通过求解严格的耦合模理论建立的三包层结构长周期光纤光栅特征方程,研究了三包层长周期光纤光栅谐振波长与第二包层(薄膜)的折射率和厚度之间的关系。结果发现,随着膜厚及折射率的增大,谐振波长偏移的变化分成三个区域,这与Nicholas D R的实验结果相符。利用HE/EH模的判据数,对三个区域的模特性进行了分析,给出了区域划分的衡量标准。给出了在不同薄膜参数时的长周期光纤光栅透射谱,发现一阶低次HE模式的耦合强度要远大于一阶低次EH模式。 The relationship between the resonant wavelength of triple-clad long-period fiber grating and the refractive index and thickness of the second cladding (thin film) was studied by solving the characteristic equation of the long-period fiber grating with three-cladding structure established by the strict coupled-mode theory. The results show that with the increase of film thickness and refractive index, the change of resonant wavelength shift is divided into three regions, which is in good agreement with the experimental results of Nicholas D R. Based on the criteria of HE / EH modules, the modalities of the three regions are analyzed and the criteria of regional division are given. The transmission spectra of long-period fiber gratings at different film parameters are given. It is found that the coupling strength of the first-order low-order HE mode is much larger than that of the first-order low-order EH mode.