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Harmonic frequency estimation method in multi-surface interferometry

The surface morphologies and thickness variation are the basic characteristics of a transparent parallel plate with multiple surfaces. Measurements of these profiles are greatly significant for the evaluation of surface quality. The measurement accuracy of phase-shifting interferometry can reach the nanometer level, which is an effective method for high-precision measurements of ultra-smooth surfaces. However, each measured surface will give its own harmonic to the captured interferograms, resulting in the information of harmonics mixed. Thus the surface topology information can not be detected directly, which can cause problems of the wavefront reconstruction of the measured surface. To solve this problem, separating the mixed harmonics is necessary. The fundamental difference in harmonics lies in their frequency. Therefore, determining these differences of harmonics by frequency is a preliminary method of phase demodulation.