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.
Aiming at the improvement of edge effect in CNC polishing. A new polishing method based on the surface extension is proposed. The basic idea and workflow are presented. The availability of this method is verified by simulation. Experimental study was carried out on 420mm×420mm caliber fused quartz optical element. The experimental results show that this method can restrain the collapse and warped edge surface. PV less than λ/3，GRMS less than 7.7nm/cm and PSD1 less than 1.8nm can be obtained combining the small scale smoothing technology.
The influence of polishing parameters such as particle size, pad material and pressure on the surface roughness of glass optics were investigated and analysed. It reveals that the surface roughness will get worse with increase of the polishing particle size. The surface roughness would remain stable in a certain period of polishing pressure, but get worse with increase of the pressure beyond the period. The surface roughness is getting better when using smooth pitch polishing pad than polyurethane pad with lots of micropores. By optimizing the polishing parameters, the surface roughness of large aperture fused silica window is improved to 0.46nm before band-pass filtering and 0.084nm after band-pass filtering.