Folded-tapered multimode-no-core fiber sensor for simultaneous measurement of refractive index and temperature.

• The sensor is constituted by multimode interferometer and Mach-Zehnder interferometer. • Simultaneous measurement of refractive index (RI) and temperature can be achieved. • The sensitivity of RI reach to 1191.27 nm/RIU in the low RI range. • It has the advantages of cost effectiveness, high sensitivity and easy fabrication. In this paper, a refractive index (RI) and temperature sensor based on a folded-tapered multimode-no-core (FTMN) fiber structure is proposed and experimentally demonstrated. The FTMN has an additional Mach-Zehnder interferometer (MZI), which is introduced in the folded-tapered multimode (FTM) fiber. And with the inherent multimode interference (MMI) and the previously mentioned MZI as foundation, a composite interference is successfully established. This synthetic composite interference greatly improves the performance of traditional optical fiber RI sensing in the low RI range. The experimental results demonstrate that a maximum sensitivity of 1191.5 nm/RIU within a linear RI ranging from 1.3405 to 1.3497 can be achieved, which is greater than the traditional modal interferometer structure. Furthermore, the temperature sensitivities at interference dips A and B are 0.0648 nm/°C and 0.0598 nm/°C, respectively. By monitoring the wavelength shifts of interference dips A and B, the sensor can simultaneously measure RI and temperature to overcome the temperature induced cross-sensitivity. [ABSTRACT FROM AUTHOR]