As freeform and conformal optics are becoming more broadly used in the optics industry, the manufacturing technology and processing conventions need to be developed to meet the requirements and challenges posed in the manufacturing of these parts. These optics reduce the number of total optics required in an assembly in addition to overcoming the limitations in rotationally symmetric optics. While these optics offer many benefits for system capabilities, these traits also make manufacturing and metrology more difficult. In order to manufacture these complex optics, OptiPro has been focusing on development of the software, hardware, and processing techniques required to meet the stringent part tolerances. For software, OptiPro has been working on the continual development of PROSurf, a CAM package geared specifically toward freeform optics, and the UltraSurf software, which controls OptiPro’s non-contact metrology system. For hardware, the main area of focus has been on fixture development: how should the part be held so that it can be transferred between machines repeatably while still providing adequate support during processing and access to the alignment features. One of the focus areas of processing improvement has been on developing the conventions for the surface datums used for locating the surface and referencing analyzed data against. This paper will present the challenges in freeform manufacturing along with the solutions that we have developed to meet these manufacturing needs.
Freeform optical components are becoming more prevalent in the precision optics industry. Metrology of freeform surfaces is traditionally limited to coordinate measuring machines, or computer-generated holograms. OptiPro has developed UltraSurf, a non-contact metrology platform that has been adapted to measuring freeform surfaces. The non-contact probing method, and multiple axes of motion allow UltraSurf to scan freeform optical surfaces and the associated datum structures. Datum structures define a coordinate system that all the freeform surfaces of a component can be referenced for metrology and manufacturing. Iterative corrective grinding and polishing is dependent on accurately locating the form error map to the surface within the machine. Measuring the datums along with the surface provides an absolute reference for the form error map. Allowing an operator to correct form error with confidence that the machine will grind or polish in the correct location. Datum and optical surface metrology together will allow referencing multiple freeform surfaces to each other, which is critical on transmissive freeform systems. Practical examples of non-contact freeform metrology with the UltraSurf will be presented along with considerations of datum features.
OptiPro Systems, LLC has been recognized as a U.S. leader in computer-controlled equipment for manufacturing high precision optical components found in medical, military, aerospace, night vision and many other industrial and commercial applications. OptiPro’s success in winning several Small Business Innovative Research (SBIR) grants through the Navy, NASA and the Army have led to the commercialization of new machines, software and processes for grinding, polishing and measuring complex optics such as windows and domes. OptiSonic machines incorporate the latest in ultrasonic machining technology by adding 20-40 kHz of adaptive oscillation to the grinding tool, which reduces force on the tool and part during processing. OptiSonic machines have proven to increase processing speed and precision when grinding various hard optical materials that are ideal for hypersonic applications. UltraForm Finishing (UFF) machines are capable of deterministically finishing windows, domes, and other complex optical elements through a patented sub-aperture polishing technique. This method involves a moving belt of polishing material wrapped around a precision compressive wheel. UltraSurf machines deliver ultra-precision non-contact metrology though the utilization of high-quality machine components and the latest non-contact probe technologies. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset. Finally, our PROSurf freeform computer-aided manufacturing (CAM) software is used to develop advanced toolpaths for each of these platforms. We will present an overview of each of these technologies and discuss how they are advancing the precision optics manufacturing world.
Advancements in optical manufacturing technology allow optical designers to implement steep aspheric or high departure surfaces into their systems. Measuring these surfaces with profilometers or CMMs can be difficult due to large surface slopes or sharp steps in the surface. OptiPro has developed UltraSurf to qualify the form and figure of steep aspheric and diffractive optics. UltraSurf is a computer controlled, non-contact coordinate measuring machine. It incorporates five air-bearing axes, linear motors, high-resolution feedback, and a non-contact probe. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset. Multiple probe technologies are available on UltraSurf. Each probe has strengths and weaknesses relative to the material properties, surface finish, and figure error of an optical component. The measuring probes utilize absolute distance to resolve step heights and diffractive surface patterns. The non-contact scanning method avoids common pitfalls with stylus contact instruments. Advancements in measuring speed and precision has enabled fast and accurate non-contact metrology of diffractive and steep aspheric surfaces. The benefits of data sampling with twodimensional profiles and three-dimensional topography maps will be presented. In addition, accuracy, repeatability, and machine qualification will be discussed with regards to aspheres and diffractive surfaces.
Metrology of freeform shapes has traditionally been difficult, especially at the sub-micron level. Sub-aperture polishing techniques and diamond turning allow optical designers to incorporate freeform surfaces into their systems. Contact measuring systems typically lack the accuracy or resolution required for optical qualification and can potentially damage the surfaces. Interferometric systems are unable to handle high spherical departures and may require complicated lateral calibration to generate feedback for deterministic grinding and polishing. OptiPro has developed UltraSurf, a noncontact coordinate measuring machine to determine the form, figure, and thickness of freeform optics. We integrated several non-contact sensors that acquire surface information through different optical principles. Each probe has strength and weaknesses relative to an optic’s material properties, surface finish, and figure error. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset. Incorporating datums from mechanical prints into the non-contact measuring method is especially important for freeform surfaces. UltraSurf has the ability to measure a wide range of surface roughness and has the degrees of freeform needed to scan datums and surfaces. The metrology method of UltraSurf and the non-contact probes will be presented. Form, figure, and thickness data will highlight the capabilities of UltraSurf to measure freeform surfaces.
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. <p> </p>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. <p> </p>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.
Aspheric cylinders have the ability to improve optical performance over standard cylindrical surfaces. Over the last several years there has also been development into the application and functionality of utilizing freeform surfaces to improve optical performance. Freeforms have the ability to not only improve image quality over a greater field of view, but can open up the design space of an optical system making it more compact. Freeform geometries, much like aspheric cylinders, may not have an axis of rotation to spin the optic about during manufacturing. This leads to costly fabrication processes and custom metrology set ups, which may inhibit their use.<p> </p> Over the last several years, OptiPro Systems has developed and optimized our eSX grinding, UFF and USF polishing, UltraSurf metrology, and ProSurf software programming technologies to make the processing of these complex geometries much easier and deterministic. In this paper we will discuss the challenges associated with manufacturing complex shapes like aspheric cylinders as well as freeform geometries, and how several technologies working together can overcome them. The technologies focus on metrology feedback to a grinding and polishing machine that is controlled through an iterative computer aided manufacturing software system. We will also present examples of these hard to manufacture shapes with results.
Advancements in optical manufacturing technology allow optical designers to implement steep aspheric or high departure surfaces into their systems. Accurate metrology during the grinding and polishing stages of asphere manufacturing will reduce time and cost. Measuring these surfaces with common interferometers or profilometers can be difficult due to large surface slopes or unpolished surface texture. OptiPro has developed UltraSurf to qualify the form, figure, and thickness of steep aspheric and freeform optics. UltraSurf is a computer controlled, non-contact coordinate measuring machine. It incorporates five air-bearing axes, linear motors, high-resolution feedback, and a non-contact probe. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset. There are multiple probe technologies available on UltraSurf, and each probe has strengths and weaknesses relative to the material properties, surface finish, and figure error of an optical component. Validation of the system accuracy, repeatability, and methodology must be performed to trust the measurement data. Form and figure maps of a flat, a sphere, and an asphere using UltraSurf will be presented with comparisons to interferometry. In addition, accuracy, repeatability, and machine qualification will be discussed.
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. Premature tool wear or tool loading during the grinding process is a common result of these tough mechanical properties. Another challenge is the requirement to create geometries that conform to the platforms they reside in, but still achieve optical window tolerances for wavefront. These shapes can be complex and require new technologies to control sub aperture finishing techniques in a deterministic fashion. In this paper we will present three technologies developed at OptiPro Systems to address the challenges associated with these materials and complex geometries. The technologies presented will show how Ultrasonic grinding can reduce grinding load by up to 50%, UltraForm Finishing (UFF) and UltraSmooth Finishing (USF) technologies can accurately figure and finish these shapes, and how all of them can be controlled deterministically, with utilizing metrology feedback, by a new Computer Aided Manufacturing (CAM) software package developed by OptiPro called ProSurf.
Advancements in optical manufacturing technology allow optical designers to implement freeform and conformal shapes in their systems. Metrology of the shapes has traditionally been difficult, especially at the sub-micron level. Contact measuring systems typically lack the accuracy required for optical qualification and can damage the surface. Interferometric systems are unable to handle high spherical departures and may require complicated lateral calibration to generate feedback for deterministic grinding and polishing. OptiPro has developed UltraSurf, a noncontact coordinate measuring machine to determine the form, figure, and thickness of freeform and conformal optics. We integrated several non-contact sensors that acquire surface information through different optical principles. Each probe has strength and weaknesses relative to an optic’s material properties, surface finish, and figure error. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset. Measurements of freeform and conformal shapes will be presented. The scanning method of UltraSurf and the non-contact probes will also be shown. The form, figure, and thickness data will highlight the capabilities of UltraSurf to measure freeform surfaces. Comparisons between accuracy and measureable surface departure will be made with current metrology systems such as coordinate measuring machines, interferometers, and profilometers. Additionally, methods for defining a freeform or conformal surface for metrology analysis and manufacturing will be discussed.
Recently, the desire to use freeform optics has been increasing. Freeform optics can be used to expand the capabilities of optical systems. 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, and, as freeform optics become more prevalent in the industry, tolerances will become more stringent. OptiPro Systems has developed a method of deterministic freeform polishing to be used with its UltraForm Finishing (UFF) process. This method uses the error map of the surface to determine the appropriate feed rates for removing a portion of the error from the surface of the optic. The material removed varies across the surface of the optic to allow for the error to decrease across the surface at a uniform rate. The flexibility of this method allows for the deterministic polishing of surfaces that can be mathematically modeled. In addition to deterministic polishing, OptiPro is also developing a software package for generating freeform tool paths. This software can be used for both grinding and polishing freeform optics. It has the ability to generate the freeform tool paths for deterministic polishing. This software will make is easier to manufacture and polish complex freeform surfaces.
Manufacturing aspheric optics can present challenges depending on the complexity of their shape. This is especially true during the finishing stage. To tackle this challenge, OptiPro Systems has developed two technologies for deterministic optical polishing: UltraForm Finishing (UFF) and UltraSmooth Finishing (USF). UFF is a deterministic sub aperture polishing process that polishes spherical, aspheric, and free form surface geometries. In contrast, the USF process is a deterministic mid to large size aperture polishing process that works with a conforming lap. These two technologies have the ability to tackle a wide range of optical shapes by removing sub-surface damage, removing various mid-spatial frequency artifacts that might be left from a grinding process, and correct the optic’s figure error in a controlled fashion. This presentation will describe these technologies, present performance information as to their capabilities, and show how OptiPro is developing these technologies to push the state of the art in manufacturing.
We have performed a round-robin study of surface irregularity measurements of a free-form toroidal window. The measurement tools were a Leitz scanning CMM at Optimax Systems, Inc., an UltraSurf, a non-contact measuring system at OptiPro Systems, a Zeiss scanning CMM at OptiPro Systems, a F25 micro-CMM at Carl Zeiss Industrial Metrology, and an ASI(Q)™ at QED Technologies. Each instrument resulted in a 2.5D surface error map. The measurements were compared with multiple analysis settings. The different analysis settings removed some low frequency height errors, which varied amongst the measurements. This highlights the need for more study to determine the reasons for the differences in the low frequency errors. With the low frequency errors removed, the measurements compared very well, to within 0.2 μm rms.
Metrology of asphere surfaces is critical in the precision optics industry. Surface metrology serves as feedback into deterministic grinding and polishing platforms. Many different techniques and devices are used to qualify an asphere surface during fabrication. A contact profilometer is one of the most common measurement technologies used in asphere manufacturing. A profilometer uses a fine stylus to drag a diamond or ruby tip over the surface, resulting in a high resolution curved profile. Coordinate measuring machines (CMM) apply a similar concept by touching the optic with a ruby or silicon carbine sphere. A CMM is able to move in three dimensions while collecting data points along the asphere surface. Optical interferometers use a helium-neon laser with transmission spheres to compare a reflected wavefront from an asphere surface to a reference spherical wavefront. Large departure aspheres can be measured when a computer generated hologram (CGH) is introduced between the interferometer and the optic. OptiPro Systems has developed a non-contact CMM called UltraSurf. It utilizes a single point non-contact sensor, and high accuracy air bearings. Several different commercial non-contact sensors have been integrated, allowing for the flexibility to measure a variety of surfaces and materials. Metrology of a sphere and an asphere using a profilometer, CMM, Interferometer with a CGH, and the UltraSurf will be presented. Cross-correlation of the measured surface error magnitude and shape will be demonstrated. Comparisons between the techniques and devices will be also presented with attention to accuracy, repeatability, and overall measurement time.
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
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.
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.
There is a need for precisely figured large sapphire windows with dimensions of up to 20 inches with thicknesses of 0.25 inches that will operate in the 1- to 5-micron wavelength range. In an effort to reduce manufacturing cost during grinding and polishing, OptiPro Systems is developing technologies that provide an optimized deterministic approach to making them. This development work is focusing on two main areas of research. The first is optimizing existing technologies, like deterministic microgrinding and UltraForm Finishing (UFF), for shaping operations and precision controlled sub-aperture polishing. The second area of research consists of a new large aperture deterministic polishing process currently being developed at OptiPro called UltraSmooth Finishing (USF). The USF process utilizes deterministic control with a large aperture polishing tool. This presentation will discuss the challenges associated with manufacturing large sapphire windows and present results on the work that is being performed to minimize manufacturing costs associated with them.
<p> 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. </p>
<p> 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. </p><p> 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. </p>
OptiPro Systems has been developing the UltraSurf, a non-contact measuring system using state of the art, precision
motion control. The goal is to precisely scan standard optical shapes such as concave and convex spherical surfaces, as
well as the complex geometries of aspheric, ogive, and freeform shapes without the limitations associated with other
measurement methods. Common optical measurement methods have limitations with surface roughness, slope error, and
deviation from best-fit sphere. Optipro designed the UltraSurf to further the manufacturing capabilities of companies
generating complex precision optics.
The UltraSurf measures with sub-micrometer non-contact point sensors to collect surface information. Various sensors
are commercially available from multiple companies, each with their own distinct optical measuring technology. One
optical sensor uses white light confocal chromatic imaging to measure individual optical surfaces. Another optical sensor
uses low-coherence interferometry with a near infrared laser, and is able to measure the inside, outside, and thickness of
optical materials at a single point.
The UltraSurf scans the optical sensors over the surface of the part under test, keeping it normal to the surface. The
single point measuring method coupled with computer-controlled motion gives the UltraSurf flexibility to measure
greatly varied geometries. Ultimately, a point cloud of the measured surface is generated. The cloud can be used to
calculate deviation from the desired shape, as well as various surface parameters. Applications, definitions, and
measurement results of freeform and conformal shapes using UltraSurf will be presented.
UltraForm Finishing (UFF), OptiPro Systems' five axis sub-aperture polishing machine has evolved from an initial
prototype into a robust aspheric manufacturing system that can rapidly produce finished aspheres directly from a ground
surface. UFF utilizes a belt of polishing material 50" long supported by a polyurethane wheel to polish a wide variety of
materials ranging from traditional glasses to IR materials. This belt polishing system provides a tuned stiffness that is
capable of conforming to the polishing surface without replicating the surface roughness. When combined with state of
the art figure correction algorithms, the UFF is capable of robust and deterministic figure correction for aspheric
Recently, OptiPro Systems has expanded the capability of the UFF to include deep concave ogive and free-form
surfaces. Although these types of surfaces can be beneficial from an optical or aerodynamic standpoint they pose
additional challenges both from their steep geometry as well as from a polishing tool path perspective. A brief
description of these challenges as well as possible solutions to these problems will be presented. In addition, the current
figure correction capability of the system utilizing feedback from OptiPro's five axis non-contact free-form metrology
system will be presented.
OptiPro Systems has developed a robust 5-axes computer controlled platform, for implementation of the sub-aperture
UltraForm Finishing (UFF) process specifically focused on finishing AlON, spinel and transparent polycrystalline
alumina (PCA) steep concave, convex and ogive shaped infrared domes and aspheres. Traditional manufacturing of
optical components typically involves a three-stage process: grinding, lapping and polishing. The lapping and polishing
stages are focused at reducing the surface roughness while preserving the integrity of the form acquired during grinding.
Polishing of non spherical and irregular shapes is nearly impossible using traditional full aperture techniques. However,
finishing these non-spherical and irregular shapes is possible using UltraForm Finishing.
A brief description of the evolution of the UltraForm hardware and processes will be presented, with the current
hardware developments. A review of the results with regard to form/figure and roughness improvements on glass, AlON
and transparent PCA will be presented using a variety of grinding and finishing abrasives. Differences in the abrasive
materials, some bound, and others loose in a slurry have a large impact on the process cycle time and resultant surface
OptiPro Systems is developing a non-contact measurement system using state of the art motion control while minimizing the axes of motion during the measurement. The goal is to precisely scan concave and convex surfaces of aspheric, deep parabolic, and ogive shapes without the limitations associated with other measurement methods. The metrology systems will use different computer controlled slicing techniques to create a topographical surface map of the surface form with a high accuracy non-contact probe.
To achieve this precise scan the measurement system will incorporate sub-micrometer precision air bearings for the linear and rotary axes motion to minimize the effect of non-repeatable mechanical errors. Calibration of the measurement system will use high precision reference spheres. Finite element modeling and estimate has been used to predict and possibly compensate for mechanical flexures.
OptiPro has built a "breadboard" measurement system using a Professional Instruments air bearing and a STIL white light measurement pen. The results from the measurement of a near full hemispheric dome measurement will be presented as well as a comparison to the same dome measured using a stitching interferometer. The final system will incorporates complete computer controlled axes requiring as little operator training and set up as possible. The prototype system will utilize a non-contact pen for measurement. Current developments include the utilization of the STIL white light pen and the OptiGauge optical probe which utilizes invisible 1310 nm infrared light. The current system design and performance will be presented.
The primary focus of the course is to explain the methodology of measuring precision freeform and conformal optics. The goal of the course is to provide a basic understanding of current industrial techniques for freeform metrology. Special consideration is given to datum and fiducial structures, along with associated International Organization for Standardization (ISO) standards. The course will consist of classroom lecture, followed by application of concepts at OptiPro’s booth in the exhibition hall.