Defect suppression: Laser-assisted low-defect manufacturing of fused quartz components

Fused silica glass

Fused silica glass is widely used in aerospace, fusion energy, laser attack and defense, high-energy physics and other major national optical engineering and defense fields due to its stable chemical properties, high softening temperature, low coefficient of thermal expansion, strong radiation resistance and good light transmission performance from ultraviolet band to infrared band. The laser fusion device represented by the National Ignition Device (NIF) of the United States is of great strategic significance to nuclear explosion simulation, fusion energy and cutting-edge scientific research, and has always been one of the large scientific devices promoted by the main forces of world powers.

In NIF, the total number of windows, lenses and diffractive elements made of fused quartz glass amounts to more than 2,000. However, the problem of UV laser-induced damage caused by processing defects at low laser flux (~8J/cm2, 3w, 3ns) seriously affects the performance and lifetime of fused quartz elements, although the laser-induced damage threshold (LIDT) of the intrinsic surface of fused quartz reaches >100J/cm2. In order to improve the damage resistance of fused quartz components, research institutions such as Lawrence Livermore National Laboratory (LLNL) have carried out a lot of defect elimination/suppression work in the past few decades from the laser-induced surface damage mechanism, traditional processing technology improvement, post-processing technology introduction, new processing concepts and methods, etc., but until now, LLNL still has to adopt optical cycling strategies to ensure the operation of NIF.

The main difficulties in preparing fused quartz elements with high UV LITT are:

1. As a typical hard and brittle material, the current contact processing method based on mechanical scratch mechanism based on material brittleness and plastic removal will inevitably introduce cracks, scratches and other processing defects on the surface/subsurface of the component.

2. There is a lack of three-dimensional full-caliber subsurface defect characterization methods in the grinding stage, and the current quantitative subsurface defect characterization methods are mostly local detection, and can only be detected indirectly through the sample, which is difficult to accurately reflect the maximum subsurface defect depth in actual processing. The lack of accurate estimation of the maximum subsurface defect depth can lead to insufficient material removal, residual subsurface defects after polishing, and difficulty in grasping the final processing quality.

3. The residual subsurface defects and polishing pollution introduced by polishing require post-treatment, but the current post-treatment technology will introduce new defects while removing/passivating defects.

Aiming at the problem of preparing fused quartz elements with high resistance to ultraviolet laser-induced damage, the team of Wei Chaoyang of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences proposed a laser-based defect characterization and removal process, and realized the manufacturing of high resistance to ultraviolet laser-induced damage fused quartz elements through full-link laser non-contact processing. The microsecond pulsed CO2 laser tomography ablation technology overcame the problem of three-dimensional full-caliber characterization of subsurface defects of fused quartz elements, and the CO2 laser process chain was further proposed, and the UV laser-induced damage thresholds of the prepared samples were 41% (0% probability) and 65.7% (100% probability) higher than those prepared by the traditional process chain, respectively. This laser-based defect characterization and removal method provides a new tool and direction to guide the optimization of traditional grinding processes and the preparation of highly laser-resistant fused quartz elements.

The results were published in Light Advanced Manufacturing under the title “Laser-based defect characterization and removal process for manufacturing fused silica optic with high ultraviolet laser damage threshold”.

In order to prepare fused quartz elements with high UV LITT, subsurface defects arising during the grinding process must be effectively managed. How to accurately obtain the spatial distribution and maximum depth of subsurface defects is the key. Considering the limitations of the existing subsurface defect characterization methods, the researchers first obtained the microsecond pulsed CO2 laser uniform ablation method based on the combination of theory and experiment, and then proposed the microsecond pulsed CO2 laser chromatographic ablation technology to characterize subsurface defects. This technique removes the material layer by layer by ablation, so that the subsurface defects are directly exposed to the observable surface, and then the subsurface defect distribution evolves with depth. This technology can achieve longitudinal ablation resolution from nm to UM, with a minimum longitudinal ablation resolution of < 5nm. Through the comparative analysis of the process image of subsurface defect evolution with depth, it is verified that the laser ablation process does not cause crack propagation or introduce new microcracks, which can accurately obtain subsurface defect information. At the same time, due to the unlimited ablation caliber and depth, this technique enables three-dimensional, full-caliber characterization of subsurface defects. To verify the practicality of this method, the researchers also characterized subsurface defects under different grinding processes.

Figure 1: Schematic diagram of the process for characterizing subsurface defects using microsecond pulsed CO2 laser tomographic ablation

Although microsecond pulsed CO2 laser tomographic ablation is also a destructive characterization technique, it integrates defect characterization and material removal, and can be coupled into the material removal process as a “grinding” process that does not introduce subsurface defects. To this end, the researchers further proposed a CO2 laser processing link including laser ablation “grinding”, laser conformal cleaning, and laser polishing. In this process link, laser ablation “grinding” is used to completely remove the subsurface defects introduced by the pre-process, and then laser NM-level ablation (laser conformal cleaning method) is used to clean and remove the residual re-deposited ablation products after laser “grinding”, and finally laser polishing and melting are used to smooth the ablation trajectory generated by laser “grinding”. Through the suppression of defects in the whole link, the zero-probability LITT and 100% LITT of the prepared components are increased by 41% and 65.7% respectively compared with the components prepared by the traditional process.

Figure 2: Schematic diagram of the CO2 laser processing link

The three-dimensional full-aperture characterization characteristics of microsecond pulsed CO2 laser tomographic ablation to characterize subsurface defects can realize the acquisition of global defect information of components, which provides an effective guidance tool for the formulation of subsurface defect removal strategies in the traditional process chain. At the same time, the characteristics of this technology integrating defect characterization and material removal highlight the potential of applying laser ablation “grinding” process to the grinding process of large-diameter components, and the introduction of machine vision system will realize the online characterization and removal of subsurface defects. At the same time, the proposal of CO2 laser processing link also provides a new idea for the preparation of high resistance to ultraviolet laser-induced damage molten quartz elements. The proposed technology and method can also be extended to the processing of other materials with defect control requirements. (Source: Advanced Manufacturing WeChat public account)

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