EFPI Demodulation Algorithm Based on the Combination of Nuttall Convolution Window and Discrete Gap Transformation

A novel extrinsic Fabry–Perot interferometer (EFPI) spectrum demodulation method based on four terms of 3rd-order Nuttall convolution window (NCW) and discrete gap transformation (DGT) is adopted to solve the phase jump problem in optical path difference (OPD) high-precision demodulation, which is also suitable for non-ideal EFPI sensing structure. Ideal EFPI spectral demodulation assumes the sensor reflective end face to be flat, with no lateral misalignment or angular offset, and ignores the effect of additional phase. But these factors need to be taken into account for some structures. The proposed method is capable of reducing the influence of variable additional phase difference and lateral misalignment offset, and the demodulation resolution can reach 1.2 nm. Meanwhile, it possesses fine real-time performance which the maximum demodulation time does not exceed 0.024 s. Finally, the effectiveness and superiority of this algorithm are verified by comparing it with the three-parameter cosine fitting (TPCF) algorithm through simulation and experiment.