Papers by Author: Yi Fan Dai

Abstract: Surface roughness plays an important role on optical performances for optics in high-energy laser systems. In this study, optical surface of fused silica were polished by the Magnetorheological Finishing (MRF) processes. The polishing factors in term of Magnetorheological fluid (MR fluid) flow rate, polishing wheel rotational speed, electromagnet current, and polishing ribbon penetration depth, were carried out using an self-developed MRF machine to determine optimum conditions for surface roughness. The settings of the MRF processing parameters were determined by using Taguchi’s experimental design method. Taguchi’s orthogonal array, signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were employed to investigate the optimal processing parameters. The experimental results indicate that surface with smaller roughness could be machined under the conditions with slower rotating speed and higher flow rate and current, and nearly independent of penetration depth.

Abstract: Local wet etching (LWE) is a non-conventional deterministic surface figuring and finishing technique in ultra-precision optics fabrication fields. The general removal function in LWE is cylinder, so fringe of the removal function is sharp and scale of the removal function is determined by inner diameter of the nozzle head. When fabricating some specimen with high frequency figure error, ideal designed shape can’t be achieved easily. Compared with general LWE removal function, Gaussian removal function is more suitable for figuring owing to its smoother fringe and the centralization of its energy. At the same time Gaussian removal function can improve the efficiency in calculation of the dwelling time, because it’s very suitable for Fourier transform. What’s more, theoretical residual figure error can also be reduced for Gaussian removal function’s high spatial resolution. Ideal Gaussian function is difficult to obtain in LWE, so we have proposed near-Gaussian removal function by eccentric rotation of the nozzle head. Through controlling offset of the eccentric rotation, we achieve the optimal near-Gaussian removal function in LWE. Aims of the introduction of near-Gaussian removal function in LWE are to improve the fabrication efficiency and to remove the surface’s high frequency residual figure error.

Abstract: Numerical controlled local wet etching is a novel non-contact deterministic figuring method in ultra precision optics fabricating and functional material manufacturing fields, and the cross-sectional shape of the traditional removal spot is a simple cylinder, so the removal function has no adjustability. In order to create more practical and regular removal function, an eccentric rotation system is introduced to improve the LWE system. By controlling the eccentricity, it can achieve varied shapes removal function. When the rotary axis is controlled to a proper eccentricity, the removal function can be close to the Gaussian function. Moreover, the theoretical calculation and experimental validation are coincident and can give the research a steady foundation. The improvement not only can increase the adjustability of the removal function in LWE, but also can expand its applied field and provide reference for other ultra precision machining methods whose removal function does not have circular symmetry.

Abstract: Ion beam figuring (IBF) is a novel technology for Ultra-precise optics. Material is removed from optic surface in atomic or molecular form by physical sputtering. Due to non-contact between the tool and the work piece, the problems involved in the conventional process are avoided, such as edge-effect and tool-wear. The ion beam figuring process is of high determinacy and high efficiency. All these properties make ion beam figuring one of the promising methods for producing mirrors of high precision with nm-rms accuracy. In this article, a new ion beam figuring system which contains doubled vacuum chambers is set up. Optics can be exchanged by a transport vehicle shuttling between the two vacuum chambers without opening the primary vacuum chamber and waiting for the ion source to cool completely, which means the efficiency can be increased greatly. A high performance processing robot contains three linear axes and two angular axes of motion, providing 5-axis ion source positioning capability with high accuracy. The angle can be up to 50° to figure very steep spherical and aspherical surfaces. Then, the beam removal function of Gaussian shape is obtained by an experimental method and it is extremely stable for a long time. Finally, two sample mirrors are figured by the ion beam figuring system: one is a fused silica flat mirror with a 100 mm diameter (90% effective aperture) and an ultra-precise flat mirror with a surface error of 0.89 nm rms, 14.7 nm PV is obtained; the other fused silica concave spherical mirror with a 100 mm aperture (90% effective aperture) and 420 mm radius of curvature is figured and a concave spherical mirror with 1 nm rms, 16.9 nm PV is obtained, which prove that the ion beam figuring system is favourable for the figuring process.

Abstract: The surface figure of the primary mirror in a large aperture telescope is one of the key factors influencing imaging quality. Space telescopes are used in weightless environments, which is different from the ground environment for optical machining and testing. Therefore the effect of the deformation induced by gravity on the imaging quality is not negligible and the mirror support is to be carefully designed to unload the gravity during the course of machining and testing. In this paper, finite element analysis software is adopted to simulate and compare the deformation induced by gravity of a metre-scale aspheric lightweight primary mirror with different support methods and different working conditions. The results are then imported into MetroPro software supplied by ZYGO Corporation, to remove the tilts and rigid body motion of the mirror and calculate the primary aberrations from the deformation. Finally, sensitivity analysis of support deformation is implemented and the principles of mirror support design are summarized. The research is instructive for design, machining and testing of large aperture mirrors.

Abstract: This research is aiming at exploring a feasible machining method for ultra-high slope surface with deterministic figuring technology. We first successfully realize the nanometer-precision machining of optical hemispherical surface by using ion beam figuring (IBF) under the guidance of the reconstructed three-dimensional (3D) surface error. For figuring such a high-slope surface, we proposed some technical details, the modified algorithm for dwell time, the nonlinear developing method of surface error and the stitching machining technology, to address some important problems in process. Simultaneity a fused silica hemispherical surface is figured with our proposed method on the experimental installation IBF700-5V developed by ourselves. The root-mean-square (RMS) surface error of this sample is reduced from 19.6 nm to 9.6 nm after four iterations and about 37.8min. The experimental results indicate that IBF is a technique of high efficiency for the nanometer-precision machining of high-slope surface.

Abstract: Microgroove processes are carried out on Potassium Dihydrogen Phosphate (KDP) crystals of different crystalline orientation using diamond tool, the speed of groove is very low in order to avoid the influence of temperature. The main process characteristics are examined including the groove geometry, cutting forces and critical underformed chip thickness at the onset of ductile-to-brittle cutting transition. Additionally, the cutting pressure is calculated from the cutting force and grooving geometry. The experimental results show that as the groove depth increase, the groove geometry clearly revealed that ductile-to-brittle cutting transition occurred, and the transition are well reflected by changing in the cutting pressure. Further, it is shown that the critical undeformed chip thickness varies greatly with the workpiece KDP crystalline orientation.

Abstract: Potassium Dihydrogen Phosphate (KDP) crystals are used for the key components in high power density solid-state laser for Inertial Confinement Fusion. KDP crystals are mainly machined in the dry cutting condition to avoid ‘Fogging’ of the crystals. The main difficulty identified in dry machining of KDP is chip removal from the machined surface. A vacuum sucking device based on venturi vacuum pump is used to suck the chips during cutting, and the relationship between level of vacuum in cutting zone and the comply air pressure was established. An empirical model for chip emission during turning processes is used to analyze the influence of cutting parameters on the chip emission. The influence of cutting parameters on the removal of chips is investigated. Finally, a face turning of KDP crystals is carried out with the turning parameters of feed rate 1um/rev, depth of cut of 0.8 um/rev and the cutting speed from 1.82m/s to 3.9m/s. A super-smooth surface with chips free in the whole sample is achieved, having the surface roughness of 2.994nm (Ra) measured by AFM. The surface quality achieved satisfies the requirements of KDP crystals implemented in high power lasers.

Abstract: Optical components with complex surfaces or microstructures are more and more widely used, but it is very difficult to manufacture these components by using traditional mechanical fabrication methods. Fast Tool Servo (FTS) system driven by piezoelectric ceramic (PZT) can manufacture these complex surfaces or microstructures efficiently and accurately, owing to its high response frequency, good dynamic performances and high stiffness. In this paper, the cutting characteristics are studied in the FTS machining process, and the identifying algorithm of tool interference is given. A set of high-powered FTS system is developed, which consists of PZT, fast feeding device, DSP28335 control panel and ultraprecision machine tool. The testing results indicate that the FTS system’s motion resolution is 1nm, the stroke is 90μm, and the response frequency is 220Hz while moving distance is 36μm. On the basis of above work, three kinds of typical workpieces with complex surfaces are manufactured by using FTS system. The measuring results indicate that surface accuracy can reach PV 0.14μm, the roughness is less than Ra 12nm, and the means are presented to improve the machining accuracy.

Abstract: Ion beam Figuring (IBF) of optical mirror is a polishing process based on CCOS with the application of ion sputtering effects. The material removal mechanism is in the atom scale, which can enhance the precision of the target surface to tens of nanometers. Without contact, this process can also promote the subsurface quality. In this paper, the Ion Beam Figuring Machine (IBFM) is introduced，which realizes the relative motion between the target surface and the ion source. Firstly, we describe the design method, the machine configuration and principal technical parameters. With the configuration and motion ability described, we introduce different scanning modes for surfaces of different scales respectively. Secondly presented is the precision analysis method for IBFM. Based on the CCOS principle, the relation between the convergence rate and positioning errors are deduced. This relation describes the precision requirement of machining process. Finally, the postposition algorithm of IBFM is analyzed. According to coordinate transition and rotation, the general transfer model for different scanning modes is given.