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采用严格的耦合模理论,通过求解表面镀层长周期光纤光栅的特征方程,给出了双峰谐振波长的确定方法,以及它与光栅周期和模式序数之间的关系。结果表明,对应于较高次的包层模式,存在双峰谐振现象,且包层模式序数越高,与芯模产生谐振耦合所需的光栅周期越小。进一步讨论了双峰谐振波长的间距随薄膜参量与光栅参量变化的关系,描绘了这些参量对透射谱衰减谐振峰的影响,理论分析结果与X.W.Shu的无镀层长周期光纤光栅实验结果一致。这些研究为建立高灵敏的双峰谐振薄膜传感器提供了结构优化的理论支持。
Using the strict coupling mode theory, the method of determining the bimodal resonance wavelength and the relationship between the grating period and mode ordinal number are given by solving the eigenvalue equation of long-period fiber grating with surface coating. The results show that there is a bimodal resonance phenomenon corresponding to the higher order cladding modes, and the higher the order of the cladding modes, the smaller the grating period required for resonant coupling with the core modes. The relationship between the parameters of the bimodal resonant wavelength and the parameters of the grating is also discussed. The effects of these parameters on the resonant peak of the transmission spectrum are described. The theoretical analysis is consistent with the experimental results of the X.W.Shu uncoated long period fiber grating. These studies provide structural optimization theoretical support for the establishment of highly sensitive bimodal resonant thin film sensors.