Analysis of optical damage in the final optics assembly induced by transport mirror defects

Laser induced damage in the final optics assembly (FOA) is one of the bottleneck problems in high power laser systems for the inertial confinement fusion. In the online experiment, a correlation between the transport mirror defects and the optics damage in the final optics assembly has been found. A physical model is built to analyze the influence of defect size and modulation depth on the light intensification in the FOA. Basically, small size defect on the mirror has a tiny influence on the downstream modulation in the FOA. Optical propagation simulation is also carried out with the real defect phase information got from the interferometer. Results show that there could be strong light intensification caused by the upstream mirror defect. So the local high enough intensity is the main reason for the optics damage. The abnormal intensification is mainly caused by the irregular defect contour, which is produced in the laser induced damage process on the mirror. It is best to eliminate this kind of coating defect on the transport mirrors. When this kind of damaged defect on the transport mirror is inevitable, a mitigation strategy for the optics damage in the FOA is proposed. An inverse design of the best distance between the transport mirror and the FOA can be used to reduce the influence of the transport mirror defects on the downstream FOA. The study can provide some reference for the improvement of the damage resistance of final optics assembly in the high power laser facility.