Jeffrey A. Seckold

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.

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.

Design of a lithium niobate Fabry–Perot étalon-based spectrometer

The design of a tunable Fabry-Perot étalon-based filter that has a passband of 0.01 nm and a free spectral band larger than 50 nm when operated in the center of the visible spectrum is described. The filter consists of two Y-cut lithium niobate étalons having thicknesses in a vernier ratio. The polarization state of light passing through the tandem étalons is rotated 90 degrees before again being transmitted through the étalon pair. If the components are arranged in a symmetrical manner, the filter will operate with unpolarized incident light. Each étalon in double pass will have a greater transmittance than two individual étalons of the same average optical thicknesses, since variations in the physical thickness due to fabrication errors are correlated.

The design and construction of a prototype lateral-transfer retro-reflector for inter-satellite laser ranging

The Gravity Recovery and Climate Experiment (GRACE) mission, launched in 2002, is nearing an end, and a continuation mission (GRACE Followon) is on a fast-tracked development. GRACE Follow-on will include a laser ranging interferometer technology demonstrator, which will perform the first laser interferometric ranging measurement between separate spacecraft. This necessitates the development of lightweight precision optics that can operate in this demanding environment. In particular, this beam routing system, called the triple mirror assembly, for the GRACE Follow-on mission presents a significant manufacturing challenge. Here we report on the design and construction of a prototype triple mirror assembly for the GRACE Follow-on mission. Our constructed prototype has a co-alignment error between the incoming and

Certification of the full size double corner cube fiducials for the Space Interferometer Mission-PlanetQuest test bed

A full size Double Corner Cube (DCC) assembly was delivered recently to NASAs Space Interferometer Mission (SIM) PlanetQuest testbed at JPL. The DCC was developed at CSIROs Australian Centre for Precision Optics (ACPO) to demonstrate the fabrication of the flight size DCC fiducials. The DCC was assembled from three 30°, high precision ULE glass wedges and a 132 mm diameter base plate. After alignment to sub arc-second angular tolerances, the three wedges were chemically bonded to the base-plate. Comprehensive testing was performed on the assembly to certify the compliance of several parameters including the dihedral angle errors, figure of all reflecting surfaces and the Non Common Vertex Error (NCVE) of the DCC. This paper elaborates on some of the metrology and the certification results of the delivered DCC assembly as well as the chemical bond strength tests.

Development of precision double corner cubes for the Space Interferometer Mission

NASAs 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.

Testing the GRACE follow-on triple mirror assembly

We report on the successful testing of the GRACE follow-on triple mirror assembly (TMA) prototype. This component serves to route the laser beam in a proposed follow-on mission to the Gravity Recovery and Climate Explorer (GRACE) mission, containing an optical instrument for space-based distance measurement between satellites. As part of this, the TMA has to meet a set of stringent requirements on both the optical and mechanical properties. The purpose of the TMA prototype testing is to establish the feasibility of the design, materials choice and fabrication techniques. Here we report on co-alignment testing of this device to the arc second (5 μrad) level and thermal alignment stability testing to 1 .

Long-term stability of optical surfaces

It is not uncommon in high-precision testing of optical surfaces to find that repeated measurements produce inconsistent results. The most obvious cause is where the test system is not in thermal equilibrium, or where, for example, in the case of massive telescope mirror substrates, the support structure loading has changed. But even where the ambient temperature is precisely controlled and the test setup, procedure, and overall environment are apparently unchanged, it is a matter of experience that unexplained discrepancies can sometimes remain. Dew reported the results of long-term tests of a 30.5cm-diam optical glass flat with a thickness of 5.1 cm. The glass type was Chance hard crown ~HC 524589!, classified as ‘‘fine annealed,’’ with no birefringence evident in a strain viewer when viewed normal to the two flat surfaces. During a period of 8 yr it was found that the optically flat surface changed systematically with time to give a cumulative error of 0.4l (l5546.1 nm), the rate of change decreasing toward the end of the period. Other flats measured by this author also revealed changes with time. No apparent correlation was found between the level of observed strain bire-

Coating requirements for the reference flat of a Fizeau interferometer used for measuring from uncoated to highly reflecting surfaces

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.

LIGO optics manufacture: figuring transmission core optics for best performance

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.