Bridges, A; Yacoot, A; Kissinger, T; Humphreys, D A; Tatam, R P (2021) Correction of periodic displacement non-linearities by two-wavelength interferometry. Measurement Science and Technology, 32 (12). 125202

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Abstract

Non-linearities in interferometric displacement measurements commonly affect both homodyne and heterodyne optical interferometers. Unwanted back reflections (ghost reflections) or polarisation leakage introduce non-linearity terms at harmonics of the illuminating wavelength that cannot be fully corrected for with standard non-linearity correction techniques. A two-wavelength interferometric approach, operating at 632.8 nm and 785 nm, is presented here that is capable of correcting such non-linearities. Non-linearities are separated from the difference between two displacement measurements made at differing wavelengths with a Fourier approach. Compared to a standard Heydemann ellipse fitting correction, the proposed approach reduces estimated residual non-linearities from 84 pm to 11 pm in the case of a linear displacement profile. In particular this approach is applicable to the correction of higher order non-linearities that are caused by multiple reflections, and that are therefore very sensitive to alignment conditions.

Item Type:	Article
Keywords:	interferometry, non-linearity, dimensional metrology
Subjects:	Engineering Measurements > Dimensional
Divisions:	Engineering
Identification number/DOI:	10.1088/1361-6501/ac1dfa
Last Modified:	12 Jan 2022 14:54
URI:	http://eprintspublications.npl.co.uk/id/eprint/9312

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