A Sagnac Loop Sensor for Refractive Index Measurement

A Sagnac loop sensor with a long-period grating (LPG) inscribed in polarization maintains fiber (PMF) for refractive index (RI) and temperature measurement is proposed. Taking using of the different responses of the LPG and the Sagnac loop to RI and temperature, simultaneous measurement can be achieved by monitoring the wavelength shifts and the intensity changes of the resonance peaks. One of the transmission minimum of the Sagnac loop sensor was measured, whose wavelength varied with temperature and the intensity changed with refractive index. The experiments show that sensitivity of RI and temperature are 49.9 dB/RIU and -0.65 nm/°C, respectively. The sensor system shows advantages of high sensitivity, small size and low cost.