Marc Tricard

Magnetorheological Jet (MR JetTM) Finishing Technology

Conformal (or free form) and steep concave optics are important classes of optics that are difficult to finish using conventional techniques due to mechanical interferences and steep local slopes. One suitable way to polish these classes of optics is by using a jet of abrasive/fluid mixture. The energy required for polishing may be supplied by the radial spread of a liquid jet, which impinges a surface to he polished. Such fluid flow may generate sufficient surface shear stress to provide material removal in the regime of chemical mechanical polishing. Once translated into a polishing technique, this unique tool may resolve a challenging problem of finishing steep concave surfaces and cavities. A fundamental property of a fluid jet is that it begins to lose its coherence as the jet exits a nozzle. This is due to a combination of abruptly imposed longitudinal and lateral pressure gradients, surface tension forces, and aerodynamic disturbance. This results in instability of the flow over the impact zone and consequently polishing spot instability. To be utilized in deterministic high precision finishing of remote objects, a stable, relatively high-speed, low viscosity fluid jet, which remains collimated and coherent before it impinges the surface to be polished, is required. A method of jet stabilization has been proposed, developed, and demonstrated whereby the round jet of magnetorheological fluid is magnetized by an axial magnetic field when it flows out of the nozzle. It has been experimentally shown that a magnetically stabilized round jet of magnetorheological (MR) polishing fluid generates a reproducible material removal function (polishing spot) at a distance of several tens of centimeters from the nozzle. The interferometrically derived distribution of material removal for an axisymmetric MR Jet™, which impinges normal to a plane glass surface, coincides well with the radial distribution of rate of work calculated using computational fluid dynamics (CFD) modeling. Polishing results support the assertion that the MR Jet finishing process may produce high precision surfaces on glass and single crystals. The technology is most attractive for the finishing of complex shapes like free form optics, steep concaves, and cavities.

Magnetorheological finishing of large and lightweight optics

Magnetorheological finishing (MRF) is a production proven, sub-aperture polishing process for flat, spherical, aspherical, and cylindrical optics in the size range of 10 - 400 mm. Surface figure accuracy of better than 30 nm peak-to-valley (better than 5 nm rms), and microroughness better than 1 nm rms is routinely achieved on a variety of glasses, glass ceramics and single crystal materials. Recent work has demonstrated the applicability of MRF for larger apertures and lightweight optics. A platform capable of finishing 1000 mm apertures has already been built. Engineering studies for extending the aperture size further are underway. Finishing of large, lightweight mirrors has additional challenges because the non-uniform support of the face-sheet requires special efforts to avoid quilting errors caused by print-through of the cell structure due to fabrication processes, gravity and/or temperature effects. Unique characteristics of MRF such as a competitively high, stable removal rate, the conformal nature of the sub-aperture tool and a shear mode of material removal give it advantages in finishing this class of optics. Specifically, MRF avoids generating print-through errors and has a high rate of convergence in correcting quilting errors created by other processes, gravity or temperature effects. An additional important quality is that it has been shown that inserting MRF into a manufacturing process can substantially reduce the subsurface damage (SSD), increasing the laser damage threshold of a surface, providing advantages for use in mirror fabrication for high-energy applications. Supporting results will be given in this paper.

High precision metrology of domes and aspheric optics

Many defense systems have a critical need for high-precision, complex optics. However, fabrication of high quality, advanced optics is often seriously hampered by the lack of accurate and affordable metrology. QEDs Subaperture Stitching Interferometer (SSI®) provides a breakthrough technology, enabling the automatic capture of precise metrology data for large and/or strongly curved (concave and convex) parts. QED’s SSI complements next-generation finishing technologies, such as Magnetorheological Finishing (MRF®), by extending the effective aperture, accuracy and dynamic range of a phase-shifting interferometer. This workstation performs automated sub-aperture stitching measurements of spheres, flats, and mild aspheres. It combines a six-axis precision stage system, a commercial Fizeau interferometer, and specially developed software that automates measurement design, data acquisition, and the reconstruction of the full-aperture figure error map. Aside from the correction of sub-aperture placement errors (such as tilts, optical power, and registration effects), our software also accounts for reference-wave error, distortion and other aberrations in the interferometer’s imaging optics. The SSI can automatically measure the full aperture of high numerical aperture surfaces (such as domes) to interferometric accuracy. The SSI extends the usability of a phase measuring interferometer and allows users with minimal training to produce full-aperture measurements of otherwise untestable parts. Work continues to extend this technology to measure aspheric shapes without the use of dedicated null optics. This SSI technology will be described, sample measurement results shown, and various manufacturing applications discussed.

Deterministic, precision finishing of domes and conformal optics

In order to enhance missile performance, future window and dome designs will incorporate shapes with improved aerodynamic performance compared with the more traditional flats and spheres. Due to their constantly changing curvature and steep slopes, these shapes are incompatible with most conventional polishing and metrology solutions. Two types of a novel polishing technology, Magnetorheological Finishing (MRF®) and Magnetorheological (MR) Jet, could enable cost-effective manufacturing of free-form optical surfaces. MRF, a deterministic sub-aperture magnetically assisted polishing method, has been developed to overcome many of the fundamental limitations of traditional finishing. MRF has demonstrated the ability to produce complex optical surfaces with accuracies better than 30 nm peak-to-valley (PV) and surface micro-roughness less than 1 nm rms on a wide variety of optical glasses, single crystals, and glass-ceramics. The polishing tool in MRF perfectly conforms to the optical surface making it well suited for finishing this class of optics. A newly developed magnetically assisted finishing method MR JetTM, addresses the challenge of finishing the inside of steep concave domes and other irregular shapes. An applied magnetic field coupled with the properties of the MR fluid allow for stable removal rate with stand-off distances of tens of centimeters. Surface figure and roughness values similar to traditional MRF have been demonstrated. Combining these technologies with metrology techniques, such as Sub-aperture Stitching Interferometer (SSI®) and Asphere Stitching Interferometer (ASI®), enable higher precision finishing of the windows and domes today, as well as the finishing of future conformal designs.

New mold manufacturing techniques

Typically, optical molds have been made from silicon carbide (SiC) or tungsten carbide (WC). Magnetorheological Finishing (MRF) polishing results of SiC and WC molds will be reviewed. Impressive figure corrections have been demonstrated on both types of materials. The roughness performance of CVD-SiC, WC and binderless WC will be compared. However, the hardness and polycrystalline nature of these materials make them difficult to manufacture. In this paper we report positive initial results using an alternate mold material, glassy carbon. Test samples have been ground, pre-polished and finish polished to a 38 nm surface figure peak-to-valley (PV) and a 6 Å rms surface roughness, with improved cycle times versus SiC and WC. Glassy carbon is a promising mold material candidate as an amorphous material of lower hardness. The lower hardness leads to more effective diamond grinding process and results in a better surface rms roughness following MRF. After reviewing key material properties of glassy carbon material, this paper will describe some collaborative activities between Toshiba Machine Co., Ltd. and QED Technologies (QED) to manufacture representative examples of glassy carbon. Details of the grinding, pre-polishing and final polishing process will be provided along with the resultant metrology results after key steps. Molding experiments based on these developments will also be presented.

Developments in the finishing of domes and conformal optics

The final finish and characterization of windows and domes presents a number of difficult challenges. Furthermore, there is a desire to incorporate conformal shapes into next generation imaging and surveillance systems to provide significant advantages in overall component performance. Unfortunately, their constantly changing curvature and steep slopes make fabrication of such shapes incompatible with most conventional polishing and metrology solutions. Two novel types of polishing technology, Magnetorheological Finishing (MRF®) and Magnetorheological Jet (MR JetTM), along with metrology provided by the Sub-aperture Stitching Interferometer (SSI®) have several unique attributes that give them advantages in enhancing fabrication of hemispherical domes and even conformal shapes. The advantages that MRF brings to the precision finishing of a wide range of shapes such as flats, spheres (including hemispheres), cylinders, aspheres and even freeform optics, has been well documented. The recently developed MR Jet process provides additional benefits, particularly in the finishing the inside of steep concave domes and other irregular shapes. Combining these technologies with metrology techniques, such as the SSI, provides a solution for finishing current and future windows and domes. Recent exciting developments in the finishing of such shapes with these technologies will be presented. These include new advances such as the ability to use the SSI to characterize a range of shapes such as domes and aspheres, as well as progress in using MRF and MR Jet for finishing conventional and conformal windows and domes.

Cost-effective subaperture approaches to finishing and testing astronomical optics

The fabrication and metrology of astronomical optics are very demanding tasks. In particular, the large sizes needed for astronomical optics and mirrors present significant manufacturing challenges. One of the long-lead aspects (and primary cost drivers) of this process has traditionally been the final polishing and metrology steps. Furthermore, traditional polishing becomes increasingly difficult if the optics are aspheric and/or lightweight. QED Technologies (QED(r)) has developed two novel technologies that have had a significant impact on the production of precision optics. Magnetorheological Finishing (MRF(r)) is a deterministic, production proven, sub-aperture polishing process that can enable significant reductions in cost and lead-time in the production of large optics. MRF routinely achieves surface figure accuracy of better than 30 nm peak-to-valley (better than 5 nm rms) and microroughness better than 1 nm rms on a variety of glasses, glass ceramics and ceramic materials. Unique characteristics of MRF such as a comparatively high, stable removal rate, the conformal nature of the sub-aperture tool and a shear-mode material removal mechanism give it advantages in finishing large and lightweight optics. QED has, for instance, developed the Q22-950F MRF platform which is capable of finishing meter-class optics and the fundamental technology is scalable to even larger apertures. Using MRF for large optics is ideally partnered by a flexible metrology system that provides full aperture metrology of the surface to be finished. A method that provides significant advantages for mirror manufacturing is to characterize the full surface by stitching an array of sub-aperture measurements. Such a technique inherently enables the testing of larger apertures with higher resolution and typically higher accuracy. Furthermore, stitching lends itself to a greater range of optical surfaces that can be measured in a single setup. QEDs Subaperture Stitching Interferometer (SSI(r)) complements MRF by extending the effective aperture, accuracy, resolution, and dynamic range of a standard phase-shifting interferometer. This paper will describe these novel approaches to large optics finishing, and present a variety of examples.

Improvement of figure and finish of diamond turned surfaces with magneto-rheological finishing (MRF)

Single Point Diamond Turning (SPDT) has been a cost effective technique to achieve the required figure and roughness specification on a wide range of infrared (IR) optics. SPDT is one of the few technologies that can efficiently generate aspherical surfaces, and as recent developments such as fast-tool servos mature, “free-form” surfaces are becoming feasible as well. Optical end-user requirements for a wide range of industries are continuing to tighten, driven, for example by multi-spectral systems that require good performance at shorter wavelengths in addition to IR. In many cases, specified shape tolerances can exceed SPDT capabilities. Additionally, SPDT typically leaves “turning marks” (affecting micro-roughness) that can be detrimental to performance. In some cases, surface integrity (e.g. sub-surface damage) can also be of concern. Magneto-Rheological Finishing (MRF®) has the proven ability to simultaneously improve roughness, figure, and surface integrity in a fast and cost effective manner. MRF is a deterministic, sub-aperture polishing technology, and is typically employed as the last manufacturing step. MRF can deterministically remove from tens of nanometers to microns worth of material, while efficiently “converging” to the specified requirements. Conversely, SPDT has proven to be very effective in removing the hundreds of microns (if not mm) sometimes required to “pre-shape” an aspheric surface before its final polish. After a brief introduction of MRF, this paper will discuss how SPDT and MRF processes can complement one another very effectively. Examples of MRF results on a wide range of IR materials will be presented.

Subaperture approaches for asphere polishing and metrology

This paper summarizes some of QED Technologies’ latest developments in the field of high-precision polishing and metrology. Magneto-Rheological Finishing (MRF) is a deterministic sub-aperture polishing process that overcomes many of the fundamental limitations of traditional finishing. MRF has demonstrated the ability to produce optical surfaces with accuracies better than 30 nm peak-to-valley (PV) and surface micro-roughness less than 0.5 nm rms on a wide variety of optical glasses, single crystals, and glass-ceramics. The MR fluid forms a polishing tool that is perfectly conformal and therefore can polish a variety of shapes, including flats, spheres, aspheres, prisms, and cylinders, with either round or rectangular apertures. QED’s Sub-aperture Stitching Interferometer (SSI) complements MRF by extending the effective aperture, accuracy, resolution, and dynamic range of a phase-shifting interferometer. This workstation performs automated sub-aperture stitching measurements of spheres, flats, and mild aspheres. It combines a six-axis precision stage system, a commercial Fizeau interferometer, and specially developed software that automates measurement design, data acquisition, and the reconstruction of the full-aperture map of figure error. Aside from the correction of sub-aperture placement errors (such as tilts, optical power, and registration effects), our software also accounts for reference-wave error, distortion, and other aberrations in the interferometer’s imaging optics. By addressing these matters up front, we avoid limitations encountered in earlier stitching work and significantly boost reproducibility beyond that of the integrated interferometer on its own.

New industrial applications of magnetorheological finishing (MRF)

MRF is a deterministic, sub-aperture finishing process with system stability, high material removal rates, and a shear mode of material removal that makes it uniquely well-adapted to a variety of optical and industrial applications.