Simultaneous measurement of refractive index, strain, and temperature based on a four-core fiber combined with a fiber Bragg grating

A fiber optic sensor capable of measuring refractive index (RI), strain, and temperature simultaneously is proposed and demonstrated. The sensor is formed by the integration of a four-core fiber (FCF) with a fiber Bragg grating (FBG). When the FCF is kept straight, a pronounced interference pattern appears in the transmission spectrum. Compared with previously reported optical fiber modal interferometers, higher fringe visibility can be obtained in our scheme. The maximum fringe visibility of the interference resonance dips exceeds 21dB. By monitoring the wavelength shifts of three dips, simultaneous measurement of RI, strain, and temperature can be achieved. For 10pm wavelength resolution, the resolution of the sensor is 0.0004RIU, 11.06με, and 0.17°C in RI, strain, and temperature, respectively. The proposed sensor has the advantages of easy to construct, higher fringe visibility, and the capability to measure RI, strain, and temperature simultaneously, which are desirable features in RI measurement.