Research on simultaneous measurement of refractive index and temperature comprising few mode fiber and spherical structure

A simple sensor for simultaneous measurement of refractive index (RI) and temperature based on Mach-Zehnder interference (MZI) is proposed and studied experimentally and theoretically. It consists of a section of few mode fiber and two spherical structures. The two spherical structures act as couplers that can excite high-order core modes and recouple the high-order core modes with the cladding modes back into the core. Two selected dips of the interference spectrum shift with the variation of RI or temperature, and simultaneous measurement of temperature and RI can be realized by means of a sensitivity coefficient matrix. The temperature sensitivity is 0.054 nm/°C as high as in the range from 25 °C to 80 °C, and the RI sensitivity is −27.77 nm/RIU in the range from 1.335 to 1.398. The proposed MZI is characterized by low cost, easy fabrication, and good robustness.