Simultaneous measurement of refractive index and temperature using a cascaded core-offset in-line fiber Mach–Zehnder interferometer

Abstract

In-line fiber Mach–Zehnder interferometers (MZIs) have been widely investigated for refractive index (RI) sensing; however, variations in temperature affect the accuracy of RI measurement due to the thermo-optic effect. In this study, a cascaded core-offset in-line fiber MZI was simulated and experimentally demonstrated. The experimental results indicated that the sensor enabled simultaneous measurement of RI and temperature with low measurement error, effectively eliminating the effect of temperature on RI measurement. The sensor was constructed by connecting two core-offset in-line fiber MZIs with core-offset displacements of 6 μm and 40 μm, which exhibited distinct responses to RI variations. Measurements were performed using both wavelength-based and phase-based demodulation method, and sensitivity matrices were established to determine RI and temperature. In the wavelength-based measurement, the maximum temperature and RI sensitivities were 0.0649 nm/°C and 108.326 nm/RIU, respectively, while in the phase-based measurement, the higher temperature and RI sensitivities were obtained which are −0.0634 rad/°C (0.4543 nm/°C) and −20.199 rad/RIU (144.6791 nm/RIU), respectively. The smaller measurement errors of 0.1584 °C and 1.4439 × 10−4 RIU were obtained in the wavelength-based measurement, respectively. The proposed sensor thus enables highly sensitive, low-error, and low-cost RI sensing using only single-mode fiber.