Numerical Analysis of High-Sensitivity Refractive Index Sensor Based on LPFG With Bandpass Transmission

In this paper, a novel surrounding refractive index (SRI) sensor based on the long period fiber grating (LPFG) with cladding refractive index perturbation (C-LPFG) working around the turning-around point is proposed and investigated theoretically. By introducing the incident light into the fiber cladding, the bandpass transmission spectrum of the C-LPFG can be obtained (bandpass C-LPFG). The modified full-vector complex coupled mode theory is then derived to analyze the sensing performance of the bandpass C-LPFG-based SRI sensor. The average sensitivity and the local sensitivity up to 4757.6 and $\sim 6000$ nm/RIU are achieved, respectively, in the SRI range (from 1.33 to 1.36) for the bandpass C-LPFG-based SRI sensor having a small cladding radius. It is demonstrated that the bandpass C-LPFG is more sensitive to the SRI and exhibits an ultra-high sensitivity in contrast to the conventional uncoated LPFG.