Publisher’s Note: This paper, originally published on 8 July 2019, was replaced with a corrected/revised version on 13 August 2019. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
The application of fused silica in the field of High power laser requires that the formation of sub-surface damage be reduced in the process of grinding and polishing. Subsurface damage is unavoidable in traditional processing methods. Laser smoothing, as a non-contact polishing method, has attracted more and more attention in the surface treatment of fused silica. Laser smoothing is capable of producing smooth surface without incurring serious mechanical defects. Thus it is employed to polish fused silica in the hope of reducing mechanical defects on the optical components. In this paper, aiming at the ground surface of fused silica, the characteristics of mid-far infrared laser treatment and modification are studied. The surface smoothness under different laser power are studied, and the optimal laser power and action time for laser smoothing are obtained. This technology can reduce the ground surface roughness from above 100 nanometer to nanometer.
In this paper, the polishing properties of the zirconia slurry on the fused silica were studied, comparing with the effect of the ceria polishing. The polyurethane without dopant was applied as polishing pad. The experiment results show that it is easier to produce surface scratches when compared with ceria polishing owing to the high Mohs hardness of zirconia. The surface roughness(Rq) of the workpiece was less than 1nm after polishing with submicron size polishing slurry and suitable polishing pad. The R-on-1 test of 355 nm laser-induced damage threshold for fused silica show that zirconia polishing performance is 3.02% higher, however the exposed subsurface damages with HF etching were more than ceria polishing.The subsurface defect density is 0.10def/cm2 (@1μm)with ceria polishing and 1.19def/cm2 (@1μm)with zirconia polishing. Based on the fine polishing performance and abundant resources in nature, the zirconia slurry may be an alternative for fused silica polishing in prospect with the appropriate technological solution.
MRF is capable of producing smooth surface without incurring serious mechanical defects. Thus it is employed to machine fused silica in the hope of reducing mechanical defects on the optical components. The MRF-polished surface was damage-tested with 355nm 8ns pulsed laser and it is found that the laser induced damage threshold was not improved (31.2J/cm2 ) even if the surface contains almost no mechanical defects. However, the damage threshold is increased to 45J/cm2 after slight HF-solution etching (~1μm material removal). On the other hand, the ion beam etching (IBE) was also investigated to find out the potential effects on the laser damage performance of fused silica. The laser damage threshold of IBE processed fused silica is 26.5J/cm2 while the threshold rose to 55J/cm2 after slight chemical etching (~1μm material removal) with HF solution. For comparison, the control samples finished with conventional pad polishing process were also tested. The thresholds prior to and following HF wet etching (~1μm material removal) are 29 J/cm2 and 42 J/cm2 , respectively. From the experimental results, it is clear that slight HF wet etching can enhance the damage resistance of fused silica irrespective of the finishing techniques. Neither MRF nor IBE finished fused silica surface behave better than conventionally polished surface whilst IBE-finished surface appears to have stronger damage resistance after HF etching. HF etching can improve the laser damage threshold by >107% for IBE finished fused silica.
The Atmospheric pressure plasma jet (APPJ) is a very efficient tool that can be employed in the damage-free optical manufacturing. The deep understanding of the temperature field and flow pattern inside the discharge can be a useful tool for optimizing these devices. In this paper, simulation analysis and experiment research on the surface temperature distribution are presented. Through FLUENT finite element analysis software and the heat transfer theory in the flow field, the surface jet characteristics of components are analyzed. Combined with the temperature measurement experiments, the characteristics of surface temperature field modeling are verified, which provides an analysis basis for the comparative study of subsequent temperature and thermal deformation.
Fused silica is widely used in high-power laser systems because of its good optical performance
and mechanical properties. However, laser damage initiation and growth induced by 355 nm laser illumination
in optical elements have become a bottleneck in the development of high energy laser system. In order to
improve the laser-induced damage threshold (LIDT), the fused silica optics were treated by two types of
HF-based etchants: 1.7%wt. HF acid and buffer oxide etchant (BOE: the mixture of 0.4%wt. HF and 12%wt.
NH4F), respectively, for varied etching time. Damage testing shows that both the etchants increase the damage
threshold at a certain depth of material removal, but further removal of material lowers the LIDT markedly.
The etching rates of both etchants keep steady in our processing procedure, ~58 μg/min and ~85 μg/min,
respectively. The micro-surface roughness (RMS and PV) increases as etching time extends. The hardness (H)
and Young’s modulus (E) of the fused silica etched for diverse time, measured by nano-indenter, show no
solid evidence that LIDT can be related to hardness or Young’s modulus.
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.
Magnetorheological finishing (MRF) is a key polishing technique capable of rapidly converging to the required surface figure. Due to the deficiency of general one-polishing-head MRF technology, a dual polishing heads MRF technology was studied and a dual polishing heads MRF machine with 8 axes was developed. The machine has the ability to manufacture large aperture optics with high figure accuracy. The large polishing head is suitable for polishing large aperture optics, controlling large spatial length’s wave structures, correcting low-medium frequency errors with high removal rates. While the small polishing head has more advantages in manufacturing small aperture optics, controlling small spatial wavelength’s wave structures, correcting mid-high frequency and removing nanoscale materials. Material removal characteristic and figure correction ability for each of large and small polishing head was studied. Each of two polishing heads respectively acquired stable and valid polishing removal function and ultra-precision flat sample. After a single polishing iteration using small polishing head, the figure error in 45mm diameter of a 50 mm diameter plano optics was significantly improved from 0.21λ to 0.08λ by PV (RMS 0.053λ to 0.015λ). After three polishing iterations using large polishing head , the figure error in 410mm×410mm of a 430mm×430mm large plano optics was significantly improved from 0.40λ to 0.10λ by PV (RMS 0.068λ to 0.013λ) .This results show that the dual polishing heads MRF machine not only have good material removal stability, but also excellent figure correction capability.