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Modeling and design of a parallel demodulation system used for EFPI and FBG sensor

Extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors can construct one of most promising sensor group for simultaneous temperature and strain measurement in structure health monitoring. The demodulation of the sensor group is critical. Here, a parallel demodulation system used for the goal is presented, which is based on a Michelson interferometer and combines the method of low coherence interference of EFPI sensor and Fourier transform spectrum of FBG sensor. The parallel demodulation method is modeled with Fourier transform spectrum theory, which shows the common base that existing between low coherence interference and Fourier transform spectrum. Three data processing methods are proposed according to different application situations, and separation method for EFPI and FBG overlapping signals is emphasized. The design of system is described, and the optical path difference (OPD) scanning and sampling method without reference light are discussed in detail. Then a series of experiments were carried out. The results showed the parallel demodulation system has good performance of spectrum demodulation, low coherence interference demodulation and parallel demodulation. It can realize simultaneous strain and temperature measurement while keep the whole system configuration less complex.