表面膜层参数对长周期光纤光栅特性的影响

采用严格的耦合模理论建立了表面涂覆弱吸收薄膜长周期光纤光栅（LPFG）的理论模型，用微扰法对复数超越特征方程进行了求解。研究发现，对应某一膜厚，存在一折射率范围，有效折射率的变化和谐振波长偏移特别明显；但消光系数对有效折射率和谐振波长的影响则很小。从功率密度分布曲线可看出低阶奇次包层模式与纤芯模式的耦合要强于低阶偶次包层模式与纤芯模式的耦合，消光系数引起的功率损耗较小。薄膜折射率对透射谱的变化（幅度和谐振峰偏移）影响很大；消光系数对谐振峰偏移影响很小，对谐振峰幅度的影响与具体的薄膜折射率有关。结果表明，敏感薄膜折射率和消光系数与传感器的灵敏度高低有直接关系，传感器的结构优化非常必要。

Effect of Surface Film Optical Parameters on the Characteristic of Long-Period Fiber Grating

Based on rigorous coupled-mode theory,a theoretical model is established for the long-period fiber grating(LPFG) with weakly absorbing film coating,and the complex characteristic equation is resolved by perturbation method.The effects of the refractive index and extinction coefficient of sensing film on the cladding mode effective index,resonant wavelength,mode field distribution and transmission spectrum are studied.It is found that the change of the cladding mode effective index and the shift of the resonant wavelength are especially obvious in some region of refractive index for given film thickness;but the influence of extinction coefficient on the cladding mode effective index and resonant wavelength is small.From the distribution curves of power intensity it could be seen that the coupling between the low-order odd cladding modes and core mode is stronger than that between the low-order even cladding modes and core mode,and the power loss caused by the extinction coefficient is small.The film refractive index influences the transmission spectrum(magnitude and shift of resonant peak) greatly.The extinction coefficient has a small effect on the shift of the resonant peak,and its effect on the magnitude of resonant peak is dependent on the film refractive index.All of the results are also demonstrated to show that the thin film parameters(refractive index and extinction coefficient) have direct relation to the sensitivity of the sensors and the structure optimization is essential.