The development of fully coherent free electron lasers and diffraction limited storage ring x-ray sources has brought to focus the need for higher performing x-ray optics with unprecedented tolerances for surface slope and height errors and roughness. For example, the proposed beamlines for the future upgraded Advance Light Source, ALS-U, require optical elements characterized by a residual slope error of <100 nrad (root-mean-square) and height error of <1-2 nm (peak-tovalley). These are for optics with a length of up to one meter. However, the current performance of x-ray optical fabrication and metrology generally falls short of these requirements. The major limitation comes from the lack of reliable and efficient surface metrology with required accuracy and with reasonably high measurement rate, suitable for integration into the modern deterministic surface figuring processes. The major problems of current surface metrology relate to the inherent instrumental temporal drifts, systematic errors, and/or an unacceptably high cost, as in the case of interferometry with computer-generated holograms as a reference. In this paper, we discuss the experimental methods and approaches based on correlation analysis to the acquisition and processing of metrology data developed at the ALS X-Ray Optical Laboratory (XROL). Using an example of surface topography measurements of a state-of-the-art x-ray mirror performed at the XROL, we demonstrate the efficiency of combining the developed experimental correlation methods to the advanced optimal scanning strategy (AOSS) technique. This allows a significant improvement in the accuracy and capacity of the measurements via suppression of the instrumental low frequency noise, temporal drift, and systematic error in a single measurement run. Practically speaking, implementation of the AOSS technique leads to an increase of the measurement accuracy, as well as the capacity of ex situ metrology by a factor of about four. The developed method is general and applicable to a broad spectrum of high accuracy measurements.

To preserve the brightness and coherence of x-rays produced by diffraction-limited-storage-ring (DLSR) and freeelectron-laser (FEL) light sources, beamline optics must have unprecedented quality. For example, in the case of the most advanced beamlines for the DLSR source under development at the Advanced Light Source (ALS), the ALS-U, we need highly curved x-ray mirrors with surface slope tolerances better than 50-100 nrad (root-mean-square, rms). At the ALS X-Ray Optics Lab (XROL), we are working on the development of a new Optical Surface Measuring System (OSMS) with the required measurement accuracy. The OSMS is capable for the two-dimensional (2D) surface slope metrology over the spatial range from the sub-mm scale to the clear aperture. Usage of different arrangements of the OSMS sensors allows measuring the mirrors in the face-up or side-facing orientation, corresponding to the beamline application. The OSMS translation system and data acquisition software are designed to support multi scan measurement runs optimized for automatic suppression and compensation of instrumental drifts and major angular and spatial systematic errors. Here, we discuss the recent results of the OSMS research and development project. We provide details of the OSMS design and describe results of experimental performance tests of the gantry system. In particular, we show that the system is capable for measurement repeatability with strongly curved mirrors on the level of 20 nrad (rms). The high angular resolution of the OSMS rotational tip-tilt stage is adequate for implementation of instrumental calibration with using the mirror under test as a reference. The achieved measuring accuracy is demonstrated via comparison to metrology with the carefully calibrated Developmental Long Trace Profiler, also available at the XROL.

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 advents of fully coherent free electron lasers and diffraction limited synchrotron storage ring sources of x-rays are 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 (PBI). These devices have been consistently improved upon by the x-ray optics metrology community, but appear to be approaching their metrological limits. Here, we revise the traditional optical schematic of the LTP. We experimentally show that, for the level of accuracy desired for metrology with state-of-the-art x-ray optics, the Dove prism in the LTP reference channel appears to be one of the major sources of instrumental error. Therefore, we suggest returning back to the original PBI LTP schematics with no Dove prism in the reference channel. In this case, the optimal scanning strategies [Yashchuk, Rev. Sci. Instrum. 80, 115101 (2009)] used to suppress the instrumental drift error have to be used to suppress a possible drift error associated with laser beam pointing instability. We experimentally and by numerical simulation demonstrate the usefulness of the suggested approach for measurements with x-ray optics with both face up and face down orientations.

Super high quality aspherical x-ray mirrors with a residual slope error of ∼100 nrad (root-mean-square) and a height error of ∼1-2 nm (peak-to-valley), and even lower, are now available from a number of the most advanced vendors utilizing deterministic polishing techniques. The mirror specification for the fabrication is based on the simulations of the desired performance of the mirror in the beamline optical system and is normally given with the acceptable level of deviation of the mirror figure and finish from the desired ideal shape. For example, in the case of aspherical x-ray mirrors designed for the Advanced Light Source (ALS) QERLIN beamline, the ideal shape is defined with the beamline application (conjugate) parameters and their tolerances. In this paper, we first discuss an original procedure and dedicated software developed at the ALS X-Ray Optics Laboratory (XROL) for optimization of beamline performance of pre-shaped hyperbolic and elliptical mirrors. The optimization is based on results of ex situ surface slope metrology and consists in minimization of the mirror shape error by determining the conjugate parameters of the best-fit ideal shape within the specified tolerances. We describe novel optical metrology instrumentation, measuring techniques, and analytical methods used at the XROL for acquisition of surface slope data and optimization of the optic's beamline performance. The high efficacy of the developed experimental methods and data analysis procedures is demonstrated in results of measurements with and performance optimization of hyperbolic and elliptical cylinder mirrors designed and fabricated for the ALS QERLIN beamline.

The performance of free electron laser x-ray light sources, and systems for ultrafast electron diffraction and ultrafast electron microscopy, is limited by the brightness of the electron sources used. The intrinsic emittance, or equivalently, the mean transverse energy (MTE) of electrons emitted from the photocathode determines the maximum possible brightness in such systems. With ongoing improvements in photocathode design and synthesis, we are now at a point where the physical and chemical surface roughness of the cathode can become a limiting factor. Here we show how measurements of the spatially dependent variations in height and surface potential can be used to compute the electron beam mean transverse energy (MTE), one of the key determining factors in evaluation of brightness.

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