High-precision Optical Surfaces Research Articles

Investigation of Surface Integrity Induced by Ultra-Precision Grinding and Scratching of Glassy Carbon.

Glassy carbon provides material characteristics that make it a promising candidate for use as a mould material in precision glass moulding. However, to effectively utilize glassy carbon, a thorough investigation into the machining of high-precision optical surfaces is necessary, which has not been thoroughly investigated. This research analyses the process of material removal and its resulting surface integrity through the use of nano-scratching and ultra-precision grinding. The nano-scratching process begins with ductile plastic deformation, then progresses with funnel-shaped breakouts in the contact zone, and finally concludes with brittle conchoidal breakouts when the cutting depth is increased. The influence of process factors and tool-related parameters resulting from grinding has discernible impacts on the ultimate surface roughness and topography. Enhancing the cutting speed during cross-axis kinematic grinding results in improved surface roughness. Increasing the size of diamond grains and feed rates leads to an increase in surface roughness. An achievable surface roughness of Ra < 5 nm together with ductile-regime grinding behaviour meet optical standards, which makes ultra-precision grinding a suitable process for optical surface generation.

Improved methods of binocular phase measuring deflectometry.

Phase measurement deflectometry (PMD) is an excellent method for high-precision optical surface measurement; through the simple system structure, accuracy comparable to that of interference methods can be realized. The key of PMD is to solve the ambiguity between the surface shape and normal vector. Considering all kinds of methods, the binocular PMD method is undoubtedly a method with a very simple system structure and can be easily applied to complex surfaces, such as free-form surfaces. However, this method relies on a large screen with high accuracy, which not only increases the weight of the system, but also reduces the flexibility of the system, and manufacturing errors in the large-size screen can easily become error sources. In this Letter, we have made some improvements based on the traditional binocular PMD. At first, we try to replace the large screen with two small screens to increase the flexibility and accuracy of the system. Further, we replace one small screen with a single point to simplify the system structure. Experiments show that the proposed methods not only improve the system flexibility and reduce the complexity, but can also achieve high measurement accuracy.

Интерферометр для контроля выпуклых гиперболических поверхностей малого диаметра

We used Fourier transforms to perform a theoretical analysis of how the process of interference pattern formation in a laboratory-scale Fizeau laser interferometer is affected by the error in the working wave front. The interferometer is designed to monitor convex hyperbolic surfaces characterised by small diameters and low aperture angles. We indicate their application scope. The interferometer consists of a laser illuminator, a microlens, a beam splitter, that is, a thin plane-parallel plate with a semitransparent coating on its front surface, and a screen. A thin translucent plate located perpendicular to the line connecting the geometric foci of the hyperbolic surface acts as a separator and a reference simultaneously. The paper provides the main equations describing the wave aberration of the working branch and the position of optical elements in the interferometer design. We present an example of computing aberration in the working branch of the interferometer. This calculation reveals that aberrations in the thin plane-parallel plate can be discarded at low aperture angles. The interferometer forms a high-precision optical surface map, that is, interference band distortion caused by the interferometer error does not exceed 0.1 of the distortion caused by the error inherent to the surface being monitored. The interferometer provides highly accurate measurement of hyperbolic surfaces while featuring an obviously simple design

Multi-tool optimization for computer controlled optical surfacing.

With the rapid development of precision technologies, the demand of high-precision optical surfaces has drastically increased. These optical surfaces are mainly fabricated with computer controlled optical surfacing (CCOS). In a CCOS process, a target surface removal profile is achieved by scheduling the dwell time for a set of machine tools. The optimized dwell time should be positive and smooth to ensure convergence to the target while considering CNC dynamics. The total run time of each machine tool is also expected to be balanced to improve the overall processing efficiency. In the past few decades, dwell time optimization for a single machine tool has been extensively developed. While the methods are applicable to multi-tool scenarios, they fail to consider the overall contributions of multiple tools simultaneously. In this paper, we conduct a systematic study on the strategies for multi-tool dwell time optimization and propose an innovative method for simultaneously scheduling dwell time for multiple tools for the first time. First, the influential factors to the positiveness and smoothness of dwell time solutions for a single machine tool are analyzed. The compensation strategies that minimize the residual while considering the CNC dynamics limit are then proposed. Afterwards, these strategies are extended to the proposed multi-tool optimization that further balances the run time of machine tools. Finally, the superiority of each strategy is carefully studied via simulation and experiment. The experiment is performed by bonnet polishing a 60 mm × 60 mm mirror with three tools of different diameters (i.e., 12 mm, 8 mm, and 5 mm). The figure error of the mirror is reduced from 45.42 nm to 11.18 nm root mean square in 13.28 min. Moreover, the measured polishing result well coincides with the estimation, which proves the effectiveness of the proposed method.

Effect of change in a normative approach on the evaluation of surface scratches on high-precision optical surfaces.

The evaluation of optical surface defects is an integral part of the realization of precise optical elements. In addition to the traditionally used visual inspection procedures based on MIL-O-13830A, procedures based on ISO standards, which underwent some changes in 2017, are coming to the forefront. One of these changes was the omission of equivalent radiometric sizes and their replacement by geometric dimensions. This paper focuses just on the analysis of the impact of this change on the values of specifications defining the requirements on the scratch and dig quality of optical elements. Among other things, this presents a possible way to correct specification classes based on the statistical processing of a set of parameters of imperfections detected on optical element surfaces, which have been realized in the TOPTEC Centre.

Flexible Robot-Based In-Line Measurement System for High-Precision Optical Surface Inspection

Advances in industrial manufacturing and an increasing trend towards direct integration of measurement systems in the production line require flexible, fast, precise and robust systems. This paper presents the development and evaluation of a fully automated robot-based in-line measurement system, suitable for applications incorporating process variability and uncertainty, however, requiring single micrometer resolution and low instrumentation effort. The system utilizes laser triangulation in combination with mechanical 2D-scanning for fast and precise measurements. Lissajous trajectories and a computer vision algorithm based on edge detection are used to locate relevant features within the scan area. This allows alignment of the measurement tool, by using plane fits, to optimally utilize the measurement range of the sensor system for a high resolution scan, applied to a refined area, providing robustness and flexibility. For a scan area of 25×25mm the measurement uncertainty equals 6.4μm at 2σ. The system accuracy is evaluated to 2.2μm. The combined standard uncertainty of reference sample measurements equals 1.2μm.

Minimization of CaF2 spherical surface deformation caused by annealing

We explore causes and possible techniques to minimize shape deformation of high-precision monocrystalline calcium fluoride optical surface formed during thermal annealing. Due to the anisotropic mechanical properties of calcium fluoride crystals, machining of lens shapes introduces defects into the crystal lattice. The thermal annealing thus leads to activation of the processes of recovery, resulting in the formation of characteristic surface structures causing both shape error and increased microroughness. The surface deformation can be gradually minimized by thermal treatment of the optical element and subsequent polishing steps to the order of units of nanometers, so they do not represent a fundamental problem for optical performance.

Multiposition Rotation Interference Absolute Measurement Method for High-Precision Optical Component Surfaces

Modern optical engineering requires increasingly sophisticated interferometry methods capable of conducting subnanometer scale measurements of the large aperture, high-precision optical component surfaces. However, the accuracy of interferometry measurement is limited to the accuracy with which the surface of the reference mirror employed in the interferometer system is known, and the influence of gravity-induced deformation cannot be ignored. This is addressed in the present work by proposing a three-flat testing method based on multiposition rotation interference absolute surface measurement technology that combines the basic theory of N-position rotation with the separability of surface wavefront functions into sums of even and odd functions. These functions provide the rotational symmetric components of the wavefront, which then enables the absolute surface to be reconstructed based on the N-position rotation measurements. In addition, we propose a mechanical clamping combined with computational method to compensate for the gravity-induced deformations of the flats in the multiposition rotation absolute measurements. The high precision of the proposed absolute surface measurement method is demonstrated via simulations. The results of laboratory experiments indicate that the combination compensation method provides the high-precision surface reconstruction outcomes. The present work provides an important contribution for supporting the interferometry measurement of large aperture, high-precision optical component surfaces.

Ductile-brittle coupled cutting of a single-crystal silicon by ultrasonic assisted diamond turning.

Ultrasonic assisted diamond turning (UADT) is a significant machining technology for the fabrication of a crack-free surface on single-crystal silicon. However, due to insufficient understanding of intermittent cutting characteristic, most researches have been only focused on the mechanism of ductile-regime machining rather than the improvement of surface quality and machining efficiency. Therefore, the novel machining model in UADT, ductile-brittle coupled cutting, is proposed to reveal how to realize the high-precision optical surface with larger processing parameters. Two quantitative performance indices, crack length projection and maximum tolerance length, are employed to evaluate whether a smooth surface can be achieved. And the variation of microscopic crack is analyzed and discussed in a single vibration cycle with different machining and tool parameters. In the experiments, the odd cosine surface is fabricated and the surface roughness Ra can reach 1.739 nm after measuring. The results show that better surface quality and higher machining efficiency can be achieved on single-crystal silicon by ductile-brittle coupled cutting in UADT.

Φ1.05 m轻量化反射镜设计与制造

针对Ф1.05 m空间光学系统主镜的设计指标要求，提出了轻量化反射镜结构优化设计的新方法，并建立了反射镜结构自动化仿真分析与优化设计平台，基于此平台确定了性能优异的主镜结构设计方案。主镜重量小于50 kg，轻量化率已接近国外先进水平；主镜在三球铰支撑下的第一阶模态频率为361.2 Hz，自由状态下的一阶非零模态频率为501.9 Hz；在1 ℃均匀温度变化下，不去离焦和去除离焦之后的面形RMS分别为0.55 nm和0.10 nm；主镜在30g过载加速度作用下的最大应力为16.1 MPa，均满足设计要求。采用目前最先进的第三代大口径反射镜加工工艺，路线为超精密铣磨—小磨头数控研抛—离子束精修，实现主镜面形误差的确定性去除。为保证面形检测结果的天地一致性，发展了重力卸载技术和面形误差数据后处理技术，剔除重力和其他系统误差对检测的影响。主镜最终面形精度达到0.011λ RMS，获得了高精度的光学面形，也证明了方案的合理性。

Three-dimensional surface profile measurement of a cylindrical surface using a multi-beam angle sensor

To investigate the application of the multi-beam angle sensor (MBAS) to high-precision optical aspheric and freeform surfaces, which are critical components in optical systems, we present a method of using an MBAS to reconstruct an aspheric surface from angle data. The MBAS is based on a multi-autocollimator system with a microlens array, which can split the beam into several spots and can convert centroid detection of the light intensity into an angle measurement. The MBAS is designed to address the curvature-range problem via a circumferential scan better than other methods and automatically eliminates the tilt error caused by rotation of a workpiece. Using a tracking technique, the MBAS can automatically determine focal spot positions from the centroid measurement of the light intensity. This is directly related to the accuracy of the angular difference measurement. The experimental results confirm the feasibility of using an MBAS for 3D surface profile measurements of cylindrical surfaces.

Absolute optical flatness testing by surface shape reconstruction using Zernike polynomials

Absolute measurement is an effective way to obtain high-precision optical surface measurements. This paper describes a convenient absolute testing approach that allows reconstruction of surfaces using Zernike polynomials. This method requires a classical three-flat measurement and a one-rotation measurement before reconstructing the surface. Utilizing a well-established procedure, the absolute surface profile of the testing surface can be reconstructed with more Zernike orders than are provided by Fritz’s method. In particular, simulation of the testing error through recalculation of the test surface profile at a different angle could provide the optimized angle with a minimum testing error. This implies that an additional rotation measurement for the optimized angle can improve testing accuracy. The experimental results of a 100-mm flat surface provided a reflected root mean square (RMS) of 2.6 nm and a residual RMS of 0.1 nm.

Noncontact on-machine measurement system based on capacitive displacement sensors for single-point diamond turning

On-machine measurements can improve the form accuracy of optical surfaces in single-point diamond turning applications; however, commercially available linear variable differential transformer sensors are inaccurate and can potentially scratch the surface. We present an on-machine measurement system based on capacitive displacement sensors for high-precision optical surfaces. In the proposed system, a position-trigger method of measurement was developed to ensure strict correspondence between the measurement points and the measurement data with no intervening time-delay. In addition, a double-sensor measurement was proposed to reduce the electric signal noise during spindle rotation. Using the proposed system, the repeatability of 80-nm peak-to-valley (PV) and 8-nm root-mean-square (RMS) was achieved through analyzing four successive measurement results. The accuracy of 109-nm PV and 14-nm RMS was obtained by comparing with the interferometer measurement result. An aluminum spherical mirror with a diameter of 300 mm was fabricated, and the resulting measured form error after one compensation cut was decreased to 254 nm in PV and 52 nm in RMS. These results confirm that the measurements of the surface form errors were successfully used to modify the cutting tool path during the compensation cut, thereby ensuring that the diamond turning process was more deterministic. In addition, the results show that the noise level was significantly reduced with the reference sensor even under a high rotational speed.

Modeling of surface topography in single-point diamond turning machine.

Surface roughness is an important factor in characterizing the performance of high-precision optical surfaces. In this paper, we propose a model to estimate the surface roughness generated by a single-point diamond turning machine. In this model, we take into consideration the basic tool-cutting parameters as well as the relative vibration between the tool and the workpiece in both the infeed and feeding directions. Current models focus on the relative tool-workpiece vibration in the infeed direction. However, based on our experimental measurements, the contribution of relative tool-workpiece vibration in the feeding direction is significant and cannot be ignored in the model. The proposed model is able to describe the surface topography for flat as well as cylindrical surfaces of the workpiece. It has the potential to describe more complex spherical surfaces or freeform surfaces. Our experimental study with metal materials shows good correlation between the model and the diamond-turned surfaces.

五角棱镜光学平行差的理论分析

By using matrix optics method, expressions of two types of optical parallelism in pentaprism were obtained. An relationship between optical parallelism and normal vector of working surface was analysed. An experiment device based on Agilent laser interferometer and DHC 3D rotation platform was established. With angular measurement function provided by the device, optical parallelism of a pentaprism made by Thorlabs were measured. The experimental results are consisten with the calculation of analytical expressions, whose deviation is no more than 4%. Conclusion indicates that changes of roll angle and yaw angle in practical application are equivalent to alteration of optical parallelism. It means that the jitter errors of online measurement and fabrication errors can be in-situ calibrated simultaneously by coordinates transformation. The study is useful for manufacturing, adjusting and pre-calibrating a pentaprism in optical system, and the conclusions above can be a good help in high-precision optical surface profiler metrology in practice. ©, 2015, Chinese Optical Society. All right reserved.

Influence of material removal programming on ion beam figuring of high-precision optical surfaces

Ion beam figuring (IBF) provides a nanometer/subnanometer precision fabrication technology for optical components, where the surface materials on highlands are gradually removed by the physical sputtering effect. In this deterministic method, the figuring process is usually divided into several iterations and the sum of the removed material in each iteration is expected to approach the ideally removed material as nearly as possible. However, we find that the material removal programming in each iteration would influence the surface error convergence of the figuring process. The influence of material removal programming on the surface error evolution is investigated through the comparative study of the contour removal method (CRM) and the geometric proportion removal method (PRM). The research results indicate that the PRM can maintenance the smoothness of the surface topography during the whole figuring process, which would benefit the stable operation of the machine tool and avoid the production of mid-to-high spatial frequency surface errors. Additionally, the CRM only has the corrective effect on the area above the contour line in each iteration, which would result in the nonuniform convergence of the surface errors in various areas. All these advantages distinguish PRM as an appropriate material removal method for ultraprecision optical surfaces.

Nanopatterning of optical surfaces during low-energy ion beam sputtering

Ion beam figuring (IBF) provides a highly deterministic method for high-precision optical surface fabrication, whereas ion-induced microscopic morphology evolution would occur on surfaces. Consequently, the fabrication specification for surface smoothness must be seriously considered during the IBF process. In this work, low-energy ion nanopatterning of our frequently used optical material surfaces is investigated to discuss the manufacturability of an ultrasmooth surface. The research results indicate that ion beam sputtering (IBS) can directly smooth some amorphous or amorphizable material surfaces, such as fused silica, Si, and ULE® under appropriate processing conditions. However, for IBS of a Zerodur® surface, preferential sputtering together with curvature-dependent sputtering overcome ion-induced smoothing mechanisms, leading to the granular nanopatterns’ formation and the coarsening of the surface. Furthermore, the material property difference at microscopic scales and the continuous impurity incorporation would affect the ion beam smoothing of optical surfaces. Overall, IBS can be used as a promising technique for ultrasmooth surface fabrication, which strongly depends on processing conditions and material characters.

基于经验模态分解-Wigner分布的光学元件中频误差识别

The mid-spatial frequency error of high precision optical surface is crucial to the performance of high-energy laser system. To assure the performance and stability of the system, the low frequency error and high frequency error of the optical lens surface must be strictly controlled, the mid-spatial frequency error must also be strictly limited. In this paper, the relationship between empirical mode decomposition-Wigner-Ville distribution (EMD-WVD) method and power spectrum density is analyzed, and EMD-WVD diagnosis method is applied to identifying and locating the mid-spatial frequency error of the precision optical surface. The experimental results show that, with EMD-WVD method, the mid-spatial frequency error distribution of optical surface can be located in 15-27 mm and the spatial frequency is 0.1 mm-1, the interference of cross-term caused by multi-component signal whose spatial frequency is about 1.0-1.5 mm-1 can also be reduced, which enhances the identification accuracy of mid-spatial frequency error.

Ultra-smooth polishing of high-precision optical surface

A new method of ultra-smooth uniform polishing was presented, which can avoid high-precision surface figure getting worse after ultra-smooth polishing. At first, the fundamental and process were introduced. Then the process was simulated with “Gauss” and “V” type removal function. It shows that there will be no significant influence on optical surface figure after ultra-smooth uniform polishing with any type removal function. To demonstrate the process, a high-precision Ø100mm fused silica flat optical element was polished, which was prior figured by IBF. Its surface figure accuracy root-mean-square (rms) value is improved from initial 3.624nm to final 3.393nm, the mid-spatial frequency surface roughness rms value is improved from initial 0.477nm to final 0.309nm, and the high-spatial frequency surface roughness rms value is improved from initial 0.167nm to final 0.0802nm. At last, the surface quality of the lens was analyzed by power spectral density (PSD). The result indicates that the surface roughness of high-precision optical element could be improved by ultra-smooth uniform polishing method without the surface figure destroyed.

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.