Cleanliness management is essential for the safe operation of high power laser systems. In order to maintain the transmittance and minimize the optics damage, we should protect the optics surface with coatings from contamination. Large aperture optics in the vacuum is easily contaminated by airborne molecular from material outgas. In this paper, we introduce the techniques of cleanliness management in SG-II-UP laser facility. The online monitoring of surface airborne molecular contaminant is carried out with quartz crystal microbalance. We investigated the influence of the airborne molecular contaminant on the 351nm transmittance of fused silica optics with sol-gel coatings.
Many literatures have focused on the design, fabrication and intrinsic damage of multilayer dielectric gratings (MLDGs). However, contamination may be a key factor of the MLDGs lifetime. In this paper, we compare the laser damages of the clean and contaminated MLDGs, and damage threshold of clean MLDGs is 3.12J/cm<sup>2</sup> . We obtain the curve of the critical particle size at various laser fluences, above which the MLDGs will be damaged. Intrinsic LIDT of clean MLDGs is larger than that of contaminated ones, so the cleanness control is a crucial problem for the MLDGs. The scanning electron microscope (SEM) morphologies of intrinsic and contaminant induced damages are different. The intrinsic damage is cold ablation, while contaminant induced damage is thermal ablation.
Strictly controlling the cleanness of transport mirror surface in high power laser system has an important significance. Removal efficiencies of dust in different sizes and on different positions of the transport mirror surface are studied, by using the air knife blowing method with different inlet pressures and installation positions. Full experiments and range analysis show that the air knife blowing method is an effective way to control the cleanness of the transport mirror surface. The removal efficiency of dust particles in different sizes and positions of the transport mirror surface is better when inlet pressure is 0.9 MPa and the air knife installation position is 3 mm. Besides that, some simulations on flow fields are conducted. The simulation results and the experimental results have a good consistency.