Edward Fess

Magnetorheological finishing: a deterministic process for optics manufacturing

Finish polishing of optics with magnetic media has evolved extensively over the past decade. Of the approaches conceived during this time, the most recently developed process is called magnetorheological finishing (MRF). In MRF, a magnetic field stiffens a fluid suspension in contact with a workpiece. The workpiece is mounted on the rotating spindle of a computer numerically controlled machine. Driven by an algorithm for machine control that contains information about the MRF process, the machine deterministically polishes out the workpiece by removing microns of subsurface damage, smoothing the surface to a microroughness of 10 angstroms rms, and correcting surface figure errors to less than 0.1 micrometers p-v. Spheres and aspheres can be processed with the same machine set-up using the appropriate machine program. This paper describes MRF and gives examples which illustrate the capabilities of a pre-prototype machine located at the Center for Optics Manufacturing.

Deterministic manufacturing processes for precision optical surfaces

The Center for Optics Manufacturing is developing computer-aided manufacturing technology to produce affordable high quality spherical and non-spherical optical surfaces. Advances in MRF magnetorheological finishing), a computer-controlled deterministic finishing technology, now demonstrate a capability to polish plano, spherical, aspherical, and cylindrical optics, with round or non-round apertures, to better than 0.05 wave p-v (peak-to-valley), 5.0 Å rms surface microroughness, and no subsurface damage. MRF is a paradigm shift in optics manufacturing that redefines the capabilities of the industry and will ultimately enable the affordable manufacture of any freeform optical shape.

Recent advances in aspheric and conformal grinding at the Center for Optics Manufacturing

Deterministic microgrinding (DMG) process technology, developed by the Center for Optics Manufacturing (COM) at the University of Rochester, has been extended with the development of two new computer-controlled contouring microgrinders that can produce highly accurate aspheric and conformal optical surfaces in minutes. The NanotechTM 150AG Asphere Grinder was designed and built by Moore Tool Company (Bridgeport, CT) with input from the COM Machine Technical Advisory Board. Funded by DARPA, the scope of this COM-led machine development project was to create a cost- effective, high precision machine that would deterministically microgrind aspheric optical components in brittle glass materials. A critical performance requirement dictated that the ground surfaces produced by the NanotechTM 150AG be compatible with the COM and QED-developed magnetorheological finishing (MRF) process. With the new capabilities of this grinder and the magnetorheological finishing process, COM has demonstrated a potential 10X reduction in the cost of asphere fabrication. The NanotechTM 150AG is a CNC controlled, ultra-precision machining system that is capable of deterministically generating axisymmetric aspheric optical surfaces up to 100 mm in diameter in a production environment. The contouring DMG process produces aspheric surfaces within 1 micrometer of the intended surface shape with significantly lower surface roughness and less subsurface damage than conventional grinding processes. Grinding results on a variety of glasses, crystal and polycrystalline brittle materials are reported in this paper. The NanotechTM 500FG ultra- precision Freeform Generator is the second next-generation deterministic microgrinder. Also developed under a DARPA funded program, the NanotechTM 500FG was designed and built by Moore Nanotechnology Systems, LLC with input from a COM-led Machine Technical Advisory Board. The NanotechTM 500FG is a multi-axis, deterministic microgrinding platform capable of generating non-axisymmetric and axisymmetric optical surfaces in brittle materials. This paper reports the first results from this machine.

Ultrasonic processing of hard materials for conformal optics

Hard ceramic optical materials such as sapphire, ALON, Spinel, or PCA can present a significant challenge in manufacturing precision optical components due to their tough mechanical properties. These are also the same mechanical properties that make them desirable materials when used in harsh environments. Tool wear and tool loading conditions during the grinding process for these materials can be especially problematic. Because of this, frequent dressing and reshaping of grinding wheels is often required. OptiPro systems is developing an ultrasonic grinding process called OptiSonic to minimize the forces during grinding and make the grinding process more efficient. The ultrasonic vibration of the grinding wheel allows for a grinding process that has the capacity for longer tool life and reduced tool wear for a more deterministic process. This presentation will discuss the OptiSonic process and present current results.

Developments in precision optical grinding technology

Optical systems that utilize complex optical geometries such as aspheres and freeform optics require precise control through the manufacturing process. As the preparatory stage for polishing, this is especially true for grinding. The quality of the grinding process can greatly influence the polishing process and the resultant finished product. OptiPro has performed extensive development work in evaluating components of a precision grinding machine to determine how they influence the overall manufacturing process. For example, spindle technology has a strong effect on how a grinding machine will perform. Through metrology techniques that measure the vibration characteristics of a machine and measurements of grinding forces with a dynamometer, OptiPro has also developed a detailed knowledge of how the machine can influence the grinding process. One of the outcomes of this work has led OptiPro to develop an ultrasonic head for their grinding platform to aid in reducing grinding forces. Initial results show a reduction in force by ~50%.

Deterministic processes for manufacturing conformal (freeform) optical surfaces

This paper describes the computer-controlled machines and deterministic processes developed by the Center for Optics Manufacturing (COM) at the University of Rochester to produce conformal windows and domes that have non-traditional optical surface geometry and unusual shapes. COMs DMG (deterministic microgrinding) technology produces aspheric and conformal (freeform) surfaces in minutes, versus the weeks that are required to produce the surfaces conventionally. The demonstrated techniques and equipment provide a predictable and repeatable optical production process for just about any IR, visible, or UV material. The DMG process, in concert with newly developed CNC machining equipment, typically yields 1.0 wave peak-to-valley form accuracy, 150 Angstroms rms surface finish, and subsurface damage levels low enough that some of the infrared materials do not require additional polishing.

Freeform and conformal optical manufacturing

Future optical systems are moving away from traditional spherical optics. The anticipated benefits are numerous for freeform optics as they provide better aerodynamic characteristics for aircraft, lighter weight for space missions, and smaller size for medical procedures. Currently the design and utilization of conformal and freeform shapes are costly due to the difficulties introduced with fabrication and metrology of these parts. Techniques for creating these complex optical surfaces are still in development for traditional optical materials. OptiPro has a unique opportunity create manufacturing solutions through computer controlled multi-axis optical generating, polishing, and metrology machines. OptiPro Systems is continuing to develop advanced optical manufacturing technologies. OptiPro has made toric and freeform arch shapes. OptiPro’s existing manufacturing platforms include its eSX grinding, UltraForm Finishing, and UltraSurf non-contact surface scanning system, which will be used for grinding, polishing, and measuring conformal and freeform shapes. Freeform surfaces are initially generated using deterministic micro-grinding with diamond bonded tools. Tool paths with up to five axes of simultaneous motion are required to generate and polish the optical figure of conformal surfaces. Sub-aperture corrective polishing will need to vary the amount of time the tool contacts at each location in order to remove the proper amount of material. These locations and dwell times are derived from a surface figure error map provided by OptiPro’s UltraSurf. Research and development of the freeform manufacturing process will be presented.

Advancements in asphere manufacturing

Aspheric optics can pose as a challenge to the manufacturing community due to the surface shape and level of quality required. The aspheric surface may have inflection points that limit the usable tool size during manufacturing, or there may be a stringent tolerance on the slope for mid-spatial frequencies that may be problematic for sub-aperture finishing techniques to achieve. As aspheres become more commonplace in the optics community, requests for more complex aspheres have risen. OptiPro Systems has been developing technologies to create a robust aspheric manufacturing process. Contour deterministic microgrinding is performed on a Pro80 or eSX platform. These platforms utilize software and the latest advancements in machine motion to accurately contour the aspheric shape. Then the optics are finished using UltraForm Finishing (UFF), which is a sub-aperture polishing process. This process has the capability to adjust the diameter and compliance of the polishing lap to allow for finishing over a wide range of shapes and conditions. Finally, the aspheric surfaces are qualified using an OptiTrace contact profilometer, or an UltraSurf non-contact 3D surface scanner. The OptiTrace uses a stylus to scan across the surface of the part, and the UltraSurf utilizes several different optical pens to scan the surface and generate a topographical map of the surface under test. This presentation will focus on the challenges for asphere manufacturing, how OptiPro has implemented its technologies to combat these challenges, and provide surface data for analysis.

Freeform grinding and polishing with PROSurf

Recently, the desire to use freeform optics has been increasing, including shapes such as torics and anamorphic aspheres. Freeform optics can be used to expand capabilities of optical systems. They can compensate for limitations in rotationally symmetric optics. These same traits that give freeform optics the ability to improve optical systems also makes them more challenging to manufacture. This holds true for grinding, polishing, and metrology. As freeform optics become more prevalent in the industry, tolerances will become more stringent, requiring deterministic manufacturing processes. To generate freeforms, it is crucial to have control over all aspects of the process. Controlling the surface definition is important for achieving a better surface finish during processing. Metrology will be required to adjust tool paths at various stages in manufacturing. During grinding, metrology will be used to adjust tool positions relative to the nominal tool path to compensate for repeatable machine and tooling error. For polishing, metrology will be used to deterministically adjust dwell relative to the amount of the error in different surface locations, allowing for convergence towards the desired surface at a uniform rate. OptiPro has developed PROSurf, a CAM software package for creating freeform tool paths and applying metrology-based corrections. The software can be used for both grinding and polishing freeform optics. The software has flexibility to allow for different methods of modelling the surface: mathematical equations, solid models, and point clouds. The software is designed to make it easier to manufacture and polish complex freeform optics.

Non-contact metrology of aspheres and windows of large departure

The measurement of large departure aspheres and windows is a challenge for the optics community. OptiPro systems has developed a non-contact measuring system called UltraSurf to overcome these difficulties. The UltraSurf system utilizes a single point non-contact sensor coupled with high accuracy air bearings to scan optical surfaces. Five air bearing axes allow for the optical probe to maintain normal angles with the surface under test, and provide a smooth and accurate scan. The axes of motion allow scanning of rotationally symmetric parts such as spheres and aspheres, but also give it the freedom to perform areal surface scanning and freeform metrology. By maintaining a tangent angle with the surface, this technique allows for large surface slopes and deviation from best fit sphere to easily be measured. Several commercial non-contact sensors have been integrated into UltraSurf. The sensors operate with different optical principles, allowing for greater flexibility of the types of surfaces to be measured. One sensor applies white-light confocal chromatic aberration for high resolution, single surface measurement. Another sensor that uses low-coherence interferometry with a 1310 nanometer light source is able to see through materials, enabling multiple surface and thickness measurements simultaneously. Measurement of large departure aspheres and windows will be demonstrated. Cross comparison of UltraSurf data with current metrology techniques will be shown on surfaces that can be measured with multiple methods.