Extending the range of interferometry though subaperture stitching

Abstract

Subaperture stitching is a well-known technique for extending the effective aperture or dynamic range of phase measuring interferometers. Stitching has been routinely applied to extend the spatial coverage of plano interferometers. Due to the presence of additional mechanical and optical degrees of freedom, it has been much more difficult to apply stitching techniques to spherical and aspherical interferometric testing. Particular care must be taken in mechanical alignment, motion control, and mathematical processing of subaperture phase data in order to obtain an accurate reconstruction of the full-aperture phase map. We have designed and developed an interferometer workstation specifically to perform high-accuracy subaperture stitching of spherical and flat surfaces up to 200 mm (8) in diameter. The workstation combines a commercial 100 mm (4) Fizeau interferometer, a 6-axis stage, and a software package that automates the entire stitching process. Automation of the measurement design, motion control, phase data acquisition, and data analysis process allows complex stitched measurements to be made in a shop-floor production environment by optical technicians of modest skill. Mathematical techniques were developed to compensate for several classes of systematic and random errors inherent in the measurement process, e.g. motion errors, magnification error, viewing system distortion, and reference wave error. This allows each measurement to be self-calibrated. Basic system capability is demonstrated by comparing a conventional full-aperture phase measurement of a surface to a measurement reconstructed from stitched subapertures.