Theoretical And Experimental Studies On Surface Damage Repaire Of Fused Silica Optical Components

Improvement of the damage resistance capability of fused silica opticalcomponents is very important to achieve the high fluence output of high power solidlaser facilities. However, laser induced surface damage is still a ‘bottleneck’ to limit thefluence of facilities. Mitigating the surface damage and its growth is an effective way toensure the fluence level and prolong the life-time of components. Based on thismotivation, after sufficient document research, the dissertation focuses on the mitigationof damage and growth from theoretical simulation and experimental investigations. Atthe same time, the properties of residual stress and annealing parameters to eliminatestress are obtained. In addition, the optimized mitigation protocol and parameters areacquired according to the test results of damage threshold, life-time, damage growth andlight modulation. The important investigation contents and results are summarized asfollows:1. The characters and types of scratches and damage sites in the surface/subsurfaceof fused silica are investigated. The scratches can be classified as lateral scratch, radialscratch, Hertizan cone scratch and trailing indent scratch according to the morphologyand can be classified as plastic scratch, brittle scratch and mixed scratch according tothe forming properties. The morphology of damage sites is directly related to the profileof laser beam. The damage morphology can be divided into gray haze,‘mussel’ typeand ‘pansy’ type if the beam is spatial Gaussian distribution. There will be anotherflat-top type damage if the beam is spatial flat-top distribution.2. The physical effect such as melting, evaporation and thermal stresss betweenCO₂laser and fused silica material are discussed from theoretical analysis. Thetemperature and stress field distribution expressions are obtained for the damage repaireby CO₂laser in point-to-point and scanning modes. The physical mechanism of melting,evaporation and ablation are discussed theoretically and the corresponding formulas andalso obtained.3. The mitigation methods for surface scratches in the surface of fused silica areinvestigated. The repair parameters are obtained via theoretical simulation together with experiments. In order to ensure good mitigation, the CO₂laser power should beincreased step by step and the surface should be scanned for many times.4. The mitigation protocol for various sizes of damage sites in the surface of fusedsilica is systemically investigated. The results show that it can be repaired by CO₂laserdirectly when the lateral size of damage site less than400μm and depth less than200μm. If the lateral size of damage site is ranging from400μm to600μm, and the depthranging from200μm to300μm, it should be mitigated with HF etching combined withCO₂laser. How to repair the damage site less than400μm and the depth less than200μm is the principal investigation content in this dissertation. Two mitigation methods,high-power, short-time, single-shot （Method Ⅰ） irradiation and low-power, long-time,multi-shot （Method Ⅱ） irradiation, are presented. Meanwhile, the temperature fielddistribution for the corresponding method is also obtained by Ansys simulation.5. The formation mechanisms and elimination methods of the ablation debris andbubbles are studied since they seriously influence the damage threshold of mitigatedsites. The ablation can be classified as heavy ablation and light ablation and the debriscan be eliminated by large-beam passivation or HF etching. The bubbles can be alsoclassified as two types: sphere and ellipsoid. Meanwhile, the relationships between thenumber and size of bubbles and damage threshold are obtained.6. The property and elimination method of residual stress are investigated bytheoretical simulation and experiments. Isothermal annealing and isochronous annealingare investigated. It is shown that the influence of temperature on annealing effect ismore obvious. For the residual stress obtained with same mitigation technique on samecomponents, it can be completely eliminated as long as the residual stress resulting fromthe maximum size of CO₂laser beam can be eliminated with the annealing parameters.7. The effects of annealing and residual stress on the damage threshold of mitigatedsites are investigated. It is shown that the mitigated site has the highest damagethreshold at the center of site and the lowest damage threshold at the location ofmaximum residual stress for the mitigated sites before annealing. The damage thresholdof mitigated sites can recover to or exceed the original substrate threshold aftereliminating the residual stress completely.8. The mitigation effect is studied from three aspects: damage threshold, life-timeand damage growth. It is shown that the damage threshold obeys the normal distribution, which is independent of the size or profile of the test laser beam. A lower damagethreshold will be obtained with a larger test laser beam for the same profile of laserbeam. The life-time test results show that the life-time will be enhanced if increasing theirradiation fluence gradually. The damage growth test results show the growthcoefficient is the largest for the unannealing mitigated site. There is no evidentdifference on growth coefficient for the mitigated site with retardance of residual stressless than5nm comparing with that of fused silica substrate.9. The modulation effects of mitigated site obtained by different sizes of CO₂laserbeams and methods are investigated. It is shown that the modulation obtained byMethod Ⅰ is larger than that of Method Ⅱ for any sizes of CO₂laser beam. Meanwhile,it should control or eliminate the raised rim around the mitigated site to prevent itproducing a higher modulation. Otherwise, it potentially endangers the downstreamoptical components.Based on the theoretical simulation together with experimental investigations, inthis the dissertation the method and parameters are successfully obtained to mitigate thedamage site with lateral size less than600μm and depth less than300μm. However,further investigation is need to validate the mitigation technique on large damage site,wave-front quality control and the mitigation parameters transfer from small optics tolarge-scale optics. Meanwhile, theoretical simulation also needs further investigation.