We present a two-carriage, air-bearing system on a common ceramic support beam designed to utilize multiple modes of long-trace-profiler (LTP) operation, with movable and stationary optical sensors for both coherent and incoherent light probes, dubbed the LTP-2020. Measurements with different movable and stationary sensors integrated in the LTP-2020 system allow on-bench round-robin comparisons for ensuring high-accuracy metrology of x-ray optics. In the case of variable-line-spacing (VLS) gratings, one sensor can characterize the zero-order surface, and the second, in Littrow configuration, can record diffraction angle changes without introducing uncertainty of the mutual alignment between tools. We also aspire to preserve the major advantage of the current ALS LTP-II with the capability of raising and lowering the ceramic beam with the carriages and sensors. This design allows characterization of unmounted optical substrates, as well as multi-element optical systems and large mirror assemblies, such as bendable x-ray mirrors. The modular design of the LTP-2020 gantry system together with reconfigurable optical sensors mounted to separate carriages allows operation for scanning optical surfaces at three native orientations: face-up, side-facing, face-down. We also discuss the gantry system motion control algorithms and software that enable us to perform sophisticated data acquisition based on advanced optimal scanning strategies for anti-correlation of temporal drift and systematic errors. Experimental data illustrating the high performance of the developed gantry system is also presented. This work was supported in part by the U. S. Department of Energy under contract number DE-AC02-05CH11231.
The thorough realization of the advantages of the new generation x-ray light sources, such as the Upgraded Advanced Light Source (ALS˗U) under construction, requires near-perfect x-ray optics, capable of delivering light without significant degradation of brightness and coherence. The stringent requirements of beamline optics drive the state of the art in ex situ optical metrology. Here, we present the results of the ongoing efforts at the ALS X-Ray Optics Laboratory to develop a new generation long trace profiler, LTP-2020. We discuss the system design that incorporates different types of surface slope sensors. In addition to the classical pencil beam interferometry (PBI) sensor with an improved optical design, we develop a deflectometry sensor based on a customized electronic autocollimator (AC). By applying a new data processing algorithm to the AC raw image data available from the customized AC, we significantly reduce the quasi-periodic systematic error of the AC equipped with a small size aperture. We also treat the possibility to use the AC as a PBI sensor with external light beam sources based on super-luminescent emitting diode (SLED) and single-mode laser diode (SMLD). Operation modes with stationary and/or translated sensors are possible due to the two-carriage gantry system with adjustable vertical position. The variety of the available operation modes allows optimization of the LTP-2020 experimental arrangement for providing the best possible performance in measurements with state-of-the-art aspherical x-ray optics, variable-line-spacing diffraction gratings, and multi-element optical systems.
To fully exploit the advantages of fourth-generation synchrotron light sources, diffraction-limited-storage-rings (DLSR) and fully coherent free electron lasers (FELs), beamline mirrors and diffraction grating must be of exceptional quality. To achieve the required mirror and grating quality, the metrology instrumentation and methods used to characterize these challenging optics and, even more so, optical assemblies must also offer exceptional functionality and performance. One of the most widely used slope measuring instruments for characterizing x-ray optics is the long trace profiler (LTP). The easily reconfigurable mechanical design of the LTP allows optimization of the profiler arrangement to the specifics of a particular metrology task. Here, we discuss the optical schematic, design, and performance of an original multifunctional light beam source that provides functional flexibility of the LTP optical sensor. With this source, the LTP can be easily reconfigured for measurements of x-ray mirrors or diffraction gratings that have widely different source coherence requirements. Usage of a source with a low degree of coherence for mirror metrology helps to suppress the LTP systematic errors due to spurious interference effects in the LTP optical elements. A high-coherence narrow-band source is used for groove-density-distribution characterization of x-ray diffraction gratings. The systematic error and spatial resolution of the LTP with the different sources is also measured and analyzed.
X-ray optics, desired for beamlines at free-electron-laser and diffraction-limited-storage-ring x-ray light sources, must have almost perfect surfaces, capable of delivering light to experiments without significant degradation of brightness and coherence. To accurately characterize such optics at an optical metrology lab, two basic types of surface slope profilometers are used: the long trace profilers (LTPs) and nanometer optical measuring (NOM) like angular deflectometers, based on electronic autocollimator (AC) ELCOMAT-3000. The inherent systematic errors of the instrument’s optical sensors set the principle limit to their measuring performance. Where autocollimator of a NOM-like profiler may be calibrated at a unique dedicated facility, this is for a particular configuration of distance, aperture size, and angular range that does not always match the exact use in a scanning measurement with the profiler. Here we discuss the developed methodology, experimental set-up, and numerical methods of transferring the calibration of one reference AC to the scanning AC of the Optical Surface Measuring System (OSMS), recently brought to operation at the ALS Xray Optics Laboratory. We show that precision calibration of the OSMS performed in three steps, allows us to provide high confidence and accuracy low-spatial-frequency metrology and not ‘print into’ measurements the inherent systematic error of tool in use. With the examples of the OSMS measurements with a state-of-the-art x-ray aspherical mirror, available from one of the most advanced vendors of X-ray optics, we demonstrate the high efficacy of the developed calibration procedure. The results of our work are important for obtaining high reliability data, needed for sophisticated numerical simulations of beamline performance and optimization of beamline usage of the optics. This work was supported by the U. S. Department of Energy under contract number DE-AC02-05CH11231.
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