Intensity-Modulated Refractometer with Long-Period Fiber Gratings Near Turn-Around Point for Low Refractive Index Detection

We demonstrate a novel intensity-modulated refractometer based on long-period fiber gratings (LPFGs) in thin core fiber (TCF) near turn-around point. The correlation of contrast to surrounding refractive index (RI) for TCF-LPFGs with different index modulations in the low RI region was investigated. Compared with low index modulation, TCF-LPFGs with high index modulation have a larger sensing region and relatively higher sensitivity. With the index modulation of 4.5 ^x 10-4, a high sensitivity of -17.42 dB/ RIU and -163.32 dB/RIU was achieved in the RI region of 1.0-1.20 and 1.24-1.30, respectively. The proposed TCF-LPFGs based refractometer has potential application for low RI detection fields.