KEYWORDS: Atmospheric plasma, Plasma, Diffractive optical elements, High power lasers, Polishing, Chemical analysis, Chemical reactions, Surface finishing, Chemical elements
Continuous phase plate (CPP) is an important diffractive optical element, which is widely used in high power laser devices. The continuous phase plate with a small aperture period of 4 mm is processed by the atmospheric pressure plasma polishing (APPP). Through the study of the reaction mechanism, it is found that the removal volume has a non-linear relationship with the dwell time, which will lead to machining errors. Based on this, a dwell time compensation method is proposed, and the machining program is generated according to this relationship. A 70mm × 70mm × 20mm continuous phase plate was fabricated by using the processing program generated by this method. The processing time was 4.5h, and the surface residual converged to 57.188nm RMS. The experimental results show that the method can effectively calculate the removal function under different dwell time, and significantly improve the machining accuracy.
As a new type of laser gain medium, sesquioxide crystal has significant advantages in thermal conductivity, phonon energy and other properties. It has an important application in improving the performance of picosecond/femtosecond laser. However, due to its hard and brittle characteristics, the machining process is prone to pitting, cracks, microcracks, sub surface defects and other damages, which brings great difficulties and challenges to its polishing process. In order to solve the above problems, a new chemical mechanical polishing slurry is proposed, which includes alumina, cerium oxide, aluminum sulfate, urea and pure water. After polishing with this slurry, the surface roughness of the crystal is better than 0.4nm, the damage depth of the sub surface is better than 1.5 μm, the surface shape is close to 1/10λ, and the material removal rate is up to 48nm / min. Based on the analysis of the results of small angle grazing X-ray diffraction and X-ray photoelectron spectroscopy (XPS), this paper describes the production of ReOHCO3 by the reaction of urea and Re2O3 in the new polishing slurry. The hydration layer composed of ScOOH and Sc2(SO4)3 is formed on the surface of alumina/ aluminum sulfate and crystal, and the soft product is removed by ceria to form a smooth and low damage crystal surface.
Reaction-sintered silicon carbide (RS-SiC) has been widely applied in space telescope mirrors, reflector, microelectronic mechanical systems due to its excellent properties. These applications require high surface accuracy and quality, but traditional mechanical contact polishing methods are hard to process this material due to the hardness and poor machinability. In this paper, atmospheric pressure plasma processing (APPP) which is a non-contact optical manufacturing technique with highly efficient and subsurface damage-free has been proposed to process this material, and optimal process parameter mainly about influence on removal function for machining processing RS-SiC have been found. The sample was observed and analyzed by scanning white-light interferometer (SWLI), which showed the calculated Peak- Maximum removal rate (MRR) and Volume-MRR were 12.526μm/min and 0.1298mm3 /min. Meanwhile, the surface chemical composition of RS-SiC was also investigated by XPS to reveal the plasma etching processes. The results illustrated that small number of radicals CxFy were introduced onto the RS-SiC surface during the plasma process which could be generated during the process of the reaction gas CF4 being excited. The process parameters and analysis of surface chemical composition in this paper will guide the further processing of RS-SiC.
The axicon is a rotationally symmetrical optical element along the optical axis of the focal line, including concave shaft cone and convex shaft cone. It is widely used in laser beam shaping, laser drilling, optical detection, laser resonator and lithography illumination due to the property of long focal length and narrow transverse width. The axicon has only one optical axis, and the curvature radius of each point of the surface is different in the direction of the optical axis, that is, the line is a straight line, but the points of the adjacent bus are really circular torus, and the traditional equipment is difficult to polish. A new process suitable for axicon processing is proposed, including forming, traditional polishing, Atmospheric Pressure Plasma Processing for surface correction, flexible asphalt discs for smoothing. With it, the convergence of the polishing process is significantly improved, and the PV is decreases from 27λ to 3λ. There are two main factors that affect processing efficiency: temperature and gas flow rate. An analytical method is selected to estimate the complex interaction between the temperature field analysis and the polishing efficiency during the polishing process, and it is possible to achieve efficient and determined process.
In order to study the mist phenomenon on phosphate glass, the chemical composition of the hydrolysis layer on the glass surface of phosphate glass was investigated. The solid and liquid components of the used slurry and the surface composition of polished phosphate glass were tested. The experimental results show that the SIMS test results show that the content of K does not change with depth, and the content of Mg, Al, Nd firstly increases with depth and then tends to be gentle; K element does not participate in chemical reactions during polishing, just simply mechanical removed; but Mg, Al, and Nd elements are not only mechanically removed but also participate in chemical reactions and are precipitated as ions, which is consistent with the XRF test results. It is proposed a new substance generated on the surface of phosphate glass during polishing process, a hydrolysis layer formation model was proposed. The mist phenomenon is related to the hydrolysis layer, which is different from the substrate material and properties (density, light reflectance, etc.). So different scattered light is seen and the thicker the hydrolyzed layer, the stronger the scattered light. Moreover, the severity of mist is related to the thickness of the mist layer, and the thicker the thickness, the more severe the mist.
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-6 m (about 0.7025λ), 0.249E-6 m (about 0.3925λ ), and 0.105E-8 m (about 0.00166λ), respectively. With radial temperature difference of 0.1°C, the deformation of Nd:glass, K9 and ULE are 0.831E-7 m (about 0.1313λ), 0.465E-7 m (about 0.07348λ) and 0.196E-9 m (about 3.0973E-4λ), 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.
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