Optical systems are often athermalized over large temperature ranges through the proper choice of glasses and mounting materials. However, variations in the coefficients of thermal expansion (CTE) and thermo-optical coefficients that govern thermal behavior are seldom included in the tolerance analysis. Manufacturers rarely provide these material tolerances and we can only account for their effects through custom macros in lens design software. We demonstrate that a first-order sensitivity analysis on the change in focus position at each environmental condition accurately predicts the degradation of the system performance. We verified this correlation by creating a custom catalog of identical glasses with perturbed thermal parameters and evaluating the RMS wavefront error for each material substitution.
In the design of optical assemblies, emphasis is placed on tolerancing the surface irregularity, which is a driving factor in price and manufacturing prices and time during polishing. Quite often, the default irregularity tolerance in modeling software is assumed to be a 50:50 split between astigmatism and 3rd order spherical aberration (i.e. symmetric zonal errors). In this paper, we reviewed the irregularity of over 1,000 custom fabrication optical surfaces. We looked at the relationship between the spherical and astigmatism aberrations and found generally that a surface will be either astigmatic or spherical, but rarely a mixture of the two. We also looked at the PV and rms of the surfaces and how that compares to the model and the general knowledge. One striking result of our analysis came from a closer analysis of how the optical modeling software package handles ‘power’ errors in the irregularity tolerance. It is possible that there is a mismatch between the model and the optical manufacturer.
As aircraft design advances, the requirements on the aerodynamics of such aircraft become much more stringent. The use of conformal windows on such aircraft enables better aerodynamics by eliminating the need for protruding sensor packages, thereby reducing the aerodynamic drag across the airframe. However, manufacturing conformal windows to optical specifications is difficult due to the extreme freeform shapes required for these aerodynamic design forms. Optimax has developed a robotic form correction platform designed specifically to manufacture extreme freeform optical surfaces over large areas.
This presentation will outline the manufacturing process by which a 330 mm square conformal window was created with 123 mm of sag, 98 mm departure from a best fit sphere, and a nominal 23 mm thickness. The surface specification was given as <0.5 μm RMS surface irregularity over the clear aperture. Fiducials were designed into the part to ensure accurate form generation and measurement.
Both sides were generated and the fiducials created using a commercial 5-axis ultrasonic grinding machine. The initial fiducials were determined to be inadequate and resulted in excessive and unrepeatable tilt in subsequent measurements of the generated surfaces, new fiducials were created, resulting in tilt less than the measurement uncertainty of the CMM used for the metrology. After generation, a custom robotic form correction platform based off of an industrial 6 degree of freedom robotic arm was used to polish both surfaces.
Systems like TESS require specialized components that challenge all involved. These systems consist of many sub-components, but we are focused on the refractive and reflective optical components. One purpose of this talk is to introduce optical system designers to application specific manufacturing processes. We, as manufacturers, need to tailor our processes for the optic’s specific operational environment. In addition, we want to introduce some of our more unique manufacturing capabilities to allow system designers to widen their design space. It is now feasible to manufacture a wide range of sizes, shapes, and materials for many different applications.
Additive manufacturing, or 3D printing, has become widely used in recent years for the creation of both prototype and end-use parts. Because the parts are created in a layer-by-layer manner, the flexibility of additive manufacturing is unparalleled and has opened the design space to enable features like undercuts and internal channels which cannot exist on traditional, subtractively manufactured parts. This flexibility can also be leveraged for optical applications. This paper outlines some of the current uses of 3D printing in the optical manufacturing process at Optimax. Several materials and additive technologies are utilized, including polymer printing through fused deposition modeling, which creates parts by depositing a softened thermoplastic filament in a layerwise fashion. Stereolithography, which uses light to cure layers of a photopolymer resin, will also be discussed. These technologies are used to manufacture functional prototypes, fixtures, sealed housings, and other components. Additionally, metal printing through selective laser melting, which uses a laser to melt metal powder layers into a dense solid, will be discussed due to the potential to manufacture thermally stable opticalmechanical assembly frameworks and functional optics. Examples of several additively manufactured optical components will be shown.
Freeform and conformal optics have the potential to dramatically improve optical systems by enabling systems with fewer optical components, reduced aberrations, and improved aerodynamic performance. These optical components differ from standard components in their surface shape, typically a non-symmetric equation based definition, and material properties. Traditional grinding and polishing tools are unable to handle these freeform shapes. Additionally, standard metrology tools cannot measure these surfaces. Desired substrates are typically hard ceramics, including poly-crystalline alumina or aluminum oxynitride. Notwithstanding the challenges that the hardness provides to manufacturing, these crystalline materials can be highly susceptible to grain decoration creating unacceptable scatter in optical systems. In this presentation, we will show progress towards addressing the unique challenges of manufacturing conformal windows and domes. Particular attention is given to our robotic polishing platform. This platform is based on an industrial robot adapted to accept a wide range of tooling and parts. The robot’s flexibility has provided us an opportunity to address the unique challenges of conformal windows. Slurries and polishing active layers can easily be changed to adapt to varying materials and address grain decoration. We have the flexibility to change tool size and shape to address the varying sizes and shapes of conformal optics. In addition, the robotic platform can be a base for a deflectometry-based metrology tool to measure surface form error. This system, whose precision is independent of the robot’s positioning accuracy, will allow us to measure optics in-situ saving time and reducing part risk. In conclusion, we will show examples of the conformal windows manufactured using our developed processes.
Sapphire presents many challenges to optical manufacturers due to its high hardness and anisotropic properties. Long lead times and high prices are the typical result of such challenges. The cost of even a simple 'grind and shine' process can be prohibitive. The high precision surfaces required by optical sensor applications further exacerbate the challenge of processing sapphire thereby increasing cost further. Optimax has demonstrated a production process for such windows that delivers over 50% time reduction as compared to traditional manufacturing processes for sapphire, while producing windows with less than 1/5 wave rms figure error.
Optimax's sapphire production process achieves significant improvement in cost by implementation of a controlled grinding process to present the best possible surface to the polishing equipment. Following the grinding process is a polishing process taking advantage of chemical interactions between slurry and substrate to deliver excellent removal rates and surface finish. Through experiments, the mechanics of the polishing process were also optimized to produce excellent optical figure. In addition to reducing the cost of producing large sapphire sensor windows, the grinding and polishing technology Optimax has developed aids in producing spherical sapphire components to better figure quality.
In addition to reducing the cost of producing large sapphire sensor windows, the grinding and polishing technology Optimax has developed aids in producing spherical sapphire components to better figure quality. Through specially developed polishing slurries, the peak-to-valley figure error of spherical sapphire parts is reduced by over 80%.
Freeform optical surfaces, which have little to no symmetry, are gaining popularity with lens designers and optical system integrators. These freeform optics can enhance optical performance by reducing the number of elements, enabling lighter and more efficient systems, and by reducing aberrations. Most traditional manufacturing techniques cannot yield polished freeform optical surfaces. Likewise, traditional metrology equipment has difficulty accurately measuring the deviation of freeform surfaces from their nominal shape, the surface form error. The inclusion of mechanical fiducials on freeform optics can ease some of this difficulty. Well placed fiducials can provide alignment features for contact metrology equipment such as coordinate measuring machines (CMMs). Mechanical fiducials allow for precise locating of all surfaces of an optic in reference to one another and provide a reference from which the freeform surface can be measured against. This allows a CMM to output the deviation of a surface from its nominal shape, as well as the wedge and center thickness of the optic. Alignment fiducials also enable more precise locating of the surface during the manufacturing process, shortening the time required for production and lowering the cost of fabricating freeform optics. This paper will explore the advantages of including locating fiducials on freeform optics and how they can aid in the manufacture and measurement of optical surfaces.
The blistering pace of recent technological advances has led lens designers to rely increasingly on freeform optical components as crucial pieces of their designs. As these freeform components increase in geometrical complexity and continue to deviate further from traditional optical designs, the optical manufacturing community must rethink their fabrication processes in order to keep pace. To meet these new demands, Optimax has developed a variety of new deterministic freeform manufacturing processes. Combining traditional optical fabrication techniques with cutting edge technological innovations has yielded a multifaceted manufacturing approach that can successfully handle even the most extreme freeform optical surfaces.
In particular, Optimax has placed emphasis on refining the deterministic form correction process. By developing many of these procedures in house, changes can be implemented quickly and efficiently in order to rapidly converge on an optimal manufacturing method. Advances in metrology techniques allow for rapid identification and quantification of irregularities in freeform surfaces, while deterministic correction algorithms precisely target features on the part and drastically reduce overall correction time. Together, these improvements have yielded significant advances in the realm of freeform manufacturing. With further refinements to these and other aspects of the freeform manufacturing process, the production of increasingly radical freeform optical components is quickly becoming a reality.
With the ongoing advancements in aspheric manufacturing and metrology, companies have to overcome processing challenges and from time to time learn costly lessons along the way. Optimax Systems, Inc., a leader in quick delivery prototype optics, has been manufacturing aspheric lenses for over 20 years. Along the way, we have learned many lessons, some the hard way. In this paper, I will share a few stories of how aspheres have humbled us, how we overcame the problem, and provide takeaways for other manufactures and designers.
Freeform optical shapes or optical surfaces that are designed with non-symmetric features are gaining popularity with lens designers and optical system integrators. Tolerances on a freeform optical design influence the optical fabrication process. Case studies and soft tolerance limits for easier fabrication will be discussed. This paper will also give a high level overview of a freeform optical fabrication process that includes generation, high speed VIBE polishing, sub-aperture figure correction and testing of freeform surfaces.
The evolution from spherical, to aspheric, to freeform optics is quickly progressing towards more complex freeform surfaces. Freeform surfaces typically have little to no symmetry making the alignment of the surfaces difficult. The alignment of such freeform surfaces relative to the other features on the optic has been little considered. A typical alignment specification like wedge (edge thickness difference) is not well defined for freeform optics, nor is the wedge measurement. We show that by using fiducials during the manufacturing of freeform surfaces, the alignment and locating of the freeform surface can be specified and measured.
Freeform surfaces on optical components have become an important design tool for optical designers. Non-rotationally symmetric optical surfaces have made solving complex optical problems easier. The manufacturing and testing of these surfaces has been the technical hurdle in freeform optic’s wide-spread use. Computer Numerically Controlled (CNC) optics manufacturing technology has made the fabrication of optical components more deterministic and streamlined for traditional optics and aspheres. Optimax has developed a robust freeform optical fabrication CNC process that includes generation, high speed VIBE polishing, sub-aperture figure correction, surface smoothing and testing of freeform surfaces. Metrology of freeform surface is currently achieved with coordinate measurement machines (CMM) for lower resolution and interferometry with computer generated holograms (CGH) for high resolution irregularity measurements.
Freeform optical shapes or optical surfaces that are designed with non-symmetric features are gaining popularity with lens designers and optical system integrators. This enabling technology allows for conformal sensor windows and domes that provide enhanced aerodynamic properties as well as environmental and ballistic protection. In order to provide ballistic and environmental protection, these conformal windows and domes are typically fabricated from hard ceramic materials which challenge the optical fabricator. The material hardness, polycrystalline nature and non-traditional shape demand creative optical fabrication techniques to produce these types of optics cost-effectively. This paper will overview a complete freeform optical fabrication process that includes ultrasonic generation of hard ceramic surfaces, high speed VIBE polishing, sub-aperture figure correction of polycrystalline materials, finishing and final testing of freeform surfaces. This paper will highlight the progress made to each of the processes as well as the challenges associated with each of them specifically focusing on the use of fiducials in the manufacturing and measurement process and the adaptation of stitching interferometry to the measurement of a freeform conformal window.
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.
Freeform applications are growing and include helmet-mounted displays, conformal optics (e.g. windows integrated into airplane wings), and those requiring the extreme precision of EUV. These non-rotationally symmetric surfaces pose challenges to optical fabrication, mostly in the areas of polishing and metrology. The varying curvature of freeform surfaces drives the need for smaller, more “conformal”, tools for polishing and reference beams for interferometry. In this paper, we present fabrication results of a high-precision freeform surface. We will discuss the total manufacturing process, including generation, pre-polishing, MRF®, and metrology, highlighting the capabilities available in today’s optical fabrication companies.
Conformal windows pose new and unique challenges to manufacturing due to the shape, measurement of, and requested hard polycrystalline materials. Their non-rotationally symmetric shape and high departure surfaces do not lend themselves to traditional optical fabrication processes. The hard crystalline materials are another challenge due to increased processing time and possibility of grain decoration. We have developed and demonstrated a process for manufacturing various conformal windows out of fused silica, glass, zinc-sulfide multispectral, and spinel. The current process involves CNC generation/grinding, VIBE polishing, and sub-aperture figure correction. The CNC generation step incorporates an ultrasonic assisted grinding machine; the machine settings and tool are being continuously optimized for minimal sub-surface damage and surface form error. In VIBE, polishing to less than 5 nm rms surface roughness while maintaining overall form error is accomplished with a full aperture conformal polishing tool and with rapid removal rates. The final sub-aperture polishing step corrects the overall form error. Currently we utilize our CMM for surface form measurement and have shown that we can produce spinel conformal windows with form error within ±10 micrometers of the nominal shape, without grain decoration. This conformal window manufacturing process is continuously optimized for cost reduction and precision of the final optic.
For over 100 years, optical imaging systems were limited to rotationally symmetric lens elements, due to limitations in processing optics. However, the present rapid development and application of CNC machines has made fabrication of non-rotationally symmetric lenses, such as freeform surfaces, economical. The benefit of using freeform surfaces is that the lens designer has more flexibility to create innovative 3D imaging packages, while correcting for aberrations. This report details capabilities at Optimax for manufacturing freeform surfaces, with a specific example towards creation of freeform ZnS-multispectral optics for application as a corrector element. In addition to fabricating freeform optics, advances have been made in producing smooth surfaces on polycrystalline materials. In the past, achieving a smooth surface on polycrystalline materials during sub-aperture polishing has proven challenging, because of a phenomenon called grain highlighting. Significant progress has been made at Optimax in this field through utilization of proprietary pads, slurries, and processes.
Freeform optical shapes or optical surfaces that are designed with non-symmetric features are gaining popularity with lens designers and optical system integrators. This enabling technology allows for conformal sensor windows and domes that provide enhanced aerodynamic properties as well as environmental and ballistic protection. In order to provide ballistic and environmental protection, these conformal windows and domes are typically fabricated from hard ceramic materials. Hard ceramic conformal windows and domes provide two challenges to the optical fabricator. The material hardness, polycrystalline nature and non-traditional shape demand creative optical fabrication techniques to produce these types of optics cost-effectively. This paper will overview a complete freeform optical fabrication process that includes ultrasonic generation of hard ceramic surfaces, high speed VIBE polishing, sub-aperture figure correction of polycrystalline materials and final testing of freeform surfaces. This paper will highlight the progress made to each of the processes as well as the challenges associated with each of them.
Many aspheres and free-form optical surfaces are measured using a single line trace profilometer which is limiting
because accurate 3D corrections are not possible with the single trace. We show a method to produce an accurate fully
2.5D surface height map when measuring a surface with a profilometer using only 6 traces and without expensive
hardware. The 6 traces are taken at varying angular positions of the lens, rotating the part between each trace. The output
height map contains low form error only, the first 36 Zernikes. The accuracy of the height map is ±10% of the actual
Zernike values and within ±3% of the actual peak to valley number. The calculated Zernike values are affected by errors
in the angular positioning, by the centering of the lens, and to a small effect, choices made in the processing algorithm.
We have found that the angular positioning of the part should be better than 1, which is achievable with typical
hardware. The centering of the lens is essential to achieving accurate measurements. The part must be centered to within
0.5% of the diameter to achieve accurate results. This value is achievable with care, with an indicator, but the part must
be edged to a clean diameter.
A Gaussian model of the radius measurement of micro-optics has been developed and tested using simulations. The model is based on the propagation distances in the interferometer, a heretofore uninvestigated effect. The goal of the model is to determine the bias error in the radius due to the Gaussian Beam propagation model. After testing the model with varying conditions, we have concluded the following: the measured part is smaller than the input, the cat's eye and confocal positions have approximately the same error, radius error increases with smaller test parts, decreasing the numerical aperture increases the errors, and the propagation distances do not affect the radius. The outline of the experimental plan to be used to verify the results is given.
Under specific conditions, light will undergo a phase change upon reflection. This phase change affects the results of
many precision dimensional measurements. The phase change on reflection was investigated at a glass-metal interface
using samples with evaporated metal strips on the back surface of a wedged glass substrate. The samples were measured
on a phase shifting interferometer and the phase change was calculated from the apparent measured step heights from the
internal reflection at the back glass-metal interface. The background subtraction process was the largest contributor to
the phase change uncertainty. The phase change was measured for gold, copper and aluminum at normal incidence and
for gold at varying angle of incidence. The measured phase change for gold, copper, and aluminum is 131.4° ±3.8°,
173.7° ±3.8°, and 200.7° ±3.8°, respectively. The measured phase change for gold at varying incident angle is shown.
In addition, the effect of the phase change on the radius measurement was investigated. A bias in the radius
measurement due to the phase change was found that is interferometer dependent.
We have focused on measurement needs for micro-refractive lenses and have developed a flexible and compact micro-interferometer that can be used to measure lens radius of curvature and form errors. Transmitted wavefront and back focal length measurements can be easily added to the instrument. This instrument addresses measurement needs for micro refractive lenses. The interferometer is based on a Mitutoyo metallurgical microscope and operates with a 633 nm helium neon source. The radius of curvature measurement is directly traceable, meaning an external artifact is not required for calibration. This requires a careful mechanical design, a detailed alignment procedure with estimates of alignment uncertainties, and stage error motion characterization with estimates of uncertainties. The instrument can also be used to measure some diffractive components and mold form errors. We describe the instrument in this paper and discuss design goals and measurement specifications.