Acidic magnetorheological finishing of infrared polycrystalline materials.

Chemical-vapor-deposited (CVD) ZnS is an example of a polycrystalline material that is difficult to polish smoothly via the magnetorheological finishing (MRF) technique. When MRF-polished, the internal infrastructure of the material tends to manifest on the surface as millimeter-sized "pebbles," and the surface roughness observed is considerably high. The fluid's parameters important to developing a magnetorheological (MR) fluid that is capable of polishing CVD ZnS smoothly were previously discussed and presented. These parameters were acidic pH (∼4.5) and low viscosity (∼47 cP). MRF with such a unique MR fluid was shown to reduce surface artifacts in the form of pebbles; however, surface microroughness was still relatively high because of the absence of a polishing abrasive in the formulation. In this study, we examine the effect of two polishing abrasives-alumina and nanodiamond-on the surface finish of several CVD ZnS substrates, and on other important IR polycrystalline materials that were finished with acidic MR fluids containing these two polishing abrasives. Surface microroughness results obtained were as low as ∼28 nm peak-to-valley and ∼6-nm root mean square.

Thermomechanical model to assess stresses developed during elevated-temperature cleaning of coated optics.

A thermomechanical model is developed to estimate the stress response of an oxide coating to elevated-temperature chemical cleaning. Using a hafnia-silica multilayer dielectric pulse compressor grating as a case study, we demonstrate that substrate thickness can strongly affect the thermal stress response of the thin-film coating. As a result, coatings on large, thick substrates may be susceptible to modes of stress-induced failure (crazing or delamination) not seen in small parts. We compare the stress response of meter-scale optics to the behavior of small-scale test or "witness" samples, which are expected to be representative of their full-size counterparts. The effects of materials selection, solution temperature, and heating/cooling rates are explored. Extending the model to other situations, thermal stress results are surveyed for various combinations of commonly used materials. Seven oxide coatings (hafnia, silica, tantala, niobia, alumina, and multilayers of hafnia-silica and alumina-silica) and three glass substrates (BK7, borosilicate float glass, and fused silica) are examined to highlight some interesting results.

Fracture mechanics of delamination defects in multilayer dielectric coatings.

During the fabrication of multilayer-dielectric (MLD) thin-film-coated optics, such as the diffraction gratings used in OMEGA EP's pulse compressors, acid piranha cleaning can lead to the formation of chemically induced delamination defects. We investigate the causes of these defects and describe a mechanism for the deformation and failure of the MLD coating in response to hydrogen peroxide in the cleaning solution. A fracture mechanics model is developed and used to calculate the crack path that maximizes the energy-release rate, which is found to be consistent with the characteristic fracture pattern observed in MLD coating delamination defects.

Experiments and observations regarding the mechanisms of glass removal in magnetorheological finishing.

Recent advances in the study of the magnetorheological finishing (MRF) have allowed for the characterization of the dynamic yield stress of the magnetorheological (MR) fluid, as well as the nanohardness (H(nano)) of the carbonyl iron (CI) used in MRF. Knowledge of these properties has allowed for a more complete study of the mechanisms of material removal in MRF. Material removal experiments show that the nanohardness of CI is important in MRF with nonaqueous MR fluids with no nonmagnetic abrasives, but is relatively unimportant in aqueous MR fluids or when nonmagnetic abrasives are present. The hydrated layer created by the chemical effects of water is shown to change the way material is removed by hard CI as the MR fluid transitions from a nonaqueous MR fluid to an aqueous MR fluid. Drag force measurements and atomic force microscope scans demonstrate that, when added to a MR fluid, nonmagnetic abrasives (cerium oxide, aluminum oxide, and diamond) are driven toward the workpiece surface because of the gradient in the magnetic field and hence become responsible for material removal. Removal rates increase with the addition of these polishing abrasives. The relative increase depends on the amount and type of abrasive used.

Nanoindentation hardness of particles used in magnetorheological finishing (MRF).

Knowledge of the hardness of abrasive particles that are used in polishing is a key to the fundamental understanding of the mechanisms of material removal. The magnetorheological-finishing process uses both magnetic and nonmagnetic abrasive particles during polishing. The nanohardnesses of the micrometer-sized magnetic carbonyl iron and nonmagnetic abrasive particles have been measured successfully by use of novel, to our knowledge, sample-preparation and nanoindentation techniques. Some of the results reported compare favorably with existing microhardness data found in the literature, whereas other results are new.

Angular-scattering characteristics of ferroelectric liquid-crystal electro-optical devices operating in the transient-scattering and the extended-scattering modes.

The angular distribution of forward-scattered light in transient-scattering-mode (TSM) and extended-scattering-mode (ESM) ferroelectric liquid-crystal (FLC) devices was evaluated by use of circularly polarized incident light. For both modes the intensity and the distribution of forward-scattered light depended primarily on the FLC birefringence, spontaneous polarization, and the cell path length. In the FLC materials examined, the forward-scattering intensity under ESM drive conditions increased with longer FLC pitch lengths, whereas under TSM conditions stronger forward scattering was observed with increasing FLC spontaneous polarization. Although both TSM and ESM drive conditions displayed a similar angular distribution for forward-scattered light, the intensity of ESM scattering over a 0 degrees -6 degrees range was considerably smaller than that observed in earlier experiments with linearly polarized incident light.

Bound-abrasive polishers for optical glass.

Polishing abrasives that have been bound in a solid matrix can offer several potential advantages over loose-abrasive processes with pitch or polyurethane laps for finishing of optics. These advantages include polishing efficiency, temperature stability, cost of consumables, and compatibility with computer numerically controlled generating machines. Unfortunately, little has been published on bound-abrasive polishers, and very few commercially available products exist. We have developed several compositions and manufacturing techniques that show promise for polishing a variety of optical glasses. We establish the various criteria for a successful bound-abrasive polisher. The numerous variables to be considered in designing these polishers are identified, and the results of screening experiments are used to find successful compositions. Polishing experiments with bound abrasives in arrays of pellets, as ring tools, and as full-contact laps are described.

Polarization asymmetry in waves backscattering from highly absorbant random media.

Within the range in which light penetration depth is approximately the same as or less than the diameter of the particles in the medium, particulate media with considerable absorption behave as two-dimensional, rough-surface structures. As penetration depth increases, a complicated transition between volume and surface effects is seen. For these media, low-order scattering sequences have small spatial extent, making observation of polarization characteristics difficult. We present an experimental technique to access the low-order scattered photons by artificially reinjecting them through total internal reflections. Using a dielectric layer in contact with the high-absorption medium, we are able to observe fourfold polarization asymmetry in backscattering from highly absorbant media. We discuss the origin of the polarization patterns in a ray-optics approximation and suggest possibilities for solving practical problems encountered in characterizing composites with appreciable absorption.

Surface microroughness of optical glasses under deterministic microgrinding.

Deterministic microgrinding of precision optical components with rigid, computer-controlled machining centers and high-speed tool spindles is now possible on a commercial scale. Platforms such as the Opticam systems at the Center for Optics Manufacturing produce convex and concave spherical surfaces with radii from 5 mm to ∞, i.e., planar, and work diameters from 10 to 150 mm. Aspherical surfaces are also being manufactured. The resulting specular surfaces have a typical rms microroughness of 20 nm, 1 µm of subsurface damage, and a figure error of less than 1 wave peak to valley. Surface roughness under deterministic microgrinding conditions (fixed infeed rate) with bound abrasive diamond ring tools with various degrees of bond hardness is correlated to a material length scale, identified as a ductility index, involving the hardness and fracture toughness of glasses. This result is in contrast to loose abrasive grinding (fixed nominal pressure), in which surface microroughness is determined by the elastic stiffness and the hardness of the glass. We summarize measurements of fracture toughness and microhardness by microindentation for crown and flint optical glasses, and fused silica. The microindentation fracture toughness in nondensifying optical glasses is in good agreement with bulk fracture toughness measurement methods.

Slurry particle size evolution during the polishing of optical glass.

The particle size distribution of aqueous metal-oxide slurries can evolve during the polishing of optical glass in response to changes in mechanical and chemical process factors. The size-evolution phenomenon and its consequences were systematically studied in a planar continuous-polishing process. The concurrent application of electrokinetic techniques to characterize common optical shop materials has contributed new insight into the nature of silicate glass polishing by demonstrating the pivotal role of fluid chemistry, particularly pH, in maintaining electrokinetically favorable conditions for a welldispersed polishing agent. According to the proposed slurry-charge-control effect, a well-dispersed polishing agent is the key to obtaining the smoothest possible glass surfaces, especially when a recirculated slurry is used.

Midinfrared modulation through the use of field-induced scattering in ferroelectric liquid crystals.

The feasibility of the use of modulation devices based on field-induced transient scattering in ferroelectric liquid crystals (LC) to replace mechanical choppers used in uncooled infrared-imaging systems was investigated. Devices fabricated with ITO-coated ZnSe substrates and a ferroelectric LC path length of 25 µm were able to modulate optical radiation by transient forward scattering at rates approaching 20 kHz. Through the use of a commercial arbitrary waveform generator and associated PC-based software, drive waveforms were developed that produced a variable, square-wave optical-modulation pattern by the extension of the duration of the scattering state to periods ranging from hundreds of microseconds to milliseconds. The ability of these extended-scattering-mode (ESM) devices to modulate radiation in both the visible and midinfrared regions was verified in a simple experiment through the use of a Fourier-transform infrared spectrometer, in which an unoptimized ESM device displayed a 40% modulation dep th for IR radiation in the 8-12-µm region.

Nematic polymer liquid-crystal wave plate for high-power lasers at 1054 nm.

A nematic polymer liquid crystal is used to construct wave plates for use at 1054 nm. Three methods of wave-plate construction are discussed: double substrate with fiber spacers in homogeneous distribution, double substrate with fiber spacers in annular distribution, and single substrate. The polymer liquid crystal shows high laser-damage resistance, making it particularly useful for high-peak-power laser applications. Alignment techniques and measurement of birefringence for the highly viscous polymer are described.

Model for laser damage dependence on thin-film morphology.

The high laser damage thresholds often reported for porous thin films are discussed in terms of point defects or small absorbing inclusions as sites of thin-film damage initiation. The model is based on the internal pressure built up upon laser heating at short times. The competing effects of short pressure relaxation distances and low thermal conductivity inherent in porous films are discussed. The model predicts that at thicknesses less than or equal to the neck or column diameter of a porous film, the effect of lower thermal conductivity should dominate and cause the films to exhibit lower laser damage thresholds than their denser counterparts.

Automated spatially scanning ellipsometer for retardation measurements of transparent materials.

A highly sensitive and automated technique has been developed for measuring the birefringence in transparent optical materials. The spatially scanning modulated transmission ellipsometer maps the birefringence of a transparent material by probing it with a polarization-modulated He-Ne laser beam. Computer-controlled voltage biasing of a Pockels cell permits self-calibration and background subtraction of the system retardance. The technique is capable of resolving differential retardances as small as 0.1 nm (λ/6328) through a range of ±λ/2, where λ = 632.8 nm. Samples typically range in size from 50 µm to 10 cm in diameter within the sample plane and as much as 400 mm along the optical axis.

Physics of loose abrasive microgrinding.

This study examined the physics of loose abrasive microgrinding (grinding with micron and submicron sized abrasives). More specifically, it focused on the transition from brittle to ductile mode grinding which occurs in this region of abrasive sizes. Process dependency on slurry chemistry was the primary area of emphasis and was studied for diamond abrasives varying in size from 3.0 to 0.75 microm on both ULE and Zerodur, with emphasis on ULE. Ductile mode grinding was achieved with smaller abrasives, as expected, however two significant discoveries were made. The first observation was that by simply changing slurry chemistry, it was possible to induce the transition from brittle fracture to ductile mode grinding in ULE. This transition point could be intentionally moved about for diamonds 3.0-0.75 microm in diameter. For any given abrasive size within these limits, either brittle fracture or ductile removal may be achieved, depending on the slurry used to suspend the diamonds. Several slurries were studied, including water, a series of homologous n-alcohols, and other solvents chosen for properties varying from molecular size to dielectric constant and zeta potential. The study revealed that this slurry dependency is primarily a Rebinder effect. The second finding was that a tremendous amount of surface stress is introduced in loose abrasive ductile mode grinding. This stress was observed when the Twyman Effect in ULE plates increased by a factor of 4 in the transition from the brittle to the ductile mode. An assessment of the cause of this stress is discussed.

TEM(00)-mode and single-longitudinal-mode laser operation with a cholesteric liquid-crystal laser end mirror.

We describe the unique properties of a cholesteric liquid crystal as a laser end mirror. We show how it gives rise to TEM(00)-mode operation as well as single-longitudinal-mode operation of a solid-state laser resonator.

Strengthened phosphate glass in a high rep rate active-mirror amplifier geometry.

Ion-exchange strengthened phosphate glass in an active-mirror geometry remained unfractured at pump power levels 3 times the average pump fracture limit of unstrengthened phosphate glass in the same geometry. In addition, pretreatment and posttreatment measurements of surface wavefront and roughness were made on a set of rectangular substrates to quantify any ion-exchange-induced surface modifications. Experimental measurements of treatment-induced wavefront deformation of strengthened blocks were shown to be less than modeled values of distortion attributable to extended treatment times.

High-repetition-rate Cr:Nd:GSGG active-mirror amplifier.

We have designed, constructed, and operated a 3.8-cm clear-aperture Cr:Nd:GSGG active-mirror amplifier. We believe this to be the first active mirror that uses a crystalline host and the largest-aperture Cr:Nd:GSGG amplifier yet reported. We have measured a small-signal gain of 1.6. The wave front has been measured and found to be less than three waves of defocus at repetition rates of up to 10 Hz. Surface displacements were measured and compared with theory. Depolarization was less than 2.5% at maximum power at any location in the clear aperture.