A customized planetary rotation stage has been fitted to a commercial ion beam sputter coater to enable the deposition of
high uniformity, multilayer optical coatings on large substrates without the use of masks.
Uniformity in this system achieved by sequentially depositing each layer in two fixed locations in the sputtered particle
plume where the geometry of the natural thickness distributions on a rotating substrate in these locations are of
complementary shape and add to produce an overall uniform layer. The modified planetary stage allows substrate
rotation about its own axis at any fixed position of the substrate centre about the axis of the planetary system. The
suitable locations in the plume of each material that allow maximum uniformity are found by trial and error refinement
of locations obtained by modelling of the plume distribution and expected thickness distributions. Ellipsometric
monitoring of the thickness of the layer in each fixed position is used to determine the precise ratio of thicknesses in each
location needed to obtain the correct total layer thickness simultaneously with high uniformity.
The system has thus far enabled single wavelength antireflection coatings of less than 0.001% reflectance to be
fabricated over 270 mm diameter substrates. This requires the film thickness uniformity on all layers to be less than ± 0.2%. In addition, 4-layer, dual wavelength antireflection coatings have been fabricated with less than 0.01% reflectance
on both wavelengths over similar substrate dimensions.
The process of fabricating optical surfaces has remained relatively unchanged for hundreds of years and only within the
last 10-20 years have significant improvements been made to the basic techniques of manual polishing. Various
deterministic grinding, etching, and polishing machines are now available that can generate free-form optical surfaces by
sub-aperture removal of surface material. This presentation will concentrate on a new technique for deterministic
surface correction that is achieved using selective deposition of an index matched material simultaneously over the entire
surface of the optic. The technique uses a multi-aperture mask to selectively occlude the plume from a physical vapour
deposition system. The mask design has two functions; firstly to remove the inherent variation in the plume itself; and
secondly, to create the required spatially varying layer to correct or form the final optical shape. This ability to correct
the entire surface of the optic simultaneously has the considerable advantage that the corrections are achieved in a very
short space of time and deposition times of 100-300 seconds are typical. A further advantage of this technique is that the
surface roughness does not seem to be affected by the deposition process, and so the surface quality of the optic is
maintained throughout the correction process. The technique has been successfully applied to both transmissive and
reflective optics and has achieved uniformities of λ/1000 rms. During the talk I will present our latest results and
progress towards selective etching, full edge correction, and spatially varying thin-film filter deposition.
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.
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.
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.
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