Effect of Polishing-Induced Subsurface Impurity Defects on Laser Damage Resistance of Fused Silica Optics and Their Removal with HF Acid Etching

Abstract

Laser-induced damage on fused silica optics remains a major issue that limits the promotion of energy output of large laser systems. Subsurface impurity defects inevitably introduced in the practical polishing process incur strong thermal absorption for incident lasers, seriously lowering the laser-induced damage threshold (LIDT). Here, we simulate the temperature and thermal stress distributions involved in the laser irradiation process to investigate the effect of impurity defects on laser damage resistance. Then, HF-based etchants (HF:NH 4 F) are applied to remove the subsurface impurity defects and the surface quality, impurity contents and laser damage resistance of etched silica surfaces are tested. The results indicate that the presence of impurity defects could induce a dramatic rise of local temperature and thermal stress. The maximum temperature and stress can reach up to 7073 K and 8739 MPa, respectively, far higher than the melting point and compressive strength of fused silica, resulting in serious laser damage. The effect of impurity defects on laser damage resistance is dependent on the species, size and spatial location of the defects, and CeO 2 defects play a dominant role in lowering the LIDT, followed by Fe and Al defects. CeO 2 defects with radius of 0.3 µm, which reside 0.15 µm beneath the surface, are the most dangerous defects for incurring laser damage. By HF acid etching, the negative effect of impurity defects on laser damage resistance could be effectively mitigated. It is validated that with HF acid etching, the number of dangerous CeO 2 defects is decreased by more than half, and the LIDT could be improved to 27.1 J/cm 2 .

Laser-induced damage threshold based on thermal effects in high-purity silica optical fiber

“…Compared with the gray dotted line in Fig. 3(c), the tendency of our temperature curve with 355 nm wavelength is similar to the results calculated by Cheng et al 45 By changing laser irradiation sites, the temperature of the center is consistently very high under the stable-state calculation. Therefore, we can find that the center of the front surface is more possible to get damage caused by high temperature.…”

Impact of Temperature and Etching Methods on Surface Roughness, Topography, and Composition of Glass Micromodels

“…Pápa et al showed an increasing trend up to 10 min by HF (38%), followed by a constant trend upto 20 min . Cheng et al reported a periodic trend of increasing–decreasing–increasing in the surface roughness of fused silica etched by a mixture of 5% HF and 10% NH 4 F, over 80 min . Koga et al observed a reduction in the surface roughness in the first 15 min of etching and a slow increment in the etching times greater than 20 min when using HF (5%) for etching the soda–lime glass surface .…”

“…54 Cheng et al reported a periodic trend of increasing−decreasing− increasing in the surface roughness of fused silica etched by a mixture of 5% HF and 10% NH 4 F, over 80 min. 55 Koga et al observed a reduction in the surface roughness in the first 15 min of etching and a slow increment in the etching times greater than 20 min when using HF (5%) for etching the soda−lime glass surface. 56 Therefore, the trend of roughness variation with etching time is not specific and depends on several parameters such as surface composition, HF concentration, and etching time.…”

Special Issue on Solid State Lasers Materials, Technologies and Applications

“…A detailed study on the impact of subsurface impurity defects introduced in the polishing process of fused-silica optics, and the beneficial effect of HF acid etching in the defect removal and laser-induced damage threshold enhancement, is presented by J. Cheng, J. Wang, J. Hou, H. Wang, and L. Zhang in [10].…”