The transmitted wavefront mid-spatial-frequency(MSF) errors of spherical lens has a great influence on the quality of the transmitted beam. Aiming at the problems of poor convergence precision and low convergence efficiency in mid-spatial frequency errors polishing of large square spherical lenses, this paper proposes a mid-spatial-frequency errors correction technology by using full-aperture rigid polishing combined with numerically controlled sub-aperture smooth polishing. In the full-aperture polishing stage, the surface shape distribution that is conducive to subsequent sub-aperture polishing is obtained through radius compensation technology to reduce the sudden change of surface shape in the corner area of the component. The more rigid polishing pad is used to smooth the whole surface then, so that the component has better MSF errors condition before the sub-aperture high-precision surface correction. In the stage of small tool CNC polishing, the transmission structure and mass distribution of the polishing disc are optimized, and the ideal transmission characteristics and size parameters of the polishing disc are obtained through mechanical simulation analysis to reduce the overturning moment of the polishing disc when the direction changes suddenly. This optimization also improves the pressure distribution of the polishing interface. Stability, a flexible polishing disc combined with a high dispersion polishing slurry is used to correct the surface errors. The concentration of the polishing slurry is optimized and the supply method is well changed. So when the surface shape errors convergence process is more efficient and controllable, it will not cause the deterioration of the MSF errors. The smooth tool is applied in the last stage with reducing the temperature change of the polishing interface. All these measures are aimed to increase the stability of smooth polishing and to achieve high-efficiency, high-precision and stable convergence of MSF errors. The experimental verification was carried out on four square spherical lenses with a size of 440mm×440mm. The final PSD1:RMS values have all reached within 1.8nm. Additionally the overall processing time has been greatly shortened.
Automatic measurement of single points schema by coordinate measuring machine(CMM) is used to measure the Ultra-Long curvature radius of spherical optical element. The removal quantity of each measuring point can be calculated through contrasting the measure value and the theoretical value. A removal model of spherical optical element polishing is established based on Preston equation, and the required machining parameters are predicted by removal simulation in MATLAB. A processing test on a fused silicon with an aperture of 440mm×440mm was performed and the result shows that the model is effective in Ultra-Long curvature radius control of spherical optical element during full aperture polishing.
In large-scale high-power laser devices, the mid-spatial-frequency(MSF) error of the transmitted wavefront of the large-aperture spherical lens has a direct impact on the energy scattering of the high power laser. This paper proposes a technology about correction of MSF error of large-aperture spherical lens based on computer numerical control polishing. A smooth polishing theoretical model is established for spherical lens and the removal function morphology is optimized. To make a better MSF error convergence, the rigid conformed tool is designed and assembled. The polishing tool and the main axle are connected by a high-precision universal joint, so that the polishing pad can be flexibly attached to the surface of the workpiece when the tool is running. This makes it able to polish the square spherical workpiece with large curvature radius by the processing method of planar workpiece. In the earlier stage, two kinds of path are applied to converge the low- frequency error by crossing each other. Then the random path is applied for MSF error convergence. By the experimental varification of Four fused silica spherical lens with aperture of 440 mm × 440 mm, the RMS value of the PSD1 frequency band of the lens’ transmitted wavefront error is finally converg-ence to 2.2 nm through once MSF error correction.
In view of the problem of projection distortion in the result of aberration-free point test of off-axis paraboloid mirror, this paper analyzes its formation principle, puts forward a transformation method from the detection coordinate system to the processing coordinate system, according to the mathematical relationship between the two, reconstructs the surface shape of the detection result, realizes the transformation of projection distortion image, and analyzes the error of the transformation result by using the fucial function. It is proved that this method is feasible by using the reconstructed surface results to guide the NC Polishing.
In order to improve the stability of the removal function for aspheric surface and realizes its machining. A polishing tool based on non-Newtonian fluid (silly putty) is designed. The influence of shear stress of silly putty in polishing tools on the pressure distribution on the aspheric surface is analyzed. According Preston equation, the removal function of non- Newtonian fluid polishing tool is optimized theoretically based on its surface pressure distribution with the polishing tool executing a planetary motion of the polishing tool. The influence of the structure, materials and driving mode of polishing tool on the removal function shape was analyzed. Then, the polishing tool was improved and the marks on the removal spot were removed. The effect of filling rat io of silly putty and process parameters on removal function was studied by experiments. A method of obtaining stable removal function based on non-Newtonian fluid polishing tools is presented.
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