Binary Pseudo-Random Array (BPRA) Standards for Inspection and Calibration of Cylindrical Wavefront Interferometry

High-accuracy metrology is vitally important in manufacturing and optimally using ultra-high-quality free-form mirrors designed, for example, for space X-ray telescopes to manipulate X-ray light with nanometer-scale wavelengths. Due to the shorter wavelength, requirements on the surface figure (shape) and finish (roughness) of X-ray mirrors are many orders of magnitude more stringent than for visible-light optics. Themetrology integrated into X-ray mirror manufacturing must ensure the accuracy of optical surface fabrication on the sub-nanometer level over a large area (on the scale of a meter and even more) strongly aspherical optical elements with the sagittal ROC on the order of a meter and less, whereas the tangential ROC can reach a few hundred meters. The absence of the required metrology is the major limitation of modern technology used for the fabrication of X-ray mirrors. As an adage says, If you cant measure it, you cant make it. Binary Pseudo Random Array test artifacts and accompanying data processing software offer unparalleled advantages to quantitatively characterize the instrument transfer function (ITF) of the metrology tools and enable data reconstruction to reveal the true surface. Previously, BPRA Based methodology has been validated with planar wavefront.In this Project, BPRA-based test artifacts data processing methodology is adapted for the Cylindrical Wavefront Interferometry for inspection and calibration for mid-long range spatial frequencies. In Phase II, we will develop BPRA test artifacts with adjustable Radius of Curvature, and Computer Generate Holograms combined with BPRA for thorough calibration, and then data reconstruction based on the measured calibration data. The final product will be the BPRA test artifact with user-friendly and GI-controlled software. High-accuracy metrology is vitally essential in manufacturing and optimally using ultra-high-quality free-form mirrors designed, for example, for space X-ray telescopes to manipulate X-ray light with nanometer-scale wavelengths. Due to the shorter wavelength, requirements to the surface figure (shape) and finish (roughness) of X-ray mirrors are many orders of magnitude more stringent than for visible-light optics. The metrology integrated into X-ray mirror manufacturing must ensure the accuracy of optical surface fabrication on the sub-nanometer level over large-area (on the scale of a meter and even more) strongly aspherical optical elements. Metrology technology has not kept up with the advancement in fabrication technologies. The deficiencies in the metrology, rather than in the fabrication technologies, primarily limit the optical quality. As an adage says, “If you can’t measure it, you can’t make it.” We will develop novel turn-key technology and methodology for high-precision calibration and sophisticated data processing directed to advance Cylindrical Wavefront Interferometry. The key novelty of our approach is the development and application for the calibration of different test patterns (including the highly randomized, HR, uniformly redundant, URA, BPRAs, and traditional checkerboard patterns) imprinted to shape variable (bendable) substrates. We will also develop a test artifact with a cylinder computer-generated hologram (CGH) with an encoded BPRA. This allows the use of a CGH /BPRA  for measurements of the instrument transfer functions of interferometers that are equipped .with Fizeau objectives..  Realizing the proposed goals will open a new avenue for fabrication and performance characterization of large-area, strongly aspherical, grazing-incidence x-ray mirrors that are critical optical elements of the high-performance space x-ray telescopes and beamline systems at modern x-ray facilities. State-of-the-art will be achieved using the easy-to-adapt ITF characterization and data processing methodology. The final product will consist of the BPRA test standard on cylindrically haped and bendable substrates, and analysis software, which can be easily integrated into the existing CWI-based metrology systems.

This product improves the integrated metrology process. The ability to assess the high accuracy and high-efficacy metrology is key to further improvement of the optical fabrication and lowering the cost of the X-ray optics.  Both precision optics manufacturers and metrology tool makers are strongly interested in this technology. They all agree on the importance of metrology to further improve the product quality in optics and provide better metrology tools (including interferometers) to the customers. 

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