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.
The spectral performance requirements of optical thin film coatings continue to be increasingly demanding. Optical thin film design programs are now so sophisticated that they can meet the majority of these specifications. However, producing these designs is often very difficult. Many of the resulting designs have many layers (>50) of unequal optical thicknesses (non-quarter-waves) including both very thin layers (<5 nm) and thick layers. The tolerances on layer thicknesses and refractive indices are usually very tight (<1 nm per layer). Techniques for the monitoring of optical coatings range from crystal oscillator thickness and rate monitoring, time monitoring for sputtered films, single- wavelength turning point monitoring (particularly narrow-band interference filters such as DWDMs), and multiwavelength monitoring for multilayers having complex designs with possible real time re-optimization. This presentation will describe the use of ellipsometry for optical monitoring. Advantages of the technique are that both refractive index and thickness are measured during deposition, meaning that accurate design re-optimization is possible after each layer is terminated. Very thin layers can be measured accurately, and unlike photometric monitoring the technique has high sensitivity for all layer thicknesses. Examples will be given that illustrate the advantages of ellipsometric monitoring, including a laser notch plus band-blocker filter and a very broadband antireflection coating.
Narrow bandpass Fabry-Perot etalons are widely used in solar astronomy for spectroscopic imaging. Solid electro-optically tunable filters made of thin, single-crystal lithium niobate are presented in this article. The pass-band is typically ~0.02nm at 550nm. We describe customized corrective and high-reflectivity optical coatings designed and manufactured to tailor the filter for the specific application. Spectral reflectance is calculated to satisfy wavelength requirements and to achieve optimal optical performance. The measured optical thickness of the lithium niobate wafer is an important factor in determining the optimal design of the etalon mirrors. Out-of-band rejection and bandwidth requirements are also taken into account, as well as the influence of the spectral properties of a high-order filter which blocks adjacent etalon orders. Design customization is particularly important in the case of tandem and double-pass etalons.
The Space Interferometry Mission (SIM) PlanetQuest is managed by the Jet Propulsion Laboratory for the National Aeronautics and Space Administration. SIM requires, among other things, high precision double cube-corner retroreflectors. A test device has recently been fabricated for this project with demanding specifications on the optical surfaces and gold reflective coatings. Several gold deposition techniques were examined to meet the stringent specifications on uniformity, optical properties, micro-roughness and surface quality. We report on a comparative study of optical performance of gold films deposited by resistive and e-beam pvaporation, including measurements of the scattering from the coated surfaces. The effects of oxygen bombardment and titanium under-layer on optical properties and adhesion were evaluated. The influence of surface preparation on the optical properties was examined also.
Roger Netterfield, Mark Gross, Fred Baynes, Katie Green, Gregory Harry, Helena Armandula, Sheila Rowan, Jim Hough, David Crooks, Martin Fejer, Roger Route, Steven Penn
A significant limiting factor on the sensitivity of interferometric gravitational wave detectors has been identified as thermal noise generated by mechanical loss in the high reflectivity dielectric mirror coatings on the test masses. The development of coatings which maintain high optical performance and minimize mechanical loss is therefore vital if the current designs of interferometers are to achieve adequate sensitivity. While the origins of the mechanical loss are yet to be fully elucidated, some progress has been made toward minimizing it, although there is still some way to go before specifications can be met. The work reported here is progress made toward achieving low mechanical loss coatings on behalf of the LIGO consortium. The current directions include attempts to reduce the loss in the coating materials by control of the coating stoichiometry and intrinsic stress. This includes such methods as ion bombardment of the growing films and optimization of post-deposition thermal treatments.
A technique is described for modifying the spectral profile of a Fabry-Perot etalon by introducing a spatial variation to the etalon thickness and reflectivity. The viability of the technique is demonstrated by grinding and polishing a solid lithium niobate etalon to give a controlled thickness variation. A flat-topped transmission profile is generated from the measured thickness map, and further modification of the profile is predicted by introducing customized variations in thickness and reflectivity of the etalon mirrors
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.
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.
A novel resonant grating filter has been designed and constructed for the red part of the visible spectrum. A multilayer waveguide structure underneath a subwavelength grating has been used to reduce the reflectivity of the grating in the vicinity of the resonant peak.
Angular selective films have been reactively deposited onto an oblique glass substrate by vacuum arc evaporation. The deposition beam is magnetically filtered before striking the glass. The structure and optical properties of these films are discussed for targets of aluminum, chromium, and titanium at various partial pressures of oxygen and various final thicknesses. The most promising films are cermets, with aluminum giving superior angular selectivity for the deposition conditions studied. The angle of incidence dependence of solar and luminous transmittance is analyzed for some Al/Al2O3 films and their performance is compared to simple isotropic solar control films.
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