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基于耦合模理论,研究了镀吸收型薄膜长周期光纤光栅(LPFG)的模式转换及其折射率响应特性。讨论了镀膜LPFG包层模有效折射率随薄膜厚度的响应特性,从模场分布和波导传输本质上统一了模式转换与模式跳变两种表述。在此基础上分析了镀吸收型薄膜情形下LPFG包层模有效折射率实部与虚部随膜厚及薄膜折射率的变化,给出并解释了高阶和低阶包层模发生的一步、二步模式转换情况,进一步考察了薄膜消光系数对模式转换区域的影响。结果表明,薄膜消光系数对模式转换区域变化的影响很小。最后研究了转换区及其附近区域的镀吸收型膜LPFG透射谱的折射率响应特性,发现靠近模式转换区相较于模式转换区,LPFG对薄膜折射率分辨率高出近一个数量级,而模式转换区域采用简易的透射率相对变化进行检测,对薄膜折射率的分辨率也高达10-5以上。
Based on the coupled-mode theory, the mode conversion and refractive index response of LPFG are studied. The response of the effective refractive index of the LPFG cladded film to the thickness of the film is discussed. From the distribution of the mode field and the transmission of the waveguide, the two modes of mode conversion and mode transition are essentially unified. On this basis, the real part and the imaginary part of the effective refractive index of LPFG cladding mode with the absorption-absorbing film are analyzed with the change of the film thickness and the refractive index of the film, and a step of high-order and low-order cladding modes is given and explained , Two-step mode conversion, and further investigated the impact of film extinction coefficient on the mode conversion region. The results show that the influence of film extinction coefficient on the change of mode transition region is small. Finally, the refractive index response characteristics of the LPFG transmission spectra of the absorption-absorbing film LPFG in the transfer zone and its vicinity are studied. It is found that the refractive index resolution of the LPFG film is nearly an order of magnitude higher than that of the mode transfer zone, The conversion area is measured using a simple relative change in transmittance, and the resolution of the film index is as high as 10-5.