Developments in Precision Asphere Manufacturing Jay Tierson, Ed Fess, Greg Mathews OptiPro Systems LLC, 6368 Dean Parkway, Ontario NY 14519

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1 Developments in Precision Asphere Manufacturing Jay Tierson, Ed Fess, Greg Mathews OptiPro Systems LLC, 6368 Dean Parkway, Ontario NY ABSTRACT The increased use of aspheres in today s optical systems has led to specialized manufacturing equipment and processes that are needed to meet component specifications. Due to their sub-aperture nature, each stage of these processes can leave behind a signature that could adversely affect the asphere s overall performance. Utilizing a variety of grinding and polishing techniques can help minimize residual artifacts that are left in an asphere. OptiPro has performed extensive process development work to understand how to grind and polish aspheres at production speeds with minimized process signatures. For example, the amount of stock removed from a substrate using a sub aperture polishing process can increase the amount of mid-spacial frequencies that can be detected. Through precise grind control, sub aperture, and mid-aperture polishing process research, OptiPro developed a detailed knowledge of asphere process control. One of the outcomes of this work has led OptiPro to develop an asphere polishing head for their 160A polishing platform which allows more process flexibility and control. Keywords: optics, aspheres, polishing, deterministic INTRODUCTION In 1990, OptiPro Systems was one of the first companies to commercialize the revolutionary Opticam series of optical grinding platforms as part of its collaboration with the Center for Optics Manufacturing at the University of Rochester [1]. In 2003, OptiPro Systems began development of the Ultra Form Finishing (UFF) platform to polish large departure aspheres through a Small Business Innovative Research grant sponsored by the Department of the NAVY. The UFF is a CNC deterministic sub-aperture polisher that became commercially available in These two technologies used together created a unique asphere manufacturing cell. The UFF platform uses a belt with fixed or loose abrasive media to deterministically polish a ground surface to an optically finished surface. The esx is a 5 axis CNC platform that utilizes fixed abrasive grinding tools for processing brittle materials. The UFF & esx create a process that allows for precise and repeatable production of traditional optical shapes (spherical lenses, flat windows) as well as axially symmetric, and high departure aspheric optics. This process provides an affordable and deterministic manufacturing process for these shapes over a diverse range of materials such as optical glasses, IR crystals, and hard optical ceramics.

2 Figure 1: OptiPro PRO 80 UFF (left), PRO GTS contouring grinding an asphere (center), PROP 80 USF polishing an asphere (right). Deterministic Microgrinding The deterministic microgrinding (DMG) process can be utilized in a variety of ways depending on the type of surface being manufactured (Figure 3). If the surface is spherical, the most common utilization is to use the edge of a ring shaped tool aligned over the center of the optic, tilted at the appropriate head angle and fed in a single axis motion to generate the sphere. In contrast, if aspheres or freeform optics are being manufactured, the typical approach is to use hemispherical balls or toric shaped grinding wheels that traverse across the surface to create the desired profile. This requires multiple axes of simultaneous motion. This also makes the grinding process sensitive to vibration errors, which can result in undesirably excessive tool marks getting imprinted on the surface that may present challenges to remove during polishing. If such errors occur on the surface of an optic one can generally adjust process parameters to reduce the severity or eliminate the spatial frequency entirely [2] [3]. If the shape of the optic has localized small concave radii that limits tool diameter, or the material being ground is a hard optical ceramic, then the process may become unstable. This may be in part to excessive tool wear, or tool loading conditions that may not be corrected for through process parameter adjustments. To combat these issues, OptiPro is developing an ultrasonic grinding system. The ultimate goal of which would be to reduce grinding forces making the process more stable and productive.

3 Figure 3: Modes of deterministic microgrinding. a.) ring tool used for spherical grinding, b.) hemispherical ball tool for aspheric grinding, and c.) raster tool path using a toric grinding wheel for freeform optics. Background ULTRAFORM FINISHING (UFF) PLATFORM UltraForm Finishing (UFF) is a sub aperture polishing process that has been developed by OptiPro systems to polish spherical, aspheric, and free form surface geometries. It is a computer numerically controlled (CNC) deterministic process that uses measured surface data in conjunction with a measured removal function to generate a tool path. This tool path will move the UltraForm wheel across the part s surface in a controlled manner to reduce the form error of the surface being processed. UFF is capable of processing planos, spheres, aspheres, and freeform optics. The UltraForm wheel consists of a rubber wheel that may be of various diameters and hardness. This allows control of the size of the removal function and system stiffness. Wrapped on the outside of the wheel is a belt of polishing material. This belt may be bound with abrasive materials (i.e. cerium oxide, diamond), or may be of a polyurethane material. Mathematics The primary assumption that the UltraForm algorithm makes is that the material removal rate is linearly related to the dwell time when the contact pressure and relative velocity between the work piece and polishing belt remain constant. This assumption is based off of the Preston equation for polishing material removal rate [4], (1)

4 where dh/dt is the material removal rate, C p is the experimentally determined Preston, V is the relative tool and workpiece velocity, and P is the contact pressure between the tool and work piece. In the following sections, results will be provided which show that the mechanics of the UFF process correspond to Preston s equation. With the assumption of a linear material removal with time, it is possible create a linear mathematical model that represents the material removal of the tool as it traverses across the work piece per unit time by convolving a map of the tool s material removal profile. With UFF, this tool removal profile, or removal function, is generated by compressing the polishing tool in contact with a representative part. A map of the material removed from the surface is then generated by scanning the surface of the work piece with a fully integrated optical pen. Figure 4: Measured UFF Removal Function where RM is an array that represents the removal map of the polishing tool per unit time, v c is the dwell time for the polishing tool at each of the specified dwell positions, and dr is the depth of material removed at each of the specified calculation positions. Eq. 2 can then be used to correct the figure error that is present in the work piece by specifying the desired material removal profile to correct the figure error present in the work piece. Eq. 2 can then be inverted to solve for the optimized v c. In our case, additional constraints are applied to the solution of the optimum dwell map such that the machine maximum velocity and acceleration constraints are not violated. The output from this process is shown in the figures below. (2) (a) (b) (c) Figure 5: Initial Figure Error (a), Calculated Figure Error after Polishing with UFF (b), and Polishing Tool Velocity (c) Verification of this algorithm has been conducted and it was shown to accurately correct the figure error in a surface from ~0.276 λ (at 633 nm) peak to valley error to ~0.098 λ (at 633) peak to valley error. Figure 6 shows the results of this verification after 4 min correction run.

5 Figure 6: Color Map of Initial Figure Error before Correction (left) Measured Figure Error after UFF Correction (right) 4.1 Stock removal 4. RESIDUAL ERRORS The amount of stock removed on the substrate has a direct correlation to the amount of signature the RF will leave on the surface of the optic. In Figure 7, the same RF was used to calculate two different tool paths with the only difference being the amount of center thickness that will be removed. The first solve has a predicted thickness removal of 1.5 microns and the second has 10 microns of removal, the difference in the predicted surface quality of the work pieces is noticed. Figure 7: Predicted residual error of microns rms after 1.5 microns of stock removal (left) predicted residual error of microns rms after 10 microns of stock removal. 4.2 RF Geometry The geometry of the RF plays a role in the amount of residual error that will be left in the finished piece. In the first RF (Figure 8), the shape is very round and the predicted solve for that RF shows a surface of microns PV. The second RF (Figure 9) is elliptical and produces a solve that has much higher residual error. Notice that even though the removal rates only vary by -.01mm 3 /min, the difference in quality is substanial.

6 Figure 8: The RF was very round which produced a solve with microns of error after 10 microns of material removal. Figure 9: The RF was eliptycal which produced a solve with microns of error after 10 microns of material removal. 5 USF (UltraSmooth Finishing) Due to the large size of the USF tool, and the large contact zone between the tool and the optic, a high removal rate can be achieved. This increased removal rate speeds up polishing time significantly. An additional benefit due to the large size of the polishing lap is the ability to smooth out a wide range of mid-spatial frequencies that are traditionally problematic for sub-aperture polishing processes. Shown below in Figure 10 is one of the types of tools used in the USF process. Figure 10: USF Tool used on 50mm asphere

7 The USF system is capable of smoothing mid-spacial frequencies or polishing a ground asphere while maintaining a reasonable figure error. This allows the USF to be utilized in two ways. If the tool is used to pre-polish an asphere prior to UFF, this will minimize the stock that has to be removed in the UFF which will minimize residual errors. The other method is to polish an asphere that has been figure corrected in UFF but has some slight signatures that are not acceptable as shown in Figure 12. Figure 12: Large asphere polished with UFF (left); asphere smoothed with USF after UFF polishing (right) CONCLUSIONS The process of applying the strength of each technology (DMG, UFF, & USF) has led to the ability to produce aspheres faster and to a higher quality than previously demonstrated. These tests have led to a combination of UFF and USF being merged into one CNC asphere platform, the 160A. The 160A allows OptiPro to offer an asphere polishing machine that gives optical component manufactures the flexibility needed for today s fabrication challenges. At OptiPro, we are continuing to refine the asphere manufacturing process so that it is easier and faster than ever before. REFERENCES [1] Lambropoulos JC, "Surface microroughness of optical glasses under deterministic microgrinding," in Applied Optics, [2] S. Gracewski, Y. Li, Y. Zhou, P. Funkenbusch and J. Ruckman, "Relationship between microgrinding parameters and lens surface features," SPIE Vol. 3134, pp , [3] Y. Li, S. Gracewski, P. Funkenbusch and J. Ruckman, "Minimizing tool marks in deterministic microgrinding," in Current Developments in Optical Design and Engineering VII, San Diego, CA, [4] Preston, F. W., The theory and design of plate glass polishing machines, Journal of the Society of Glass Technology,