Measuring the radius of curvature of spherical surfaces with actively tunable Fizeau and Twyman-Green interferometers

• Minjae Kim, Arjent Imeri, and Syed Reza
• received 01/21/2024; accepted 04/22/2024; posted 04/24/2024; Doc. ID 519083
• Abstract: Accurate and repeatable measurements of radius of curvature (RoC) of sphericalsample surfaces is of great importance in optics. This importance lies in the ubiquitous useof spherical optical elements such as curved mirrors and lenses. Due to a high measurementsensitivity, interferometric techniques are often deployed for accurate characterization of thesample surface RoC. One method by which a typical Commercial Fizeau or Twyman-Green(TG) interferometer measures surface RoC is via scanning between two principal retroreflectiveoptical configurations - namely the confocal and catseye configurations. Switching betweenthese two configurations is typically achieved by moving an optical head along the axis of thepropagating laser beam and the RoC is estimated by measuring the magnitude of mechanicalmotion to switch between the two principal configurations. In this paper, we propose a motion-freecatseye/confocal imaging-based sample RoC measurement system. The necessity of bulk motionto switch between the two configurations is circumvented via the use of an actively-controlledvarifocal lens. We demonstrate the usefulness of the proposed innovation in RoC measurementswith either the TG of the Fizeau interferometers. Furthermore, we converted a commercialmotion-based Zygo ⃝c RoC measurement system into a motion-free one by introducing a tunablelens inside the apparatus and using it to accurately characterize the RoC of different test samples.We also compute the wavefront aberrations for all spherical samples surfaces from the recordedmeasurement data.