Full Aperture Polishing Research Articles

Study on the Influence of Temperature Distribution in the Optical Elements on the Surface Shape and Its Control Method during Full Aperture Polishing

Study on the Influence of Temperature Distribution in the Optical Elements on the Surface Shape and Its Control Method during Full Aperture Polishing

Method to improve the surface shape of BK7 glass in full-aperture polishing.

Full aperture polishing is a significant process to fabricate optical workpieces with nano figure precision. It has the characteristics of excellent full spectrum uniform removal ability, fast convergence speed, high machining accuracy, and low production cost, which makes it the first choice for the processing of large-aperture optical elements. However, with BK7 glass it is difficult to achieve deterministic processing due to its large thermal expansion coefficient and other factors, and the surface shape accuracy is difficult to improve. In this paper, the thermal deformation of BK7 glass is analyzed first, and then the temperature distribution in the element is measured during full-aperture polishing. The influence of the temperature field in the optics on the accuracy of the final surface shape is analyzed, and the quantitative relationship between the temperature difference and the surface shape is established. In addition, the methods of deterministically controlling the surface shape of optics such as polishing liquid temperature control and immersion polishing are proposed. Finally, the model of quantitative control of the surface shape of optics is verified by experiments, which improve the surface shape accuracy of 600 mm BK7 optics to 0.2λ.

Comprehensive study of the rapid stressed mirror polishing method for off-axis aspheric SiC thin-plate mirrors.

In this study, the stressed mirror polishing technique is used to perform off-axis aspheric silicon carbide (SiC) mirror full-aperture polishing for the first time. Mechanical and optical parameter analysis methods have been proposed. A medium-diameter off-axis aspheric SiC thin-plate mirror is used as a scaling model for an optical system mirror. A full diameter polishing simulation was completed, and a conceptual design for stress loading equipment is presented. An initial aspheric surface method for stressed mirror polishing of an off-axis aspheric SiC thin-plate mirror, providing a reference for rapid stress mirror polishing of SiC mirrors, is also proposed.

Full aperture super-smooth polishing and surface topology evolution of the mandrels for X-ray optics used in eXTP mission

The X-ray focusing mirror used in enhanced X-ray Timing and Polarimetry(eXTP) mission are manufactured by the electroformed nickel replication process, and the machining accuracy of the mandrels are directly related to the focusing quality of the mirror. The optical performance of the focusing mirror is not only affected by the shape error, but also closely related to the mid and high-spatial frequency errors. In this research, the full-aperture polishing process of NiP die was investigated, and the evolution of surface waviness and the surface roughness under subsequent polishing processes were examined. The 7 × 4 h of full-aperture polishing experiment was conducted on the self-developed two-axis polishing machine with optical grade pitch as the polishing tool. The experimental results showed that the mid-spatial frequency error of the mandrel was effectively reduced and the root mean square value of the surface roughness was decreased to 0.266 nm, which meets the stringent requirements for the surface quality of mandrel in eXTP mission.

Deterministic control of material removal distribution to converge surface figure in full-aperture polishing

Full-aperture polishing is one of the most important processes in fabricating large flat optical elements. Control of the elements’ surface figure to a high precision in a determined manner has been a major challenge in this process. This study focuses on the converging mechanism and deterministic control of the surface figure in the full-aperture polishing process. A novel balance equation for material removal is proposed to correlate the primary aspects of the polishing condition to the surface figure. The removal balance equation reveals that the final surface figure is determined by the spatial distributions of three factors, including the relative velocity, polishing pressure, and removal coefficient. The dependence of these factors’ distribution on the polishing condition is detailed in theory and experiments, and systematic methods to deterministically control these factors’ distribution are proposed. This study is a significant attempt to reveal the converging mechanism of the surface figure and develop a deterministic full-aperture polishing process.

Measurement of Contact Pressure Distribution Map at Workpiece/tin Lap Interface and Process Parameters Optimization During Full-Aperture Polishing With Tin Lap

Contact pressure at the workpiece/tin lap interface during full-aperture polishing is of great significance to workpiece’s material removal process. To promote the determinacy and efficiency of polishing process, a novel wireless contact pressure measurement system is developed. Further, the contact pressure distribution map is solved by the interpolation algorithm using the acquired contact pressure data at workpiece/ tin lap interface. Subsequently, a material removal amount model within given polishing time is proposed. Based on this model, process parameters are optimized with the genetic algorithms using contact pressure distribution map under specific workpiece’s initial surface figure and polishing conditions. Compared with random process parameters, the optimized process parameters can greatly improve the accuracy and convergence efficiency of workpiece’s surface figure in full-aperture polishing with tin lap, which is validated by the polishing experiments.

Effects of kinematics and groove parameters on the mid-spatial frequency error of optics induced during full aperture polishing

The mid-spatial frequency (MSF) error is a key controlled specification of optical elements for many important applications (e.g., high power laser systems, extreme ultraviolet lithography systems and other high-resolution imaging systems). However, the effective controlling of MSF error is one of the difficulties in high precision optical materials processing, especially in the full aperture polishing process. In this study, a theoretical model of optical element MSF error induced during full aperture polishing process was established. The effects of kinematics and groove parameters on workpiece surface MSF error were studied by numerical simulation. According to the theoretical analysis results, several process optimization measures for reducing workpiece MSF error during full aperture polishing process were proposed. The practical effects of these optimization measures were verified in the following polishing experiments. The theoretical analysis and polishing experimental results suggested that the period of MSF error is obviously influenced by the grooving spacing of the polishing pad, and the MSF error value (PSD-RMS) can be significantly reduced by optimizing the polishing speed and swing parameters.

Theoretical model and experimental analysis of non-uniform material removal during full-aperture polishing

Full-aperture polishing is a key process in the fabrication of large flat optical elements with a high-precision surface figure. Controlling of the surface figure, which is primarily dependent on the material removal distribution, during the polishing process is challenging. In this study, a novel model is proposed to calculate the material removal distribution and the resultant surface figure. The model determines the material removal amount of points on the workpiece by considering the kinematic parameters and pressure distribution along the sliding trajectory relative to the pad. Moreover, the pressure distribution during the polishing process is acquired from the mechanical and morphological characteristic of polishing pad. With this model, the final surface figures under several polishing conditions were simulated and were found to be in close agreement with the experimental results.

Closed-loop laser polishing using in-process surface finish metrology.

This paper lays out the trail onto a closed-loop polishing process of optical elements enabling the application of the optimum polishing time needed. To that aim, an in-process testing method for monitoring an inclusive microsurface quality (e.g., comprising surface roughness and scratch-and-dig) within the polishing spot is analyzed, and its applicability to closed-loop polishing for classical loose-abrasive full-aperture polishing as well as for computer-controlled laser polishing is experimentally tested and verified. This enables the determination of the optimum local dwell time resulting in stable and cost-optimized polishing.

Kinematic model for material removal distribution and surface figure in full-aperture polishing.

Full-aperture polishing is a significant process for fabricating large ultra-precision optic flats. The surface figure is one of the key specifications required of the optic flats, which is determined by the material removal distribution during polishing. To date, the most frequently referred to equation, the Preston equation, only provides a solution for qualitative calculation of material removal of a single point on the optic surface. In this study, we present a kinematic model for deterministic calculation of the removal amount at every local optic position. The model is based on the sliding track of each local optic position on the lap, and it incorporates local pressure and most key kinematic parameters and considers the effect of the lap grooves. With this model, we analyzed the impacts of various kinematic parameters and groove features on the distribution of the removal amount in terms of sliding distance, assuming a uniform pressure distribution at the lap/optic interface. Several polishing experiments have been carried out in which the model is validated.

Experimental Investigation of Material Removal and Surface Roughness during Optical Glass Polishing

It has been a challenge to finish optical glass surfaces due to their hard and brittle nature. Moreover, tight tolerances of surface figure and finish make polishing a more critical operation. This work reports the results of an experimental study performed for full aperture polishing of BK7 optical glass. Flat samples of borosilicate (BK7) glass are polished using an optical pitch polisher and cerium oxide (CeO2) slurry. Taguchi's L9 orthogonal array is used for the design of experiments. Abrasive concentration, pressure and overarm speed are considered as variable process parameters. Polishing is performed for duration of 120 minutes for each combination of parameters. Material removal is measured using the precision weighing balance. Surface roughness was measured using a Form Talysurf PGI 120 profiler. Abrasive slurry concentration is observed to be one of the most significant parameters in the optical polishing process. It affects both the material removal rate (MRR) and the surface roughness. Pressure applied at the workpiece–polisher interface affects the MRR, but the variation of pressure is not found to affect the surface roughness significantly. Relative motion at the workpiece–polisher interface is also observed to be significant in defining the final polishing outputs.

Deterministic measurement and correction of the pad shape in full-aperture polishing processes

Full-aperture polishing is a significant process in fabricating large optical flats because it restrains Mid-Spatial Frequency errors and removes material quickly on the whole optic surface. Nevertheless, optical flats fabricated by full-aperture polishing generally fail to meet the stringent requirement of surface figure, which has to be corrected by sub-aperture polishing processes. Surface figure of optical flats in full-aperture polishing processes is primarily dependent on the pressure distribution uniformity which correlates intensively with the lap shape. At present, practical and precise means are urgently desired for measuring and correcting the lap shape, especially the polyurethane pad lap. In the study, we present a novel method for deterministic measurement of the pad shape. The method obtains the height of the pad at spirally distributed locations implemented by the revolution of the pad and translation of the laser displacement sensor. The pad shape in terms of matrixes whose elements representing the heights at the corresponding locations is then calculated by interpolation algorithm based on the obtained data. Further, we propose a method for deterministic correction of the pad shape utilizing a small conditioning tool. The dwell time algorithm and implementation strategy for the dwell time are provided for common full-aperture polishers. These solutions for the deterministic measurement and correction of the pad shape have been validated on a full-aperture polisher with polyurethane pad. The polishing experiments revealed that the optic surface figure was obviously improved.

Convergent polishing: a simple, rapid, full aperture polishing process of high quality optical flats & spheres.

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial shape (i.e., surface figure), will converge to final surface figure with excellent surface quality under a fixed, unchanging set of polishing parameters in a single polishing iteration. In contrast, conventional full aperture polishing methods require multiple, often long, iterative cycles involving polishing, metrology and process changes to achieve the desired surface figure. The Convergent Polishing process is based on the concept of workpiece-lap height mismatch resulting in pressure differential that decreases with removal and results in the workpiece converging to the shape of the lap. The successful implementation of the Convergent Polishing process is a result of the combination of a number of technologies to remove all sources of non-uniform spatial material removal (except for workpiece-lap mismatch) for surface figure convergence and to reduce the number of rogue particles in the system for low scratch densities and low roughness. The Convergent Polishing process has been demonstrated for the fabrication of both flats and spheres of various shapes, sizes, and aspect ratios on various glass materials. The practical impact is that high quality optical components can be fabricated more rapidly, more repeatedly, with less metrology, and with less labor, resulting in lower unit costs. In this study, the Convergent Polishing protocol is specifically described for fabricating 26.5 cm square fused silica flats from a fine ground surface to a polished ~λ/2 surface figure after polishing 4 hr per surface on a 81 cm diameter polisher.

Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats & Spheres

Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats & Spheres

Multiplex path for magnetorheological jet polishing with vertical impinging.

We report a way to shape surfaces by optimizing the path instead of changing the removal function of a polishing tool for magnetorheological jet polishing (MJP). The M-shaped removal function of MJP generates a track with a W-shaped profile along one path. However, applying two parallel paths with appropriate line spacing can obtain a track with V-shaped profile, which has a removal distribution similar to that by using the Gaussian removal function along one path. Based on this, a multiplex path applying an M-shaped removal function is constructed in an actual process. A transformation model describing the relationship between the M-shaped removal function and the Gaussian removal function is established, which is crucial to determine the velocity function on the multiplex path. By using the M-shaped removal function, we have planned new processing steps by applying the multiplex path and the velocity function for full aperture polishing. Polishing performance is designed and demonstrated on two K9 work-pieces with different multiplex paths. The form error on 23 mm diameter is decreased from 0.256λ PV (λ=632.8 nm) and 0.068λ RMS to 0.038λ PV and 0.005λ RMS with scanning multiplex path. Results indicate that this method of path optimization is suitable for optical manufacturing.

Mid-Spatial Frequency Error (PSD-2) of optics induced during CCOS and full-aperture polishing

Mid-Spatial Frequency (MSF) Wavefront Error of optics divided into the PSD-1 and PSD-2 ranges plays an important role in the performance of high power laser systems. The present work focuses on the PSD-2 range in terms of short ripples which haven’t been well studied in the literature. Characteristics and origins of these short ripples were detailed, whereafter small tool computer controlled polishing (CCP) and conventional full aperture polishing experiments were conducted on fused silica. It is revealed that PSD2 error is independent of the main process parameters including lap rotating rate and polishing pressure in continuous polishing and tool path pitch and crossfeed velocity in small tool CCP processes. Whereas the type of polishing lap has a decisive effect on PSD2 error of the optics. The pitch lap shows superiority in restraint of short ripples over polyurethane pad. By introducing diamond conditioner for dressing polyurethane pad, the PSD2 error has been greatly decreased.