Single crystal calcium fluoride (CaF2) is the excellent transparent optical substance that has extremely good permeability and refractive index from 120nm wavelength ultraviolet range to 12μm wavelength infrared range and it has widely used in the applications of various advanced optical instrument, such as infrared optical systems (IR), short wavelength optical lithography systems (DUV), as well as high power UV laser systems. Nevertheless, the characteristics of CaF2 material, including low fracture toughness, low hardness, low thermal conductivity and high thermal expansion coefficient, result in that the conventional pitch polishing techniques usually expose to lots of problems, such as subsurface damage, scratches, digs and so on. Single point diamond turning (SPDT) is a prospective technology for manufacture the brittle material, but the residual surface textures or artifacts of SPDT will cause great scattering losses. Meanwhile, the roughness also falls far short from the requirement in the short wavelength optical systems. So, the advanced processing technologies for obtaining the shape accuracy, roughness, surface flaw at the same time need to put forward. In this paper, the authors investigate the Magnetorheological Finishing (MRF) technology for the high precision processing of CaF2 material. We finish the surface accuracy RMS λ/150 and roughness Rq 0.3nm on the concave aspheric from originate shape error 0.7λ and roughness 17nm by the SPDT. The studying of the MRF techniques makes a great effort to the processing level of CaF2 material for the state-of-the-art DUV lithography systems applications.
The corrective calibration of the removal function plays an important role in the magnetorheological finishing (MRF) high-accuracy process. This paper mainly investigates the asymmetrical characteristic of the MRF removal function shape and further analyzes its influence on the surface residual error by means of an iteration algorithm and simulations. By comparing the ripple errors and convergence ratios based on the ideal MRF tool function and the deflected tool function, the mathematical models for calibrating the deviation of horizontal and flowing directions are presented. Meanwhile, revised mathematical models for the coordinate transformation of an MRF machine is also established. Furthermore, a Ø140-mm fused silica plane and a Ø196 mm, f/1∶1, fused silica concave sphere samples are taken as the experiments. After two runs, the plane mirror final surface error reaches PV 17.7 nm, RMS 1.75 nm, and the polishing time is 16 min in total; after three runs, the sphere mirror final surfer error reaches RMS 2.7 nm and the polishing time is 70 min in total. The convergence ratios are 96.2% and 93.5%, respectively. The spherical simulation error and the polishing result are almost consistent, which fully validate the efficiency and feasibility of the calibration method of MRF removal function error using for the high-accuracy subaperture optical manufacturing.
Extreme optical fabrication projects known as EUV and X-ray optic systems, which are representative of today’s advanced optical manufacturing technology level, have special requirements for the optical surface quality. In synchroton radiation (SR) beamlines, mirrors of high shape accuracy is always used in grazing incidence. In nanolithograph systems, middle spatial frequency errors always lead to small-angle scattering or flare that reduces the contrast of the image. The slope error is defined for a given horizontal length, the increase or decrease in form error at the end point relative to the starting point is measured. The quality of reflective optical elements can be described by their deviation from ideal shape at different spatial frequencies. Usually one distinguishes between the figure error, the low spatial error part ranging from aperture length to 1mm frequencies, and the mid-high spatial error part from 1mm to 1 μm and from1 μm to some 10 nm spatial frequencies, respectively. Firstly, this paper will disscuss the relationship between slope error and middle spatial frequency error, which both describe the optical surface error along with the form profile. Then, experimental researches will be conducted on a high gradient precise aspheric with pitch tool, which aim to restraining the middle spatial frequency error.
Nickel is the unique material in the X-ray telescopes. And it has the typical soft material characteristics with low hardness、high surface damage and low stability of thermal. The traditional fabrication techniques are exposed to lots of problems, including great surface scratches, high sub-surface damage and poor surface roughness and so on. The current fabrication technology for the nickel aspheric mainly adopt the single point diamond turning(SPDT), which has lots of advantages such as high efficiency, ultra-precision surface figure, low sub-surface damage and so on. But the residual surface texture of SPDT will cause great scattering losses and fall far short from the requirement in the X-ray applications. This paper mainly investigates the magnetorheological finishing (MRF) techniques for the super-smooth processing on the nickel optics. Through the study of the MRF polishing techniques, we obtained the ideal super-smooth polishing technique based on the self-controlled MRF-fluid NS-1, and finished the high-precision surface figure lower than RMS λ/80 (λ=632.8nm) and super-smooth roughness lower than Ra 0.3nm on the plane reflector and roughness lower than Ra 0.4nm on the convex cone. The studying of the MRF techniques makes a great effort to the state-of-the-art nickel material processing level for the X-ray optical systems applications.
In order to reduce the quality of overall optical system and improve the capability of correcting aberration, amending
image quality and expanding the field of view. The optical components of optical systems are trending to the lightweight
aspheric with larger radius to thickness ratio. For improving the processing efficiently and restraining the mid-high
frequency error, Computer Control Active Lap (CCAL) technology has become a significant method for processing.
However, when the surface accuracy RMS of lightweight mirrors is lower than λ / 40 (λ = 632.8nm ), the lightweight
mirror surface will appear “imprinting effect” stripes caused by Active Lap, and this phenomena has become the barriers
for high-precision processing and the development of lightweight mirrors. In this paper, based on the Finite
Element Analysis (FEA) software, taking an innovative way that applies reverse air to the internal structure of
lightweight mirrors to resist the elastic deflection derived from the lap pressure and self-weight. And, comparing two
methods: cavity inflation and immersed inflation, simulation results show that the immersion inflation method not only
can eliminate the “imprinting effect, but also the surface accuracy of mirrors is much better than that only caused by selfweight
during polishing process.
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