Polishing Characteristics of ELID-Ground Surface of Nano Precision Optical Elements

This paper describes an ultra precision polishing method of aspherical mirrors, and the fundamental research on polishing characteristics. The aspherical mirrors with a diameter of about 30mm made by fused silica glass and CVD-SiC were ELID (electrolytic in-process dressing)-ground to high form accuracy with #4000 cast iron bonded diamond wheel, and then polished with a small polishing tool. As the result, final surface roughness of 1.4nmRa and form accuracy of 1.2 μm was obtained. Introduction Over the recent years, there are growing demands for various asymmetric aspherical surfaces to decrease energy losses and enhance condenser efficiency in optics systems such as X rays and lasers [1]. Aspherical optical elements which are a typical example of these ultra-precision parts require not only excellent surface roughness but high profile accuracy as well. Until now, the authors have developed high precision and high efficiency ELID grinding methods and realized stable high accuracy and high quality grinding of complicated products [2]-[5], and demonstrated that the cost and time for mirror fabrication can be reduced by applying ELID grinding to the ultra-precision grinding of such mirrors, and by adding compensation grinding using on-machine measuring device developed by the authors [6]-[8]. Experiments have already verified that the use of ELID grinding wheels with abrasive size finer than #4000 (for example above #8000) produces high quality mirror surfaces, and it is believed that the addition of a polishing method using free abrasives is efficient for super smooth polishing of mirrors surface. The combination of EILD grinding and other finishing techniques is expected to contribute to efficient finishing. The authors therefore proposed ELID nano mirror surface grinding processes of brittle materials with the help of the high profile precision fabrication ability of ELID grinding and the mirror surface finish provided by polishing, and conducted fundamental verification experiments. We have already developed a high precision high quality X-ray mirror by combining isolated abrasive polishing. In this paper, fundamental polishing experiments on fused silica and CVD-SiC aspherical mirrors fabricated by ELID for laser optics condenser mirrors are described, and investigations on pre-grinding surface roughness which enables mirror surface polishing while maintaining high precision shapes are also described. ELID Grinding of Off-Axis Paraboloidal Surfaces ELID grinding is now able to achieve an accuracy level which has been impossible so far with the development of micro abrasives, the development of ultra-high precision grinding systems with higher system control accuracy, and the combination of nano precision grinding systems with stick slip-free drive mechanism which can control the order of 1nm. Metal bonded grinding wheels used