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Absolute distance measurements with micrometer resolution using white-light spectral interferograms processed by a phase-locked loop method

A new spectral-domain interferometric method of measuring absolute distances is utilized when the effect of low dispersion in an interferometer is known and the spectral interference fringes are resolved over a wide spectral range. First, processing the recorded spectral interferograms by a phase-locked loop (PLL) method, which is a special simplified version of the general recurrence non-linear data processing method, the unmodulated spectrum, the spectral fringe visibility function and the unwrapped spectral fringe phase function are obtained. Then, knowing the dispersion relation for the material present in the interferometer, the material effective thickness is determined. Finally, the positions of the interferometer mirror are determined precisely by fitting the recorded spectral interferograms to the theoretical ones knowing all the mentioned spectral functions.