Design of a high-sensitivity cascaded long-period fiber grating sensor operating at PMTP

Abstract

This paper presents a novel cascaded long-period fiber grating (CLPFG) sensor which is cascaded by two identical coated long-period fiber gratings (LPFGs) operating at the phase-matching turning point (PMTP). This CLPFG sensor operates at the PMTP, so it has the highest sensitivity compared with those LPFGs operating near the PMTP. In addition, the spectrum of this CLPFG sensor is composed of a series of narrow-bandwidth loss peaks, which are easy to be detected by an ordinary wavelength demodulator. In theory, the transmission spectrum of the CLPFG can be calculated by the transfer matrix method. We select two loss peaks with highest sensitivity from the transmission spectrum of the CLPFG for dual-peak sensing. The simulation results show that the wavelength spacing of the two peaks changes from 24.6 to 74.8 nm with the surrounding refractive index changing from 1.330 to 1.334. The average sensitivity is 12550 nm/RIU, which is about 6 times higher than traditional LPFG sensors where the LPFG is operating near the PMTP, so this kind of CLPFG sensor is suitable for high-sensitivity sensing.