The inadequate laser-induced damage threshold (LIDT) of optical elements limits the future development of high-power laser systems. With the aim of raising the LIDT, the elastic passivating treatment mechanism and parameter optimization of a combined magnetorheological finishing (MRF) and HF etching process are investigated. The relationships among the width/depth ratio of defects and parameters of the passivating treatment process (MRF and HF etching), relative intensity (RI), and LIDT of fused silica (FS) optics are revealed through a set of simulations and experiments. For high-efficiency improvement of LIDT, in an elastic passivating treatment process, scratches or other defects need not be wiped off entirely, but only passivated or enlarged to an acceptable profile. This combined process can be applied in polishing high-power-laser-irradiated components with high efficiency, low damage, and high LIDT. A $100\mathrm{mm}\times 100\mathrm{mm}\times 10\mathrm{mm}$ FS optic window is treated, and the width/depth ratio rises from 3 to 11, RI decreases from 4 to 1.2, and LIDT is improved from 7.8 to $17.8\mathrm{J}/{\mathrm{cm}}^2$ after 385 min of MRF elastic polishing and 60 min of HF etching. Comparing this defect-carrying sample to the defect-free one, the MRF polishing time is shortened, obviously, from 1100 to 385 min, and the LIDT is merely decreased from 19.4 to $17.8\mathrm{J}/{\mathrm{cm}}^2$. Due to the optimized technique, the fabricating time was shortened by a factor of 2.6, while the LIDT decreased merely 8.2%.

Surface damage precursor evolution has great influence on laser-induced damage threshold improvement of fused silica surface during Ion beam etching. In this work, a series of ion sputtering experiment are carried out to obtain the evolutions of damage precursors (dot-form microstructures, Polishing-Induced Contamination, Hertz scratches, and roughness). Based on ion sputtering theory, surface damage precursor evolutions are analyzed. The results show that the dot-form microstructures will appear during ion beam etching. But as the ion beam etching depth goes up, the dot-form microstructures can be mitigated. And ion-beam etching can broaden and passivate the Hertz scratches without increasing roughness value. A super-smooth surface (0.238nm RMS) can be obtained finally. The relative content of Fe and Ce impurities both significantly reduce after ion beam etching. The laser-induced damage threshold of fused silica is improved by 34% after ion beam etching for 800nm. Research results can be a reference on using ion beam etching process technology to improve laser-induced damage threshold of fused silica optics.

In this paper, the influence of ion beam sputtering (IBS) process on laser damage resistance as a function of sputtering depth and typical defects which associated with laser damage performance was investigated. Damage test results reveal that the damage resistance of HF etched surface can be further enhanced about 30% by appropriate IBS removal depth (less than ~1000nm). Within this removal depth, the IBS process can remove the redeposited reaction products during HF acid etching process, improve surface quality and reduce chemical structure defects concentration. However, further ion sputtering often results in a decrease rather than an increase of damage threshold with enhanced surface densification and increased chemical structure defects of ODC and NBOHC which generated from sputtering damage. Moreover, the sputtered surface will accelerate the chemical reaction of surface atoms with water molecules. Thus the newly obtained hydroxylation layer rich in highly absorptive products can result in the decrease of laser damage resistance. The study reveals the improvement mechanism by IBS process, and provides both technical guidance and theoretical basis for the optimization of the post-process of fused silica.

Near-surface nanoscale damage precursor generated from the fabrication process has great influence on laser-induced damage threshold improvement of fused silica. In this work, high-resolution transmission electron microscopy (HRTEM) is used to characterize the arrangement of material particles near surface. The nanoscale defects in the Beilby layer could be clearly distinguished. And we find ion beam etching (IBE) has little effect on the arrangement of material particles. This microscopic phenomenon makes IBE a promising technique for the detection of nanoscale near-surface damage precursors. To further investigate the nanoscale near-surface damage after chemical mechanical polishing, a trench is generated by ion sputtering to contain the nature and characteristics of nanoscale precursors in different depths. The evolutions of chemical structure defects and nanoparticles are measured and their laser-induced absorption performance are tested. The results show that there is a nanoscale defect layer (~360nm) beneath the Beilby layer. A model for nanoscale defect layer of fused silica after CMP is offered. In the model, the quantitative density of nanoparticles falls exponentially with increasing the depth and the contents of ODC and NBOHC decreases linearly, respectively. Research results can be a reference on characterizing nanoscale defects near surface and conducting post-processing technologies to improve the laser damage resistance property of fused silica.

Nanoscale laser damage precursors generated from fabrication have emerged as a new bottleneck that limits the laser damage resistance improvement of fused silica optics. In this paper, ion beam etching (IBE) technology is performed to investigate the evolutions of some nanoscale damage precursors (such as contamination and chemical structural defects) in different ion beam etched depths. Surface material structure analyses and laser damage resistance measurements are conducted. The results reveal that IBE has an evident cleaning effect on surfaces. Impurity contamination beneath the polishing redeposition layer can be mitigated through IBE. Chemical structural defects can be significantly reduced, and surface densification is weakened after IBE without damaging the precision of the fused silica surface. The photothermal absorption on the fused silica surface can be decreased by 41.2%, and the laser-induced damage threshold can be raised by 15.2% after IBE at 250 nm. This work serves as an important reference for characterizing nanoscale damage precursors and using IBE technology to increase the laser damage resistance of fused silica optics.