Thermodynamic And Kinetic Simulation On Fused Silica Materials Irradiated By CO₂ Laser

Fused silica is a preferred optical material for high-fluence optics in many laser facilities. Its laser induced damage is a ‘bottleneck’ to limit the fluence of applications. Mitigating the surface damage and growth and extending the life-time is an important means to enhance damage resistance of high power solid laser facilites. Among the methods to mitigate surface damage and growth of fused silica, CO₂ laser repair has been widely recognized. In this dissertation, the physical mechanism is analyzed and calculated theoretically. The main investigation contents and results are as follows:1. Three-dimensional temperature distribution and evolution of fused silica irradiated by Gaussian continuous CO₂ laser are calculated. At the end of the laser irradiation, temperature is the highest in the spot center and gradually decreases toward the surrounding area. At the surface of the component isotherm is circular and temperature gradient is small while isotherm is Gaussian-shaped and temperature gradient is large at the internal component. That means the isotherms have larger width and smaller depth. Both during heating and cooling process, temperature change rate decreases with time and there is large temperature gradient near the spot edge.2. The effect of pulsed laser parameters on the repair size is analysed firstly with the finite element method. Temperature distribution and evolution of pulsed laser irradiation is similar to continuous laser irradiation. The biggest difference is that the temperature increases during the pulse irradiation and decreases during the pulse interval. Furthermore, the repair size of pulsed CO₂ laser is studied. The profile of evaporation and molten region are both shaped Gaussian pit. The influences of laser power, irradiation time and beam radius on repair size are big, especially on repair depth. However, the influences of pulse frequency and duty cycle are relatively small. With the increace of frequency, evaporation size is significantly reduced while molten size is slightly decreased. It is advantageous to suppressing evaporation and smoothing the repair surface as observed in experiment.3. Residual stress distribution of fused silica irradiated by Gaussian continuous CO₂ laser are calculated. Normal stress distribution on the surface is annulus whose center is laser spot center. However, the maximum stress is not located in the center where the material had melted, but with a certain distance from the center. During irradiation, compressive stress is generated near the center and tensile stress is generated at peripheral region due to heating and expansion. The reverse situation occurs during cooling process. Surface shear stress is symmetrical. Maximum shear stress does not appear in the center, but close to the edge of the spot. There are four high-stress area, shaped like a spindle. Furthermore, radius of the maximum shear stress increases with the increasing beam size which agreement with the experiment.4. Creep theory is introduced into high-temperature annealing and destressing effect is calculated. Creep factors of fused silica at 800℃ are identified initially by comparison with the experimental results repeatedly. The results show that the residual stress can be effectively reduced after high-temperature annealing. However, creep is closely related to the initial residual stress before annealing, which may have a negative impact on the morphology. In addition, three support schemes for large-scale fused silica component during high-temperature annealing are designed. By comparison, side-face support with vertical optical surface can effectively control the impact of weight on the stress and morphology of the component.5. A two-dimensional melt flow model is established and a quantitative description of CO₂ laser repair morphology is given with heat transfer and flow coupling mode. Three kinds of driving force leading to flow are analyzed respectively. The results show that surface tension is the dominant driving force. A unique morphology, Gaussian pit plus raised ring, forms due to melt flow of the surface material, which has been observed in the experiments.6. The influences of laser power, irradiation time, beam radius on melt flow are analyzed. The result shows that the flow velocity increases rapidly with the increase of irradiation time. Meanwhile, the depth of Gaussian pit and height of raised ring also increases rapidly while the radius of raised ring increases slightly. On the other hand, the flow velocity is almost constant at the same maximum temperature with different laser power and beam size. As laser power is reduced or beam size increases, the depth of Gaussian pit, height and radius of raised ring are all increased. Moreover, the depth of Gaussian pit and height of raised ring are sensitive to laser power while the radius of raised ring is sensitive to beam size.7. Based on common surface damages and defects, physical models are established for scratch, pits and damage crater, after that, the processes repaired by CO₂ laser are simulated by two-dimensional model. The result shows that repair morphologies are all Gaussian pit plus raised ring no matter what type of initial damage. Besides, the repair process is more difficult for large original damge size, especially the damage depth and the transverse width at the damage bottom.8. The process of CO₂ laser polishing is studied by elastoplastic structural analysis coupled with thermal analysis. A new laser beam, uniform line focused spot, is presented and simulated to polish fused silica surface. For polishing by round Gaussian spot, scan path is complex. Meanwhile, undulating morphology like ripples appears on the surface. In addition, laser power fluctuation less than 3% has little influence on the repair morphology. For polishing by uniform line focused spot with same length of component, scan path is simple. Furthermore, residual stress is small and dispersed while surface deformation is relatively flat. However, laser power fluctuation has a larger influence on the repair morphology. In general, polishing by uniform line focused spot is better than round Gaussian spot fundamentally if laser power is stable enough.