Simultaneous Surface Profiling of Transparent Optical Flats on Wavelength Tuning Interferometry Using Dual-Window All-Phase Harmonic Demodulation

The fringe analysis method, using phase modulation with wavelength tuning, is widely used in profiling the surfaces and thickness variation in optical flat interferometry. However, the measurement accuracy may deteriorate owing to the phase demodulation errors resulting from the offsets in frequency estimation due to the unavoidable spectrum leakage. To address this challenge, a dual-window all-phase harmonic demodulation on wavelength tuning interferometry (Dwa-WTI) is developed to improve the measurement accuracy of surface profiles and thickness variation of the optical flat. The Dwa-WTI compensates for the effect of the sampling sequence on the spectrum by synthesizing the new interferograms, which effectively suppresses the undesirable error of phase demodulation. With the developed Dwa-WTI, the initial phase of the wavefront distribution at the spectrum peak of the synthesized sequence is independent of the frequency offset. The reliability of the Dwa-WTI has been validated by numerical simulations and experiments. In experiments, the optical flats with thicknesses of 5, 10, and 30 mm are measured. The difference of peak-to-valley (PV) and root-mean-square (rms) for the profiles of the thickness variation, front and rear surfaces are compared in repeated experiments. Meanwhile, the measurements under different spatial carriers are compared to assess the stability of the Dwa-WTI and the relative standard deviation (RSD) for the root-mean-square of the theoretical error (RMSTE) is about 0.085.