Ruiqing Xie

A method for evaluating subsurface damage in optical glass

An alternative method for evaluating subsurface damage (SSD) in ground fused silica is presented. The method can acquire the knowledge of depth and morphology of subsurface damage at the same time. The fundamental support lent to the method is the fact that the depth of field reduces as the numerical aperture (NA)/magnification increases in optical microscopes. Large depth of field without undermining NA is preferred in most applications while the narrow range of focus depth is desired for our method. Using this method, we experimented on fused silica which was ground with bound-abrasive diamond wheels and the results show good agreement with the traditional method. The consistency indicates that the proposed method is practicable and effective in inspecting the subsurface damage in optical components.

Producing fused silica optics with high UV-damage resistance to nanosecond pulsed lasers

The laser induced damage to optics has been an issue of paramount importance in laser research community. The low damage threshold of fused silica surfaces predominantly restricts the development of high power and high energy systems. This paper is aimed at improving the surface damage threshold of fused silica substrates by researching the effect of mechanical and chemical defects on laser damage: cracks/scratches and metallic impurities. The cracks were found to close, at least in part, after thermal processing and the damage threshold of the indented region was little affected by the thermal processing. In contrast, the cracks were enlarged after chemical etching and the damage threshold was improved slightly. Concerning scratches, the damage threshold can be recovered significantly after different HF-based etching. The metallic contamination can be removed by HF-based etching and acid leaching. The etched surface shows that the damage threshold increased first to ~30J/cm2 and then decreased with etching time while the damage threshold stabilized at ~30J/cm2 for leaching >45min. The surface roughness may degrade after etching, from <1nm to 3~5nm RMS, but that is ~1nm after leaching. The leaching may be a potential method for dissolving metallic contaminants on the glass surface in order to get a smooth surface with high damage resistance.

The Surface Layer of Fused Silica Finished by Various Polishing Techniques

Our recent experiments show that the surface layer of fused silica manufactured by the magnetorheological finishing(MRF) is different from those finished by conventional polishing technique (e.g. pitch polishing or polyurethane polishing) in mechanical properties. We examined the hardness of the surface layer on fused silica polished by pitch/pad polishing and MRF respectively, and the results indicate that the hardness of traditionally polished layer is smaller than bulk material, decreasing with the distance from the sample surface, while the nanohardness of MRF-manufactured sample is greater than bulk, gradually increasing to the bulk value. We attribute the differences to the unique material removal mechanism of MRF. The distinctions among these polishing techniques may make contribution to varied laser-induced breakdown topography.

In-situ shape measurement technology during large aperture optical planar continuous polishing process

Continuous polishing technology is an important means to realize batch processing of large aperture and high precision planar optical components. However, traditional continuous polishing process largely depends on the operators experience, with poor controllability of component surface figure and unstable processing efficiency. In order to solve this problem, the in-situ shape measurement technologies including measurement of pitch lap surface figure and workpiece surface figure have been proposed in this paper. The real-time states of the pitch lap flatness and the workpiece surface figure in polishing process are obtained by in-situ measurement technologies, which provide the quantitative informations for adjusting process parameters. In the experiment, a large aperture mirror (material as K9; size as 800mm×400mm×100mm) was polished. The results show that the surface figure of the component was improved from λ/2 (1λ=632.8nm) to λ/6 by using the in-situ measurement technologies during the continous polishing process.

Research on high-efficiency polishing technology of photomask substrate

A method of photomask substrate fabrication is demonstrated ,that the surface figure and roughness of fused silica will converge to target precision rapidly with the full aperture polishing. Surface figure of optical flats in full aperture polishing processes is primarily dependent on the surface profile of polishing pad, therefor, a improved function of polishing mechanism was put forward based on two axis lapping machine and technology experience, and the pad testing based on displacement sensor and the active conditioning method of the pad is applied in this research. Moreover , the clamping deformation of the thin glass is solved by the new pitch dispensing method. The experimental results show that the surface figure of the 152mm×152mm×6.35mm optical glass is 0.25λ(λ=633nm) and the roughness is 0.32nm ,which has meet the requirements of mask substrate for 90~45nm nodes.

Research and progress of rapid polishing technology for large aperture optics

In view of the ultra-precision machining for large aperture optics developing rapidly but lacking effective rapid polishing methods, the research problems worth further studying are pointed out. The current situation, removing mechanism and processing difficulties of large aperture ultra-precision machine are introduced. The quantity and diameter of plane optics are increasing year by year, but currently the rapid removing of ultra-precision machine based on deterministic control of surface shape of the meter size optical components is still a blank. The development of related field is seriously affected huge losses caused by the low precision and low processing efficiency. The research situation of the high accuracy and rapid polishing methods are introduced for the main factors in the polishing processing system, such as movement patterns, polishing pad, polishing slurry and process parameters. According to the characteristics of the polishing machining process that bad deterministic control of the surface shape and the low removal efficiency, the research emphasis should be the methods that deterministic control and rapid removing of the machine, a new generation of high precision and high efficiency rapid polishing equipment and application process suitable for the large aperture and flat optical components.