As the third-generation wide band gap semiconductor material, single crystal silicon carbide has high electron saturation mobility and excellent thermal properties. It has broad application prospects in the manufacture of high temperature and radiation resistant high frequency and high-power devices. Magnetorheological polishing of silicon carbide wafers has the characteristics of high efficiency and non-destructive processing, but there is a lack of research on magnetorheological polishing of Si and C surfaces at this stage. Therefore, this paper studies the following aspects. Firstly, through the removal function experiment of multi-parameter comparison, the difference between the residence time of magnetorheological polishing and the removal effect of SiC Si and C surfaces is analyzed, and the efficiency evolution law of magnetorheological polishing of different crystal surfaces is obtained. The influence of different immersion depth of polishing ribbon on the removal efficiency of different crystal faces was explored, and the relationship between polishing depth and material removal rate under different crystal faces was obtained. The results show that with the increase of residence time, the removal efficiency of Si and C surfaces increases, and the growth rate of Si surface is much larger than that of C surface. With the increase of immersion depth, the growth rate of material removal efficiency of C surface is more sensitive than that of Si surface. Therefore, this study explores the technical feasibility of magnetorheological polishing of SiC, and provides a new idea for high-efficiency and high-precision polishing of SiC.
Fused silica optics, as key components in high - power laser systems, are prone to produce laser damage with different sizes under laser irradiation. For large - size laser damage, CO2 laser can be used for repairing process. For small - size laser damage, magnetorheological finishing (MRF) technique is appropriate. This work focused on the MRF repairing process of small - size laser damage on fused silica optical surface, the optimization of MRF processing parameters and the absorption characteristics of optics are investigated. First, the MRF processing parameters were determined by experimental method. Under the conditions of a wheel speed 200 rpm, flow rate 110 L/min, current 7.5 A, press depth 0.3 mm and polishing abrasive CeO2, the volumetric removal efficiency of removal function was relatively high, up to about 1.108 mm3/min. Appling the optimized parameters in small - size laser damage repairing process, no "comet tail" defects occurred on the optical surface. After MRF repairing process, the surface roughness was restored by immersed CCOS (computer control optical surfacing) technique. For immersed CCOS process, it contained two stage: coarse polishing and fine polishing. With 120 min of coarse polishing and 40 min of fine polishing, the roughness Ra decreased from the initial 6.179 nm to 0.7 nm, which was basically restored to the initial state. Finally, the photo - thermal absorption of the optics before and after repairing process was detected on a weak absorption testing platform. The results showed that the surface absorption decreased from the initial 425.38 ppm to 125.92 ppm, and the laser energy absorption of the repaired fused silica optics was significantly reduced, which verified the validity of the combined process of MRF and immersed CCOS process. In a conclusion, the research results in this paper can provide important technical support for the rapid repairing of small - size laser damage on fused silica optical surfaces, which was conducive to improving the service life of high - power laser systems.
The high-power laser system imposes stringent demands on the complete spatial frequency range, particularly the mid-spatial frequency errors of its optical elements. The presence of mid-spatial frequency errors significantly compromises the optical performance of the system. To mitigate the mid-spatial frequency waviness error in high-power laser elements following magnetorheological polishing, this study utilized a proprietary rotatable magnetorheological polishing apparatus. The aim was to simulate the impact magnetorheological polishing on the surface's mid-spatial frequency waviness errors in the absence of rotation, as well as to evaluate the effectiveness of random rotation angles in suppressing the surface's mid-spatial frequency waviness errors. Ultimately, the fused quartz element was simulated and modified. The simulation results demonstrate that utilizing the rotatable magnetorheological polishing equipment with a rotating magnetorheological polishing head effectively suppresses the low-spatial frequency surface accuracy, resulting in reduced mid-spatial frequency waviness on the processed element's surface. The RMS of surface errors decreases from 11.142 nm to 0.765 nm, eliminating noticeable mid-spatial frequency waviness errors and completely eradicating the characteristic peak in the Power Spectral Density (PSD) curve. Consequently, the surface accuracy of the element is significantly enhanced. Therefore, by adjusting the rotational angle of the magnetorheological polishing head, it is possible to effectively suppress mid-spatial frequency waviness errors on the element's surface and achieve deterministic shaping, thereby meeting the manufacturing requirements for high-power laser elements.
Fused silica, as an excellent optical material, was widely used in the fabrication of laser optics applied in short-pulse/high-power laser systems. Fused silica containing structural defects was easy to be destroyed under laser irradiation. Deeply understanding the impact of different structural defects on the damage characteristics was of great significance for improving the laser damage threshold of fused silica optics. This work focused on structural defects including Oxygen Deficiency Center (ODC), Non-Bridging Oxygen Hole Center (NBOHC), E’ center, Peroxy Linkage (POL), Peroxy Radical (PDR), and elaborated on its formation laws in detail. The changes of damage characteristics and optical property of fused silica with different structural defects were calculated by first principle method. Taking E’ center defect as example, the band gap of defective silica was 3.1242 eV while that of β silica was 5.61 eV, which meant that electrons were more likely to cross the band gap under laser irradiation, led to electron avalanche and induced laser damage. Meanwhile, the optical properties of fused silica with different structural defects also showed significant difference. The reflectivity of β silica was 0.045, while that of silica with E’ center defects was 0.0715. Under 355nm laser irradiation, the absorption of β silica was 0 cm-1 while that of E’ center defects was 16600 cm-1, the absorptivity increased significantly. The change of optical properties also increased the probability of laser damage. In this work, certain support could be provided for the laser-damage theory of fused silica, and the relevant results could also provide important reference for suppression of structural defects.
The main factor affecting the laser damage resistance of optical components is the damage precursor introduced in the manufacturing process. Some studies have shown that the ideal intrinsic matrix of fused quartz has a laser radiation damage threshold of up to 100J/cm2, but the manufactured optical components are far below this index. Therefore, effectively inhibiting or even eliminating the damage precursor is the key to improve the anti-laser damage performance of optical components. Abrasive Water Jet Polishing (AWJP) technology mainly removes materials through the collision and shear between abrasive particles and the workpiece surface, which has the characteristics of non-damage and non-contact processing. However, due to the small size of Tool Influence Function (TIF) and low efficiency, it is still challenging to achieve widespread application. In this paper, we analyzed the flow field characteristics near the workpiece surface when different nozzle tilt angles were used for processing under two different machining methods: maintaining the jet length unchanged and maintaining the standoff distance unchanged and corrected the pressure distribution. It was found that the changes in pressure distribution and shear stress distribution in the impact zone under different nozzle tilt angle conditions will result in a change in the width of the Gaussian-shaped TIF obtained after nozzle rotation machining. In addition, under the condition of the same nozzle tilt angle, the TIF of Gaussian shape with larger size can be obtained by using the processing method that keeps the standoff distance constant.
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