The large and thin plane optical windows are used in large high power laser devices. Typically conventional methods such as stressed-lap polishing and small-tool pitch polishing are used to manufacture these optics. Nevertheless, the required wavefront accuracy cannot be achieved by the last smal-tool pitch polishing process which can lead to middle spatial frequency errors and high-slope errors because of the edge effect , unstable removal rate and pressure -loaded deformation. Ion Beam Figuring (IBF) technology is an optical fabrication method which can highly correct different spatial frequency errors due to the highly deterministic, highly stable, very small tools and noncontact. In this paper, IBF was employed to correct different spatial frequency errors of a large and thin plane optical windows. Before IBF, transmission wavefront error of the substrate was 0.51λ PV, 32.3nm/cm GRMS, 2.91nm PSD1, 0.27nm PSD2, 0.38nm Rq after being polished by double-sided polishing machine, and was improved to 0.07λ PV, 2.1nm/cm GRMS, 1.76nm PSD1, 0.13nmPSD2, 0.33nmRq after only two IBF(about thirty six hours processing time). All spatial frequency errors reached the required values.
Based on the theoretical model of small-tool polishing, this paper studied the influence of polishing parameters on the results of computer controlled optical surfacing. We compared different parameters of the removal function and the tool path and analyzed the residual mid-spatial frequency errors in the polishing process. The simulation results indicated that raster path combined with Gaussian removal function, contributing to high accuracy of PSD1 RMS, but leading to serious peaks of 1D PSD1 curves in some frequency points. In contrast, pseudo-random path combined with flat-topped removal function, resulting in lower accuracy of PSD1 RMS, but could effectively restrain the amplitude of the 1D PSD1 curves. Finally, experimental study was carried out on both of the 610 mm × 440 mm K9 material transmission mirror and 430 mm × 430 mm fused silica plane window, the RMS value of PSD1 converged from 3.72 nm to 1.61 nm, and decreased from 2.30 nm to 1.37 nm, respectively, which demonstrated the correctness of the model.
The in-situ monitoring of subsurface defects and laser damages initiation using high resolution on-line microscope is performed on medium aperture fused silica optics manufactured by different procedures to investigate the specific damage precursors. The digital camera, Nomarski microscope and white light interferometer are used to characterize the subsurface defects. With shallow HF etching depth, the laser induced damages are mostly initiated on indents or invisible defects under the fluence of 8~10 J/cm2@355nm. The laser induced damages initiated on indents is gradually decreased with the increased etching depth and the laser induced damage density is also decreased. Besides, decrease of the indents by optimizing the polishing process could also make the laser induced damage density sharply decrease. These results prove that the indents are important damage precursors and the laser induced damage performance of fused silica optics could be substantially improved by decreasing the indents or deep HF etching.
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