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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。
The theoretical model of long-period fiber grating (LPFG) coated with weakly absorbing film was established by using the strict coupled-mode theory. The complex transcendental characteristic equation was solved by perturbation method. It is found that there is a range of refractive index corresponding to a certain film thickness. The change of effective refractive index and resonant wavelength shift are particularly obvious. However, the effect of extinction coefficient on effective refractive index and resonant wavelength is very small. From the power density distribution curve, it can be seen that the coupling between the low-order odd-order cladding mode and the core mode is stronger than the coupling between the low-order even-mode cladding mode and the core mode, and the power loss caused by the extinction coefficient is small. The refractive index of the film has a great influence on the change of the transmission spectrum (the amplitude and the resonance peak shift). The extinction coefficient has little effect on the resonance peak shift. The influence on the resonance peak amplitude is related to the specific refractive index of the film. The results show that the refractive index and the extinction coefficient of the sensitive film are directly related to the sensitivity of the sensor, and the structure optimization of the sensor is very necessary.