## 4 Conclusions  
To evaluate the effects of impurity defects on the laser damage resistance of fused silica, the distributions of temperature and thermal stress caused by absorptive defects are investigated by numerically solving the heat conduction and thermo-elastic equations involved in the laser irradiation process. The simulation results indicate that the presence of impurity defects would induce a dramatic rise in temperature and thermal stress. The maximum temperature and thermal stress in fused silica can exceed its melting point and compressive strength, resulting in optical breakdown of optical materials. The effect of impurity defects on the laser damage resistance is dependent on the species, size and spatial position of defects. The CeO\({}_{2}\) defect plays the dominant role in lowering the laser damage resistance, followed by Fe and Al defects. The defects located more closely to the silica surfaces would incur higher local temperature. It is further concluded that the CeO\({}_{2}\) defects with radius of roughly 0.3 \(\upmu\)m, which reside 0.15 \(\upmu\)m beneath the silica surfaces, are the most dangerous defects affecting the laser damage resistance of fused silica optics.  
With the HF-based etching method, the impurity defects on fused silica are removed and the surface quality, defect content and laser damage threshold of etched surfaces are experimentally tested to evaluate the etching effectiveness. The results show that the negative effect of impurity defects  
Figure 14: Comparison of the laser damage performance for fused silica optics before and after HF etching. (**a**) Variation of laser-induced damage thresholds (LIDTs) of etched silica surface under various etching times. The typical laser damage morphologies on un-etched (**b**) and etched (**c**) silica surfaces. The microscopic images in (**b,c**) are both magnified by 40 times.  
on the laser damage resistance can be effectively mitigated by HF acid etching. As the acid etching proceeds, the re-deposition and subsurface defect layers are successively removed, resulting in the evolution of surface roughness in three phases. The EDS and laser damage experiments validate that the number of dangerous CeO\({}_{2}\) defects on fused silica can be decreased by more than a half and the LIDT can be improved to 27.1 J/cm\({}^{2}\) (355 nm, 10 ns) by removing the re-deposition layers. It is proved that the HF-based acid etching is able to effectively control the bad effect of impurity defects on the laser damage resistance and the further improvement of LIDT is limited by the deep scratches and cracks. This work can not only contribute to the understanding of laser-induced damage mechanisms on large-aperture ultraviolet optical components, but also provide theoretical foundations for the post-processing of ultra-precision machined fused silica optics.  
The authors gratefully acknowledge the financial support from the Science Challenge Project (Grant No. JCKY2016212A506-0503) and the Key Laboratory Fund of Ultra-precision Machining Technology at the China Academy of Engineering Physics (Grant No. KF14007).  
[PERSON] and [PERSON] conceived the work and supervised the preparation of the manuscript. [PERSON], [PERSON] and [PERSON] performed the FEM simulations and analyzed the data. [PERSON], [PERSON] and [PERSON] designed the experiments of acid etching and laser damage tests. [PERSON], [PERSON], [PERSON] and [PERSON] fabricated and characterized the sample surfaces. [PERSON] wrote the manuscript with the input from other coauthors. All authors reviewed the manuscript.  
The authors declare no conflict of interest.