Spectral modulated interference fringes are observed in the form of the periodical modulation of a broadband spectrum at the output of an interferometer provided with a subsequent spectrometer. The group optical path difference of interfering light waves corresponding to the distance from the surface to be measured is characterized by the spectral fringe phase function. To recover the phase functions, a standard, Fourier-transform method, or parametric methods like a phase-locked loop (PLL) method, can be used. In the former case, the Fourier spectrum in the wavelength domain is computed and filtered out to obtain the reference spectrum and the overall phase information using a phase-unwrapping algorithm. In the latter case, the fringe phase deviations are traced dynamically in the independent variable domain, i.e. in the wavelength domain when the PLL method is applied to spectral interferometry. The PLL method was used to demodulate spectral fringes iteratively. A spectral fringe signal with a priori unknown carrier fringe frequency is considered and at the first iteration step a fringe phase equal to zero is supposed. The second iteration takes the demodulated phase found from the first iteration etc. As a result, the unwrapped phase function of the spectral fringes is found.

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Spectral modulated interference fringes are observed in the form of the periodical modulation of broadband spectrum at the output of interferometer provided with a subsequent spectrometer. Group optical path difference of interfering light waves corresponding to the distance from the surface to be measured is characterized by the phase function or fringe frequency of the spectral modulated interferogram. The phase locked loop (PLL) method was used to demodulate spectral fringes. It is supposed a priori unknown carrier fringe frequency, and PLL technique is realized in iterative mode to calculate full fringe phase. At the first iteration step the fringe phase equal to zero is supposed. The second iteration takes the demodulated phase found from the first iteration, etc. As a result, the phase function of the spectral fringes is found. Method was verified experimentally with application to the analysis of the spectral modulated interferograms inherent in Michelson interferometer excited by a low-coherent source