Characteristics of plasmon coupling mode in SPR based LPFG

Based on mode coupled theory, this paper investigates the coupled characteristics and the excitation conditions of the surface plasmon mode in the structure of SPR based LPFG coated with metal film, and analyzes the strain sensing properties of this LPFG. Firstly, according to the properties of surface plasmon wave (SPW) in a cylindrical waveguide, SPW electromagnetic field distribution in the LPFG coated with metal film can be obtained by solving the electromagnetic wave equation. Using the coupled-mode theory, the coupled-mode equations of SPR based LPFG is derived and set up, and the coupled conditions between the core mode and SPW are given. Further, transmission spectra of LPFG coated with metal film can be obtained by solving coupled-mode equation, the results show that the SPR peak is generated by the coupling of core mode and SPW. By analyzing the influence of grating period on the SPR peak, it can be known the tiny change of the grating period will cause obvious deviation of the SPR peak. So this structure of SPR based LPFG is very suitable for strain sensor. Further calculation shows the strain sensitivity can reach 2.04 pm/με, superior to the traditional LPFG sensors.