Analysis of the Influence of External Refractive Index on Long-Period Fiber Grating Coupler

The influence of external refractive index on long-period fiber grating coupler consisting of two parallel long-period fiber gratings is analyzed by coupled-mode theory considering three-layer geometry of the fiber. The self-scattering of guided mode and the shift of resonant wavelength caused by the core index modulation are taken into account for precise simulation. The variation of coupling coefficients, resonant wavelength, transmissivity and coupling efficiency are discussed respectively. Finally, the experiments are implemented with different external indexes which prove the validity of our model.