[2025] Improving aspheric metrology with computer-generated holograms (CGH) on an automated interferometric workstation

Paul E. Murphy, Shelby Ament, Cormic Merle, Tyler Steele, Johnathan M. Davis, Jacob Siegel, and Lee Sortore “Improving aspheric metrology with computer-generated holograms (CGH) on an automated interferometric workstation”, Proc. SPIE 13728, Optifab 2025, 137280Q (3 December 2025); https://doi.org/10.1117/12.3071950

Abstract:

Aspheric surfaces provide significant benefits to optical systems. In asphere fabrication, metrology is key to enabling efficient production. Several precision metrology capabilities exist, including interferometric null testing and interferometric subaperture stitching, each with distinct benefits. Interferometric null tests require a custom nulling optic, and the computer-generated hologram (CGH) is commercially available to fulfill this need. But there are important considerations for alignment, calibration, and analysis required to achieve maximum benefit. The Aspheric Stitching Interferometer (ASI™), a multi-axis automated interferometric workstation, is a highly capable and proven asphere metrology system, but acquisition time can limit throughput, and measurable surface asphericity is limited by slope departure from best-fit sphere. We aim to simplify the user experience in aligning, acquiring, and calibrating CGH null measurements, and improve the throughput and aspheric departure that can be achieved by the ASI. We achieve this by integrating a CGH module into an ASI, combining the accuracy and alignment features of an Arizona Optical Metrology (AOM) CGH with the automated multi-axis workstation and software of the ASI. We detail the basic tasks in making a CGH null measurement using the ASI, including the alignment of the test asphere, vertex radius calculation, and error compensation. We demonstrate software-assisted alignment techniques for the CGH and test surface, as well as automatic compensation of systematic errors. Integrating the CGH null element and ASI motion system and software leverages the strengths of each technology and streamlines the measurement process – ideal for production environments with operators and technicians. It also provides a path to subaperture stitching with a CGH, enabling interferometric measurement of larger aperture convex aspheres.

Link to publication.