Fused Quartz Sub-surface Defects Induced By The Fdtd Study Of Laser Damage

With the concept of high power laser used in inertial confinement fusion, the manufacture of various high power lasers is progressing like a raging fire. However, there is a lack of load ability on optical elements. It becomes a big bottleneck of outputting the high power. The defects on the subsurface are the main reason for reducing the damage threshold of optical elements. Aiming at this question, we do some researches as follow in theory. Firstly, the mechanisms of laser induced damage are studied. The influencing factors of laser induced damage are discussed. Secondary, because of high amount of computation and complicacy, there is nobody at home has simulated the three-dimension light field distribution neighborhood the defects on the subsurface of fused silica. The differences between the results of three-dimension and two-dimension are researched. The necessity of three-dimension simulation is discussed. Finally, the three-dimensional defect models are made, and writing and debugging the calculating program is an important work too. Lastly, the safe size of various defects is quantificational analyzed and academic evidences for safe judging of fused silica in experiment is provided.In my postgraduate period, we do researches on fused silica subsurface damage in theory. The main work and results as followed.1) Lots of achievements about subsurface damage (SSD) at home and overseas are perused. Finding out several primary factors on laser induced damage (LID). Including parameters of laser such as laser power, wavelength, pulse length, pulse count, and the characters of subsurface defects such as geometry, size, angle and so on. Based on pattern characteristics on fused silica subsurface, we conclude several typical defects, including transverse cracks, radial cracks, Hertz cone scratches (HCS), and impurities.2) We studied the three-dimension finite-difference time-domain (FDTD) method. The three-dimension is much more complicated than the two-dimensional case. We put three-dimensional defects model, FDTD calculating formula, total boundary conditions (including surface boundary conditions and edge boundary conditions) and perfectly matched layer (PML) absorbability boundary conditions. This method consumes more time than two-dimension case. Significantly, we do three-dimensional simulation and two-dimension respectively for the same size defects and same parameters of incidence laser. The results show that the three-dimensional defects have stronger effect. Therefore, it's necessary to do three-dimensional simulation.3) Based on the three-dimensional FDTD method, the three-dimensional cubic defect model, transverse defect model, HCS model on the fused silica subsurface was made, and simulated respectively. The modulation of the incident laser caused by sorts of defects is analyzed in detail. The results show that the exit surface is fused silica blind side in laser induced damage. The light intensity enhancement caused by radial crack can reach about 10. The enhancement about 57 caused by HCS is much higher than transverse crack. But the modulation of incident light field by impurities is the most observably. The light intensity enhancement factor (LIEF) can reach 100 easily.