Fiber modal interferometer with embedded fiber Bragg grating for simultaneous measurements of refractive index and temperature

A highly sensitive and compact fiber-optic sensor for simultaneous measurement of refractive index (RI) and temperature has been proposed and experimentally demonstrated. The device is based on a fiber modal interferometer assisted by an embedded fiber Bragg grating, which helps to resolve the crosstalk issue. The fiber modal interferometer consists of an Up-Fusion-Bitaper (UFBT) pair to excite high-order cladding-modes and re-couple them to the fiber core. For high order cladding mode involved in the interference, the implemented sensor yields high spectral sensitivities of −197.03 ± 7.35 nm/RIU (refractive index unit) for a refractive index (RI) range of 1.365–1.415 and 63.7 ± 1.84 pm/°C for a temperature range from 20 to 80 °C, respectively. Simultaneous measurement of ambient RI and temperature has been experimentally demonstrated. The maximum RI and temperature measurement errors are ±0.00054 RIU and ±0.77 °C, respectively. These results demonstrate the feasibility of this configuration for reliable RI-based chemical and biochemical monitoring.