High Sensitivity Refractometer Based on Tapering Two-Mode Fiber Embedded Long-Period Fiber Grating

A novel tapered two-mode fiber embedded long-period fiber grating (TTMF-LPFG) sensor is proposed. The long-period fiber grating (LPFG) is fabricated by periodically splicing the two-mode fiber (TMF) and single-mode fiber (SMF), and then the LPFG is tapered to make a refractive index (RI) sensor. By comparing the sensors with different waist diameters, we found that the smaller the diameter of the sensor is, the higher the sensitivity is. When the taper waist diameter is 36$\mu \text{m}$, the maximum sensitivity reaches 1360.43 nm/RIU. TTMF-LPFG shows low sensitivity to the temperature, avoiding the influence of temperature. For the traditional taper fiber, a very thin taper waist is required to excite strong evanescent field (EF) in order to obtain a large RI sensitivity. This situation creates a sensor fragile. For our structure, through the taper, LPFG can be formed and the sensitivity can be improved at the same time. The sensor has great potential in chemical composition analysis and biomedical applications.