A Compact Refractive Index and Temperature Simultaneous Measurement Sensor Based on Coaxial Dual-Waveguide Fiber Embedded No-Core Fiber Structure

A new cascaded structure is proposed and investigated as a sensor for simultaneous measurement of refractive index (RI) and temperature. This sensor is fabricated by periodically embedding the coaxial dual-waveguide fiber in the no-core fiber to form a long-period fiber grating (CEN-LPFG). The simulation analysis of the transmission spectrum of the proposed CEN-LPFG sensor is carried out to obtain the appropriate period. The periodic arrangement induces the transmission spectrum with two resonance peaks. Therefore, the simultaneous measurement of RI and temperature is realized by observing the wavelength shift of the two peaks. The total length of the sensing unit is only 4.4 mm. Experimental results show that the RI sensitivities of the two resonance peaks of the CEN-LPFG sensor are 183.62 and 278.44 nm/RIU with the RI range of 1.33–1.41, respectively. Meanwhile, the temperature sensitivities of the two resonance peaks of the CEN-LPFG sensor are −27.67 and 37.5 pm/°C with a temperature range of 20 °C–100 °C, respectively. The feasibility of the CEN-LPFG sensor has been experimentally demonstrated for simultaneous measurement.