Simulation Of Light Intensification Induced By Defects And Impurities In Subsurface And Their Inspection And Control

Being as bulk material in transmissive and diffractive optics for high-power lasersystems, research of vitreous silica has been motivated prominently by its ubiquity ininertial confinement fusion. Since the amorphous silica bulk material is manufacturedfree of impurities and defects, laser-induced damage （LID） events rarely occur insidethe bulk. Instead, most damage is found on the surfaces. Up to now, the surface damageof fused silica has been a predominate issue subjecting to extensive studies. Once adamage site initiates, it grows catastrophically under further irradiation. Moreover, itnot only generates the optical scattering, but also imposes added modulation on thedownstream optical devices. Thereby, it dramatically shorten the service lifetime ofoptics. On the other hand, produced from high-quality SiO₂, these components arerelatively expensive and it is of great interest in terms of damage repair and recycling.In this thesis, we detailedly investigated the light intensification around defects byusing finite-difference time-domain （FDTD） algorithm, associating with relevantexperimental results of optical microscope, atomic force microscope （AFM）, and stylusprofilometer. The aforementioned defects are cracks in subsurface, repaired damagesites, and contaminants on anti-reflective film optics. In experimental department, laserconditioning of sol-gel SiO₂films, laser shockwave cleaning （LSC）, and measuration oforganic film thickness have been investigated. Main conclusions are made as follows:（1） According to fracture geometry during grinding and polishing steps, subsurfacecracks can be divided into lateral crack, discontinuous crack, Hertzian-conical crack（HCC）, trailing indent crack （TIC）, et al. FDTD numerical studies show that lightintensity enhancement induced by HCC is the most remarkable of all. There will be ahighest modulation when inclination angle θ is around π/6. When θ ranges from π/8toπ/4, the light intensity enhancement factor （LIEF） can easily break through100, and themodulation follows a decreasing trend in further etching. The intensification induced bylateral crack is the second strong, and the LIEFmaxis18.49when the breadth depth ratioR belongs to the range of1³. The modulations formed by discontinuous crack and TICare the weakest and close to each other, and the LIEFmaxare12.9and11.2, respectively. （2） The modulation of HfO₂/SiO₂high reflector deposited on lateral cracks hasbeen discussed when1053nm laser irradiates on the input surface. Results show thatthe catface of the reflectance coatings generate much larger intensity enhancement thanother sites. Moreover, the intensification extends to the substrate inside. The defectmentioned above can cause LIEF greater than30. When the crack width is larger than5λ, the modulation is equal to nodular defects.（3） As for repaired damage sites, it is shown that the unrepaired cracks cangenerate notable modulation, whose LIEFmaxreaches24.3. With increasing the numberof bubble in subsurface, a larger modulation acquired. When crack or bubble distributesin the near-surface area （<3λ） and close to the edge of crater ring, the field modulationis obvious. Damage sites irradiated by CO₂laser at different parameters give birth totwo nonideal craters, with raised rim as well as with concave rim. When the concaverim converts to the raised rim, the modulation undergoes decreasing firstly and thenincreasing. That is to say, the order of laser intensification power is: craters with a raisedrim> craters with a concave rim> ideal Gaussian craters.（4） The light intensity enhancement induced by particulate silica contaminants onanti-reflective film has been studied by utilizing the FDTD method. Result showsparticles on the input surface generate much larger intensity enhancement than on theoutput surface. Peak field modulation inside the component manifests an attenuationtrend, and the attenuation length are3λ and2λ for the input-and output-surface particlesrespectively. Comparing with the ellipsoidal structure, the spherical particles cangenerate larger ehancement. For irradiation under3ω laser, silica particles can causeLIEF as high as77.5.（5） The influence of sol-gel SiO₂films after raster scanning laser conditioning witha1064nm Nd:YAG pulsed laser has been investigated. The results show no obviouschange of the laser-induced damage threshold （LIDT,“R-on-1” regime） of clean films.Damage characteristics of transparent impurities films have been improved. The LIDTsof the samples contained absorbing impurities are less than half of the clean ones.（6） LSC has been used to remove the particulate contamination from SiO₂sol-geloptical film. Compared with dry laser cleaning （DLC）, LSC with shockwave initiatedby plasma formation under a focused laser beam pulse has much better efficiency.Furthermore, it has been demonstrated that the transmittance of contaminated SiO₂film can restores to the non-contaminated after LSC.（7） The spectroscopic ellipsometry is a precision instrument in measuring filmthickness. The inhomogeneity of the film is the most notable obstacle for precisemeasurement. Thickness measured by AFM is mostly determined by the prepared steps.The step size should be within the range of the AFM. In AFM analysis, the thickness ofa point is corresponding to one-line step height difference. And the average thickness ofan area is corresponding to the median pixel value difference on both step sides.Above results indicate that the HCC can generate the most obvious lightintensification of all, followed by silica particles impurities. The theoretical simulationsand experimental investigations progress greatly and achieve the anticipative objectives.However, related defects in the far-field modulation, and thermal induced damage is yetto be verified. In the LSC experiments, dependences of the parameter variables （particlesize, laser energy, gap distance） and cleaning efficiency need to be quantified, andtheoretical research and technology research also need further development andvalidation. In addition, it is necessary to carry out the analysis of the chemicalcomposition in pollutants detection, and then take advantage of the ellipsometerdynamic monitoring the pollutant deposition process.