CSIRO's 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.
NASA's Space Interferometer Mission (SIM) PlanetQuest requires, among other things, very precise retroreflectors. The CSIRO Australian Centre for Precision Optics (ACPO) has developed Double Corner Cubes (DCCs) to meet the requirements. The DCC consists of an assembly of three 30o wedged prisms optically contacted to a 132 mm diameter flat base plate. The material for all components was Zerodur. The specifications for the DCC were extremely challenging and posed considerable difficulties in the fabrication, coating, assembly, alignment and metrology. Some of the key specifications included: flatness of all reflecting surfaces to be ~ 10 nm peak to valley (P-V); dihedral angle errors < 0.5 arc seconds; collocation of the vertices of the two corner cubes within a circle of 5 um radius; all reflecting surfaces to be gold coated for a final microroughness < 0.5 nm rms; the clear or working aperture extended to within 0.2 mm of all physical edges; and the assembly had to withstand large vibrational forces. CSIRO delivered to JPL a DCC that was used as the primary unit in the so-called Kite testbed to satisfactorily meet the demonstration requirements of the SIM Milestone 8. This paper will discuss some of the procedures used to realize the DCCs and will show examples of results achieved.
There is significant sophistication in the individual fields of fabrication, coating, and metrology. Uncoated optics are characterized accurately by a wide array of techniques, as are optical coatings. However, often the coating process can change the intrinsic properties of the polished substrate such as figure, microroughness, defect density and so scattering properties. Optical components can often be distorted out of specification during assembly by contacting or cementing, and during mounting. This presentation will give examples of the interplay of all processes from fabrication, cleaning, coating, assembling and mounting on the measured performance of some precision optical components and assemblies.
Surface properties of single crystal silicon are of great interest to many people in research and development as well as in industry, particularly the semiconductor industry. In this paper we present and discuss the results of a series of nanoindentations as they relate to the elasticity and microhardness properties on the surface of a large single crystal silicon sphere. The sphere is 94 mm, it has a total roundness error of <213 nm P-V, and a super-polished surface <0.4 nm rms. The cubic structure of a silicon crystal displays different properties of hardness and elasticity in the different crystallographic planes. We present a study of the measurable differences in these mechanical properties as they relate to the <111> and the <100> directions. We show how these differences affect the overall shape of the sphere during the surface finishing process.
In order to improve the detection sensitivity of the Laser Interferometer Gravitational-wave Observatory (LIGO) the use of 40-kg sapphire test masses is being considered for the next instrument upgrade. Currently, sapphire material of adequate size is only available with the optical axis aligned with the m axis of the crystal. To determine the material's suitability it is necessary to characterize the refractive index inhomogeneity of the sapphire substrates for two orthogonal directions of polarisation, to a fraction of a part per million (ppm). We report on a method used to measure the refractive index inhomogeneity which requires three separate measurements of the polished sapphire blank in a Fizeau interferometer. These measurements are of the surface shapes or figures of the two polished sides of the blank and that of the wavefront entering side one propagating through the blank, reflected off side two and exiting through side one. The phase maps corresponding to these three measurements are combined to obtain the refractive index inhomogeneity map distribution. Measurements were carried out on two sapphire substrates (m axis) produced by the heat exchange method. The inhomogeneity maps show features which depend on polarisation direction. The physical origin of the inhomogeneities is discussed as well as the probable impact on the detection of a gravitational wave signal.
The Laser Interferometer Gravitational-wave Observatory (LIGO) is a long baseline Michelson interferometer, with arms of up to 4 km in length each containing a Fabry Perot cavity. CSIRO has manufactured 32 core optical components for the LIGO interferometer consisting of five different groups of optical elements. Long radii of curvature (7 km - 15 km) and tolerances in the order of plus or minus 200 m in the radius are specified. Although the components are made of hyper pure fused silica there are some residual inhomogeneities in the material. The optics used in transmission must be figured so that the influence of these material inhomogeneities on the transmitted wave front is compensated for. This was done by correcting the surface figure on side 2 of the optics. The approach we took to manufacturing the transmission optics was to calculate the quadratic component of refractive index gradient (Delta) n of the substrate from the measurements of the transmitted wavefront and the surface profile of the two substrate surfaces, determine what shape had to be produced on side two of the substrates to compensate for this gradient and then produce this by optical polishing. The surfaces were polished on rigid solid laps of Zerodur coated with a thin layer of Teflon as the polishing matrix, a technique developed by CSIRO for super-polishing very flat surfaces.
The wavefronts reflected by and transmitted through a coated substrate will be influenced by the non-uniformities of the coatings and distortion of the substrate produced as a result of coating stress. In this paper we describe the characterization procedure and results of a coated substrate for the Laser Interferometer Gravitational wave Observatory (LIGO) project. The fused silica substrate is 250 mm in diameter, 40 mm thick and on one side a multilayer anti- reflection coating is deposited and a 50% reflectivity multilayer coating on the other. To characterize the coatings, reflected and transmitted wavefront measurements were carried out with a 300 mm aperture phase-shifting Fizeau interferometer in combination with ellipsometric measurements of the coated surfaces. The interferometric measurements allowed the deformation of the substrate by the coatings to be assessed while the ellipsometric measurements allowed the coatings' thickness variation to be measured and the resulting phase variation in the reflected and transmitted wavefronts to be estimated. The measurements revealed substrate deformation of about 45 nm with a coating relief non-uniformity of about 5 nm over a working aperture of 200 mm.
The manufacture and testing of the 'core' optical substrates for the Laser Interferometer Gravitational-wave Observatory (LIGO) are described in this paper. These substrates are for use in long baseline Michelson interferometers with Fabry Perot cavities up to 4 km in length in each arm. The optical surfaces of the substrates (250 mm diameter by up to 100 mm thick) are specified either flat or curved, with radii of curvature varying between 7 and 15 km and tolerance bands on the radius equivalent to variations in the sag (over 200 mm) of twenty nanometers or so. Very strict tolerances were also placed on the astigmatism of the surfaces and the surface errors in two spatial frequency bands, one at low frequencies ('waviness') and another at high frequencies ('roughness'). In some cases the radius of the wavefront emerging from the substrate was also specified (for a collimated test beam).
Fizeau interferometers are traditionally used to determine the surface figure of uncoated optics. Since the reflectances from the uncoated reference and test surfaces are usually equal, the intensity of the interference fringe minimum is zero, so the fringe contrast is unity. If the same reference surface is used to measure the figure of a higher reflecting surface, the fringe minimum intensity is no longer zero and the fringe contrast decreases, eventually reaching zero for 100 percent reflecting optics. The problem can be overcome if the reference surface is coated with a lossy reflecting surface. We describe a coating to measure the figure of optical components having reflectance from 100 percent to 3.5 percent at 1064 nm. The spatial variations across the 150 mm working aperture in physical thickness of the two materials used to coat the reference surface were determined to an accuracy better than 1 nm using a single wavelength ellipsometer operating at 633 nm. Variations across the aperture of the phase changes on reflection from both sides of the reference flat substrate and on transmission were estimated. These results are presented together with calculations designed to determine the relative positions of the outside of the coating with respect to substrate surface as well as the position as seen optically. Substrate distortion due to stress in the coatings has also been measured.
CSIRO is manufacturing the `core' optical substrates for LIGO, a Michelson interferometer with arms up to 4 km in length each containing a Fabry Perot cavity. The beam splitter and input test mass mirrors (the entrance mirror to each cavity) have specifications not only for the optical surfaces but also for the radius of curvature of the wave front transmitted through the optical substrate. Our approach to manufacturing the substrates is to calculate the quadratic component of refractive index gradient (Delta) n from measurement of the transmitted wave front and the surface relief of the two substrate surfaces. After one of the surfaces (S1) is polished to specification, the radius on the second side required to achieve the specification on the transmitted wave front is calculated (using the measured value of (Delta) n, the actual value of S1 and the target value of the transmitted wave front). Results of this work and complications of the measurement procedure due to the thermal inertia and poor thermal conductivity of the silica substrates will be presented.
The Laser Interferometer Gravitational-wave Observatory (LIGO) core optical components have been manufactured by CSIRO. These optical substrates are optically polished on a lap surface that is made of Teflon coated onto a thick rigid faceted Zerodur base. To produce the km-scale radii (> 10 km) on these substrates the lap surface is shaped by abrading it with a fine ground silica plate whose radius of curvature corresponds to the one specified for the LIGO component. The plates are measured by a commercial phase stepping interferometer which is used in a grazing incidence arrangement. We describe the process of shaping and measuring the conditioning plates and laps.
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.
Optical interferometers are being built to operate over longer and longer baselines, in some cases more than a few kilometers. Examples include laser interferometers to detect gravitational waves, and stellar interferometers which use starlight to determine the angular diameter of the source. The optics used in these interferometers must satisfy demanding performance criteria. In this paper we review some of those criteria and present results which demonstrate that such optical surfaces can be successfully fabricated and measured.
Metrology procedures for determining the power, astigmatism, low and high spatial frequency variations in the surface profile on flat and curved optical surfaces are described. The procedures are applied to the characterization of optics produced for the Pathfinder program of the laser interferometer gravitational observatory and demonstrate that in the case of low spatial frequency surface errors measured by optical interferometry, measurements to a resolution down to (lambda) /2000 are possible in the measurement of the standard deviation of surface variations.
Fabrication procedures employed for producing optical components for the laser interferometer gravitational-wave observatory are described and discussed. Two ultra-pure fused silica substrates, 250 mm diameter and 100 mm thick, were polished to very high surface quality and precision. One side of each substrate is flat with the other side having a very long concave radius of curvature of 6000 m. The performance of the Teflon lap technique in polishing both surfaces will be reviewed.
We describe the optical grinding and polishing, and the measurement of sphericity by multiple roundness profiles, of large single crystal silicon spheres (balls) (93.6 mm diameter and 1 kg mass). The balls are used in direct measurements of volume and density in the determination of the Avogadro constant. A typical peak to valley error of sphericity achieved was < 60 nm. The arithmetic mean deviation of the error surface relative to a least square sphere was around 16 nm. The microroughness was < 0.2 nm rms when measured with a WYKO TOPO 3D noncontact surface profiler. Defects in the optical finish were negligible when compared to the total allowable uncertainty specified for the volume of the balls of 0.1 ppm.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.