Calibrating interferometric imaging distortion using subaperture stitching interferometry

Abstract

Optical surfaces are routinely measured using phase-shifting interferometry. The fringe imaging and other interferometer optics introduce distortion into the measurements. Distortion causes a change in magnification as a function of field position, and is often not quantified and calibrated during measurements of optical surfaces. When calculating the figure of an optical surface, systematic errors such as distortion will ultimately limit the accuracy of the measurement. We present a method for improving the accuracy in interferometric measurements using subaperture stitching interferometry. QED's Subaperture Stitching Interferometer (SSI®) is a six-axis computer-controlled workstation that incorporates a standard Fizeau interferometer with our own stitching algorithms. The SSI is a commercially available product that automatically performs inline calibration of systematic errors such as reference wave and distortion. By measuring an optical surface in multiple orientations both on and off-axis, our stitching algorithms are shown to have the ability to measure the distortion (and other systematic errors) in an interferometer, and compensate for these errors automatically. Using the compensators obtained from stitched measurements, distortion values are calculated and plots are shown for several different transmission optics. Theoretical simulations displaying the effects of distortion on surface metrology are shown. Measurements are taken with and without distortion compensators, and the residual difference is analyzed.