Optically Flat Research Articles

Focus on lasers and optics

Focus on lasers and optics

Measurement of a <inline-formula><math altimg="none" display="inline" overflow="scroll"><mrow><mn>2</mn><mtext>-</mtext><mtext>meter</mtext></mrow></math></inline-formula> flat using a pentaprism scanning system

Precise manufacturing of optical flats requires precise characterization of the surface. A scanning pentaprism system is ideal for use as an absolute and precise test for an optical flat. Such a system was built and used to test a 2-m diameter flat mirror. This system uses light from an autocollimator that is reflected from two pentaprisms to project reference beams of light onto the flat mirror. The light reflected from the mirror back through the pentaprisms provides information on low order optical aberrations in the flat mirror. We report results of the test on a 2-m flat, characterizing the errors and their sources. There is enormous potential for our system to be used to test larger flats and even curved surfaces, made of either a glass or a liquid.

Three Gaussian beam interferometric profilometer applied to the characterization of an optical flat

A three-beam scanning optical interferometric microscopic technique applied to roughness characterization of optical flats is described. The technique is based on the heterodinization of three coherent optical beams. One of the beams, the probe beam, is focused on the surface under test. A second beam is obtained after being reflected by a reference surface. Finally, the last beam consists of one of the first orders of diffraction that emerges of a Bragg-cell. The three beams are coherently added at the sensitive surface of a photodetector that integrates the overall intensity of the beams. We show analytically that, the electrical signal at the output of the photodetector, is a time-varying signal whose amplitude is proportional to the surface local vertical height. We characterize experimentally the frequency response of the system by measuring the profile of three different gratings. We show measurements of the roughness of an optical flat processed by means of the frequency response of the system.

The flexure-based microgap rheometer (FMR)

We describe the design and construction of a new microrheometer designed to facilitate the viscometric study of complex fluids with very small sample volumes (1–10μl) and gaps of micrometer dimensions. The flexure-based microgap rheometer (FMR) is a shear-rate-controlled device capable of measuring the shear stress in a plane Couette configuration with directly controlled gaps between 1 and 200μm. White light interferometry and a three-point nanopositioning stage using piezo-stepping motors are used to control the parallelism of the upper and lower shearing surfaces, which are constructed from glass optical flats. A compound flexure system is used to hold the fluid sample testing unit between a drive spring connected to an “inchworm” motor and an independent sensor spring. Displacements in the sensing flexure are detected using an inductive proximity sensor. Ready optical access to the transparent shearing surfaces enables monitoring of the structural evolution in the gap with a long working-distance video microscope. This configuration then allows us to determine the microgap-dependent flow behavior of complex fluids over 5 decades of shear rate. We demonstrate the capability of the FMR by characterizing the complex stress and gap-dependent flow behavior of a typical microstructured food product (mayonnaise) over the range of gaps from 8to100μm and stresses from 10to1500Pa. We correlate the gap-dependent rheological response to the microstructure of the emulsion and changes induced in the material by prolonged shearing.

High-Resolution Electrostatic Analog Tunable Grating With a Single-Mask Fabrication Process

We present the design, modeling, fabrication, and characterization of the microelectromechanical systems (MEMS) analog tunable diffraction grating with the concept of transverse actuation. In contrast to the vertically actuated digital tunable grating, our prototype design trades angular tunable range for tuning resolution. The prototype shows an angular tunable range of 250 murad with 1-murad resolution at 10 V. Grating pitch changes corresponding to the full range and resolution are 57 nm and 2.28 Aring, respectively confirmed by experimental measurement and theoretical calculation. Simulation shows that subradian tunable range is feasible with better lithographic design rules or higher actuation voltage. The single-mask fabrication process offers several advantages: 1) Excellent optical flatness; 2) ease of fabrication; and 3) great flexibility of device integration with existing on-chip circuitry. Tunable gratings such as the one presented here can be used for controlling dispersion in optical telecommunications, sensing, etc., applications

Optical surface flatness recognized by discrete wavelet transform and grey level co-occurrence matrix

Optical surface flatness evaluated by the discrete wavelet transform and grey level co-occurrence matrix (GLCM) is presented. A PC-based measurement system can be used to detect the interference fringe of an optical reflective surface captured from a Twyman–Green interferometer. Wavelet analysis and GLCM process associated with the entropy criterion appears to be a good method for recognizing automatically the flatness of the optical surfaces. Three-dimensional plots of the GLCM and surface deformation contour for various captured interference patterns of glass substrates have been compared and discussed. The parameter of entropy has been calculated from the GLCM and then can be used as an indicator for optical surface characteristics.

Simple Technique for Polishing Optical Components Made from KDP Group of Crystals

The present paper describes a technique for block-polishing end surfaces of optical components made from KDP family of crystals to high optical quality surface finish and flatness. Relevant considerations for designing of polishing holders for soft, hygroscopic crystals have been discussed. Fabrication of polishing holders, setting andfixing of optical components in the holder, grinding/polishing of the block, testing of surface figure and parallelism during the polishing process and de-blocking of the optical components after the polishing operation have been described.

Sub-nanometre double shearing heterodyne interferometry for profiling large scale planar surfaces

A novel interferometric method, called double shearing interferometry, is presented for profiling large-scale quasi-planar surfaces, such as semiconductor wafers, optical flats and x-ray mirrors. The surface profile is measured even if there is an inclination in the scanning stage. By adopting the common-path optical configuration and heterodyne detection, the proposed method achieves excellent resolution at the subnanometre scale and allows a robust measurement in the presence of unwanted disturbances in the measurement environment. A height resolution of 0.1 nm was achieved experimentally. The standard deviation of the analytical surface profiles was found to be 1.3 nm, even when using a conventional screw-lead scanning stage. The measured results show that the surface profile and stage inclination are determined separately. We confirmed that the resultant profile was quite consistent with that measured with a Fizeau interferometer.

Studies on optical pumping cells (OPC) to polarize 3He

The technique applied at HMI to obtain nuclear-spin-polarized 3He, used in neutron spin filters (NSFs), is metastability-exchange optical pumping. To prepare efficient NSF, one must highly polarize 3He nuclei in the optical pumping volume (OPV) and reduce the polarization losses during the compression phase. Great progress has been achieved in reducing of depolarization due to the recent development of both, large polarization preserving piston compressors and long relaxation time filter cells. It is even more important to significantly enhance the 3He polarization rate during optical pumping in order to increase NSF efficiency. Different cells materials were tested, such as Duran and quartz glass. In order to use the laser light more efficiently and to decrease the risk of 3He depolarization due to unfavorable reflections, antireflection (AR) coatings were used on cell windows made of quartz glass. They were compared with the ones without coating, made of quartz, Duran and BK7 glass. The comparison of various techniques to mount the windows such as blowing, gluing or molecular diffusion was also conducted. It indicated that the molecular diffusion is the most suitable technique because of a better purity of the gas in the cell and the preservation of the optical flatness of the windows. Cells, for practical reasons each entirely made from the same material (Duran, Quartz glass) with windows mounted using this method, showed the best polarization performance.

An interactive correlating–filtering approach to remove distortion errors in interferometric optical testing

Distortion errors resulting from backside reflections are found to be the major troubles encountered in interferometric testing of optical transparent flats. In order to remove those unwanted reflections, an interactive correlating–filtering approach is introduced in this study. Its parallel data processing is pursued in the time and frequency domains simultaneously. The experimental results show that the new approach can meet the requirements for on-line interferometric optical testing.

Surface-plasmon-resonance-enhanced cavity ring-down detection.

The cavity ring-down technique is used to probe the absolute optical response of the localized surface plasmon resonance (SPR) of a gold nanoparticle distribution to adsorption of trichloroethylene (TCE) and perchloroethylene (PCE) from the gas phase. Extended Mie theory for a coated sphere with a particle-size-dependent dielectric function is used to elucidate size-dispersion effects, the size-dependence of the SPR sensitivity to adsorption, and the kinetics of adsorption. An approximate Gaussian distribution of nanospheres with a mean diameter of 4.5 nm and a standard deviation of 1.1 nm, as determined by atomic force microscopy, is provided by the intrinsic granularity of an ultrathin, gold film, having a nominal thickness of approximately 0.18 nm. The cavity ring-down measurements employ a linear resonator with an intracavity flow cell, which is formed by a pair of ultrasmooth, fused-silica optical flats at Brewster's angle, where the Au film is present on a single flat. The total system intrinsic loss is dominated by the film extinction, while the angled flats alone contribute only approximately 5 x 10(-5)/flat to the total loss. Based on a relative ring-down time precision of 0.1% for ensembles averages of 25 laser shots from a pulsed optical parametric oscillator, the minimum detectable concentrations of PCE and TCE obtained by probing the SPR response are found to be 2 and 7 x 10(-8) mol/L, respectively, based on a 30 s integration time.

Wet-etch figuring for precision optical contouring.

Wet-etch figuring utilizes free surface flows driven by surface tension gradients (the Marangoni effect) to confine and stabilize the size and shape of an etchant droplet attached to the underside of a glass surface. This droplet, or wetted zone, is translated on the surface, etching where it contacts and leaving behind no residue, to facilitate an etching-based small-tool figuring process that is free of mechanical and thermal stresses. The optic needs no backing plate, and its back side is free for inspection by optical means. When transmissive optics is figured, the optical thickness between the front and the rear surfaces of the optic is measured interferometrically and used in real time to control the local dwell time of the etchant zone. This truly closed-loop figuring process is robust, environmentally insensitive, and fully automated. It is particularly suited for figuring patterns such as phase plates, corrective elements, and optical flats on very thin (<< 1-mm) substrates that are difficult to figure with traditional abrasive polishing methods.

A NEW METHOD FOR ADJUSTMENT OF THE LEVEL: ADJUSTMENT IN TRIPLES

Surveyors are farniliar with the principle of reversal: for example, in checking a straightedge or a setsquare and in routine observing strategies to eliminate instrumental errors. An extension of this idea, lthich may be used in the production of optical flats, allows testing for ‘perfect’ flatness when the surfaces are manufactured in triples. The paper describes a technique which allows ‘perfect’ adjustment when levels are compared in triples.

A NEW METHOD FOR ADJUSTMENT OF THE LEVEL: ADJUSTMENT IN TRIPLES

Surveyors are farniliar with the principle of reversal: for example, in checking a straightedge or a setsquare and in routine observing strategies to eliminate instrumental errors. An extension of this idea, lthich may be used in the production of optical flats, allows testing for ‘perfect’ flatness when the surfaces are manufactured in triples. The paper describes a technique which allows ‘perfect’ adjustment when levels are compared in triples.

Flattening the flats

Calculating the expansion of the Universe requires accurate measurements of distances to nearby galaxies, which may be easier with true optical flats.

Curvature compensation in micromirrors with high-reflectivity optical coatings

High-reflectivity coatings on micromirrors are critical to reduce reflection losses and absorptive heating. Unfortunately, coating stress induces an unwanted curvature in micromirrors. This effect is much more serious than in bulk optics because the thin coatings are similar in thickness to the structural material of the mirror. This paper describes a method to apply coatings that simultaneously achieve high reflectivity and optical flatness. The design theory is based on one-dimensional (1-D) static analysis and achieves curvature compensation with only a single additional coating layer The technique is appropriate for any number of coating layers and includes both the elastic and plastic behaviors of the thin film layers. Plastic deformation is modeled using an empirically determined strain versus dielectric thickness curve. Experimental measurements of 200 /spl mu/m/spl times/200 /spl mu/m/spl times/3.5 /spl mu/m polysilicon plates show mirror flatness better than /spl lambda//10.

Polishing techniques for optical grade sapphire

Sapphire is an important crystal that finds application in a number of high technology optical applications. For optical applications a high transparency is essential. Several defects/imperfections are present in the surface layer of ‘as grown’ sapphire crystal. These defects/imperfections can be removed by polishing the sapphire crystals. The high hardness and strength of sapphire necessitates the use of high machining and polishing forces. Conventional polishing techniques are unable to meet this requirement. Specialised techniques have been developed to obtain optical flatness and low scatter surface finish. These techniques are reviewed and compared in this article.

Notes on Surface: toward a genealogy of flatness

This essay revisits the issue of flatness, typically associated with teleological readings of modernist painting. At its outset, the text highlights a paradox: that some of the most powerful and prestigious accounts of postmodernism, including Frederic Jameson’s, identify flatness –the modernist trope par excellence – as postmodernism’s distinguishing feature. In an effort to untangle this contradiction, a new genealogy of flatness is proposed. Firstly, a powerful link is noted in Greenbergian modernism between optical flatness and psychological depth. Secondly, a preoccupation with the appropriation and reframing of stereotypes is identified in more recent art. Here, the psychological depth tied to optical flatness in modern art is itself deflated, producing a marriage of optical and psychological flatness. These transformations in the meaning surface and superficiality are traced through the art of Jackson Pollock, Jasper Johns, David Hammons and Kara Walker.

Intergranular Pressure Solution in Nacl: Grain-To-Grain Contact Experiments under the Optical Microscope

Intergranular Pressure Solution (IPS) is an important geologic lithification, compaction and deformation mechanism in a wide variety of crustal rocks. Experimental studies of IPS in quartz aggregates have not been very successful due to the low rate of IPS, and IPS experiments performed using wet halite as a rock analogue (Spiers and Schutjens, 1990; Hickman and Evans, 1991) have left uncertainty about the detailed IPS mechanism and grain contact structure/wetting in this material. The present study reports four contact dissolution experiments performed under the optical microscope to study the mechanism and kinetics of IPS at single halite/halite and halite/glass contacts loaded under brine (room temperature). Normal constant contact forces in the range 1. 0 to 2. 6 N were applied in the presence of NaCl-saturated brine, exerting stresses of 0. 8 to 7. 4 MPa. Time-dependent mass removal and convergence were observed at all contacts. In all cases, loading of the contact (or increasing the load on the contact) led to instantaneous formation of a rough contact morphology, composed of a crystallographically-controlled pattern of islands and channels with a length scale of several micrometers. This nonequilibrium microstructure evolved with time to an optically flat contact face while contact broadening and convergence continued. The smoothing/convergence process must therefore have involved diffusion of mass out of the contact, and expulsion of brine, through a connected brine phase within the contact. Whether a fine-scale rough structure persisted in contacts which evolved to optical flatness is not known, though post-test SEM (Scanning Electron Microscopy) observations suggest that it may have. If so, its amplitude was less than 500 nm. Measurements of dissolution rates enabled comparison with a model for IPS. The analysis suggests that solute diffusion through the contact boundary was probably rate-controlling, with the contact structure and effective diffusivity varying with contact force and on-going convergence. The results agree broadly with those of previous compaction creep experiments performed using wet halite powder. Discrepancies with other workers results for single-contact dissolution experiments can be explained in terms of differences in experimental configuration and competition between driving forces.

Absolute figure measurements with a liquid-flat reference

We describe a variation of the liquid-flat technique fordetermining the absolute flatness of a 240-mm-diameter optical surfaceto an accuracy better than 1/100lambda in both its horizontal(three-point support) and vertical orientations. Using theappropriate mathematics to calculate the surface deformation of a diskdue to gravity, we achieved verification of the method by comparingmeasurements carried out on a pair of optical flats and a liquidreference surface.