Research On Damage Precursor Characterization And Mitigation Of High-performance Ultra-violet Laser Irradiated Fused Silcia Optics

In the high-energy laser system such as inertial confinement fusion device,the requirement of anti-laser damage performance of optical components is increasing.How to improve the laser damage performance of optical components under the premise of controlling the optic surface shape accuracy stably becomes a great challenge for fused silica optical manufacturing.In order to realize low-defect manufacturing of high-performance ultra-violet laser irradiated optics,the laser damage precursors in optical manufacturing process need to be re-recognized,and the ability to mitigate the damage precursors should be improved.These problems are difficult to be solved only by optimizing traditional optical manufacturing processes.In order to overcome the difficulties optical manufacturing for high-performance ultra-violet laser irradiated optics and realize the breakthrough of nano-fabrication theory and technology,this thesis introduces ultra-precision grinding into the initial stage of optical manufacturing to control the subsurface damage of fused silica,and proposes a new characterization method for subsurface damage based on laser damage performance analysis.A short optical manufacturing process chain containing ultra precision grinding（UPG）and magnetorheological finishing（MRF）and smoothing polishing（SP）is proposed,meanwhile the evolution of laser damage performance on fused silica surface in the short process chain will be investigated.The characterization and mitigation of wet chemical etching and ion beam etching（IBE）on nanoscale damage precursors is studied.The research results can be a reference on improving laser-induced damage threshold（LIDT）of high-performance ultra-violet laser irradiated fused silica optics and providing support for the implementation of major high-energy laser systems of the country.Our research contents include:1.The surface quality and subsurface damage（SSD）distribution achieved with a fine-grained grinding wheel under different depth-of-cut and cutting speed is experimentally studied.the material removal mechanism under different grinding parameters is revealed by calculating undeformed chip thickness,and the mechanism of surface morphology and subsurface crack produced in brittle-ductile mode is analyzed.Affected by ductile-regime removal,the ground surface integrity is high,and surface defects caused by brittle fracture can be alleviated.By optimizing the grinding parameters,a ductile-like surface can be realized and there is only a plastic flow layer in the subsurface observed by transmission electron microscope.Due to the change of material removal mechanism in UPG,the model between subsurface crack depth and surface roughness is re-established.The brittle-ductile surface roughness is not only affected by lateral crack generation,but also by plastic flow.A linear relationship between the SR and subsurface crack depth is in accord with the formula SSD_（crack）=410×Ra-0.68 for brittle-ductile surfaces.With the help of photothermal weak absorption test and laser confocal test,the mutual relationship model among subsurface depth,SSD and subsurface laser damage performcance after UPG is constructed.The result show that there is a deformed layer below the subsurface crack,which will cause additional absorption.The subsurface crack layer and the deformed layer together form the SSD layer.After UPG,the depth of deformed layer is similar to that of subsurface crack layer.The SSD depth is usually 2～4.22 times the SSD_{（cluster）}depth.A linear relationship between the Ra and SSD depth is in accord with the formula SSD_{（cluster）}=195×Ra-0.13 for brittle-ductile surfaces.2.A short optical manufacturing process chain containing UPG and MRF and SP is proposed.Based on Preston equation,the principles of MRF and SP are analyzed.MRF is used to quickly improve the ground surface shape accuracy,and SP is conducted to correct the ground surface mid-spatial frequency error.The transformation condition between MRF and SP is determined by power spectral density（PSD）curve.The evolution of laser damage performance of fused silica in the short process chain is experimentally studied.The laser damage performance in different polishing process is determined by photothermal weak absorption test and LIDT test.The SSD after UPG is removed by MRF,the average photothermal absorption value of fused silica surface rapidly decreases from hundreds of ppm to 0.67 ppm and the LIDT increases to 10.96J/cm².After MRF and SP,the optic surface is contaminated by the Beilby layer,the average photothermal absorption value of the surface is 1.56 ppm and the LIDT is 9.08J/cm².3.The characterization and mitigation on nanoscale damage precursors of wet chemical etching technology is investigated.The effect of wet chemical etching on the nanoscale damage precursors are analyzed,and the the reaction products during wet chemical etching are studied.The results show that the fracture defects and chemical structure defects can be effectively mitigated by HF-based etching with an appropriate etching depth,and the LIDT of fused silica increases by 43%.However,after HF-based etching,the surface roughness decreases significantly,and there are a lot of reaction products on the surface.The scanning electron microscope-Energy Dispersive Spectrometer showed that the main components of residual products in HF-based etching are sodium,calcium and potassium salts.HF-based etching as the only post-process technology is not sufficient to mitigate all damage precursors.In addtion,it has been proven that KOH-based wet etching can increase laser damage resistance and maintain surface roughness at once.KOH-based etching has no obvious mitigation effect on chemical structure defects.After etching,the ions generated from the prior polishing had a dramatic decrease in concentration except K,Fe and H.Reaction product K₂Si O₃ on the etched surface is also an important obstacle for further improving laser damage resistance via AFM-IR（The combination of infrared spectroscopy and atomic force microscopy）and TOF-SIMS（Time of flight secondary ion mass spectrometry）.They do not exist locally but spread across the whole etched surface.KOH-based wet etching can reduce most impurity contamination and keep a good surface roughness of fused silica optical elements,which can make it a useful supplement to the existing post-process technology.4.The clean effects of IBE on fused silica optical components is investigated.To solve the problem on cleaning sol-gel Si O₂ coating for the recovery and reuse of fused silica optical elements,IBE is used to remove the sol-gel coating.A series of IBE experiments is conducted to investigate the removal efficiency of sol-gel Si O₂ coating and substrate material.The best removal parameters during IBE are determined based on the maximum removal efficiency ratio between sol-gel coating and substrate material.The surface roughness and shape of fused silica are both improved during IBE process.The optical transmission of etched surface is reduced to the substrate level and the surface chemical structure remains the same.IBE is expected to replace wet chemical etching technology to realize green and safe cleaning for sol-gel Si O₂ coating.In order to achieve clean manufacturing for high-performance ultra-violet laser irradiated optics,the theory of IBE technology in the characterization and mitigation on nanoscale damage precursors are investigated.A series of IBE experiments is performed to investigate the evolutions of some nanoscale damage precursors（such as nano-particles,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.Nano-particls and 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 shape.The photothermal absorption on the fused silica surface can be decreased by 41.2%,and the LIDT can be raised by 15.2%after IBE at 250 nm.This work can serve as a reference for characterizing nanoscale damage precursors and using IBE technology to increase LIDT of fused silica optics.