Effect Of Surface Defects Of Fused Silica On Laser Damage And Its Application In Processing

With the increase of laser energy density,the damage of optical elements becomes more and more serious,which becomes a key problem limiting the further development of high power laser system.Laser induced damage of optical components is a process of interaction between laser and matter in essence,and its research involves the fields of optics,photoelectric,photothermal,thermodynamics and plasma,etc.,which is a complex crossdisciplinary problem.Under laser pulse irradiation,surface defects of fused silica have become an important factor that can not be ignored to reduce laser damage threshold of fused silica optical elements.The damage of fused silica optical element not only reduces its beam transmission quality and load capacity,but also causes damage growth in the damaged area under subsequent pulse irradiation,which seriously reduces the service life of laser system and limits the development of high power laser technology.Therefore,it is very important to study the effect of surface defects on laser damage resistance of fused silica.In this paper,the influence of surface defects of fused silica on its laser damage threshold is studied.Aiming at the damage problem of surface defects of fused silica under laser pulse irradiation,a numerical model of interaction between nanosecond laser pulse and fused silica is established.The light intensity distribution,temperature distribution,stress distribution and damage morphology of fused silica with surface scratches were studied by simulation and experiment.The damage mechanism of surface defects of fused silica under nanosecond laser irradiation and the influence of defects on laser damage resistance of fused silica were further revealed.At the same time,a combined pulsed laser processing method is proposed,which uses the pre-pulse to create surface defects on fused silica to improve the absorption of the material to the subsequent main pulse and thus increase the material ablation efficiency.The influence laws of temperature accumulation,light field modulation caused by damage pit defect and light field modulation caused by stress birefringence on the ablation morphology of fused silica were studied by using control variable method.The main work is as follows:Firstly,aiming at the damage of fused silica surface defects under nanosecond laser pulse irradiation,based on electromagnetic field theory,heat conduction theory and elastoplastic mechanics theory,a numerical model of interaction between nanosecond laser pulse and fused silica suitable for surface defects was established,and the light field modulation of incident laser induced by scratch defects on fused silica surface was simulated.Starting with the distribution of temperature and stress around the surface scratch defect,the laser damage situation was analyzed,and the distribution of light intensity,temperature field and stress field inside the laser irradiated non-surface defect fused silica and surface scratch defect fused silica were simulated.The influence of different surface defect sizes on the light intensity,temperature and stress inside the fused silica was explored.Secondly,a laser damage measurement and stress online measurement system was established to study the influence of surface defects on laser damage of fused silica.The surface scratch defect was artificially created on the surface of fused silica.The laser damage threshold of fused silica without surface defect and fused silica with surface scratch defect were measured experimentally and compared with the simulation results of the constructed model.Meanwhile,the stress distribution of fused silica without surface scratch and fused silica with surface scratch after being irradiated by nanosecond laser pulse was measured by photoelastic method.The damage characteristics of surface defects of fused silica irradiated by nanosecond laser pulse were analyzed from several angles.Finally,based on the influence of surface defects of fused silica on its laser damage threshold,a combined pulsed laser processing method was proposed.Laser pulses with a certain energy density were used to create surface defects on fused silica to improve the material’s absorption of subsequent main pulses and thus increase the material’s ablation efficiency.A damage growth model of fused silica was constructed.By using the control variable method,the influence laws of temperature accumulation,light field modulation caused by damage pit defect and light field modulation caused by stress birefringence on the ablation morphology of fused silica were studied,and the ablation laws of fused silica by combined triple pulse laser were further analyzed.The research results of this paper not only help to deepen the understanding of the damage characteristics and damage mechanism of the surface defects of fused silica under the action of nanosecond laser pulse,analyze the influence of surface defects of fused silica on its laser damage resistance,but also provide a theoretical basis for improving the ablation efficiency of laser processing.