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 (CNC) 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 A rms, and correcting surface figure errors to less than 0. 1 im 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 (COM).
A high-resolution, high-precision beam-shaping system for high-power-laser systems is demonstrated. A liquid-crystal-on-silicon spatial light modulator is run in closed-loop to shape laser-beam amplitude and wavefront. An unprecedented degree of convergence is demonstrated, and important practical issues are discussed. Wavefront shaping for the applications in OMEGA EP laser is demonstrated, and other interesting examples are presented.
Magnetorheological finishing (MRF) is a subaperture lap, deterministic process developed at the Center for Optics Manufacturing (COM). MRF can remove subsurface damage from an optical component while correcting figure errors and smoothing small scale microroughness. The "standard" magnetorheological (MR) 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 LW, ZnSe, CaF2, AMTIR-l, ZnS, MgF2 sapphire, and CVD diamond. MRF OF OPTICAL GLASSESIn MRF, a magnetic-field-stiffened ribbon offluid is used to polish out a workpiece. Removal occurs through the shear stress created as the ribbon is dragged into the converging gap between the part and some reference or carrier surface. The zone of contact is restricted to a spot which is smaller than the part diameter. The removal spot constitutes a subaperture lap that conforms perfectly to the local topography ofthe part. Deterministic finishing offlats, spheres, and aspheres is accomplished by mounting the part on a rotating spindle and sweeping it through the spot under computer control, such that dwell time determines the amount ofmaterial removed. The MR fluid lap is unique, because 1) its compliance is adjustable through the magnetic field, 2) it carries heat and debris away from the polishing zone, 3) it does not load up, and 4) it does not loseits shape. A preliminary theory and an extensive review ofthe MRF process are given elsewhere.1'2 1.1 "Standard" MR fluid Most MRF work done to date utilizes an MR fluid consisting ofmagnetic particles of carbonyl iron (CI) in water, with small concentrations of stabilizers added to inhibit oxidation. Although this formulation will polish glass, removal rates are accelerated with the addition of cerium oxide (CeO2). A "standard" formulation that has been used in many ofthe experiments at the COM consists of(given in vol. %) 36/C!, 6/CeO2, 55/water, and 3/stabilizers. This high solids content fluid exhibits an apparent viscosity3 of 0.5 Pasec outside of a magnetic field. In a ragnetic field of 160-240 kA/m (2-3 kG), the apparent viscosity exceeds 10 000 Pasec at a shear rate of 8/sec4, making it stiff enough to support loads for finishing. Figure 1 gives initial size histograms and median particle sizes for the solid particles5. The CI (left top) @ 4.5 tm is in the middle size range for commercially available magnetic powders6, whereas the CeO2 (left bottom) 3.5 im is a coarse, impure, and aggressive commercial polishing abrasive7. The scanning electron micrograph (SEM) of the MR fluid shown on the right in Fig. 1 was taken after a week of use. The round CI particles are essentially unaltered from their initial median particle size, but the irregularly shaped CeO2 particles have been broke...
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