thresholds (LIDTs) are measured. The results show that with the increase of etching time, the LIDT increases slightly at first and then decreases gradually for the glass with RPs, and the LIDT increases at first and then stabilizes for the glass without RPs. Using finite-difference time-domain method, the light intensities around crack, RP and their combination are simulated, respectively. The results indicate that the light intensity enhancement factor (LIEF) increases at first and then decreases with the decrease of crack aspect ratio, and the LIEF increases with RP radius. The LIEF for the combination is generally larger than that for one crack or one RP, which greatly depends on the relative distance between the crack and the RP. Experimental and simulated results complement each other, revealing the influence mechanism of crack and RP on the LIDT. This work would contribute to improving the LIDT of optical glass by chemical etching.
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