Improved demodulation method of white light interference peak interval for simultaneous measurement of thickness and group refractive index

To simultaneously measure thickness and group refractive index using white light interferometry, it is often required to precisely obtain the interval between interference peaks. There are many demodulation algorithms to calculate the position of a single interference peak. However, these algorithms are rarely adapted for peak interval demodulation. In this article, we propose a method of calculating the interval of the white light interference peaks based on frequency domain analysis (FDA). By subtracting the phase-frequency curves of two interference peaks before linear fitting, the interval of the peaks can be accurately calculated. The demodulation procedure is simplified compared with traditional FDA method. Simulations and experiments show that the noise-induced 2π phase jump phenomenon can be effectively suppressed by this method. Simultaneous measurements of physical thickness and group refractive index of specimens of different materials and thicknesses are realized using a home-made fiber-optic white light scanning interferometer. Compared with traditional centroid method, slope FDA method and intercept FDA method, the proposed method shows better robustness to noise experimentally, while the measurement data are in good agreement with the nominal values of thickness. In a well-controlled environment, the standard deviations of repeated measurements reach 3.12 nm and 7.66 × 10−6 refractive index unit (RIU) for thickness and group refractive index respectively, which have been significantly improved compared with our previous work measuring the same specimen [1].