Abstract
X-ray active mirrors, such as bimorph and mechanically bendable mirrors, are increasingly being used on beamlines at modern synchrotron source facilities to generate either focused or "tophat" beams. As well as optical tests in the metrology lab, it is becoming increasingly important to optimise and characterise active optics under actual beamline operating conditions. Recently developed X-ray speckle-based at-wavelength metrology technique has shown great potential. The technique has been established and further developed at the Diamond Light Source and is increasingly being used to optimise active mirrors. Details of the X-ray speckle-based at-wavelength metrology technique and an example of its applicability in characterising and optimising a micro-focusing bimorph X-ray mirror are presented. Importantly, an unprecedented angular sensitivity in the range of two nanoradians for measuring the slope error of an optical surface has been demonstrated. Such a super precision metrology technique will be beneficial to the manufacturers of polished mirrors and also in optimization of beam shaping during experiments.
Highlights
The method presented here is based on the near-field speckles, which arise from the mutual interference of light
The average particle size of the abrasive paper is usually a few microns, which is smaller than the transverse coherence length
When the abrasive paper is translated transverse to the X-ray beam direction, the speckle pattern moves across the detector so that the detector records the same speckle at different pixel positions in different frames
Summary
The method presented here is based on the near-field speckles, which arise from the mutual interference of light. The membrane or abrasive paper is placed downstream of the mirror’s focal position and the second derivative of the wavefront, namely the inverse of its local radius of curvature is measured This takes account of imperfections introduced to the wavefront by the mirror under test and by any of the optics upstream of the point of measurement. It is more appropriate to evaluate the effective local radius of curvature as the reflective optics is located upstream of the diffuser In this case, the signal delays obtained by the cross-correlation can be related to the inverse of radius of curvature using the following relation:. This approach permits faster and more reliable optimization of optics
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