Fused quartz material demonstrates strong ultraviolet light transmission and resistance to laser damage, making it the primary choice for selecting large-diameter optical components in high-power laser systems. The challenge now lies in achieving low damages and high resistance to laser damage. A significant number of large-scale (with characteristic depths ranging from 10μm to 100μm) Sub Surface Damages (SSD) are generated in grinding stage, requiring substantial material removal during the subsequent polishing stage to ensure the damage resistance performance of the components. Therefore, it is crucial to study damage control in grinding stage for achieving optimal subsurface quality in optical components. This paper focuses on the processing state of fused quartz components under various grinding processes. Initially, a comparative experiment is conducted using loose abrasive and bonded abrasive with the same grain size (6μm), followed by an examination of three different grain sizes (6μm, 9μm, 20μm) of bonded abrasive grinding processes. After grinding, all processed samples are appropriately etched to reveal subsurface damages. The damage distribution and morphological information of the experimental samples are observed using optical microscopy and scanning electron microscopy. The value of surface roughness (Rz) of the fused quartz samples is measured using white light interferometry, and the corresponding SSD is calculated using the empirical formula of Kun Xiao. The findings indicate that the bonded abrasive processed samples exhibit higher removal efficiency and smaller SSD compared to the loose abrasive samples.
The PSD2 index of 3mm ultrathin large aperture optical components is difficult to converge effectively, which is influenced by various factors. In order to solve the problem, the measurement results of the PSD2 index were analyzed in two parts. One part was the results of the PSD2 index after removing the defocus and astigmatism, which we used to evaluate the impact of intermediate frequency errors. The other part was the surface shape PV value of the measurement area, which we used to evaluate the impact of low frequency errors. In the double-side polishing stage, the oscillating smooth polishing technology was used to smooth intermediate frequency errors of the optical surface, which eliminated the fine ripples in the PSD2 processing results. The surface shape local correction technology was used to improve the surface shape PV value of the measurement area. Through the synchronous control of the intermediate frequency and low frequency errors, the PSD2 index control for the whole surface of the optical components was realized. The RMS value of the the PSD2 index was converged from 1.38nm to 0.70nm on average.
With the increasing energy of high power laser devices, the laser-induced damage of optical components, especially fused quartz components, has become one of the core problems in the development of high power laser devices. Studies show that pure fused quartz glass has a high intrinsic damage threshold, but fused quartz glass will inevitably introduce a large number of subsurface damage during the process of grinding and polishing, and these subsurface damage is one of the important factors leading to the decline of laser damage resistance of optical components. It is of great significance to study the subsurface damage of solid abrasive for improving the damage resistance threshold of optical components. In this experiment, 3M's abrasive pad is used as a solid abrasive. The subsurface damage of fine grinding elements are directly observed and analyzed by combining HF pickling and optical microscopy. The results show that various subsurface damage can be detected by optical microscopy after HF pickling. Profilometer is used to measure the surface roughness of the sample, the comparative analysis of the subsurface damage of the fine grinding elements shows that the elements with solid abrasive lapping have smaller roughness when the abrasive size is the same, which has a shallower depth of subsurface damage layer correspondingly.
For the high demand of large aperture optical element, the regular trajectory errors in machining marks of double-side polishing need to be determinately controlled. The mechanism and control method of the regular trajectory errors in machining marks were deeply studied. The process was simulated and compared with the experiment. The method of active translation and pendulum motion and polishing plate correction were proposed, proved to be efficacious on eliminating the regular machining marks by the groove of the polishing pad and local surface figure errors of the polishing plate. The method of dynamic loading and motion combination was adopted, retaining the independence of the original fast convergence process on surface figure. For the optical element with 430mm×430mm×10mm, the surface figure was controlled below 1λ(PV, λ=632.8nm). Meanwhile, the regular machining marks repeatedly produced were eliminated, which provided the essential condition for the intermediate frequency index in the rear stage, small tool precision polishing, and the high efficiency and stable machining of the optical element in the index system was realized.
Conventional used ceria polishing would induce both of Ce contaminants and subsurface damages, which mainly restricts the laser induced damage resistance of fused silica optics. To control the near surface defects, nanometer sized colloidal silica are used to polish fused silica optics after the normal ceria polishing process. Then the contaminant elements and subsurface damages of the polished samples were analyzed by secondary ion mass spectrometry and Nomarski microscopy. It reveals that ceria polishing would introduce lots of subsurface damages whereas colloidal silica polishing induces much fewer subsurface damages especially no fracture induced severe subsurface damages. The laser damage tests reveal that subsequent colloidal silica polishing of the ceria pre-polished samples could gradually eliminate the ceria polishing induced subsurface damages and lower the laser induced damage density accordingly with the increased polishing time. But unlike the damage density, only the severe subsurface damages are totally eliminated could the damage threshold be substantially improved. These results incline to indicate that the subsurface damages have great influence on the laser induced damage density and the fracture related severe subsurface damages will greatly restrict the damage threshold in polished optics.
Light weighted multi-angle multi-surface mirror is made of glass-ceramic, with the structural characteristic of multicavity thin wall, high precision of surface figure and angle between surfaces, has very different processing technology with traditional solid mirror. Based on the 460mm×434mm×80mm multi-angle multi-surface mirror, glass combination manufacture method and relevant interferometry of angle measurement was designed. The process technology was studied, then the consistency of angle between surfaces and the influence on multi-cavity thin wall deformation of lateral surface and material of polishing pad on the surface figure processing were controlled. The model between the variation of angle and load was established, from which the hysteresis of angle variation was analyzed, then the prototype workpiece was finished. The difficult problem on synchronically controlling the surface angles and surface figure of the mirror, which has high center of gravity and multi-cavity thin wall, was solved. The lateral surface figure was controlled below λ/6 (PV, λ=632.8nm), and the perpendicularity of lateral surfaces were controlled below 5''.
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