Simultaneous measurement of the surrounding refractive index and temperature using a local-micro-structured long-period fiber grating sensor operating near the phase-matching turning point

Abstract

We have proposed a local-micro-structured long-period fiber grating (LMSLPFG) sensor near the phase-matching turning point (PMTP) for simultaneous measurement of the surrounding refractive index (SRI) and temperature. By using the HF solution or a femtosecond laser to partially change the cladding radius of LPFG, the LMSLPFG is realized. Such change affects the effective refractive index of cladding mode and further causes a new resonant peak in the spectrum. By using the transfer matrix method, the formation mechanisms of the peak in the middle of the LMSLPFG transmission spectrum corresponding to a little off-resonant state (ALOR) near PMTP and the other two peaks on both sides corresponding to the dual-peak resonance are studied. On this basis, the sensing characteristics of such three resonant peaks to SRI and temperature are analyzed. When the SRI and temperature change, the transmissivity of the peak in ALOR changes without a response occurring at resonant wavelength, while the wavelength of the other two dual peaks will respond proportionately. The numerical simulation results demonstrate that the sensitivities of the distance between the dual peaks to SRI and temperature are 777.14 nm/RIU and −0.22nm/∘C, respectively, and the sensitivities of single peak transmissivity to SRI and temperature are −394.06dB/RIU and 0.05 dB/°C, respectively. Therefore, the sensitivity matrix can be used to measure SRI and temperature simultaneously with a single LMSLPFG.