Optical Surface Quality Research Articles

Crystal characterization by use of birefringence interferometry

An infrared birefringency interferometer capable of evaluating the bulk quality of crystals has been designed and constructed. The birefingency interferometer, like the more familiar Mach-Zehnder interferometer, is a sensitive tool for measuring the refractive index homogeneity in bulk materials, but it does not require the optical surface quality and parallism needed for Mach-Zehnder measurements. It is, ideally suited for the preliminary screening of large quantities of material without the need for precision fabrication. This paper describes the design and operation of the instrument and its use for evaluating the optical quality of Tl 3 AsSe 3, PbBr 2, and Hg 2 Cl 2 single crystals.

Moderate Axisymmetric Deformations of Optical Membrane Surfaces

Large deformable membranes are currently used as optical reflector surfaces for solar concentrators. In this paper, we study moderate axisymmetric deformations of optical membrane surfaces and their impact on the macroscopic optical quality of the deformed surface. A uniform pressure loading applied to an initially flat membrane surface is assumed in attaining the final concentrator surface shape. The problem is important in the design of focusing stretched-membrane heliostat modules. Insights into the design of stretched-membrane parabolic dish reflectors also result from this work. This paper presents several simple design and sizing relationships, based on a nonlinear variational principle, in terms of common engineering parameters. Good approximate predictions of the nonlinear load-deformation response and the stress state in the membrane are derived; simple but accurate predictions for membrane surface slope and nominal focal length also result. The phenomena that lead to deviation of the surface shape from the desired parabolic reflector shape are identified and design procedures to minimize those deviations are deduced. Results show that the macroscopic optical surface quality, for a given nominal f/D (concentrator focal length/diameter), is highly dependent on the non-dimensionalized membrane stiffness parameter, Et/To (membrane modulus × thickness/initial tension). Further, for the heliostat cases of interest, the focal-length variations relative to the desired parabolic shape were shown to be about ±5 percent about the nominal value.

Use of an optical profiling instrument for the measurement of the figure and finish of optical quality surfaces

In this paper the use of a WYKO non-contacting optical profiling microscope for the measurement of the figure and finish of optical quality surfaces is discussed. The original instrument and our hardware and software modifications are described together with recent and current studies which make use of this facility.

New Materials For Precision Optics

A novel family of germania-modified cordierite ceramics has been developed for the fabrication of precision optical components. Compositions in this family possess zero average coefficient of thermal expansion (CTE) over selected temperature regions. Moreover, the zero CTE can be tailored to a desired temperature range of operation by varying the relative proportions of Ge02 and Si02 in the modified cordierite structure. Thermal diffusivity of these ceramics is more than twice that of the ULE glass, a current state-of-the-art material for precision mirror substrates. In addition, germanium-cordierites possess high specific stiffness and low thermal-moment/rigidity ratio. Control-lable fine-grained structure enables good polishability and offers the potential for an optical quality surface. These thermal and mechanical properties make Ge-cordierites an excellent family of materials for optical and structural applications. The theoretical basis for germania substitution for silica in the pure cordierite structure has been examined. Material characteristics including solid solubility and sinterability have been considered. All the germanium-cordierite compositions presented in this study are morphologically single-phase and microstructurally homogeneous materials. These properties along with tailorable zero CTE characteristics offer the potential of employing cordierites for high precision optics.

OPTICAL-SHOP TESTING OF LIQUID MIRRORS

ABSTRACT Borra (1982) has argued that it is scientifically useful and should be technically feasible to build very large optical telescopes (diameters over 15 meters) having as primary mirror a rotating container filled with mercury. The authors report here the results of optical tests on prototype liquid mirrors. Knife-edge and Hartmann tests, as well as direct imagery, show that optical quality surfaces are attained. The largest mirror produced has a diameter of 1.65-m but the authors tested thoroughly only a 1-m diameter mirror.

Characterizations of optical surfaces by measurement of scattering distribution

The micropolish of good quality optical surfaces can be characterized by measuring the scattered light distribution. Very often the surface defects are not isotropic but display preferred orientations that are translated into an anisotropy of the scattered light distribution. The total amount of light scattered by very high quality surfaces, coated or uncoated, scarcely exceeds a few hundred parts per million. Precise measurement of the distribution of the scattered light is always a task requiring great care and attention to detail. The apparatus is described. All the necessary degrees of freedom have been included so that the scattering may be completely analyzed. It is possible to make measurements out of the plane of incidence so that the complete spatial distribution of the scattered light can be obtained, whatever the angle of incidence of the primary beam. Thus to characterize the geometry of the system we use four fundamental parameters: the angle of incidence i, the two angles θ and ϕ that define the scattering direction, and the angle α that defines the orientation of the scattering surface in its own plane. Only two free parameters need exist because the surface roughness itself, which is the source of the scattered light, only depends on two variables. We have verified experimentally the validity of the relationships linking i, θ, ϕ, and α. In these relationships the expression for the intensity scattered in a particular direction (θ,ϕ) for an uncoated surface at angle of incidence i can be written in the form of the product of a coefficient, depending only on illumination and observation conditions, and of the 2-D Fourier transform of the autocorrelation functions of the surface roughness. Experimental measurements with uncoated surfaces of black glass have accorded with the theory. When the surfaces are coated with one or several layers the problem is more complicated, but it should be possible to derive information on the autocorrelation functions of each of the interfaces and the degree of correlation between them.

A polarization-independent optical circulator for 1.3 µm

A compact polarization-independent optical circulator for 1.3μm is described. The device utilizes birefringent plates (rutile) for beam splitting, YIG plus a single-order quartz half-wave plate for polarization rotation, and a slotted 45° glass prism for beam steering. The fiber-to-fiber insertion loss is 2.0 or 2.5 dB and the isolation is 25 or 35 dB, depending upon the ports in question. The use of birefringent plates for beam splitting and combining eliminates the need for multiple dielectric coatings for polarization beam splitting, requires only optical plate processing, virtually eliminates sensitivity to translational misalignment of individual beam-splitting/combining components, and reduces the required number of optical-quality surfaces.

Scattering from multilayer thin films: theory and experiment

A theoretical method is described for calculating the angular scattering curve for any given thin-film multilayer in which the various surfaces and interfaces are assumed to be rough. The energy that is scattered in any particular direction depends on two factors. The first is solely a function of the characteristics of the ideal multilayer and the observation conditions. The second depends on the roughnesses of the various surfaces, that is, on their autocorrelation and cross-correlation functions. The expressions that are obtained are completely general because no restrictive hypothesis has been developed on the relationship that may exist between the roughnesses of the various interfaces. The principal features of the apparatus used for the scattering measurements are briefly described. Next, several results of calculation and measurement are given as illustrations. The measurements show that polished surfaces of high optical quality often show marked anisotropy.

Proposed Use Of Visible Optical Techniques To Evaluate The Surface Finish Of X-Ray Optics

Arguments leading to the tentative conclusion that production and measurement techniques developed for low scatter optical surfaces should be directly applicable to low scatter x-ray optics are briefly reviewed, and some of the more useful methods for making and testing high quality optical surfaces are discussed.

Description Of A Unique Machine Tool Permitting Achievement Of <15-A Rms Diamond-Turned Surfaces

A new machine tool now in the final stages of development at Battelle, Pacific Northwest Laboratories uses a unique tool motion to produce diamond-turned surfaces of exceptionally high quality. Copper surfaces of revolution have been produced with a 12.3-A rms surface finish and a contour accuracy of 75 nm. The cutting tool is programmed to move in 4 nm increments along two axes: one linear (X) axis and one circular (Omega) axis. Exceptionally stiff and accurate control of the tool is possible with this system. In conjuction with a unique thermally stabilized air bearing spindle and machine calibration equipment, the Omega-X system permits a significant advance in the fabrication of optical-quality surfaces.

The PROM - A Status Report

The PROM is a solid-state, rapidly recyclable, image storage device having a number of applications in image and signal processing. Some of its important characteristics include 1/10-wave optical surface quality, 100-1p/mm three-bar resolution, 10 ergs/cm2 light sensitivity, and image plane contrast of 10 4 :1. One of the unique features of the PROM is that the bias level of stored patterns can be adjusted through application of an external voltage, resulting in image contrast inversion or enhancement. This same operation (baseline subtraction) is used to null the zero order in an optical Fourier transform, achieving a Fourier plane signal-to-noise ratio approaching 106:1. This paper reports on the current status of this device and a number of applications for which it has been tested in several areas of image and signal processing. Results are shown for coherent optical processing by computer-controlled Fourier plane filtering and real-time image correlation, and signal processing systems are described which couple the PROM with an acousto-optic raster recorder to perform spectrum analysis and correlation on radio frequency signals.

A microcomputer-based electronic error corrector bar for a numerically-controlled machine tool

The production of optical-quality surfaces for machined laser mirrors demands machining accuracies that are beyond the mechanical limits of even the best machine tools. An Electronic Corrector Bar was developed at LLL, using a microcomputer and a correction look-up table, to make corrections every 10 mils for both displacement and straightness errors to a 1 microinch Diamond Turning Lathe. The correction is predicted to improve the positional accuracy from 60 microinches to better then 5 microinches and straightness from 25 microinches to better then 5 microinches. Preliminary test results show that this goal is attainable. 4 figures.

Lateral Interferometry-Its Characteristics, Technology and Applications

This article is a survey of the characteristics, technology and applications of lateral interferometry, LI. Some of the early uses of LI in laser Doppler velocimetryl have been greatly expanded to cover new important applications in industrial instrumentations and precision, noncontact measurements. In general, LI uses a high precision optical interference pattern that is projected onto a volume in which the measurement takes place. The measurement technique involves the use of a low power laser beam (milliwatts) which is split into two beams of slightly differing frequency using a Bragg cell. When the beams are brought back together, they form an accurate fringe pattern in space. This fringe pattern is then used as a precision optical scale against which-items to be measured are compared. The fringe pattern can be adjusted to have a spatial period from submicron spacing up to several hundred microns and wider. Receiver optics and the electronics that follow detect the relationship of the fringe pattern to the item being measured. The technique is sensi-tive to the angular orientation of the item, so small angles or variation in angular orientation of an object with high resolution can be measured without making physical contact. In addition, the accurate fringe pattern can be used to measure the optical fidelity (transfer function) of lenses and optical imaging systems. One of the newest developments highlights important applications in the areas of rapid, non-contact monitoring of submicroscopic surface defects on high quality surfaces, such as semiconductor blanks that are used in IC component manufacturing, high quality optical surfaces and the like.

Optical parameters of partially transmitting thin films 2: Experiment and further analysis of a novel method for their determination

In Part 1 of this work a theory was developed for the measurement of the thickness, refractive index, and absorption coefficient of a partially transmitting thin film. In the present work, an experimental system is described and the basic theory verified by actual measurements. It is shown how the measurement of intensity ratios in a single setup is adequate for the determination of the above mentioned parameters. In addition, the application of the system for the examination of optical surface quality is demonstrated. Finally, the theory is extended to nonnormal incidence.

Fabrication and Test of 1.8- Meter-Diameter High Quality ULE Mirror

The purpose of this investigation was to determine the feasi-bility of producing large diameter, high quality optical surfaces similar to those that will be required for some of the planned orbital observing units such as the Large Space Telescope (LST). Current requirements for the Large Space Tele-scope require that a A /20 rms wavefront be produced at the Cassegrain focus. A reasonable distribution of error sources would, therefore, demand that the primary mirror be figured to a tolerance of A/64 rms on the surface. NASA has funded the initial step toward demonstrating the ability to meet that requirement. An optical fabrication demonstration program was undertaken using an available 1.8-meter-diameter ULE monolithic mirror blank weighing on the order of 1,200 pounds. The mirror that was used had no central perforation and was approximately 12 inches thick with 1-inch front-and hack-plates. In order to accomplish this task, a variety of support and test equipment was designed and fabricated and the actual optical fabrication and test were conducted over a period of approximately eight months. The results of this effort yielded a mirror with a surface quality of 0.015 wave rms or A/65.3. = 632.8 nanometers.)

Materials problems in the production of high quality optical surfaces

The production of optical surfaces of high perfection is still largely dependent on manual skills and on an empirical technology which has been developed over the past two hundred years. It is becoming more difficult to meet the demands for optical components of even higher surface perfection and more complex surface figure, by the traditional glass working techniques. An understanding of the techniques employed, the materials used and the conflicting polishing theories should enable the materials scientist to contribute to the advancement of the science and technology of glass polishing.

A new technique for growing epitaxial films of Pb 1−xSn xTe

An improved technique has been found for the vacuum deposition of epitaxial films of Pb 1−xSn xTe on cleaved KC1 substrates. In order to nucleate a crystalline film which adheres well to the substrate, the substrate temperature is initially raised to only 280°C. After a one-micron film has grown, the substrate temperature is then raised to 400°C and maintained there for all subsequent film growth. This increase in substrate temperature is needed to achieve the highest quality optical surfaces. Results on both as-grown and annealed films are presented. One result of our annealing program has been the achievement of the lowest reported carrier concentration in films with Sn fractions greater than 0.3.

Thick Epitaxial Films of Pb1−xSnxTe

During the past several years attempts have been made to grow thick epitaxial films of PbS, PbTe, PbSe, and their alloys with tin on alkali halide substrates. The thickest films which have been achieved using conventional evaporation techniques were limited to 10–15 μ m. We have developed a quasiequilibrium system which has yielded thick films of Pb1−xSnxTe ranging up to 328 μm. When combined with appropriate substrate preparations, this technique yields films having optical quality surfaces and good adherence to the substrate. In addition, the films may be easily removed from their substrates, intact and undistorted, by dissolving the substrates. Various electrical, optical, and thermal measurements on these thick films show that they are homogeneous. X-ray data indicate that the films are of high crystalline quality.

Evaluating the quality of optical surfaces by the width of interference fringes

Evaluating the quality of optical surfaces by the width of interference fringes

Ionic Polishing of Optical Surfaces

The controlled removal of material from the surface of insulators has been observed by using a positive ion beam. In the energy range 0.5-2.0 MeV the sputtering yield was observed to be unity. The application of this technique to the manufacture of high quality optical surfaces is discussed.