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基于耦合模理论,研究了镀金属两层膜系长周期光纤光栅(LPFG)的模式转换及其折射率响应特性。从表面等离子体共振(SPR)的激励条件和模式转换发生条件出发,指出镀金属两层膜系LPFG中的SPR和模式转换不会同时发生,在此基础上分析了镀金属两层膜系LPFG包层模有效折射率随敏感薄膜厚度的变化,发现镀金属两层膜系LPFG的模式转换区域比镀膜LPFG的转换区域要宽,且模式转换区域内第一次转换时的有效折射率变化斜率比镀膜LPFG的大,意味着在该区域其对敏感膜层的变化有更高的响应。考察了金属薄膜厚度对镀金属两层膜系LPFG包层模有效折射率的影响,结果表明,随着金属薄膜厚度的增加,第一次转换时有效折射率的变化斜率逐渐增加。分析了第一次转换时镀金属两层膜系LPFG透射谱对敏感薄膜折射率变化的响应特性,结果表明,镀金属两层膜系LPFG传感器对敏感薄膜折射率的灵敏度明显高于镀膜LPFG,对敏感薄膜折射率的灵敏度的分辨率可达10-6。
Based on the coupled-mode theory, the mode conversion and refractive index response characteristics of metallized two-layer long period fiber grating (LPFG) were investigated. Based on the conditions of SPR excitation and mode transition, it is pointed out that the SPR and mode transition in the metallized two-layer LPFG do not occur at the same time. Based on the analysis, the metallized two-layer LPFG The effective refractive index of the cladding mode changes with the thickness of the sensitive thin film. It is found that the mode conversion region of the LPFG with metallized two-layer film is wider than the conversion region of the coated LPFG and the slope of the effective refractive index at the first conversion in the mode conversion region Larger than the coated LPFG, which means that it has a higher response to changes in sensitive layers in this area. The effect of metal film thickness on the effective refractive index of LPFG cladding mode was investigated. The results show that the slope of effective refractive index increases with the increase of metal film thickness. The response characteristics of LPFG transmission spectra on the refractive index changes of sensitive films were analyzed. The results show that the sensitivity of LPFG sensors with metallized two layers is higher than that of LPFG. Sensitivity of the refractive index of the sensitive film resolution of up to 10-6.