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Analysis and mitigation of final optics damage caused by continuous phase plate

Laser induced damage in the final optics system is one of the bottleneck problems in the high-power laser system. For almost all the eight beams of final optics assembly (FOA), there are usually damages in the middle areas of the focusing lens in SG-II laser facility. Through detailed analysis, we find a correlation between the damage in the focusing lens and defect in the continuous phase plate (CPP). The main mechanism for downstream damage is regarded as the defect induced light intensification. The phase distribution of the CPP is characterized by coherent diffraction imaging method. Through simulation, we can clearly see a much stronger light intensification in the middle of the beam caused by the real CPP than the theoretical designed CPP. Compared with the designed CPP, there are defects on the CPP causing the downstream intensification. Meanwhile, there are unexpected periodic modulation on the surface of CPP. We assume the central defect is a kind of laser induced defect because the defects caused by the optical processing are randomly distributed. Through ray tracing analysis, we find a ghost image near the center of the CPP position. So the CPP is slightly damaged or modified in the middle area of the ghost image ray, thus forming a defect with strong modulation. A stray light management is proposed base on ground glass to mitigate the ghost image problem. The periodic modulation is possibly formed by the manufacturing process of CPP. Small-period modulation can cause greater downstream modulation. It should be controlled with power spectral density specification in the manufacturing process. Once the laser induced defect problem is solved, the laser induced damage in the middle of focusing lens is greatly mitigated.