Defects Induced Damage Behavior And Mechanism Of Fused Silica And KDP Crystal With High Power Laser Radiation

Fused silica and KDP(KH2PO4)crystals are two major functional ultraviolet optics materials used for terminal optical components of high power solid laser facility.To meet the requirements of inertial confinement fusion(ICF)research,the output flux(UV laser energy density)of high power solid laser facility is increasing,ultraviolet pulse laser energy density in terminal optical components is as high as 8 J/cm2(351 nm,5 ns),and which could be improved further.Under this extreme working condition,the problem of ultraviolet laser induced damage of fused silica and KDP crystal becomes increasingly serious,which has become the bottleneck problem restricting the development of laser inertial confinement fusion.Ideal fused silica and KDP crystal has almost no absorption to 351 nm UV pulse laser,the calculated results based on energy band theory and multiphoton ionization model show that the damage threshold is also much higher than the operation flux of high power solid laser facility at present stage.Because of inevitable defect introducing at material manufacturing and processing of fused silica and KDP crystal,which becomes the main cause of lower fluence laser induced damage.Surface defects from surface process and bulk defects from crystal growth are responsible for laser inducing surface damage of fused silica and bulk damage of KDP crystal,respectively.However,the specific correlation between the characteristic parameters of these defects and the laser inducing damage has not been obtained so far.In this paper,the surface defects of fused silica were controlled by mechanical polishing,magneto-rheological polishing and dynamic chemical etching,the bulk defects of KDP crystals were modified by thermal annealing and laser conditioning,by which,we studied laser inducing damage behavior characteristics of fused silica and KDP crystals under different defect distribution characteristics,and explored the influence of defect characteristic parameter changes on laser inducing damage.In addition,the bulk damage behavior of fused silica material and bulk defect induced bulk damage mechanism were also investigated.The main research contents and results of this paper are as follows:1.For mechanical polishing,magnetic rheological polishing and dynamic chemical etching,we used Time-of-Flight secondary ion mass spectrometry,laser induced fluorescence imaging,fluorescence spectrum and photo-thermal weak absorption to analyze and characterize evolution and transformation trend of fused silica surface defects under different process conditions.The results show that the mechanical polishing process can retain metal impurities in the shallow surface of dozens of nanometers,and the distribution depth of non-metallic impurities is more than 10 ?m,dynamic chemical etching can effectively control the pollution defects and fragmentized defects in the surface of fused silica.By adding dynamic chemical etching depth,it can also realize significant inhibition of structural defects such as absorptive defect,E',ODC and STE.The magnetorheological polishing can lead to a significant increase in surface fluorescence defects and weak absorption coefficient,which is related to the increase of absorption metal impurities and organic contamination in the process.2.Based on damage threshold and damage density,the damage resistance of the fused silica surface with different process condition was compared,laser-induced fluorescence imaging and photo-thermal weak absorption were performed to characterize the absorptive defects with different process condition,then we analyzed the quantitative correlation of absorptive defect parameters and damage resistance.The results suggest that there is no significant correlation between peak absorption coefficent and laser damage resistance,the percentage of fluorescent defects and average absorption coefficient has a significant correlation with zero probability damage threshold and damage density,spearman rank correlation coefficients are over 0.9 and the quantitative fitting curves correspond to the exponential attenuation relationship and power exponent growth relationship.By high fitting degree relation curves between the average weak absorption coefficient and the damage density,we presented a method to indirectly evaluate laser-induced damage of fused silica surface at 351 nm pulse laser.3.By focusing a high energy laser outside the bulk material,the bulk damage thresholds of typical fused silica samples,including synthetic fused silica and fused quartz were gained.The results show that the bulk damage thresholds among the synthetic fused silica samples manufactured by different vendors are between 75 J/cm2 and 90 J/cm2,and the bulk damage threshold of fused quartz is only about half that of synthetic fused silica.The effects of bulk defects in the fused silica on bulk damage resistance were analyzed statistically based on the correlation principle,the results indicate that bulk bubbles have a significant impact on the bulk damage threshold of fused quartz due to the light field modulation of the incident laser.Metal impurities and hydroxyl have weak correlation with the bulk damage threshold of synthetic fused silica.There is a strong correlation between weak absorption and the bulk damage threshold,the spearman rank correlation coefficient is-0.72,suggesting that 355nm UV laser absorption is a main factor to induce bulk damage.In the presence of low threshold damage precursors induced by surface process processing,there is no significant correlation of fused silica bulk damage threshold with surface damage threshold.4.Aim at bulk damage of KDP crystal,we studied initial damage and damage growth behavior of KDP crystal with UV pulse laser radiation,and obtained the bulk damage morphology characteristics,functional damage thresholds and damage growth thresholds of KDP crystal.By laser conditioning and thermal annealing,the material defects of KDP crystal were modified,then we characterized the defect distribution characteristics of KDP crystal under different process conditions by fluorescence spectra,absorption spectra,positron lifetime annihilation spectra and photothermal weak absorption,and studied the mechanism of defect induced damage of KDP crystal material.The result shows that thermal annealing can decrease point defect content such as interstitial hydrogen and hydrogen vacancy,and that reduce the scale of vacancy defects and UV weak absorption,which is one of the reasons for improvement of KDP crystal laser damage resistance by thermal annealing.In addition,there is a strong correlation between vacancy defects characterized by the positron annihilation lifetime spectroscopy and damage resistance of KDP crystals,vacancy defects with r2 scale is complete correlative with damage density under 8 J/cm2 flux;laser conditioning can reduce the scale of the vacancy defect,decrease UV weak absorption of KDP crystal and improve the homogeneity of defect modification,which is an important mechanism of ultraviolet pulse laser conditioning to significantly improve the damage resistance of KDP crystal.The work of this paper is helpful to further improve damage resistance of fused silica and KDP crystal in high power solid laser facility,it can provide the necessary references for selection of ultraviolet optical material and optimization of application strategy.