Proceedings of the
9th International Conference of Asian Society for Precision Engineering and Nanotechnology (ASPEN2022)
15 – 18 November 2022, Singapore

Reduction of Spherical Aberration Measurement Error in High-Numerical-Aperture Spherical Test with Synthetic-Aperture Fizeau Interferometry

Toshiki Kumagai1,2,a, Keita Tomita1, Kenichi Hibino3 and Katsumi Wasaki2

1Olympus Corporation, Tatsuno, Kamiina-gun, Nagano 399-0495, Japan

2Shinshu University, Wakasato, Nagano 380-8553, Japan

3National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8567, Japan


In spherical surface tests with conventional optical Fizeau interferometers, phase modulation within the observing aperture becomes spatially nonuniform due to geometrical effects. The phase increment decreases by -50% in the marginal regions when the numerical aperture of the test spherical surface reaches 0.86, which causes systematic errors in the measured surface profile. Even the recently developed error-compensating phase-shift algorithms suffer from induced spherical aberration with a magnitude of a few nanometers. We divided the aperture into several annular regions and applied different phase-shift analyses for different phase steps to each region. The new technique decreases the modulation nonuniformity from -50% to -8%. To prove the validity of the new method, the source wavelength was slightly shifted to change the initial phase of the object interference fringes. The induced spherical aberration errors were measured and compared for the conventional algorithm, the present synthetic method, and a wavelength tuning method.

Keywords: Phase measurement; Phase-shift algorithm, Synthetic aperture, Fizeau interferometer.

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