Interferometric profile scanning system for measuring large planar mirror surface based on single-interferogram analysis using Fourier transform method

An interferometric profile scanning system is proposed to measure straightness of a planar mirror surface. Since surface profiles of reference mirrors used in precision systems, such as a bar mirror of an X-Y stage controlled by a laser interferometer, may be distorted during installation on the systems, they need to be evaluated through on-site measurement. To implement the on-site measurement scheme, we adopted a sub-aperture interferometer configuration and Fourier transform method for evaluating each interferogram. Since the proposed system stitches multiple sub-aperture profiles obtained by analyzing the single-interferogram, it can evaluate straightness of a reference mirror during continuous scanning motion of a stage where the mirror is fixed. In the experiments, straightness of a bar mirror was measured 100 times over 383 mm range and standard deviations were less than 18 nm. The averaged profile agreed with the one obtained by a commercial profile interferometer within 15 nm in the central range of 265 mm. We also present exemplary measurement data showing surface profile variations caused by fixing force.