Temperature-Insensitive Fiber-Optic Refractive Index Sensing System With High Sensitivity by Using EFPI-Based Microwave Photonic Filter

In this article, we have proposed and experimentally demonstrated an optical fiber refractive index (RI) sensing interrogation system realized by extrinsic Fabry–Perot interferometer (EFPI) structure-based microwave photonic filter (MPF) technology. EFPI is considered as a sensing element, as well as a spectrum slicer of the MPF. The shift of the central frequency of the MPF's passband caused by the change of the RI in the Fabry–Perot (FP) cavity can be monitored to demodulate the RI variation. In our experiment, four EFPI sensors with different FP cavity lengths have been applied for the RI sensing test. The experimental results show that the sensitivity can be tailored by adjusting the length of the FP cavity, and the longer the length of the FP cavity, the higher the sensitivity can be achieved. When the length of the FP cavity is $960 ~\mu \text{m}$ , a high sensitivity of 1.2 GHz/RIU has been obtained. The proposed sensing system can transfer the changes of the interference pattern in the optical domain to the central frequency shift of the MPF's passband in the electronic domain and thus avoid the limitation of measurement range induced by the periodic characteristics of the FP interference spectrum. The proposed sensing system also has the advantages of high sensitivity, the feasibility of multiplexing, temperature insensitivity, good repeatability and stability, simple fabrication, low cost, and so on.