HF-based etching has been an effective method to increase the laser induced damage thresholds (LIDTS) of fused silica optics. In this research, the effect of etching parameters on the surface quality and laser damage performance of fused silica with Megasonic-assisted HF acid etching has been investigated systematically. The fused silica samples (50mm in diameter and 5mm thick) were maufactured through the conventional grinding and chemical mechanical polishing process, these processed samples are etched with different etching parameters. Our results show that the frequency and distribution of megasonic field will bring great effect on the surface quality of optics. The LIDTS were measured by 1- on-1 mode, results showed that the 1.3MHz megasonic field and a certain amount of etching depth will benefit the laser damage performance of fused silica optics.
The effect of ion beam etching process on the surface quality, the surface roughness and the laser-induced damage threshold at 351nm was carried out. Research results reveal that the laser-induced damage threshold of fused silica was enhanced with the increase of etching depth when the etching depth was less than 800nm, and could be further enhanced about 30% at 800nm etching depth, however the laser-induced damage threshold began to decrease with the further increase of etching depth(more than 800nm). The test results of surface microtopography, laser damage morphology, and surface roughness reveal that the ion beam etching process can remove polishing re-deposition layer without degrading the surface condition at a smaller etching depth so as to enhance the laser-induced damage threshold of fused silica, however further ion etching which can produce impurity particle often results in a decrease rather than an increase of laser-induced damage threshold.
The influence of polishing parameters such as particle size, pad material and pressure on the surface roughness of glass optics were investigated and analysed. It reveals that the surface roughness will get worse with increase of the polishing particle size. The surface roughness would remain stable in a certain period of polishing pressure, but get worse with increase of the pressure beyond the period. The surface roughness is getting better when using smooth pitch polishing pad than polyurethane pad with lots of micropores. By optimizing the polishing parameters, the surface roughness of large aperture fused silica window is improved to 0.46nm before band-pass filtering and 0.084nm after band-pass filtering.
Advanced mitigation process (AMP) has been identified as a most effective method to improve the laser induced damage threshold of the fused silica optics used in the large laser facility, and as the most important sub-process HF etching with multi-frequency megasonic agitation plays a decisive role to improve the damage threshold of fused silica. But because of the physical characteristics of megasonic, the etched surface is apt to generate striated haze which not only modulates the optical field, but also reduces the damage threshold significantly. In this paper, the generating mechanism of the striated haze is discussed, and both of the uneven acoustic field distribution and the optics' redeposited substance are recognized as the primary factor resulted this phenomenon, ultimately based on these discussions a slight swing when etching is proposed to eliminate this phenomenon.