Study On Damage Properties Induced By Processing Defects And Threshold Improvement Of KDP Crystal

KDP crystals are basic components used in the high power laser devices.Its damage performance will directly affect the output power and quality of laser beam.Benefited from the development of crystal growth and laser conditioning technology,the damage of KDP crystal material has been effectively controlled whereas the problems of surface damage become increasingly prominent.As it is easy to remain damage precursors like small-scale ripples,contaminating impurities and scratches on the processing surface of crystals,the laser damage resistance of KDP crystal is difficult to meet the need of laser facility.In order to further improve laser-induced damage threshold(LIDT)of KDP crystal,the influence of surface defects produced in the processing on damage performance of KDP crystal was studied for the purpose of exploring its damage mechanism.Meanwhile it aimed to optimize the processing technology of KDP crystal and provide theoretical guidance for optimal combined process.The main research contents are as follows:(1)The characterization and detection of surface defects and damage properties test technology of KDP crystal were introduced.Based on these techniques,a kind of method to evaluate damage performance of crystals was built,which is composed by defects characterization,photo-thermal absorption and LIDT.(2)The properties of laser damage induced by small-scale ripple on the surface of KDP crystal were studied.The influence of period and amplitude of ripples on the light intensity modulation of KDP crystal was analyzed by finite difference time domain(FDTD)method.It was concluded that in the range of 0~6μm,the period(especially when it was at 2μm)is dominant factor affecting the distribution of intensity while the amplitude is main factor from 6μm to 50μm instead.The light intensity enhancement factor(LIEF)caused by micro-waviness from 0 to 50μm could reach 3 to 5 times.In addition,the variation laws of small-scale ripples under different post-treatment processes were analyzed,and some suggestions to improve or remove ripples were proposed.(3)The effects of iron pollution and scratch defects on damage performance of KDP crystal were studied.The results of FDTD simulation showed that the intensity modulation caused by surface pollution was much larger than that of surface scratches.The experiment results of photo-thermal absorption and damage threshold test also showed that iron pollution is the dominant factor to induce laser damage.When the photo-thermal absorption of surface was up to 71.63 ppm,laser damage had occurred inKDP crystal.(4)The evolution laws of surface precursor defects and damage properties of KDP crystal under ion beam polishing and combined process were studied.When the material removed depth of ion beam reached up to 100 nm,the damage performance of crystal was the best.The process which was combined with single point diamond turning,magnetorheological water-dissolution polishing and ion beam finishing can effectively restrain the surface defects of crystal element,which turned out to increase the LIDT by nearly 50%.