Fabrication Of Freeform Optics Research Articles

Surface morphology in plasma jet polishing: theoretical description and application

Atmospheric pressure plasma jets are effective for generating optical freeform surfaces and correcting figure errors. They can also reduce high spatial frequency surface roughness, potentially replacing mechanical-abrasive polishing. Plasma jet polishing (PJP) involves thermally driven material redistribution. Current research aims to predict surface topography and roughness by analyzing initial surface topography and the local effect of the plasma jet tool. The tool interaction function was mathematically described by evaluating a microstructure pattern before and after PJP, revealing a 2D Gaussian convolution function. This function can be applied to areal topography measurements of lapped and mechanically ground surfaces to predict the polishing performance with respect to reduction of tool marks originating from pre-machining processes. Additionally, the convolution function can be used to predict the dimensions of an initial surface structure in order to produce a defined smooth microstructure using PJP. Evaluating the smoothing capability of PJP helps identify suitable pre-machining conditions in optics manufacturing, such as grinding or laser micromachining, enabling a more efficient process chain for freeform optics fabrication.

Tool path generation and optimization for freeform surface diamond turning based on an independently controlled fast tool servo

Diamond turning based on a fast tool servo (FTS) is widely used in freeform optics fabrication due to its high accuracy and machining efficiency. As a new trend, recently developed high-frequency and long-stroke FTS units are independently driven by a separate control system from the machine tool controller. However, the tool path generation strategy for the independently controlled FTS is far from complete. This study aims to establish methods for optimizing tool path for the independent control FTS to reduce form errors in a single step of machining. Different from the conventional integrated FTS control system, where control points are distributed in a spiral pattern, in this study, the tool path for the independent FTS controller is generated by the ring method and the mesh method, respectively. The machined surface profile is predicted by simulation and the parameters for the control point generation are optimized by minimizing the deviation between the predicted and the designed surfaces. To demonstrate the feasibility of the proposed tool path generation strategies, cutting tests of a two-dimensional sinewave and a micro-lens array were conducted and the results were compared. As a result, after tool path optimization, the peak-to-valley form error of the machined surface was reduced from 429 nm to 56 nm for the two-dimensional sinewave by using the ring method, and from 191 nm to 103 nm for the micro-lens array by using the mesh method, respectively.

Fabrication of freeform optical components by fluidic shaping

Freeform optical components enable advanced manipulation of light that is not possible with traditional optical systems. However, their fabrication relies on machining processes that are complex, time-consuming, and require significant infrastructure. Here we present the ability to shape liquid volumes and solidify them into desired freeform components, enabling rapid prototyping of freeform components with high surface quality. The method is based on controlling the minimum energy state of the interface between a curable optical liquid and an immersion liquid, by dictating a geometrical boundary constraint. We provide an analytical solution for the resulting topography given a predefined boundary and demonstrate the fabrication of freeform components with sub-nanometer surface roughness within minutes. Such a fabrication capability, that allows for rapid prototyping of high-quality components, has the potential to answer an unmet need in the optical design industry—allowing researchers and engineers to rapidly test freeform design concepts. It can be further envisioned to be expanded to an industrial scale, allowing for mold-less fabrication of freeform optics.

Effect of workpiece curvature on the tool influence function during hemispherical sub-aperture tool glass polishing.

The influence of workpiece curvature on the tool influence function spot during polishing of fused silica glass with cerium oxide slurry, while using a rotating hemispherical pad-foam tool for a wide variety of process conditions (tool displacement, inclination angle, and rotation rate), has been investigated. (Workpiece curvature ranged from 500 mm radius concave to 43 mm radius convex.) The TIF spot decreases in diameter and increases in the peak removal rate on more convex workpieces. In contrast, the TIF spot increases both in diameter and peak removal rate on more concave workpieces. For the range of workpiece curvatures investigated, both the spot size and the peak removal rate changed significantly, as much as 2 times. An elastic sphere-sphere contact mechanics model, which utilizes both a modified displacement (that leads to a change in the applied load) as well as a mismatch factor (that influences the pressure distribution shape), has been developed. The model was validated using both offline load-displacement measurements and finite-element analysis simulations. The model quantitatively describes the measured change in the relative contact diameter and relative pressure distribution, as well as semiquantitively describes the change in the relative volumetric removal rate on a large variety of TIF spots. The change in the volumetric removal rate for convex workpieces is a result of the balance between a decreasing spot size (reducing removal) and an increasing peak pressure (increasing removal), which usually results in relatively small changes in volumetric removal. In the case of concave workpieces, the volumetric removal rate change is also governed by a similar balance, but the spot size increase contribution dominates, resulting in a significant increase in volumetric removal rate. Understanding these trends can enable methods to add greater determinism during the fabrication of freeform optics by adjusting polishing parameters (such as dwell time) while the tool translates along a workpiece surface with different local curvatures.

Simultaneous multisurface measurement of freeform refractive optics based on computer-aided deflectometry

Freeform optics, due to the more general surface geometry that offers high degrees of design freedom to control light propagation, has already been widely used in both nonimaging optics and imaging optics. With the recent advances in design and fabrication of freeform optics, one of the remaining challenges is how to accurately measure freeform optical surfaces, especially those included in freeform refractive optics. To meet this imperative need, for the first time, we believe, present an effective simultaneous multisurface measurement method for freeform refractive optics. Instead of using a reflected optical field to reconstruct tested optical surfaces, we develop a surface reconstruction method based on a transmitted field to tackle the challenges caused by the low reflectivity and compound effect of multiple reflection of refractive surfaces. The transmitted fields from refractive elements are measured by computer-aided deflectometry in order to achieve a large measurement dynamic range and high accuracy. Using the transmitted fields, a multisurface reconstruction model based on iterative optimization is then employed to achieve the accurate multisurface measurement simultaneously. The proposed method is demonstrated to be very effective and robust in testing freeform refractive optics, with a future potential for in situ metrology.

Zernike mapping of optimum dwell time in deterministic fabrication of freeform optics.

In optics fabrication technologies such as computer controlled optical surfacing (CCOS), accurate computation of the dwell time map plays an essential role in the deterministic performance of the fabrication process. However, it is still difficult for existing methods to derive smooth dwell time maps that reduce dynamic stressing on the machine especially at the aperture edge region, while retaining fine correction capability on freeform optics. To answer these challenges, we propose a new method based on Zernike decomposition and improved differential evolution optimization of the dwell time map, which can be applied to time-dependent optics fabrication processes such as fluid jet machining. Simulations and experiments based on a bi-sinusoidal freeform design were carried out to assess the feasibility of the proposed methodology. With appropriate fluid jet pressure, a bi-sinusoidal optical surface with demanding form error target of sub-100 nm in peak-to-valley was achieved, showing a remarkable improvement on the state-of-the-art, while keeping a good average surface roughness of 2.6 nm Ra on the optical glass.

Long-stroke fast tool servo and a tool setting method for freeform optics fabrication

Diamond turning assisted by fast tool servo is of high efficiency for the fabrication of freeform optics. This paper describes a long-stroke fast tool servo to obtain a large-amplitude tool motion. It has the advantage of low cost and higher stiffness and natural frequency than other flexure-based long-stroke fast tool servo systems. The fast tool servo is actuated by a voice coil motor and guided by a flexure-hinge structure. Open-loop and close-loop control tests are conducted on the testing platform. While fast tool servo system is an additional motion axis for a diamond turning machine, a tool center adjustment method is described to confirm tool center position in the machine tool coordinate system when the fast tool servo system is fixed on the diamond turning machine. Last, a sinusoidal surface is machined and the results demonstrate that the tool adjustment method is efficient and precise for a flexure-based fast tool servo system, and the fast tool servo system works well on the fabrication of freeform optics.

Development of pseudo-random diamond turning method for fabricating freeform optics with scattering homogenization

In this paper, a novel pseudo-random diamond turning (PRDT) method is proposed for the fabrication of freeform optics with scattering homogenization by means of actively eliminating the inherent periodicity of the residual tool marks. The strategy for accurately determining the spiral toolpath with pseudo-random vibration modulation is deliberately explained. Spatial geometric calculation method is employed to determine the toolpath in consideration of cutting tool geometries, and an iteration algorithm is further introduced to enhance the computation efficiency. Moreover, a novel two degree of freedom fast tool servo (2-DOF FTS) system with decoupled motions is developed to implement the PRDT method. Taking advantage of a novel surface topography generation algorithm, theoretical surfaces generated by using the calculated toolpaths are obtained, the accuracy of the toolpath generation and the efficiency of the PRDT method for breaking up the inherent periodicity of tool marks are examined. A series of preliminary cutting experiments are carried out to verify the efficiency of the proposed PRDT method, the experimental results obtained are in good agreement with the results obtained by numerical simulation. In addition, the results of scattering experiments indicate that the proposed PRDT method will be a very promising technique to achieve the scattering homogenization of machined surfaces with complicated shapes.

Controllable fabrication of freeform optics

Freeform optics is the next–generation optics to be used widely in advanced optical systems. The approach of trial and error loops is always employed to achieve the required form accuracy. Controllable fabrication based on on–machining measurement is proposed in this study. The laser probe is mounted on an ultra–precision turning machine to measure the freeform in the spiral path. A unit decomposition algorithm (UDA) is proposed to supply the proper arrangement of measuring data for the filtering and composition. A modified robust weight function (MRWF) is also presented to improve the performance of the 3D robust Gaussian filtering (RGF). Simulation experiments and case studies are carried out to verify the validity of the controllable fabrication method.

Comparative assessment of freeform polynomials as optical surface descriptions

Slow-servo single-point diamond turning as well as advances in computer controlled small lap polishing enables the fabrication of freeform optics, or more specifically, optical surfaces for imaging applications that are not rotationally symmetric. Various forms of polynomials for describing freeform optical surfaces exist in optical design and to support fabrication. A popular method is to add orthogonal polynomials onto a conic section. In this paper, recently introduced gradient-orthogonal polynomials are investigated in a comparative manner with the widely known Zernike polynomials. In order to achieve numerical robustness when higher-order polynomials are required to describe freeform surfaces, recurrence relations are a key enabler. Results in this paper establish the equivalence of both polynomial sets in accurately describing freeform surfaces under stringent conditions. Quantifying the accuracy of these two freeform surface descriptions is a critical step in the future application of these tools in both advanced optical system design and optical fabrication.

Fabrication of Freeform Micro Optics by Focused Ion Beam

In this work, two kinds of freeform micro optics were successfully fabricated by using focused ion beam machining. A divergence compensation method was applied to optimize the machining process. Both dynamic variation of the sputter yield and the extra ion flux contributed by the beam tail were taken into consideration. Measurement results on the surface topography indicated that 3-fold improvement of the relative divergence was achieved for both optics when compared with conventional focused ion beam milling without any corrections. Furthermore, investigations on the influences of scanning strategies, including raster scan, serpentine scan and contour scan, were carried out. The serpentine scan is recommended for the fabrication of freeform optics by focused ion beam technology owing to the minimal beam travelling distance over the pattern area.

Research on the tool alignment error in fabrication of off-axis paraboloid surfaces by using slow-tool-servo technique

Fabrication of free-form optics in good quality and high efficiency is much demanded as the widely use of free-form surfaces in modern optical systems. Slow-tool-servo technique is able to fabricate free-form optics with sub-micrometric form accuracy and nanometric surface finish in high efficiency, which has broad prospects of application. The tool alignment error is an important factor to work on the form accuracy of free-forms fabricated by slow-tool-servo technique. This paper studies the tool alignment error, presents a quantitative description of tool alignment errors at both feed and height directions by using the concept of the sensitivity function. An off-axis paraboloid workpiece was fabricated, and measure results verified the point of view in the paper. The tool alignment errors at two directions were calculated by surface figure error data to guide the alignment of cutting tool.

Diamond Turning Freeform Optics

Freeform optics fabrication has become one of the hottest topics in optics industry in recent years. Although it still remains a challenge, many have tried different ways of manufacturing it. Some have achieved degrees of success. By means of a Nanotech 350-FG five axis diamond turning machine, we too have successfully produced some prototype freeform optics and lens arrays with Slow Tool Servo and Milling method. The produced freeform optics are mainly for automobile LED headlamps and the lens arrays are for LED illumination. In order to produce the freeform optics, we developed our own DT Slow Tool Servo program which is capable of generating a DT program for diamond turning a universal/general 3D freeform surface. Slow Tool Servo technique and Diamond Milling technique were mainly employed to produce these freeform surfaces. The manufacturing process and machining parameter details will be given in the paper. The two main methods we used will be compared and discussed as well. In measuring the freeform surface, a 3D white light interferometer was used to scan and obtain the surface coordinates. The software made by ourselves enabled us to compare the measure results of the work piece with that of the design drawings. The deviation of our finished forms is within 5 um from that of the nominal. The surface quality Rq is about 10 nm. Measuring equipment and methodology will also be discussed in the paper.

Fabrication of Freeform Optics in Ultra-Precision Raster Milling

Recently, the high quality and high productivity in fabrication of freeform optics has been of primary interest in manufacturing industries, such as die and mould manufacturing, aerospace part manufacturing, and so forth. However, the fabrication of freeform optics is currently expensive and vastly complex. Ultra-precision raster milling can produce non-rotational symmetric surfaces with sub-micrometric form accuracy and nanometric surface finish without the need for any subsequent post polishing. While, there is little research work focus on this kind of machining method. This paper presents a framework of a tool path generation system for freeform surface ultra-precision raster milling. This system includes model of freeform optics, tool path generator, interference monitor and an optimization model of machining parameters. The tool path generation system can generate interference free and optimal tool path for machining freeform surfaces. Some simulation results have been presented to illustrate the performance of the system.