Design and simulation of ultra-sensitivity reflective SRI sensor based on TLPFG

An ultra-sensitivity reflective surrounding refractive index (SRI) sensor structure based on tilted long-period fiber grating (TLPFG) is proposed in this paper. The sensitivity of the structure to SRI is greatly improved by the combination of the mode transition (MT) region effect and double-peak resonance effect of the high-order cladding mode in TLPFG. A metal mirror is coated at the end of the structure to change the light path and form interference fringes in the reflection spectrum, which greatly reduces the bandwidth of the monitoring peak and makes the measurement more convenient. Firstly, the response characteristics of the effective refractive indices of cladding modes with increasing overlay thickness in MT are analyzed, and a certain grating period is selected to make the TLPFG work at the phase-matching turning point (PMTP) by phase-matching curve (PMC). Then, the parameters of grating angle, grating length and connecting fiber length are optimized by theoretical simulations to make the demodulation more accurate and convenient. On the basis of the above, the reflection spectrum of this structure is simulated, and reflection interference fringes are investigated. Furthermore, the sensing characteristic of SRI of the interference fringes is analyzed. The simulation results show that the average sensitivity can reach 4.86 × 10 4 nm/RIU when SRI varies in 1.330–1.331, and the highest sensitivity is available to be 9.5 × 10 4 nm/RIU. In addition, the bandwidth of monitoring peaks of this sensor is 10 times smaller than that of ordinary TLPFG sensors.