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采用严格的耦合模理论,建立了镀金属膜和敏感膜的两层膜系长周期光纤光栅复特征方程,用微扰法对复数超越特征方程进行求解,结果和文献[5]给出的数据相符。理论模拟数据表明,金属的消光系数对功率密度和透射谱影响很大,低阶偶次模式在纤芯内的功率密度所占比重远大于奇次模式,其衰减峰幅度也比奇次模高,因此,金属镀层长周期光纤光栅实际应用时,需要选择偶次模作为研究对象。进一步理论分析了金属和敏感膜层参数对长周期光纤光栅谐振特性的影响,研究发现,金属膜厚、敏感膜厚和折射率对谐振波长有较大的影响,谐振波长随金属膜厚、敏感膜厚和折射率的增大向短波方向漂移。在某些区域谐振波长存在跳变现象,长周期光纤光栅实际应用时应避开此区域。
Using the strict coupled-mode theory, a complex eigenvalue equation of two-layer film-based long-period fiber gratings coated with metal film and sensitive film was established. The complex transcendental eigenvalue equation was solved by perturbation method. The results and the data given in [5] Match. Theoretical simulation data show that the extinction coefficient of metal has a great influence on the power density and the transmission spectrum. The proportion of the power density in the low-order even mode is much larger than the odd mode, and the attenuation peak amplitude is also higher than the odd-mode height Therefore, when the metal coating long period fiber grating is used in practice, the even mode should be selected as the research object. The effects of metal and sensitive film parameters on the resonant characteristics of long-period fiber gratings are further analyzed. It is found that the metal film thickness, the sensitive film thickness and the refractive index have a great influence on the resonant wavelength. The resonant wavelength varies with the metal film thickness and sensitivity The film thickness and the increase of the refractive index drift to the short-wave direction. In some areas there is a jump in the resonant wavelength phenomenon, the long-period fiber grating practical application should avoid this area.