Design of long period fiber grating surrounding refractive index sensor based on mode transition near phase-matching turning point

This work presents a novel design of long period fiber grating (LPFG) refractive index sensor based on mode transition near phase-matching turning point (PMTP). The dual peaks can approach to PMTP by adjusting the fiber radius, and LPFG can operate in the mode transition range by choosing the optimal film thickness. Through experiments, the LPFG based on mode transition near PMTP was fabricated by the aforesaid design, and a glycerol solution concentration monitoring test was performed to analyze its sensing properties. The results show that if the dual peaks are close to PMTP and the overlay thickness stays within the mode transition range, the sensitivity to surrounding refractive index (SRI) will be greatly improved. Compared with the regular LPFG sensors based on dual peaks resonance or the LPFG sensors based on mode transition, this LPFG has a higher sensitivity to SRI which can reach to 4900nm/RIU in the SRI range of 1.34∼1.39.