Philip S. Fairman

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.

Calibration of a 300-mm-aperture phase-shifting Fizeau interferometer

A 300-mm-aperture digital phase-shifting Fizeau interferometer has been developed in-house for precision metrology of optical components fabricated by the optical workshop at Telecommunications and Industrial Physics, Commonwealth Scientific and Industrial Research Organization. We describe the procedures used in the calibration of the instrument. A reference data file representing the deviations from flatness of the reference surface is generated, measurement uncertainty estimated, and aberrations in the instrument assessed. Measurements on 250-mm-diameter uncoated optical surfaces have consistently shown short-term repeatability of 0.3-nm rms from measurement to measurement and allowed for absolute characterization of these surfaces to within a few nanometers.

Moire based optical surface profiler for the minting industry

An Optical Surface Profiler (OSP130) has been developed for the metrology of master tooling used in the coin stamping process. The OSP130 measure, in a non-contacting manner, the surface relief of tools ranging in diameter from 10 mm to 300 mm. Rapid measurements are performed simultaneously on a large grid of equispaced points across the surface of the tool. From the relief data, many parameters such as the location of high and low features, volume of impression, background curvatures and various diameters can be quickly evaluated. The technique used is phase-shifting moire profilometry. A white light projector illuminates a periodic transmission grating which is then imaged onto the object surface. The light pattern on the object is viewed by a high resolution TV camera connected to a computer. The grating is shifted under computer control to a number of positions and corresponding intensity images of the deformed pattern on the object surface are stored in the computer. From the intensity images a phase map, representing the deformation of the periodic grating by the surface relief, is evaluated and compared with an undeformed pattern. This results in an accurate contour map of the surface relief with an uncertainty less than 1% of the relief excursion on the object. Details of the instrument and its use at the Royal Australian Mint are presented.

Measuring the thickness profiles of wafers to subnanometer resolution using Fabry-Perot interferometry

The resolution of an angle-scanning technique for measuring transparent optical wafers is analyzed, and it is shown both theoretically and experimentally that subnanometer resolution can be readily achieved. Data are acquired simultaneously over the whole area of the wafer, producing two-dimensional thickness variation maps in as little as 10 s. Repeatabilities of 0.07 nm have been demonstrated, and wafers of up to 100 mm diameter have been measured, with 1 mm or better spatial resolution. A technique for compensating wafer and system aberrations is incorporated and analyzed.

Metrology of LIGO core optics

Core optical substrates for the Laser Interferometer Gravitational Wave Observatory are being manufactured and tested at CSIRO. These substrates are for use in long baseline Michelson interferometers with Fabry Perot cavities up to 4 km in length in each arm. The optics consist of 32 high quality fused silica substrates, comprising folding mirrors, end test masses, input test masses, recycling mirrors and beamsplitters. The dimensions of the substrates are 250 mm diameter by up to 100 mm thick. The optical surfaces are either flat or curved, with radii of curvature between 7 km and 15 km and tolerance bands on the radius equivalent to variations in sag (over 200 mm) of about 20 nm.

Simultaneous measurements of vector components of displacement by ESPI and FFT techniques

To measure a number of components of displacement with electronic speckle pattern interferometry (ESPI), multiple interferograms are usually captured in succession for different sensitivity vectors. Simultaneous measurement of a number of orthogonal components of displacement using ESPI can be carried out by application of the Fourier transform method. An object is illuminated by several object beams, and the scattered light is combined with one reference beam to form a first speckle image. After providing different tilts to the object beams, a second speckle image is recorded. A third speckle image is then recorded with the object loaded. The difference of the first and second images contains a set of straight carrier fringes. The difference of the first and third images contains a corresponding separate set of modulated carrier fringes due to the loading. The Fourier transforms of these two difference images show multiple peaks which correspond to the carrier fringes of the different object beams. By appropriately masking the corresponding peaks in the two Fourier transforms, the different sets of carrier fringes for the loaded and unloaded object configurations, can be separated. The phase corresponding to the different orthogonal components of displacement can then be retrieved from the ratios of the real to imaginary parts of the two inverse Fourier transforms of each filtered peak. An example of the measurement is presented.

Development of the ASTRI heliostat

The Australian Solar Thermal Research Initiative (ASTRI) aims to develop a high optical quality heliostat with target cost – manufactured, installed and operational – of 90 AUD/m2. Three different heliostat design concepts are described, each with features identified during a prior scoping study as having the potential to contribute to cost reduction compared to the current state-of-the-art. The three concepts which are being developed will be down-selected to a single concept for testing in late 2016. The heliostat concept development work is supported by technology development streams, developing novel sandwich panel mirror facet structures, analysing and testing wind loads on heliostats in both stow and operation positions, and developing new heliostat field layouts and software tools for optical analysis of heliostats design concepts.

High-resolution metrology of high-finesse filters

The resolution of an angle-scanning technique for measuring Fabry-Perot etalons is analysed and it is shown that sub-nanometre resolution can be readily achieved in the measurement of optical thickness variations

Refractive index inhomogeneity measurement of a parallel plate in a Fizeau interferometer by a new wavelength scanning algorithm

Interferometric measurement of the refractive index inhomogeneity of a glass parallel plate has been demonstrated experimentally to a resolution of 10-6. Wavelength scanning interferometry allows the simultaneous measurement of optical thickness and surface shape of a parallel optical plate. A new sampling function suppresses the first-order refractive index dispersion and multiple-beam interference noise to give a measurement resolution of 2 nm in optical thickness.

Improved algorithms for wavelength scanning interferometry: application to the simultaneous measurement of surface topography and optical thickness variation in a transparent parallel plate

Wavelength scanning interferometry allows the simultaneous measurement of the surface profile and the optical thickness variation of a parallel plate. However, it is necessary to evaluate the modulation frequencies of the signal and noise which depend on the optical thickness and dispersion of the test plate. New nineteen-sample, wavelength scanning algorithms allow variation in these parameters and give a measurement resolution of 1-2 nanometers rms. Measurement of a BK7 near-parallel plate of 250 mm diameter and 25 mm thickness was demonstrated in a Fizeau interferometer.