Surface topography and light scattering were measured on 15 samples ranging from those having smooth surfaces to others with ground surfaces. The measurement techniques included an atomic force microscope, mechanical and optical profilers, confocal laser scanning microscope, angle-resolved scattering, and total scattering. The samples included polished and ground fused silica, silicon carbide, sapphire, electroplated gold, and diamond-turned brass. The measurement instruments and techniques had different surface spatial wavelength band limits, so the measured roughnesses were not directly comparable. Two-dimensional power spectral density ͑PSD͒ functions were calculated from the digitized measurement data, and we obtained rms roughnesses by integrating areas under the PSD curves between fixed upper and lower band limits. In this way, roughnesses measured with different instruments and techniques could be directly compared. Although smaller differences between measurement techniques remained in the calculated roughnesses, these could be explained mostly by surface topographical features such as isolated particles that affected the instruments in different ways.
Light scattered from interface imperfections carries valuable information about its origins. For single surfaces, light-scattering techniques have become a powerful tool for the characterization of surface roughness. For thin-film coatings, however, solving the inverse scattering problem seemed to be impossible because of the large number of parameters involved. A simplified model is presented that introduces two parameters: Parameter δ describes optical thickness deviations from the perfect design, and parameter β describes the roughness evolution inside the coating according to a power law. The new method is used to investigate structural and alteration effects of HR coatings for 193 nm, as well as laser-induced degradation effects in Rugate filters for 355 nm.
The light scattering of rough metallic surfaces with roughness levels ranging from a few to several hundred nanometers is modeled and compared to experimental data. Different modeling approaches such as the classical Rayleigh-Rice vector perturbation theory and the new Generalized Harvey-Shack theory are used and critically assessed with respect to ranges of validity, accuracy, and practicability. Based on theoretical calculations and comparisons with Rigorous Coupled Wave Analysis for sinusoidal phase gratings, it is demonstrated that the approximate scatter models yield surprisingly accurate results and at the same time provide insight into light scattering phenomena. For stochastically rough metal surfaces, the predicted angles resolved scattering is compared to experimental results at 325 nm, 532 nm, and 1064 nm. In addition, the possibilities of retrieving roughness information from measured scattering data for different roughness regimes are discussed.
An outstanding technique in point of ultra-precision as well as economical production of mirrors is Single Point Diamond Turning (SPDT). The unique properties of the diamonds are used to get optical surfaces with roughness values down to 5 nm rms (root mean square) and very precise form accuracy down to 70 nm rms and 500 nm p.-v. (peak to valley) value over an area of 200 mm x 200 mm. This quality level is typical for applications in the Near Infrared (NIR) and Infrared (IR) range. For applications in the VIS and UV range the turning structures must be removed with a smoothing procedure in order to minimize the scatter losses. Favorable is an aluminium base body plated with a thick-film of Nickel-Phosphorus alloy (NiP). This alloy can be polished with computer assistance. Ion Beam Figuring (IBF) is the final manufacturing step. The properties after the finishing process are better than 1 nm rms for roughness and down to 15 nm rms respectively 100 nm p.-v. regarding the surface irregularity for complex optical shapes. The techniques SPDT, polishing and IBF ensures a high quality level for large mirrors with plan, spherical or aspherical surfaces. The manufacturing chain will be analyzed by surface characterisation based on 2D profilometry and white light interferometry to measure the roughness and 3D-profilometry and interferometry to monitor the shape irregularity. Scattering light analysis deepens these investigations. This paper summarizes technologies and measurement results for SPDT and surface finish of metal mirrors for novel optical applications
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