Characteristics of plasmon coupling mode in SPR based LPFG

Abstract

Based on mode coupled theory, this paper investigates the coupling characteristics and the excitation condition of the surface plasmon mode in a structure of surface plasmon resonance (SPR) based long-period fiber grating (LPFG) coated with a metal film, and analyzes its refractive index and strain sensing properties of this structure. Firstly, according to the properties of surface plasmon wave (SPW) in a cylindrical waveguide, SPW electromagnetic field distributions in the metal coated LPFG are obtained by solving the electromagnetic wave equation. Then the coupled-mode equations of SPR based LPFG are derived and set up, and the coupled condition between the core mode and SPW is given for the first time. Further, the transmission spectrum of the metal coated LPFG is obtained by solving coupled-mode equations, the results show that the SPR peak is generated by the coupling of the core mode and SPW. The sensing characteristics of refractive index indicate the SPR peak is easily impacted by environment refractive index. The sensitivity and the corresponding resolution of environment refractive index can be available to 102 and 10−8, respectively. By analyzing the influence of grating period on the SPR peak, it can be seen that a tiny change of the grating period will cause an obvious deviation of the SPR peak. So this structure of SPR based LPFG is very suitable for a strain sensor. Further calculations show that the strain sensitivity of this LPFG can reach 0.92 pm/μe, superior to the traditional LPFG sensors.