| Laser damage resistant performance for optics is very important in high power laser systems, and laser conditioning technology can improve laser induced damage threshold (LIDT) by cleaning contamination on optical surfaces and reducing the sub-surface defects (SSD). In this thesis, UV and CO2 laser conditioning technology is introduced as well as acid etching and thermal annealing process.1. HF acid etching: Acid etching can remove both surface polishing materials (CeO2, etc.) and SSD, and also passivate the scratches and deep crater defects in the sub-surface. The LIDT increases from 6.13 J/cm2 to 6.93 J/cm2 when the 1% HF (wt.%) solution is used to etch the surface of fused silica for 10 minutes. If the etching time is longer, the threshold would be higher, but the negative effects on the optical surface roughness and wavefront can not be ignored.2. UV laser conditioning: Using laser with fluence below zero damage thresholds (Fth0) to irradiate the fused silica surface, the impurities and contaminations can be removed, and meanwhile the low-threshold initiators and electronic defects can be eliminated. It is found that the threshold of acid-etched fused silica can not be improved significantly through UV laser conditioning. Laser conditioning at high fluence even reduces the damage threshold (if the conditioning fluence is less than 60% of Fth0, the threshold approximately increases 10%. If more than 80%, the threshold decreases rapidly). However, UV laser conditioning is an essential process, because low fluence UV laser irradiation could expose surface defects, reduce the risk of system operation, and prepare for the subsequent CO2 laser conditioning.3. CO2 laser conditioning: CO2 laser conditioning is the most important process. CO2 laser (10.6μm) irradiation can result in local surface heating rapidly followed by melting, flowing and evaporation. The flow of molten material on the surface can heal the sub-surface micro-defects and improved the laser damage resistance. In this work, the CO2 laser spot size, laser fluence, and laser spot moving path, are controlled to treat different defects. For the smaller defects (< 20μm), a big laser spot with the diameter of 4 mm combined with full-aperture scanning could well repair such defects. If defects are larger (20μm-80μm), a laser spot with the diameter of 700μm fixed on the defect position and sustained 150-200 ms can heal these defects. If the defect size is 80μm-500μm, the laser spot used for repair can be 2 mm-4 mm. In practical work, each sample usually has many defects with different sizes, which requires a combination of scanning and fixed-point radiation. In a word, defects with different size distributions can be repaired, using a variety of ways to achieve the healing purpose.4. Ablation and residual stress resulted from CO2 laser conditioning are investigated. Different treating protocol and annealing process are tried to eliminate the negative effects of CO2 laser conditioning. Given the research condition, off-line thermal annealing is more convenient. The fused silica samples, repaired by CO2 laser, annealed in air for 3h at temperature 700-800℃, can release residual stress. For the CO2 laser conditioned fused silica samples, annealing in air atmosphere and preventing from contamination can obtain good performance. |
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