Fiber Optic Sensor for Simultaneous Measurement of Refractive Index and Temperature Based on Internal-and- External-Cavity Fabry–Pérot Interferometer Configuration

An internal-and-external-cavity Fabry-Pérot interferometer (IECFPI) model was mathematically constructed. Based on the IECFPI configuration, a crosstalk-free fiber optic sensor for simultaneous measurement of refractive index (RI) and temperature was proposed and experimentally demonstrated. The internal cavity was a silica cavity, while the external one consisted of an open cavity and silica cavity. The open cavity was formed by an offset single-mode fiber (SMF) sandwiched between two sections of SMF, and the silica cavity was formed by a well-cleaved SMF. A sensitivity coefficient (SC) matrix of the proposed sensor was used to determine the RI and temperature by monitoring the wavelength shifts of the envelope and fine fringes. The proposed sensor can be used for simultaneous measurement of RI and temperature based on the inverse of the SC matrix. Experimental results demonstrated that the RI sensitivities for the envelope and fine fringes were 1084.9 and 124.2 nm/RIU, respectively. Temperature sensitivities of 3.5 and 9.8 pm/°C were also achieved for the envelope and fine fringes, respectively. The crosstalk between the RI and temperature was thus eliminated. In addition, the proposed sensor has a compact size, simple fabrication, and an all-fiber structure, making it a potential candidate for many applications, such as the food industry and environment monitoring.