Advanced Metrology and Fabrication of Diffraction-Limited X-Ray Mirrors

In synchrotron beamlines, x-ray mirror figure specifications play a central role in determining beam quality and focusing performance. For partially coherent beams, mirror slope error remains the dominant specification and is typically required to be below 100 nrad RMS over spatial periods ranging from a few millimeters to the full mirror length, even for optics approaching 1 m in length. For diffraction-limited x-ray sources, such as Free-Electron Lasers and Diffraction-Limited Storage Rings, surface height specifications become increasingly important to preserve wavefront quality and achieve diffraction-limited focusing. In these regimes, figure requirements are commonly at the 1 nm RMS level or below. Meeting either slope- or height-based specifications presents significant challenges for synchrotron mirror metrology, particularly for optics with complex geometries including flat, cylindrical, off-axis elliptical, and fully two-dimensional curved surfaces.

At the NSLS-II Optics and Metrology Laboratory, Idir and his team have developed and deployed a suite of dedicated metrology instruments to address these challenges and support the characterization of high-precision synchrotron optics. These capabilities are tightly integrated with our in-house ion beam figuring (IBF) process for deterministic mirror correction. Their fabrication workflow relies on a stitching Fizeau interferometer as the primary in-process metrology tool, providing high-accuracy surface measurements that directly feedback to a custom-built ion beam figuring system. This closed-loop approach enables efficient convergence toward diffraction-limited mirror performance. Once the target specifications are reached, the optics are independently verified using complementary ex-situ metrology techniques.

Final validation is performed using multiple instruments, including stitching Shack–Hartmann sensors, nano-accuracy surface profilers, micro-stitching white-light interferometry, and atomic force microscopy, ensuring consistency across spatial frequency ranges. These tools are applied to both in-house fabricated optics and mirrors provided by external vendors prior to beamline installation.

In this talk, Idir will give an overview of our metrology and fabrication workflow, with emphasis on ex situ optical metrology and ion beam figuring for diffraction-limited synchrotron optics.

About the presenter
Mourad Idir, Ph.D., is a senior scientist at Brookhaven National Laboratory’s National Synchrotron Light Source-II (NSLS-II), where he directs the ex-situ and in-situ metrology and optics fabrication effort and serves as experimental development program manager. He received his doctorate from Pierre and Marie Curie University in 1994 and began his career at the Commissariat à l’Énergie Atomique (CEA), working on X-ray plasma diagnostics.

Following a postdoctoral appointment at the Center for X-Ray Optics (CXRO) at Lawrence Berkeley National Laboratory, he joined CNRS and later spent nearly a decade at Synchrotron SOLEIL, where he headed the Hard X-ray Optics Group and developed a test beamline. He joined the NSLS-II project in 2010 to establish and oversee optical metrology efforts.