The research and development work on the Advanced Light Source (ALS) upgrade to a diffraction limited storage ring light source, ALS-U, has brought to focus the need for near-perfect x-ray optics, capable of delivering light to experiments without significant degradation of brightness and coherence. The desired surface quality is characterized with residual (after subtraction of an ideal shape) surface slope and height errors of <50-100 nrad (rms) and <1-2 nm (rms), respectively. The ex-situ metrology that supports the optimal usage of the optics at the beamlines has to offer even higher measurement accuracy. At the ALS X-Ray Optics Laboratory, we are developing a new surface slope profiler, the Optical Surface Measuring System (OSMS), capable of two-dimensional (2D) surface-slope metrology at an absolute accuracy below the above optical specification. In this article we provide the results of comprehensive characterization of the key elements of the OSMS, a NOM-like high-precision granite gantry system with air-bearing translation and a custom-made precision air-bearing stage for tilting and flipping the surface under test. We show that the high performance of the gantry system allows implementing an original scanning mode for 2D mapping. We demonstrate the efficiency of the developed 2D mapping via comparison with 1D slope measurements performed with the same hyperbolic test mirror using the ALS developmental long trace profiler. The details of the OSMS design and the developed measuring techniques are also provided.
The development of deterministic polishing techniques has given rise to vendors that manufacture high quality threedimensional x-ray optics. The surface metrology on these optics remains a difficult task. For the fabrication, vendors usually use unique surface metrology tools, generally developed on site, that are not available in the optical metrology labs at x-ray facilities. At the Advanced Light Source X-Ray Optics Laboratory, we have developed a rather straightforward interferometric-microscopy-based procedure capable of sub microradian characterization of sagittal slope variation of x-ray optics for two-dimensionally focusing and collimating (such as ellipsoids, paraboloids, etc.). In the paper, we provide the mathematical foundation of the procedure and describe the related instrument calibration. We also present analytical expression describing the ideal surface shape in the sagittal direction of a spheroid specified by the conjugate parameters of the optic’s beamline application. The expression is useful when analyzing data obtained with such optics. The high efficiency of the developed measurement and data analysis procedures is demonstrated in results of measurements with a number of x-ray optics with sagittal radius of curvature between 56 mm and 480 mm. We also discuss potential areas of further improvement.
Recently, an original method for the statistical modeling of surface topography of state-of-the-art mirrors for usage in xray
optical systems at light source facilities and for astronomical telescopes [Opt. Eng. 51(4), 046501, 2012; ibid. 53(8),
084102 (2014); and ibid. 55(7), 074106 (2016)] has been developed. In modeling, the mirror surface topography is
considered to be a result of a stationary uniform stochastic polishing process and the best fit time-invariant linear filter
(TILF) that optimally parameterizes, with limited number of parameters, the polishing process is determined. The TILF
model allows the surface slope profile of an optic with a newly desired specification to be reliably forecast before
fabrication. With the forecast data, representative numerical evaluations of expected performance of the prospective
mirrors in optical systems under development become possible [Opt. Eng., 54(2), 025108 (2015)]. Here, we suggest and
demonstrate an analytical approach for accounting the imperfections of the used metrology instruments, which are
described by the instrumental point spread function, in the TILF modeling. The efficacy of the approach is demonstrated
with numerical simulations for correction of measurements performed with an autocollimator based surface slope
profiler. Besides solving this major metrological problem, the results of the present work open an avenue for developing
analytical and computational tools for stitching data in the statistical domain, obtained using multiple metrology
instruments measuring significantly different bandwidths of spatial wavelengths.
The advent of fully coherent free electron laser and diffraction limited synchrotron storage ring sources of x-rays is
catalyzing the development of new ultra-high accuracy metrology methods. To fully exploit the potential of these
sources, metrology needs to be capable of determining the figure of an optical element with sub-nanometer height
accuracy. Currently, the two most prevalent slope measuring instruments used for characterization of x-ray optics are the
auto-collimator based nanometer optical measuring device (NOM) and the long trace profiler (LTP) using pencil beam
interferometry. These devices have been consistently improved upon by the x-ray optics metrology community, but
appear to be approaching their metrological limits. Here, we consider a novel operational mode for the LTP. The
fundamental measuring principle of the LTP is reviewed, and a suggested mode of operation is analytically derived. This
mode of operation leads to significant suppression of the instrumental systematic errors. Via cross-comparison
measurement with the LTP in old and new modes, the performance of the profiler in the new mode is investigated. We
also discuss potential areas of further development, including the possibility for local curvature measurement.