从模式耦合理论出发,用数值计算的方法分析了长周期光纤光栅的包层模中各不同阶次模式的有效折射率及其交/直流耦合系数随波长以及阶次的变化规律.研究发现包层膜有效折射率、直流耦合系数及奇数阶次的交流耦合系数都随着波长的增加而减小.而阶次高的模式对应的有效折射率以及奇数阶交流耦合系数随波长的变化速率较之低阶次的来说要大.进一步分析了直流耦合系数对谐振峰值位置的影响,并且揭示了相邻谐振波长的峰值位置之差随波长而逐渐增大的原因.

Based on the coupling-mode theory, the changing relations of the effective refractive index and its AC/ DC coupling coefficient in different order of cladding modes in long period gratings, versus the varying of wavelength and the order, are numerically calculated and analyzed. It is revealed that the effective refractive index and the DC coupling coefficient, as well as the AC coupling coefficient of odd order modes, will be decreased with the increase in wavelengths. It is also shown that changing rates of the effective refractive index and the ac coupling coefficient of odd order modes corresponding to higher orders are bigger than that corresponding to lower orders. The effects of DC coupling coefficient on the resonant wavelength peak are further analyzed, and the reason that the difference between adjacent resonant wavelength peaks is gradually getting bigger with wavelength increase is finally revealed.