Development of a superior PS-PDI(Phase-Shifting Point Diffraction Interferometer) for Absolute Shape Measurement of Large-Diameter Surfaces

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We have developed a novel phase-shifting interferometer using a spherical wavefront, as a reference surface, come out from single-mode optical fibers. This interferometer measures an absolute shape of a distorted spherical wavefront which reflected from a sample concave mirror by using another spherical wavefront. Accuracy of the spherical wavefront is estimated to be below 10^-5-6λ. In principle, this interferometer can perform an extremely accurate measurement of concave surfaces. A key factor for the development was a low-frequency vibration of the system. The effect of the vibration restricted by accepting both phase control system in light source side and an air suspension type table. As a result, a large concave mirror(200mm diameter, 1500mm radius of curvature) was measures by this system. With a newly developed data processing program which can eliminate a systematic error caused by the optical configuration of the interferometer, the absolute shape of the mirror surface was measured with a repeatability of +/-18nmPV.