Edge effect correction using ion beam figuring.

The edge effect is regarded as one of the most difficult technical issues for fabricating large primary mirrors, as it can greatly reduce the key performance of the optical system. Ion beam figuring (IBF) has the advantage of no edge effect, so we can use it to remove high points on the edge and improve surface accuracy. The edge local correction method (ELCM) of IBF processes only the surface edge zone, and is very different from the current full caliber figuring method (FCFM). Therefore, it is necessary to study the ELCM of IBF. In this paper, the key factors of ELCM are analyzed, such as dwell time algorithm, edge data extension methods, and the outward dimension of the starting figuring point. At the same time, the distinctions between ELCM and FCFM are compared. Finally, a 142 mm diameter fused silica mirror is fabricated to verify the validity of the theoretical of ELCM. The experimental results indicate that the figuring precision and efficiency can be obviously improved by ELCM.

Figuring process of potassium dihydrogen phosphate crystal using ion beam figuring technology.

Currently, ion beam figuring (IBF) technology has presented many excellent performances in figuring potassium dihydrogen phosphate (KDP) crystals, such as it is a noncontact figuring process and it does not require polishing fluid. So, it is a very clean figuring process and does not introduce any impurities. However, the ion beam energy deposited on KDP crystal will heat the KDP crystal and may generate cracks on it. So, it is difficult directly using IBF technology to figure KDP crystal, as oblique incident IBF (OI-IBF) has lower heat deposition, higher removal rate, and smoother surface roughness compared to normal incident IBF. This paper studied the process of using OI-IBF to figure KDP crystal. Removal rates and removal functions at different incident angles were first investigated. Then heat depositions on a test work piece were obtained through experiments. To validate the figuring process, a KDP crystal with a size of 200 mm×200 mm×12 mm was figured by OI-IBF. After three iterations using the OI-IBF process, the surface error decreases from the initial values with PV 1.986λ RMS 0.438λ to PV 0.215λ RMS 0.035λ. Experimental results indicate that OI-IBF is feasible and effective to figure KDP crystals.

Fabrication of continuous phase plates with small structures based on recursive frequency filtered ion beam figuring.

Large surface gradient and extensive mid-to-high spatial frequency in continuous phase plates (CPPs) with small structures make it difficult to achieve high-precision fabrication. An ion beam figuring (IBF) technology to fabricate CPPs with such characteristics is proposed in this paper. In order to imprint CPP microstructures with smaller spatial periods even down to 1mm in shorter time, we present a multi-pass IBF approach with different ion beam sizes based on the frequency filtering method. We discuss the selection principle and when to reduce ion beam sizes for different procedures to control dwell time and adequately exert the corrective capability in detail. This filtering method can obtains better surface quality in a faster way compared to the non filtering traditional IBF method. The experimental results verify this optimized method can effectively imprint complex microstructures with spatial period as small as 0.7 mm, surface peak-to-valleys (PV) smaller than 200nm and surface gradient as large as 1.8µm/cm to within 10 nm root-mean-square (RMS) of design specifications, which displays the advantages of our fabrication method.

Improve optics fabrication efficiency by using a radio frequency ion beam figuring tool.

An ion beam with high removal rate and small diameter is expected in ion beam figuring. For an ion beam figuring tool, reducing the extraction grid opening is a feasible method to decrease the ion beam diameter, but the ion beam removal rate decreases at the same time. The ion beam removal rate depends much on the ion density in the ion source discharge room. The plasma in a hollow cathode (HC) ion source and a radio frequency (RF) ion source was simulated. The simulations suggested that the ion density in the RF ion source is higher than that of the HC one. Then, a RF ion source with an integrative matching network was developed and tested in this paper, where the ion beam removal rate reached up to 193 nm/min for 10 mm opening extraction grids.

Research on temperature field of KDP crystal under ion beam cleaning.

KH2PO4 (KDP) crystal is a kind of excellent nonlinear optical component used as a laser frequency conversion unit in a high-power laser system. However, KDP crystal has raised a huge challenge in regards to its fabrication for high precision: KDP crystal has special physical and chemical characteristics. Abrasive-free water-dissolution magnetorheological finishing is used in KDP figuring in our lab. But the iron powders of MRF fluid are easily embedded into the soft surface of KDP crystal, which will greatly decrease the laser-induced damage resistance. This paper proposes to utilize ion beam figuring (IBF) technology to figure and clean the surface of a KDP component. Although IBF has many good performances, the thermal effect control is a headachy problem for the KDP process. To solve this problem, we have established its thermal effect models, which are used to calculate a component's surface temperature and thermal gradient in the whole process. By this way, we can understand how to control a temperature map and its gradient in the IBF process. Many experiments have been done to validate and optimize this method. Finally, a KDP component with the size of 200×200×12 mm is successfully processed by this method.

Design and performance analysis of an ultraprecision ion beam polishing tool.

First, we introduce requirements for the ion beam polishing tool used in the subnanometer precision process. Based on the ion beam figuring (IBF) principle, the definitive factor of the IBF capability is analyzed, and the deficiencies of the ion beam polishing tool are identified. The effect of focused ion optics on the ion beam removal function is based on theoretic calculation and computer simulation; and focused three-grid ion optics are developed and tested. Finally, a 150 mm flat optics element is figured and results show that the contour error decreases from 15.58 nm RMS to 0.796 nm RMS, demonstrating that the ion beam polishing tool is very efficient for optical IBF.

Detailed subsurface damage measurement and efficient damage-free fabrication of fused silica optics assisted by ion beam sputtering.

Formation of subsurface damage has an inseparable relationship with microscopic material behaviors. In this work, our research results indicate that the formation process of subsurface damage often accompanies with the local densification effect of fused silica material, which seriously influences microscopic material properties. Interestingly, we find ion beam sputtering (IBS) is very sensitive to the local densification, and this microscopic phenomenon makes IBS as a promising technique for the detection of nanoscale subsurface damages. Additionally, to control the densification effect and subsurface damage during the fabrication of high-performance optical components, a combined polishing technology integrating chemical-mechanical polishing (CMP) and ion beam figuring (IBF) is proposed. With this combined technology, fused silica without subsurface damage is obtained through the final experimental investigation, which demonstrates the feasibility of our proposed method.

Structure optimization and fabricating capability analysis of an ion-beam machine for a subnanometer optical surface.

Ultraprecise and ultrasmooth surfaces become critical requirements for some high-performance optical systems. Ion-beam figuring (IBF) is a good and highly deterministic method for the final precision optical figuring. However, the uniform convergences of all spatial frequency surface errors are strongly dependent on the dynamic performance and ion-beam stability of the IBF machine. In this paper, only the dynamic performance is discussed, which is limited by the acceleration and velocity of the motion system. So we discuss these problems and their influences on figuring optical surfaces in detail. The structure optimization principle is based on the fabricating capability of ultraprecise surface errors in all spatial frequency ranges. With this requirement, the structure optimization of a quick-response platform is performed to improve its dynamic performance. Manufacturing experiments on a fused silica spherical concave surface (Φ135.7 mm, radius of curvature 340.5 mm) are accomplished, and the IBF machine can effectively correct the figure errors and improve the surface quality simultaneously. The IBF process realizes the uniform convergence of surface errors in all spatial frequency ranges, which is reduced down to 0.368 nm RMS, 0.204 nm RMS, and 0.087 nm RMS, respectively. The final results indicate that the performance of the new designed IBF machine meets the requirements well for the fabrication of a subnanometer optical surface.

Translation-reduced ion beam figuring.

A translation-reduced ion beam figuring (TRIBF) technique for five-axis ion beam figuring (IBF) plants is proposed to process large size components which cannot be processed in the traditional way. This novel technique enhances the capability of five-axis IBF plants by taking advantage of their rotation axes. The IBF kinematic model is described and the TRIBF processing technique is established by solving the motion parameters. Verification experiments are conducted on a 150 mm diameter planar mirror. This mirror was processed by TRIBF technique with only a 100 mm translation stage. The surface error was reduced from initial 10.7 nm rms to 1.3 nm rms within 97 minute processing time. The result indicates that the TRIBF processing technique is feasible and effective.

Mathematical modeling and application of removal functions during deterministic ion beam figuring of optical surfaces. Part 1: Mathematical modeling.

Ion beam figuring (IBF) is established for the final precision figuring of high-performance optical components, where the figuring accuracy is guaranteed by the stability of the removal function and the solution accuracy of the dwell time. In this deterministic method, the figuring process can be represented by a two-dimensional (2D) convolution operation of a constant removal function and the dwell time. However, we have found that the current 2D convolution operation cannot factually describe the IBF process of curved surfaces, which neglects the influences of the projection distortion and the workpiece geometry on the removal function. Consequently, the current 2D convolution algorithm would influence the solution accuracy for the dwell time and reduce the convergence of the figuring process. In this part, based on the material removal characteristics of IBF, a mathematical model of the removal function is developed theoretically and verified experimentally. Research results show that the removal function during IBF of a curved surface is actually a dynamic function in the 2D convolution algorithm. The mathematical modeling of the dynamic removal function provides theoretical foundations for our proposed new algorithm in the next part, and final verification experiments indicate that this algorithm can effectively improve the accuracy of the dwell time solution for the IBF of curved surfaces.

Mathematical modeling and application of removal functions during deterministic ion beam figuring of optical surfaces. Part 2: application.

Ion beam figuring (IBF) is established for the final precision figuring of optical components. In this deterministic method, the figuring process is represented by a two-dimensional (2D) convolution operation of a constant removal function and the dwell time, where the figuring precision is guaranteed by the stability of the removal function as well as the solution accuracy of the dwell time. However, the current 2D convolution equation cannot factually reflect the IBF process of curved surfaces, which neglects the influence of the projection distortion and the workpiece geometry. Consequently, the current convolution algorithm for the IBF process would influence the solution accuracy for the dwell time and reduce the convergence of the figuring process. In this part, we propose an improved algorithm based on the mathematical modeling of the dynamic removal function in Part A, which provides a more accurate dwell time for IBF of a curved surface. Additionally, simulation analysis and figuring experiments are carried out to verify the feasibility of our proposed algorithm. The final experimental results indicate that the figuring precision and efficiency can be simultaneously improved by this method.

Influence of local densification on microscopic morphology evolution during ion-beam sputtering of fused-silica surfaces.

Morphology evolution at microscopic scales has an inseparable relationship with surface material behaviors, especially during ultrasmooth surface fabrication. In this work, the influence of initially existing local densification on ion nanopatterning of a fused-silica surface is investigated. Our research results indicate that fused-silica surfaces will easily densify permanently under a compressive load, exhibiting an anisotropic surface at the nanoscale. During the subsequent ion-beam sputtering process, the densification-dependent sputtering would influence and even dominate surface morphology evolution, which is identified as being an important evolution mechanism. However, ion-induced relaxation mechanisms will overcome surface roughening in the absence of local densification, and an ultrasmooth surface with root mean square roughness down to 0.06 nm is obtained in our experiment.

Microscopic morphology evolution during ion beam smoothing of Zerodur® surfaces.

Ion sputtering of Zerodur material often results in the formation of nanoscale microstructures on the surfaces, which seriously influences optical surface quality. In this paper, we describe the microscopic morphology evolution during ion sputtering of Zerodur surfaces through experimental researches and theoretical analysis, which shows that preferential sputtering together with curvature-dependent sputtering overcomes ion-induced smoothing mechanisms leading to granular nanopatterns formation in morphology and the coarsening of the surface. Consequently, we propose a new method for ion beam smoothing (IBS) of Zerodur optics assisted by deterministic ion beam material adding (IBA) technology. With this method, Zerodur optics with surface roughness down to 0.15 nm root mean square (RMS) level is obtained through the experimental investigation, which demonstrates the feasibility of our proposed method.

Morphology evolution of fused silica surface during ion beam figuring of high-slope optical components.

Ultra-precision and ultra-smooth surfaces are vitally important for some high performance optical systems. Ion beam figuring (IBF) is a well-established, highly deterministic method for the final precision figuring of extremely high quality optical surfaces, whereas ion sputtering induced smoothing, or roughening for nanoscale surface morphology, strongly depends on the processing conditions. Usually, an improper machining method would arouse the production of nanoscale patterns leading to the coarsening of the optical surface. In this paper, the morphology evolution mechanism on a fused silica surface during IBF of high-slope optical components has been investigated by means of atomic force microscopy. Figuring experiments are implemented on two convex spherical surfaces by using different IBF methods. Both of their surface errors are rapidly reduced to 1.2 nm root mean square (RMS) after removing similar deep material, but their surfaces are characterized with obviously different nanoscale morphologies. The experimental results indicate that the ion incidence angle dominates the microscopic morphology during the IBF process. At near-normal incidence, fused silica achieves an ultra-smooth surface with an RMS roughness value R(q) down to 0.1 nm, whereas nanoscale ripple patterns are observed at a large incidence angle with an R(q) value increasing to more than 0.9 nm. Additionally, the difference of incidence angles on various machined areas would influence the uniformity of surface quality, resulting from the interplay between the smoothing and roughening effects induced by ion sputtering.

Deterministic ion beam material adding technology for high-precision optical surfaces.

Although ion beam figuring (IBF) provides a highly deterministic method for the precision figuring of optical components, several problems still need to be addressed, such as the limited correcting capability for mid-to-high spatial frequency surface errors and low machining efficiency for pit defects on surfaces. We propose a figuring method named deterministic ion beam material adding (IBA) technology to solve those problems in IBF. The current deterministic optical figuring mechanism, which is dedicated to removing local protuberances on optical surfaces, is enriched and developed by the IBA technology. Compared with IBF, this method can realize the uniform convergence of surface errors, where the particle transferring effect generated in the IBA process can effectively correct the mid-to-high spatial frequency errors. In addition, IBA can rapidly correct the pit defects on the surface and greatly improve the machining efficiency of the figuring process. The verification experiments are accomplished on our experimental installation to validate the feasibility of the IBA method. First, a fused silica sample with a rectangular pit defect is figured by using IBA. Through two iterations within only 47.5 min, this highly steep pit is effectively corrected, and the surface error is improved from the original 24.69 nm root mean square (RMS) to the final 3.68 nm RMS. Then another experiment is carried out to demonstrate the correcting capability of IBA for mid-to-high spatial frequency surface errors, and the final results indicate that the surface accuracy and surface quality can be simultaneously improved.

Ion beam machining error control and correction for small scale optics.

Ion beam figuring (IBF) technology for small scale optical components is discussed. Since the small removal function can be obtained in IBF, it makes computer-controlled optical surfacing technology possible to machine precision centimeter- or millimeter-scale optical components deterministically. Using a small ion beam to machine small optical components, there are some key problems, such as small ion beam positioning on the optical surface, material removal rate, ion beam scanning pitch control on the optical surface, and so on, that must be seriously considered. The main reasons for the problems are that it is more sensitive to the above problems than a big ion beam because of its small beam diameter and lower material ratio. In this paper, we discuss these problems and their influences in machining small optical components in detail. Based on the identification-compensation principle, an iterative machining compensation method is deduced for correcting the positioning error of an ion beam with the material removal rate estimated by a selected optimal scanning pitch. Experiments on ϕ10 mm Zerodur planar and spherical samples are made, and the final surface errors are both smaller than λ/100 measured by a Zygo GPI interferometer.

Ion beam figuring of high-slope surfaces based on figure error compensation algorithm.

In a deterministic figuring process, it is critical to guarantee high stability of the removal function as well as the accuracy of the dwell time solution, which directly influence the convergence of the figuring process. Hence, when figuring steep optics, the ion beam is required to keep a perpendicular incidence, and a five-axis figuring machine is typically utilized. In this paper, however, a method for high-precision figuring of high-slope optics is proposed with a linear three-axis machine, allowing for inclined beam incidence. First, the changing rule of the removal function and the normal removal rate with the incidence angle is analyzed according to the removal characteristics of ion beam figuring (IBF). Then, we propose to reduce the influence of varying removal function and projection distortion on the dwell time solution by means of figure error compensation. Consequently, the incident ion beam is allowed to keep parallel to the optical axis. Simulations and experiments are given to verify the removal analysis. Finally, a figuring experiment is conducted on a linear three-axis IBF machine, which proves the validity of the method for high-slope surfaces. It takes two iterations and about 9 min to successfully figure a fused silica sample, whose aperture is 21.3 mm and radius of curvature is 16 mm. The root-mean-square figure error of the convex surface is reduced from 13.13 to 5.86 nm.

Figuring algorithm for high-gradient mirrors with axis-symmetrical removal function.

Figuring technologies based on intracone and intercone stitching for high-gradient mirrors are discussed. Based on the conventional computer-controlled optics shaping principle, a process model for a single cone with intracone stitching is constructed. With the circular stitching property of the model, a modified Bayesian-based Richardson-Lucy (RL) algorithm is deduced to deconvolute dwell time for single cone. Building on this algorithm, with the introduction of intercone stitching, a process model for a complex cone is built. Then another modified Bayesian-based RL algorithm is deduced to deconvolute the dwell time for a complex cone from the properties of intracone stitching and intercone stitching. With a velocity realization method for dwell time on a spiral path of the cone and the determination criterion of the path parameter, figuring technologies for single and complex cones are presented. Simulation and experiment demonstrate that theories and methods discussed can solve key problems of figuring high-gradient mirrors; the figuring technologies are novel methods for high-gradient mirrors and can be used to figure mirrors finely.

Figuring technology of nonaxisymmetric errors with a spiral path.

Figuring technology of nonaxisymmetric errors using a spiral path is presented. Based on an approximation of a removal function, a finite-field nonlinear model is deduced from the computer-controlled optics shaping principle. We then present a modified Richardson-Lucy iterative algorithm to deconvolute the dwell time. With a velocity realization method for dwell time on a spiral path, the figuring technology comes into being. Simulations are made to validate these algorithms. Theoretical and simulation studies demonstrate that the figuring technology is a novel method for inexpensive fabrication of precision mirrors.

Algorithm for ion beam figuring of low-gradient mirrors.

Ion beam figuring technology for low-gradient mirrors is discussed. Ion beam figuring is a noncontact machining technique in which a beam of high-energy ions is directed toward a target workpiece to remove material in a predetermined and controlled fashion. Owing to this noncontact mode of material removal, problems associated with tool wear and edge effects, which are common in conventional contact polishing processes, are avoided. Based on the Bayesian principle, an iterative dwell time algorithm for planar mirrors is deduced from the computer-controlled optical surfacing (CCOS) principle. With the properties of the removal function, the shaping process of low-gradient mirrors can be approximated by the linear model for planar mirrors. With these discussions, the error surface figuring technology for low-gradient mirrors with a linear path is set up. With the near-Gaussian property of the removal function, the figuring process with a spiral path can be described by the conventional linear CCOS principle, and a Bayesian-based iterative algorithm can be used to deconvolute the dwell time. Moreover, the selection criterion of the spiral parameter is given. Ion beam figuring technology with a spiral scan path based on these methods can be used to figure mirrors with non-axis-symmetrical errors. Experiments on SiC chemical vapor deposition planar and Zerodur paraboloid samples are made, and the final surface errors are all below 1/100 lambda.