The response characteristics of the pitch polishing pad under the pressure of conditioner is the key factor which affect the control efficacy of surface figure in annular polishing. The effect of the environmental temperature and humidity, rotation speed of the pad, uploading ratio and eccentricity of the conditioner on the response characteristics of pitch polishing pad were experimental researched. To this aim, a control system of the environmental temperature and humidity on the 2.8m-aperture annular polishing machine is established. The control precision of the local environmental temperature upon the pad is up to ±0.1℃, and the control precision of humidity is up to ±1% RH. The experimental results indicate that the matching degree of the pad and the conditioner decides the changing rule of the surface figure of workpiece with the eccentricity of the conditioner. The new adjustable parameters including the environmental temperature and humidity and the uploading ratio of the conditioner were introduced to change the matching degree and improve the controllability of the of the surface figure. Under the optimized process, the long-term stability of the surface figure of the meter-scale optical element is realized using the 2.8m annular polishing machine, which is an important step for the final realization of the deterministic processing in the large-aperture annular polishing.
An ideal pitch button blocking process determines the level of workpiece deformation, especially the high-aspect-ratio optics, during the blocking process and process of polishing later. We have studied the pitch button blocking process by Finite Element Analysis (FEA) according to the thermoelastic equation. Meanwhile, the optimized pitch button blocking has been gotten by FEA which includes the thickness and material of blocking plate, as well as the radius, arrangement, elastic modulus and coefficient of thermal expansion of pitch buttons. The numerical simulation of Nd:glass (Ø100 mm×2 mm thickness) shows that the surface figure change (ΔPV) which is induced by the thermal stress during pitch button blocking process is influenced seriously by the thickness of blocking plate.
Different size polishing powder and different pH value ceria slurries were used to polish fused silica glass､K9 glass and Nd-doped glass on pitch plate. Material removal rates (MRR) of glass polished with different size powder and various pH value slurries, and textures of each sample were characterized. The results show that powder size has an effect on glass polishing performance: scratch densities increase with the increase of polishing powder size; surface textures become rougher with the increase of the size of polishing powder. The slurry pH value also affects glass polishing performance: MRR of fused silica glass are lowest under any pH value slurry while Nd-doped glass has the largest MRR; removal rates of all three kinds of glass will rise under both acidic and alkaline condition. Near neutral polishing environment and smaller size powder are useful for the surface polishing process. The results further reveal polishing mechanism and provide the guidance for glass surface process.
During continuous polishing, temperature is a significant source of processing uncertainty. Three work pieces of different kind material (K9, Nd:glass and ULE) were polished on 2.4m continuous polisher. It turns out that temperature difference has different influence on different material work pieces. It also indicates that temperature difference aggravates the processing uncertainy. The deformation caused by temperature difference is simulated using ANSYS. It shows that, with top-bottom temperature difference of 0.1°C, the deformation of Nd:glass, K9 and ULE are 0.444E<sup>-6</sup> m (about 0.7025λ), 0.249E-6 m (about 0.3925λ ), and 0.105E<sup>-8</sup> m (about 0.00166λ), respectively. With radial temperature difference of 0.1°C, the deformation of Nd:glass, K9 and ULE are 0.831E<sup>-7</sup> m (about 0.1313λ), 0.465E<sup>-7</sup> m (about 0.07348λ) and 0.196E<sup>-9</sup> m (about 3.0973E<sup>-4</sup>λ), respectively. To explore the top-bottom temperature difference and radial temperature difference along the polishing surface, a small aperture Nd:glass and a large aperture Nd:glass in polishing have been measured using thermal infrared imager. The results showed that for Ø 260 mm × 26 mm Nd: glass, the radial temperature difference is about 0.1°C, while the top-bottom temperature difference is about 0.1°C ~ 0.21°C. Contrastively, for 810 mm×460 mm×40 mm Nd:glass, the radial temperature difference have reached 0.4°C, while top - bottom temperature difference ranges between 0.1°C ~ 0.27°C. When element gets larger, it will suffer greater temperature difference. These temperature differences are great enough to cause deformation far beyond the polishing accuracy required. Finally, methods are proposed to diminish the effect of temperature difference.
Continuous ring polishing is the key process in large aperture optical elements. The surface figure of polishing pad is inferred by the offline testing surface figure of workpiece. The defects, low processing efficiency and uncertainty processing time in traditional continuous polishing, the real-time monitoring method of polishing is proposed. The realtime monitoring system is set up based on the computer, the dynamic interferometer, a beam expanding system and a beam reflecting system. There are a workpiece and a glass monitoring plate placing in same ring. The surface figure of workpiece, monitored by the monitoring plate, synchronize with the surface of glass monitoring plate in Peak-Valley (PV) and POWER. The new method with simple structure is fast measuring and judgmental directly to the changes of surface figures. The results of real-time monitoring and surface figure converging on the workpiece are valid for continuous polishing through experimental validation.
In the inertial confinement fusion (ICF) system, the mid-frequency errors of optical components will cause high-frequency modulation and nonlinear gain of laser beams. In this paper, theoretical simulations and experiments have been designed and operated, aiming at studying the effects of slotting methods on mid-frequency errors in Nd-doped glass continuous polishing. Based on Preston formula, theoretical simulations focus on the effects of slotting methods on the mid-frequency errors. The simulation results show that different slotting methods will cause different mid-frequency errors, and square and logarithmic shape grooves are easier to obtain smaller mid-frequency errors. On the basis of simulation results, two groups of experiments are carried out to do the Nd-doped glass continuous polishing. The results show that the mid-frequency error gradually decreases with the decrease of the spacing between grooves. The results also show that square shape groove is easier to get a smaller mid-frequency error than circular shape groove, which verifies the theoretical simulation results.