Stress analysis of a large diameter aspheric plastic lens in the variable temperature assisted injection molding process.

The technology known as precision injection molding (PIM) has shown great promise in the large-scale manufacturing of optical plastic lenses. The primary challenge with the PIM process is accurately predicting and reducing residual stress in optical plastic lenses. In this work, the finite element method (FEM) was used to analyze the residual stress distribution in plastic lenses. A three-dimensional model was created using COMSOL software to investigate how residual stress and temperature varied in optical plastic lenses during the packing and cooling stages. Based on the results, variable temperature assisted injection molding experiments were conducted. The results show that the average residual stress in the optical plastic lenses has decreased by 56%, while the minimum and maximum residual stress levels have decreased by 60% and 61%, respectively. Since this method does not require the extra heat treatment of the optical lenses, it offers considerable cost and efficiency benefits.

Heat and force coupling analysis during precision glass molding of free-form optical elements.

As an effective method for mass production of glass lenses, precision glass molding (PGM) technology has been mature in aspherical lens technology. However, glass molding of complex surfaces such as free-form optical elements is still in its infancy. For mass-produced glass lenses, the energy consumption is mainly concentrated on the heating stage, and the cost is also a key concern. In this paper, an X Y polynomial free-form optical element is simulated by the finite element method. In view of the long heating and molding time, finite element models were established, respectively, at the optimum molding temperature and a temperature point before the optimum temperature. The stress distribution and variation trend of the two methods were analyzed, and the single cycle time of the two methods was compared. The results showed that, under the premise of the maximum stress increase of 3.91 MPa, this method reduces the heating and molding time from 1000 to 910 s, which has significant advantages in terms of time and cost.

High-precision turning and ultra-smooth direct polishing of aluminum alloy mirrors.

Due to the high surface roughness requirements of aluminum alloy mirrors used in the visible light band, there are still great challenges in single point diamond turning of high-surface quality aluminum alloy mirrors. In this paper, a processing method for aluminum alloy mirrors is proposed. Based on single point diamond turning technology, the prediction model of aluminum alloy surface roughness was established. The mapping relationship between the surface roughness of the aluminum alloy mirror and each turning parameter was obtained, and the maximum possible surface quality was achieved. On the basis of the turning results, the method of small tool polishing was used to remove the turning texture generated by the copy effect of the tool arc radius, suppress errors of the medium and high-frequency, and reduce the surface roughness. The single abrasive removal efficiency model was established and mechanical removal in the polishing process was analyzed. Combined with the chemical action in the polishing process, two types of polishing liquid-acidic and neutral, were prepared and analyzed. The optimal polishing parameters were obtained through multiple single-factor experiments. On the basis of this, the surface roughness of the aluminum alloy after turning was optimized. The results show that the value was reduced from 4.811 to 1.482 nm, an increase of 69.2%. This method can effectively improve the machining accuracy of aluminum alloy mirrors and provide an important process guarantee for the application of aluminum alloy materials in visible-light systems.

Analysis on the instability of the surface profiles of precision molding chalcogenide glass aspherical lenses in mass production.

Chalcogenide glass lenses have been widely applied in infrared optical systems for their outstanding optical performance. It is a tendency for complex optical glass elements to be mass-produced with precision glass molding (PGM) technology, of course including chalcogenide glass aspheric lenses. But there is a problem that sometimes the surface profiles of the molded lenses are unstable which leads to a low pass-yield. Precision glass molding experiments and finite elements simulations are carried out to study the reasons for the mentioned problem in this paper. The results reveal that the laying error of the ball chalcogenide glass preform does not have a significant effect on the surface profile of the molded lens. However, in mass production the control of the temperature after forming stage in the PGM process is very important for obtaining the molded lenses with very similar surface profiles. The research results could help relevant researchers design the PGM processing parameters to overcome some errors in the mass production and manufacture precision glass molding machines. The increase in the yield of complex optical glass elements fabricated by PGM technology will further promote the application of such elements in various fields.

Design of adjustable multifocal diffractive optical elements with an improved smooth phase profile by continuous variable curve with multi-subperiods method.

Multifocal diffractive optical elements (MDOEs), which produce arbitrary light distribution, are widely used in lightweight and compact optical systems. MDOEs that are combined with multiple functions tend to have complex step structures, limiting their applications. We propose a facile method named continuous variable curve with multi-subperiods (CVCMS) to design adjustable multifocal single-layer diffractive optical elements. Through the analysis, the model achieved arbitrary diffraction efficiency distribution with an improved smooth continuous phase profile in each diffractive ring while retaining the periodicity. To display the high design freedom of the method, we utilized this method to design and discuss a broadband multifocal intraocular lens (MIOL) focused on the optimization of far focal point. Finally, the method was compared with other multifocal design methods. The results show that the CVCMS method achieved adjustable multifocal design with better performance and smoother profile than other MDOE design techniques. The proposed model can be applied to multifocal ophthalmic lens designs.

Theoretical model and optimization of diffractive optical elements with aspheric substrates in ophthalmology.

Diffractive optical elements (DOEs), which can produce arbitrary light distribution, are widely applied in ophthalmic lens design with spheric substrates. However, diffraction substrates tend to be designed as aspheric surfaces to eliminate aberrations. In this case, the diffraction theory of plane substrates is no longer accurate, which affects the diffraction performance. Therefore, a diffraction theory of aspheric diffraction substrates is proposed in this paper. Using the range of common parameters for aspheric substrates in ophthalmology, the influence of the substrate diopter and the aspheric surface parameters on the period radius and phase delay is analyzed. Then, through a design example of a diffraction intraocular lens (IOL), an optimization equation is proposed and discussed. The results show that the diffraction theory of aspheric substrates and the optimization equation model can analyze and reduce the effect of aspheric diffraction substrates. This research can be used in DOE design with aspheric substrates in ophthalmology.

Ultra-precision turning method efficient for optical freeform surfaces with a hybrid slow-fast tool servo.

The machining of freeform surfaces is a current research hotspot: A slow tool servo (STS) has limitations in machining accuracy and efficiency for large steep freeform surfaces. Most fast tool servo (FTS) tools are limited by their stroke and cannot manufacture freeform surfaces with a large sag. We propose a hybrid slow-fast tool servo method that combines STS and FTS to machine large steep freeform surfaces by decomposing the freeform surfaces and simultaneously turning efficiently with STS and FTS. Experimental studies were undertaken to fabricate a saddle surface. Meanwhile, a variable feedrates tool path was designed and applied to further improve the machining efficiency. The results show that this method can improve the processing efficiency by 47.5%. The arithmetic mean of surface roughness (Ra) is 2-4 nm, and the peak-to-valley (PV) value is 0.4780 µm at the hollows and 0.3884 µm at the swells.

Prediction model of residual stress during precision glass molding of optical lenses.

Precision glass molding (PGM) is an important processing technology for aspheric lenses that has the advantages of low complexity, high precision, and short processing time. The key problem in the PGM process is to accurately predict the residual stress of aspheric lenses. In this paper, we examine the residual stress relaxation model for aspheric lenses, including a creep experiment of D-K9 glass, calculating shear relaxation function, and predicting residual stress of aspheric lenses with the finite element method. Validations of the proposed model are conducted for three different process parameters, including molding temperature, molding pressure, and molding rate. The experimental and simulation results show that the errors of the residual stresses of the three process parameters are within 0.358 Mpa, which proves the validity of the model. The model can be used to predict the residual stress of the optical glass lens fabricated by PGM and analyze the processing parameters.

Material removal mechanism and MR fluid for magnetorheological finishing of an RSA-6061 aluminum alloy mirror.

We sought to improve the surface quality and the removal rate of aluminum alloy mirrors in magnetorheological finishing (MRF). Based on dense granular flow theory, we investigated the removal of magnetorheological (MR) fluid in the polishing area and adopted a new removal model to express the removal rate of MRF. Then, according to the composition and physical properties of RAS-6061 aluminum alloy, we analyzed the reaction state of it in an acidic and alkaline environment. At the same time, we gave a reasonable range of the content of MR fluid by single-factor experiment and explained the influence of MR fluid on surface quality and removal rate. Lastly, we carried out the MRF experiment on an aluminum alloy mirror and got a high surface quality. The surface roughness reached R a=2.715n m, and the tool marks formed by turning were removed, which verified the reliability of the removal model.

Deformation Analysis of the Glass Preform in the Progress of Precision Glass Molding for Fabricating Chalcogenide Glass Diffractive Optics with the Finite Element Method.

Precision glass molding (PGM) technology is a cost-efficient process for the production of micro/nanostructured glass components with complex surface geometries. The stress distribution, surface profile, and reduced refractive index of the molded lens are based on the lens being fully formed. The process of the deformation of the glass preform is rarely discussed, especially in the case of multi-machining parameters in the experiment. The finite element method (FEM) was adopted to analyze the glass preform deformation. Due to the phenomenon of incomplete deformation of the glass preforms in the experiments, two groups of finite element simulations with different boundary conditions were carried out with MSC.Marc software, to reveal the relationship between the deformation progress and the parameters settings. Based on the simulation results, a glass preform deformation model was established. The error between the model result and the simulation result was less than 0.16. The establishment method of the glass preform deformation model and the established model can be used as a reference in efficiently optimizing PGM processing parameters when the designed lens has two different base radii of curvature.

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.

Analysis of lens fracture in precision glass molding with the finite element method.

Precision glass molding (PGM) technology has recently emerged as a promising fabrication method for mass-fabricating optical glass lenses with complex surfaces. However, lens fracture as a common problem has not been analyzed in detail. In this paper, the divergent cone cracks in the molded lens were analyzed using the finite element method, because crack propagation cannot be seen in the molding process. A three-dimensional model was established in MSC Marc software for analyzing the temperature, stress components, and principal stress of the glass in different molding stages. The crack paths were analyzed using the simulation results and the fracture basis. Based on the analysis, PGM experiments with different processing parameters were carried out. The appearance of the molded lenses demonstrated the rationality and correctness of the analysis. Thus, analyses of other types of lens fractures can use the analysis method proposed in this paper rather than relying on trial and error.

Fabrication of high-precision freeform surface on die steel by ultrasonic-assisted slow tool servo.

The mold core fabrication of a freeform surface on die steel by ultra-precision machining can make the optical elements of freeform be mass-produced by plastic injection and glass mold pressing. However, because steel is a typical difficult-to-cut material, the technical limitations of existing machining methods hardly meet the current requirements of design. In this paper, a novel machining method, one-dimension ultrasonic-assisted slow tool servo (UASTS) turning, is proposed to manufacture the freeform surface with high-precision and large-steepness on the die steel. Aiming at the characteristics of UASTS turning, the tool trajectory is generated by analyzing the compensation of tool radius and confirming the position of ultrasonic displacement. The 2D surface model contours of residual tool marks are established for predicting the 3D surface topography based on considering the effects of kinematics, material elastic recovery and plastic side flow. In the experiments, the large-amplitude bidirectional sinusoidal wave grid (BSWG) surface is successfully fabricated by UASTS technology on the material, Polmax steel, for which the value Rt of surface roughness is less than 70nm and the value Ra of surface roughness can achieve to 3.298nm. The results show that the freeform surface with high-precision and large-steepness can be machined by UASTS turning technology on mold steel.

Optimization method of manufacturing for diamond turning soft-brittle materials' harmonic diffractive optical elements.

Single-point diamond turning is a high-efficiency, low-cost method for manufacturing harmonic diffractive optical elements (HDOEs). Generally, in order to ensure high diffraction efficiency of HDOEs, a half-round tool is used to reduce surface-relief profile errors and a super small feed rate is selected to control surface roughness when hard-brittle materials are turning. However, this method is no longer suitable for soft-brittle materials, which have a strict requirement for the range of the feed rate. It limits the types of materials available for HDOEs. Therefore, according to the range of the feed rate and the cutting depth for soft-brittle materials, an optimized turning model is proposed in this paper. It overcomes the high surface roughness caused by the half-round tool; meanwhile, the advantage of the half-round tool is kept in terms of surface-relief profile errors. On this basis, a mathematical model is proposed to reveal the relationship among diffraction efficiency, period widths, tool radius, and feed rate with different soft-brittle optical materials. As a typical soft-brittle material, barium fluoride (BaF2) was selected to be the material for manufacturing HDOEs. By using the optimized model, the turning experiment of BaF2 HDOEs was completed, and then the BaF2 HDOEs with Ra=2.75nm were obtained. The optimized model is verified to be effective and further provides theory guidance and engineering application in the optical manufactory field of soft-brittle materials for HDOEs. The range of application materials for HDOEs is enriched, and the freedom of advanced optical design is broadened.

Glued diffraction optical elements with broadband and a large field of view.

High diffraction efficiency is an important requirement for hybrid diffractive-refractive optical systems with a wide field of view. The issue is that diffractive optical elements cannot maintain high diffraction efficiency across a designed waveband and range of incident angles simultaneously. Glued diffractive optical elements (GDOEs) consist of two single-layer diffractive elements, and optical adhesives are presented to address the problem. Two diffractive optical elements are glued together to reduce the straylight scattered into unwanted diffraction orders. The parameters of diffractive optical elements are optimized to achieve broadband high diffraction efficiency and modulation transfer function over a wide-incident-angle range. The GDOEs enable the system to realize a diffraction efficiency of over 90% when the incident angle is no more than 58°. Through gluing two single-layer diffractive optical elements together, we can minimize the inner reflection and refraction. Diffraction efficiency losses can be compensated by the optical adhesives layer, and image quality can be improved. Our design method could make possible the use of diffraction elements in different kinds of optical systems.

Roughness model of an optical surface in ultrasonic assisted diamond turning.

In this paper, the theoretical model is established to predict the optical surface roughness of difficult-to-cut material in ultrasonic assisted diamond turning (UADT). The effects of kinematics, material elastic recovery, and plastic side flow aiming at the characteristics of vibration cutting are considered. The convincing results predicted can be obtained when main machining parameters change, such as cutting speed, cutting depth, tool feed rate, tool frequency, and amplitude. Furthermore, the qualitative analysis of the model is demonstrated on the basis of comparing the trend variation by theoretical results and simulative results with the finite element method (FEM). The arithmetic average value of the vertical coordinates of the workpiece surface nodes is regarded as the surface roughness in the FEM. The minimum mesh size of workpiece is set as 5 nm in order to gain relatively exact results and avoid exceeded element distortions. Moreover, the accuracy of the predictive model is verified by cutting the MB5 magnesium alloy with UADT. The maximum error for surface roughness Ra is merely 10.26%, and average error is only about 6% after analyzing experiment and prediction results. The optimal surface roughness Ra of magnesium alloy reflector can be 3.388 nm with UADT so that the optical application level is realized only by UADT means without subsequent abrasive machining. Therefore, the predicting model is valuable for theory guidance and engineering application in the optical manufactory field of difficult-to-cut material with UADT.

Diffraction efficiency evaluation for diamond turning of harmonic diffractive optical elements.

Single point diamond turning is a commonly used method for manufacturing harmonic diffractive optical elements (HDOEs). Diffraction efficiency of HDOEs is sensitive to surface-relief profile errors, and a half-round tool can reduce the profile error obviously. Furthermore, the diamond tools also create surface roughness errors. The two errors will produce shadowing and scattering effects. In this paper, the two kinds of errors, especially the surface roughness, are described accurately. A mathematical model is proposed to reveal the relationship among diffraction efficiency, cutting tool radius, feed rate, microstructure zone period widths, and the refractive index of the substrate material and balance the influence of shadowing and scattering effects. The simulation results show that the model can guide the acquisition of high-precision surface topography and high diffraction efficiency, which improves the imaging quality of the optical system.

Simulation of heat transfer in the progress of precision glass molding with a finite element method for chalcogenide glass.

Precision glass molding (PGM) has become a viable processing method for large-volume aspheric optical elements. The optimization of a PGM process is an obstacle to the realization of mass production. The current work is focused on optimizing the process parameters to gain satisfactory surface shape. But the machining cycle time is not optimized. When setting the process route in the machine interface, going to the next step after reaching the target temperature rather than reaching the target time is usually set for the heating and cooling phase. Thus, the time to complete the heating and cooling stages of the production cycle is known only in the actual production. As for chalcogenide glass, its physical and chemical properties are greatly dependent on temperature. So, it is necessary to effectively simulate these stages to obtain the cost in time for actual production. Due to the excellent availability of numerical simulation, the rapid development of computing technology, and the increase of task scale in data and information processing, the finite element method can be applied to simulate the whole molding process. In this paper, a heat transfer model is established with the partial differential equation toolbox in MATLAB software. MSC.Marc software is used to simulate the heating stage at the same time. The numerical results are consistent, indicating that the heat transfer model established in MATLAB is, at least to a certain extent, valid. The heat transfer model needs further improvement by considering temperature-dependent properties such as viscoelasticity to make it a more effective tool for process analysis and optimization.

Optimization method of tool path generation considering the edge of lenslets for a microlens array in FTS diamond turning.

Shape accuracy is an important parameter for evaluating the quality of microlens arrays. In fast tool servo (FTS) diamond turning, the generation of tool path has a significant influence on shape accuracy. By analyzing the distribution of the cutting points at the edge of the lenslets and the linearization error of the original tool path generated by the constant-angle method for the microlens array, there is overcut at the edge of the lenslets. Previous tool path planning focused on consideration of the entire surface, and the error on the edge of the lens was rarely considered. Therefore, an optimization method of tool path generation based on interpolation of the lens edge is proposed. Compared to the tool path generated by the conventional constant-angle method, the simulation and experimental results show that the proposed method can effectively reduce the overcut of the lens edge.

Optimization and analysis of infrared multilayer diffractive optical elements with finite feature sizes.

Infrared multilayer diffractive optical elements (MLDOEs) usually own microstructure heights of a few hundred micrometers. The design and fabrication of those elements are more difficult than MLDOEs working in the visible waveband, especially for MLDOEs with high numerical aperture and finite feature sizes. Based on scalar diffraction theory and manufacturing errors, the effective area method for improving diffraction efficiency of infrared MLDOEs is developed. Closed-form analytical relations among diffraction efficiency, microstructure heights, microstructure periods, and incident angles are derived and verified in the infrared waveband. Then, optimized microstructure heights of infrared MLDOEs with different microstructure zone widths in the infrared wavelengths 3-5 µm and 8-12 µm at normal incidence can be obtained. The results indicate that the microstructure heights of infrared MLDOEs determined by the method have higher diffraction efficiency than former design methods. The method is verified by the rigorous electromagnetic method. Finally, the influence of incident angles on infrared MLDOEs is investigated. Our results show that the suggested microstructure parameters of MLDOEs both produce higher diffraction efficiencies than that of structure designed by scalar diffraction theory and may lead to more efficient hybrid diffractive-diffractive optical systems based on MLDOEs.