Achim J. Leistner

300-mm-aperture phase-shifting Fizeau interferometer

A 300-mm-aperture digital phase-shifting Fizeau interferometer (LADI) has been developed in house for precision metrology of optical components fabricated by the Optical Workshop at CSIRO Division of Telecommunications and Industrial Physics. This paper describes the optical and mechanical configuration of the instrument as well as its calibration and performance characteristics. Recent measurements on 250-mm-diam uncoated optical surfaces have consistently shown shortterm repeatability of 0.3 nm rms from measurement to measurement and allowed absolute characterization of these surfaces to an accuracy of a few nanometers.

Power spectral density analysis of optical substrates for gravitational-wave interferometry

The power spectral density of surface-relief variations on polished optical surfaces across microscopic through to macroscopic spatial scales is calculated from measurements on substrates that are being produced for the Laser Interferometer Gravitational-Wave Observatory (LIGO). These spectra give a guide to the scattering properties of the surface, which in turn critically influence the performance of LIGO. Measurements obtained by use of a full-aperture interferometer and an interference microscope with two different objectives are combined to produce one-dimensional power spectral density representations of the surfaces across spatial frequencies ranging from 0.1 to 8000 cm(-1). These measurements from different instruments are in good agreement with an analytic power spectrum that varies as nu(-1.5), where nu is the spatial frequency. Some anomalies in the power spectral density spectra can be related to aspects of the polishing process.

Polishing study using Teflon and pitch laps to produce flat and supersmooth surfaces

Teflon polishing is compared with pitch polishing as a method for achieving supersmooth and flat optical surfaces. Because a Teflon lap wears slowly, it will retain its surface shape to produce extremely flat optical surfaces, lambda/100, consistently and reliably for extended periods of time, of the order of days. To compare the two methods, we polished 50-mm-diameter samples of various optical materials, using colloidal suspensions in water on both pitch and Teflon laps under the same polishing conditions. Flatness was maintained to better than lambda/10, and roughness less than 10 A rms was measured on all samples by two Talystep surface-profiling instruments, one in the United States and one in Australia, with excellent agreement between measurements made by the two instruments. It was possible to obtain flat andsmooth surfaces (<4-A rms roughness) on all materials (except for F4, flint glass), but only certain combinations of material, abrasive, and lap could be used to give the correct polishing conditions and s rface chemistry.

Electrooptic Fabry-Perot filter: development for the study of solar oscillations.

Observations of nonradial solar oscillations require Doppler velocity measurement at many points over the photosphere with a velocity resolution better than 1 m/s. An attractive form of imaging spectrophotometer for such a task utilizes a thin, solid, electrically tunable Fabry-Perot interference filter or etalon made of an electrooptic material such as lithium niobate (LiNbO(3)). The problems to be overcome in producing such an etalon for an imaging spectrophotometer are discussed and practical solutions demonstrated on the basis of measurements made on prototype devices.

Precision determination of the density of a single crystal silicon sphere and evaluation of the Avogadro constant

Density measurements with relative uncertainty of 1/spl times/10/sup -7/ have been made on a highly polished 1-kg single crystal silicon sphere with out-of-roundness <40 nm. Roundness was profiled using a 2-D Talyrond machine and 3-D profiles were produced. The diameter was obtained using optical interferometry for a series of breadth measurements at carefully selected points and by combining them with roundness data. The mass was obtained relative to a 1-kg stainless steel reference with appropriate corrections for air buoyancy and for convection currents due to small temperature differentials. Surface oxide thickness was measured using optical ellipsometry and the data were corrected for this oxide thickness. The molar mass, crystal quality and lattice parameter have been measured elsewhere, enabling a determination of the Avogadro constant to be made. The purpose is to obtain a definition of the kilogram in terms of a specific number of /sup 12/C atoms.

Fabrication and measurement of optics for the Laser Interferometer Gravitational Wave Observatory

The manufacture and testing of high-precision optical surfaces for the Laser Interferometer Gravitational Wave Observatory is described. Through the use of carefully shaped polishing laps made of a nondeformable polymer material coated on a rigid base, surfaces 250 mm in diameter with radii of curvature between 7 and 15 km were polished to an accuracy of several hundred meters in the curvature and with low values of waviness and microroughness. Metrology instrumentation used to measure the optical finish included a large-aperture digital interferometer calibrated to nanometer-level accuracy for measurements of curvature, astigmatism, and waviness and an interference microscope for measurements of microroughness. The power spectra of the data from both instruments were in good agreement.

Fabrication and testing of a high-precision concave spherical mirror

CSIROs Australian Centre for Precision Optics has recently finished the production of a high-precision concave spherical mirror. The specifications were very ambitious: numerical aperture 0.75; asphericity below 5.5 nm rms and 27.3 nm P-V. The available reference transmission sphere had to be calibrated to enable adequate accuracy. Due to the high numerical aperture of the mirror, sub-aperture measurements had to be stitched together to form a complete surface map of the mirror. Phase-shifting interferometry at high numerical aperture suffers from phase-step non-uniformity because of the large off-axis angles. We present what we believe to be a new interpretation of this phenomenon as a focus error, which clarifies where in the interferometer the phase-shift error occurs. We discuss the ball-averaging method for calibrating the reference transmission sphere and present results from the averaging process to ensure an uncertainty commensurate with the certification requirement. For carrying out the sub-aperture measurements, we constructed a two-axis gimbal mount to swivel the mirror around the focus of the test wavefront. If the centers of curvature of the transmission sphere and the mirror coincide, the mirror can be tilted without losing the interferogram. We present a simple and effective alignment method, which can be generally applied to optical tests where the wavefront comes to a focus. The mirror was coated with protected aluminum and tested in its mount. No effect on the sphericity error from the coating was found, and the specifications were exceeded by approximately 30%. We discuss subtleties of the stitching process on curved surfaces and report final results.

The influence of Young's modulus on roundness in silicon sphere fabrication [Avogadro constant]

Silicon single-crystal spheres for use in accurate determination of the Avogadro constant are fabricated by optical grinding and polishing. Surface profiling of the deviation from sphericity of the spheres shows a strong cubic symmetry on the scale of (20 to 40) mm, which is well correlated with the orientation of the silicon crystal axes and sets a limit on the ultimate shape that can be obtained. This deviation from sphericity can be explained in terms of the variation of Youngs modulus with crystal orientation.

Teflon laps: some new developments and results

The process of superpolishing with Teflon laps has enabled supersmooth [< 0.1 nm rms] and extremely flat (λ/100) optical surfaces to be produced on a large range of amorphous and crystalline optical materials. Stable surface conditions and the very low wear of a Teflon lap during polishing provide an opportunity to examine the effects of varying different polishing parameters. These include sample-tolap mismatch and the influence of different polishing compounds and fluid chemistry. The results show that when large optical flats are superpolished with Teflon laps, microroughness, subsurface damage, and scattering can be minimized and reliably and consistently predicted for a wide variety of optical materials, while extremely flat surfaces are simultaneously achieved.

High Precision Digital Interferometry: Its Application To The Production Of An Ultrathin Solid Fabry-Perot Etalon

Teflon polishing is capable of producing surfaces flat to 3 nm or better, but its wider application requires suitable testing methods to measure such small residual errors. Digital interferometry solves this problem and, because data can be stored and processed, makes it possible to carry out a variety of tests with high precision. The application of these techniques to the production and testing of a solid Fabry-Perot eta Ion with a diameter of 50 mm, a thickness of 0.352 mm, and a maximum deviation from parallelism of its faces of ±2 nm over the central 30 mm is described.