Fused Quartz Sub-surface Scratches On Laser Damage Induced By A Preliminary Study

Load capacity of optical components of high power laser system,during process of the ICF(inertial confinement fusion) drive by high power laser,is the key problem of limit output of high power laser system,one of the chief issues facing at present is to enhance the resistance to optical components and to prolong the using life-span.The paper presents a physical image of laser induced damage of sub-surface crack about initiation and damage growth,as well as mitigation of laser induced damage in fused silica sub-surface.Some well accepted damage mechanisms are discussed;and laser induced damage threshold of different morphology cracks are tested;the modulation of incidence light of planar radial cracks and hertzian conical cracks are simulated with finite element method;UV and CO₂ lasers are used to irradiation and pretreat the surface of fused silica to eliminate impurity and rehabilitate defect on the subsurface,so that can remove availably the initiation and mitigate damage increase rate validly.This is what we did mostly and received conclusions:1) The Coming and Scott fused silica are used in the experiences,the micro sub-surface cracks of fused silica are observed by optical microscope,which width and depth are measured by AFM.The cracks are sorted into three types according to the morphology characters:Boussinesq-point-force crack(BPFC),Hertzian-conical scratch (HCS) and plastic indent(PI).The laser induced damage threshold of cracks is measured by the SAGA-YAG solid-state laser by means of n-on-1.The results show that the damage threshold of sharp tip BPFC is less than 2.0J/cm²,that of HCS extends to 2.6 J/cm²,and that of deep deformation PI is 2.8 J/cm².The laser induced damage threshold of the shallow deformation indent is equal to that of bulk material 3.3J/cm².2) The finite element method is used to simulate the light intensity distribution in the vicinity of defect(planar and conical crack)on the fused silica subsurface.The results show light intensity distribution round the crack is resulted from several factors, including crack morphology,geometry dimension,azimuth,and incidence angle etc.. The enhancement effect of crack on the output surface is stronger than input surface, when total internal reflection occurs at both the ideal morphology planar crack and the surface,higher light intensification can occur.The light distribution in the vicinity of conical crack provides a theory seed to explain exactly why the plot adjacent diagonal of cross-crack-comer is firstly damaged.3) UV and CO₂ lasers are used to irradiation the surface of fused silica by means of raster-reticle-scan.By irradiated with UV laser,part of impurity and moisture content on the surface is ionized and decomposed,defects hided under the substrate sub-surface are exposed entirely,CO2 pretreat subsurface to make the surface of fused silica fusing and evaporating.Thereby inclusions are eliminated and defects are rehabilitated,laser induced damage threshold average enhance 30%.