Characterization of optical surfaces for the present generations of synchrotron sources

Abstract

Generalization of specific optical metrology and systematic testing of all delivered components has yield in the last decade to a significant improvement of the optical surfaces installed on synchrotron radiation (SR) beamlines around the world. Surface roughness is classically characterized by phase-shift interferential microscope, sometimes AFM. Long trace profiler (LTP)1, which measures the local slope along a line profile, has been the choice instrument to measure figure errors of large size components. Present LTPs have accuracy around 0.2 μrad RMS, and a spatial resolution around 1 mm, but they cannot provide 2D measurement nor access the small radii that modern sagitally focusing SR optics calls for. Stiching interferometry or Shack-Hartmann methods are good candidate for 2D measurement of figure errors. With increasing quality of mirrors and increasing coherence of synchrotron sources the simple specification of surface figure by the RMS slope errors of low spatial frequencies and roughness by the standard deviation of the high frequency surface height fluctuation is becoming less and less relevant. In order to achieve given performances the properties of an optical surface are better specified by the power spectrum of the surface errors in different spatial frequency ranges, the definition of which depends on the actual use of the surface. Evaluating the quality of SR mirrors used to form micro or nano X-ray probes should be checked by modeling the point spread function. It requires an accurate measure of the surface shape in the low frequency range. The next step of SR optics improvement will come from local polishing techniques guided by metrology. The required level of accuracy is calling for a panel of measuring methods adapted to different spatial frequency that are still under development.