Behavior Of UV Laser-induced Damage Growth In Fused Silica

Due to its excellent optical properties,the fused silica component has been widely used in high-power laser devices as focusing lenses,diffraction gratings,etc.However,the performance of fused silica elements can be severely degraded by initial damage or damage growth that occurs under intense laser irradiation,as the surface or bulk of the optical element undergoes permanent changes during the damage process.Once the initial damage point is present on the surface or inside the body,its growth is accelerated by the subsequent irradiation with the same laser energy.The physical properties of the component itself are severely damaged by the occurrence of the damage,with more serious consequences occurring during the subsequent growth of the damage.The operational stability of high-power laser devices is severely degraded by the laser-induced initial damage and subsequent damage growth of fused silica optical components,which has become a bottleneck limiting the load capacity of high-power laser devices.Conducting studies such as fused silica damage dynamics to investigate the mechanisms and laws of damage generation in fused silica elements and their subsequent damage growth under laser irradiation is fundamental to the improvement of high-power laser devices and a key need in the Inertial Confinement Fusion(ICF)field.Researchers and scholars at home and abroad have carried out extensive research on problems related to damage growth in fused silica and have obtained some of the mechanisms and laws of the damage growth process,but both theoretical and experimental studies have not yet investigated the behavior of energy deposition,plasma behavior,crack extension and other phenomenological evolution during the damage growth process.Based on this,this paper focuses on the damage growth on the rear surface of fused silica components.The experimental system is constructed using time-resolved pump-probe ultrafast shadow-resolved imaging technology to obtain dynamic images of damage growth,to analyze the effect of laser energy deposition on damage morphology,plasma,crack extension,and shock and stress wave phenomena during damage growth,and to further investigate the evolutionary behavior of shock waves.The experimental results show that:(1)The depth dimension of the damage site will be greater than the irradiated laser beam size when the crack size is considered throughout the damage extension.This is a further understanding of the current conclusion that the damage site size will not exceed the irradiated beam size.(2)There is a significant difference in the timing of the damaged crater and crack extension.Expansion of the head damage area precedes crack expansion during crack extension.The crack expansion time is much greater than the laser pulse width and the circumferential crack expansion time is greater than the radial crack expansion time.The combined effect of stress and shock waves during damage growth promotes the evolution of radial cracks to circumferential cracks.(3)The shock waves lead to particle deposition at the bottom of the damaged crater and on the cavity wall during the early stages of the damage,resulting in changes in laser energy deposition and the appearance of localized plasma concentration regions.The location and intensity of the plasma concentration region change with the size of the damage growth.The peak intensity of the plasma concentration region is reached when the damage grows in approximately the same amount of in-depth and radial direction.When the change in the radial dimension of the damage site is smaller than the depth dimension,the distribution of deposited particles is more dispersed.(4)As the amount of laser radiation increases,the expansion of the damaged area leads to the attenuation of the shock wave in the air,even at some delay times that cannot be observed at all.By studying the evolution of shock waves,it is found that the energy deposition caused by the expansion of the damaged area is the main reason for shock wave attenuation.The expansion of the damage region is directly related to the damage growth coefficient.By combining the damage growth coefficient and cavity depth-to-diameter ratio with the existing shock wave formula,the shock wave propagation formula suitable for this experiment is obtained.