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.
Single crystal silicon mirrors were widely used in high-energy laser system, and the surface accuracy/quality seriously restricted the output of laser system. So, how to improve the manufacturing level of single crystal silicon mirrors was particularly important, especially the polishing level. This work focus ed on the manufacturing requirements of high-load-capacity single crystal silicon mirrors and conducted a detailed study on the evolution of surface accuracy, roughness and absorption in IBF process. For IBF technique, the incident electron voltage was set to 750 eV and the beam incident angle was set to zero degree. The optical surface profile data was obtained through sub-aperture stitching method. After IBF process, the surface accuracy PV of the single crystal silicon cylindrical mirror converged from the initial 469.280 nm to 101.173 nm, and the roughness RMS diminished from 0.626 nm to 0.506 nm, the surface accuracy and quality had been significantly improved. The weak absorption of the optical surface was detected by weak-absorption platform, and absorption results of the mirror increased to a certain extent, from the initial 0.473 ppm to 0.536. The results showed that IBF technique could effectively improve the surface accuracy/quality of single crystal silicon mirror, which was of great significance to improve the performance of high-energy laser system.
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 fused silica optics are important functional ultraviolet optical elements in the high-power laser system. Contamination plays an extremely important role in laser-induced damage, which will affect the laser damage threshold of the components to varying degrees, and finally affects the output flux of high-power laser system. An organic solvent needs to be used to wipe the optical surface before applying it, and this process will introduce organic pollution. In addition, different optical films and organic solvents have different effects on the laser damage threshold of fused silica, which need to be studied further. In this work, the photo-thermal weak absorption platform was used to test the photothermal weak absorption of three samples before and after the organic solvent treatment. The first sample was tested on the surface of the fused silica substrate; the surface of the second sample was coated with aluminum oxide; and the surface of the third sample was coated with hafnium oxide. The results show that the use of ethanol can increase the photo-thermal weak absorption of the substrate by 29.6%; the photo-thermal weak absorption signal of the fused silica element coated with aluminum oxide is reduced by 20% after the surface is wiped with ethanol; the fused silica element with hafnium oxide coated on the surface reduces the photo-thermal weak absorption signal by 33%. The experimental results verify the feasibility of using ethanol to wipe fused silica components.
The laser damage threshold of fused silica optics is affected by the surface/subsurface defects in optical fabrication and is related to surface contamination. In this paper, the evolution of surface roughness and photothermal weak absorption of fused silica treated by plasma cleaning and ion beam figuring (IBF) is studied. The results show that plasma cleaning has a certain change on the surface roughness of fused silica, while the change of photothermal weak absorption depends on the initial surface quality. The surface roughness of fused silica has a certain regularity after ion beam figuring, and the photothermal weak absorption is basically consistent with the initial.
Single crystal silicon is a chemically active semiconductor material with good processing characteristics. With the development of technology, the role of single crystal silicon components in the field of ICF is becoming more and more important. Restricted by traditional processing method and MRF, damage precursors such as scratches and impurities still remain on the surface after processing. That may influence the performance of the elements. In this paper, we study the effect on the surface by two chemical etching methods and the influence on the damage precursors of nano jet polishing. We used HF solution/HNO3 and KOH/ isopropanol to etch the surface of the element. When the etching depth comes to 0.2 μm, pits and scratches could be easily found on the surface. After the etching process, the element was processed by nano jet polishing and the roughness decreased from 1.264nm to 0.986nm.We used nano jet polishing method to process the element polished by MRF in order to research the evolution of comet-tail scratch. The In-situ tracking method was also applied in this study. After the polishing process, the W-D (width to depth ratio) increased from 30.51 to 45.84. The scratch was deactivated and the PTA (photothermal absorption) decreased from 1.5413nA to 1.3500nA. The Ce impurities were also removed. Its concentration decreased from 0.1162mg/L to 0.0005mg/L and the PTA of the element decreased from 0.3044nA to 0.0652nA. From the research, we can easily know that the subsurface damage exposed after the etching process. That may lay the foundation of the nondestructive processing of single crystal silicon. After nano jet polishing process, the quality of the element became better. The roughness and concentration of Ce impurities decreased. The damage precursors were deactivated and the PTA decreased. In a word, chemical etching could expose the surface damage of single crystal silicon and nano jet polishing can improve the laser damage resistance.
This article introduces the modification of single crystal silicon cylindrical mirror that combined immersed CCOS polishing with IBF polishing. In the processing of smoothing and modifying, the quality of the mirror’s surface has been further improved. In this experiment, we optimize the CCOS polishing process and improve the traditional CCOS polishing apparatus to polish the single crystal silicon convex cylindrical mirror. During the CCOS polishing process, the parameter were set as follows: pressure 0.4kPa, speed 25 r/min, polishing liquid concentration 4.5% (pH 10), polishing time 20 min. After polishing by CCOS, IBF is used on its heels. The parameter were set as follows: energy 1000-1200eV (Ar+ ion beam with low energy), incident angle of ion beam 90°, polishing time 30 min. The immersed CCOS polishing process and IBF polishing process iterated repeatedly until the surface quality of the single crystal silicon convex cylindrical mirror meets the requirement. After the polishing process, the sub aperture stitching method and Zygo interferometer are used to detect the surface. The PV value of the single crystal silicon convex cylindrical mirror is 0.265 lambda, the roughness is 0.679nm the residual Zernike is 70.5 nm after testing. The errors over the whole frequency band were significantly reduced. It could be seen from the above that the single crystal silicon convex cylindrical mirror had high accuracy and better profile on the surface after polishing process. The error over the whole frequency band also converged obviously. Therefore, the method that had been described in this paper could be used for the surface modification of single crystal silicon cylindrical mirror.
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