Fast and simple interrogation of extrinsic Fabry–Perot interferometer sensor based on dual-wavelength intensity ratio

A fast and simple demodulation method for the variation of Fabry–Perot (FP) cavity length is proposed and demonstrated. Taking advantages of differentially detecting optical power intensities of dual-wavelength, a reliable linear correlation between the variation of FP cavity length and differential intensity ratio of dual-wavelength can be obtained. Besides, an extrinsic Fabry–Perot interferometer (EFPI) sensor with a length of 196.8 μm was used to demonstrate performance of the method, and the time for each experimental operation is within 100 ms by the demodulation method and an integrable tunable laser assembly (ITLA). The demodulation resolution of 0.726 nm was achieved for the change of cavity length in a strain sensing experiment. By selecting two wavelengths at different positions in the system, the linear ranges of FP cavity length variation are 180 nm, 150 nm, and 130 nm responding to the sensitivities of −0.01099 nm−1, −0.01485 nm−1, and −0.02253 nm−1, respectively. It shows a high flexibility between the linear range and the sensitivity. Consequently, the different requirements of the EFPI sensor system can be easily fulfilled through appropriate selections of two different wavelengths. Moreover, the advantages of simplicity, fast speed, low cost can be fully revealed in the demodulation of large-scale production EFPI sensors.