Modern optical designs often include components with shapes more complicated than simple spherical and plano
surfaces. These shapes, which include conformal, steep concave, stepped and free form surfaces, are often difficult to
finish with conventional techniques due to mechanical interference and steep local slopes. A suitable approach to
polishing these shapes is to use a jet of fluid containing an appropriate abrasive. However, a fundamental property of a
fluid jet is that it will begin to lose its coherence once it exits the nozzle. This instability results in an unpredictable
removal rate of the fluid jet, which makes it unsuitable for use in a deterministic finishing process. A method of jet
stabilization whereby a jet of magnetorheological (MR) fluid is magnetized by an axial magnetic field when it flows
from the nozzle has been demonstrated and implemented into the Magnetorheological Jet (MR JetTM) finishing process.
The magnetically stabilized jet of MR polishing fluid produces a stable and reproducible material removal function
(polishing spot) at a distance of several tens of centimeters from the nozzle making MR Jet an attractive technology for
the finishing of complex shapes such as free form optics, steep concaves, and cavities. Recent results will be presented
showing the ability to use this technology to finish a variety of shapes and materials including glass, metals, and
ceramics.
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.
Conformal, free form and steep concave optics are important classes of optics that are difficult to finish using conventional techniques due to mechanical interference and steep local slopes. The problem becomes more complicated when the optics approach millimeter size. In this presentation we will discuss some results from finishing such challenging optics to high precision using newly developed jet-based techniques.
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.
As optical components' shape becomes more complex, and requirements on the precision get tighter, new, and more versatile polishing processes are required. Conformal, free form or steep concave optics are important examples of optics that are difficult to finish using conventional techniques due to mechanical interferences and steep local slopes. One suitable way to polish such shapes is by using a jet of abrasive/fluid mixture. A fundamental property of a fluid jet is that it begins to lose its coherence as the jet exits a nozzle resulting in instability of the material removal rate, and is therefore unsuitable for deterministic finishing. A method of jet stabilization has been proposed, developed and demonstrated whereby the jet of magnetorheological fluid is magnetized by an axial magnetic field when it flows from the nozzle. It has been shown that a magnetically stabilized jet of MR polishing fluid generates a reproducible material removal function (polishing spot) at a distance of several tens of centimeters from the nozzle. The technology is most attractive for the finishing of complex shapes like free form optics, steep concaves and cavities. Results will be presented showing the ability to use this technique to finish a variety of optical components.
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.
Significant challenges are faced in the manufacturing of the complex optics for the next generation of astronomical telescopes. Process improvements are required to establish cost effective techniques to finish the optics to the tight specification required in a timely manner. An added complication is realized when the optics are lightweight. The non-uniform support of the face-sheet in this case requires special efforts to avoid a print-through of the cell structure due to fabrication processes, gravity and/or cryogenic effects. Magnetorheological finishing (MRF) is a deterministic, sub-aperture polishing process that has been a revolutionary success in the fabrication of optics in the size range of 10-1000 mm. This production proven process is capable of polishing flats, spheres, aspheres and cylinders to a 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 single crystal materials. Unique characteristics of MRF such as a high, stable removal rate, conformal nature of the sub-aperture tool and shear mode of material removal give it advantages in the finishing of large and lightweight optics. These qualities provide for a cost-effective process with a high rate of convergence that requires few iterations. Such a technology is ideally complemented by a system for the stitching of interferometric sub-aperture data. Stitching inherently enables the testing of larger apertures with higher resolution and, thanks to the built-in calibration, even to higher accuracy in many situations. While this approach enables the non-null testing of parts with greater aspheric departure and can lead to a significantly reduced non-common air path in the testing of long-radius concave parts, it is especially effective for convex optics. That is, stitching is particularly well suited to the testing of secondary mirrors and, alongside the testing of the off-axis primary segments.
It has been shown that a magnetically stabilized round jet of MR polishing fluid generates a reproducible material removal function (polishing spot) at a distance of several tens of centimeters from the nozzle. As a polishing technique, this unique tool resolves a challenging problem of high precision finishing of steep concave surfaces and cavities. Theoretical prerequisites and experimental results are discussed.
Magnetorheological finishing (MRF) is a novel process demonstrated to be effective for fine figure control and polishing of a variety of optical glasses and crystals. This paper discusses the use of MRF to stress relieve the surfaces of single crystal silicon wafers, of the type used in the semiconductor industry to fabricate integrated circuits. One hundred-mm diameter silicon wafers with a <111> crystallographic orientation were loose abrasive lapped with three different sizes of alumina abrasive to introduce compressive surface stress. The stress generated in the wafer surface was characterized by interferometrically monitoring the bending of the wafer due to the Twyman effect. The thickness of the subsurface damage (SSD) layer was characterized using a dimpling method with a fixture developed at COM. Subsequent polishing by MRF was found to be effective in removing the subsurface damage and associated residual stress generated in the wafer surface during loose abrasive lapping.
Magnetorheological Finishing (MRF) is a novel process for deterministic figure correction and polishing of optical materials that utilizes a sub-aperture lap created by moving a magnetic field-stiffened magnetorheological (MR) fluid ribbon against an optical surface. MRF has been successfully applied to a wide range of optical materials. A new research platform has been designed and built that is used to generate sub-aperture polishing profiles, i.e., polishing 'spots,' on optical flats under well-controlled conditions. This platform uses the same fluid circulation and conditioning system as the commercial computer numerically controlled MRF machine, thereby allowing fluid performance issues to be investigated. This new machine complements the capabilities of the original MRF research platform that has been in continuous use for over six years. These two machines have been used to generate polishing spots on a variety of optical materials. The spot profiles were measured to calculate material removal rates and the quality of the polished surfaces characterized by measuring the microroughness within the polishing spots. Examples are presented which illustrate how the evaluation of polishing spots was used to develop MR fluids and operating conditions for calcium fluoride, CaF2, and potassium dihydrogen phosphate, KDP.
Finish polishing of highly precise optical surfaces is one of the most promising uses of magnetic fluids. We have taken the concept of magnetorheological finishing (MRF) from the laboratory to the optical fabrication shop floor. A commercial, computer numerically controlled (CNC) MRF machine, the Q22, has recently come on-line in optics companies to produce precision flat, spherical and aspheric optical components. MRF is a sub-aperture lap process that requires no specialized tooling, because the magnetically-stiffened abrasive fluid conforms to the local curvature of any arbitrarily shaped workpiece. MRF eliminates subsurface damage, smoothes rms microroughness to less than 1 nm, and corrects p-v surface figure errors to (lambda) /20 in minutes. Here the basic details of the MRF process are reviewed. MR fluid performance for soft and hard materials, the removal of asymmetric grinding errors and diamond turning marks, and examples of batch finishing of glass aspheres are also described.
In magnetorheological finishing (MRF) the mechanical energy for material removal is generated by the hydrodynamic flow of a magnetorheological (MR) polishing suspension through a converging gap that is formed by a workpiece surface and a moving rigid wall. In addition to causing material removal, MRF also reduces the surface micro roughness of optical materials to ≤ 10 Å rms. Shape errors are corrected to a fraction of a wavelength of light and subsurface damage is removed. A theoretical analysis of MRF, based on Bingham lubrication theory, illustrates that the formation of a core attached to the moving wall results in dramatically high stress on the workpiece surface. A correlation between the shear stress on the workpiece surface and materials removal is obtained.
Magnetorheological finishing (MRF) is a subaperture lap, deterministic process developed at the Center for Optics Manufacturing. MRF can remove subsurface damage from an optical component while correcting figure errors and smoothing small scale microroughness. The 'standard' magnetorheological fluid for finishing of optical glasses consists of magnetic carbonyl iron and nonmagnetic cerium oxide particles in water. This composition works well for a variety of soft and hard glass types, but it does not perform adequately for certain single crystal materials and polycrystalline compounds used in IR applications. In this paper, we describe modifications to MRF and finishing experiments for LiF, ZnSe, CaF2, AMTIR-1, ZnS, MgF2, sapphire, and CVD diamond.
Optical polishing with magnetic media has evolved extensively over the past decade. Of the approaches conceived during this time, the newest process is called magnetorheological finishing (MRF). In MRF, all of the process parameters are controlled by utilizing the state of hydrodynamic flow of a magnetically stiffened magnetorheological abrasive fluid through a converging gap formed by a lens workpiece surface and a moving wall. The shear flow of “plastic” MR fluid results in the development of high stresses in the interface zone and material removal over a portion of the workpiece surface, referred to as the “polishing spot”. The polishing spot is an abrasive-charged, sub-aperture lap that automatically conforms to the local shape of the lens surface. Deterministic finishing is accomplished by mounting a lens on a rotating spindle and sweeping it through the MR fluid with a computer numerical controlled (CNC) machine. A computer program generates both a dwell time schedule for the MRF machine and an accurate prediction of finished surface shape, using a material removal function and initial surface condition information as input. In this paper, we describe the MRF process, a preliminary theory of material removal, properties of the MR fluid, machine configurations, software for finishing, and finishing experiments on a variety of surface shapes (spherical, flat, aspheres) and materials of interest to optics manufacturing. Advantages and current limitations to the process are also described.
Several aspheric lens elements have recently been produced using new manufacturing processes. Deterministic microgrinding and magnetorheological finishing technologies were employed to fabricate ten 47 mm diameter plano convex hyperboloidal aspheric condenser lenses. The parts had an aspheric departure of 145 microns from best fit sphere, with a final figure requirement of approximately one micron p-v. The lens elements were tested at the component and systems level and passed all end user requirements.
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.
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