长周期光纤光栅是近几年出现的新型光纤器件，表现为前向传播的纤芯导模与同向传播的各阶次包层模在特定波长时的模式耦合，因此它在温度、应变、折射率传感方面比普通的光纤光栅有更大的优越性.运用耦合模理论，首先讨论了长周期光纤光栅的谐振波长，并模拟了其透射谱.其次在理论上详细分析了长周期光纤光栅对环境折射率的敏感特性，讨论了环境折射率敏感度因子与包层模次数的关系，并比较了不同环境折射率下的灵敏度.最后模拟了在环境折射率小于光栅包层折射率的情况下，谐振波长随环境折射率变化的规律.

Long-period fiber grating is a new kind of fiber device developed in recent years, which couples light from the forward-propagating guided core mode into co-propagating cladding modes at special wavelengths, with more advantages over normal fiber gratings at the aspects of temperature，strain，refractive index sensing. Based on coupled mode theory, the resonant wavelength of LPFG was discussed and its transmission spectrum was simulated. The sensitivity of the long-period gratings to the surrounding refractive index was analyzed theoretically, and the relationship between the surrounding refractive index factors and the cladding mode orders was discussed while the sensitivities at different surrounding refractive index were compared. The changing rule of the resonant wavelength versus the surrounding refractive index was simulated, under the condition of the surrounding refractive index less than the cladding refractive index.