Use of monochromatic phase data for improved profile measurement in spectrally resolved white light interferometry

Abstract

In Spectrally Resolved White Light Interferometry (SRWLI), the white light interferogram is spectrally decomposed by a spectrometer. The interferogram displayed at the exit plane of the spectrometer has a continuous variation of wavelength along the chromaticity axis. This interferogram encodes the phase as a function of wave number. The optical phase is determined at several wavelengths simultaneously to get the surface profile. For a given optical path difference (OPD), the phase is different for different spectral component of the source. The absolute value of OPD can be determined as the slope of the phase versus wave number linear fit. Scanned over the test surface, this OPD gives an unambiguous surface profile. This profile can be improved by the monochromatic phase data which is already available from the measurement. Combining monochromatic phase data which has 2π ambiguity with the slope data a precise profile of the surface is obtained. Noisy data however leads to the misidentification of fringe order which gives unnecessary jumps in the profile. This paper addresses this problem.