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为设计高灵敏光纤光栅薄膜传感器,基于耦合模理论研究了表面镀制高折射率敏感薄膜的倾斜长周期光纤光栅(TLPFG)耦合特性和薄膜折射率传感特性。TLPFG能激发高阶(角向序数l>1)包层模耦合,高阶包层模耦合系数随倾角θ增大而增大,θ<80°时,高阶包层模耦合系数远小于1阶包层模耦合系数。研究表明,光栅倾角变化不影响包层模耦合谐振波长,但影响光谱透过率。进一步研究了TLPFG在θ<80°时1阶5次包层模耦合的光谱透过率对薄膜折射率变化的响应,分析了光栅结构参数(L、Λ、θ)和薄膜参数(h3,n3)对薄膜折射率响应灵敏度的影响,结果表明,响应灵敏度相比于非倾斜LPFG可提高一个数量级,对薄膜折射率的分辨率可达10-9。
In order to design highly sensitive FBG thin film sensors, the coupling properties of TLPFG and the refractive index sensing characteristics of the thin film are studied based on the coupled-mode theory. TLPFG can stimulate high-order (angle order l> 1) cladding mode coupling, and the higher-order cladding mode coupling coefficient increases with the increase of the inclination angle. When θ <80 °, the coupling coefficient of high-order cladding mode is far less than 1 Order cladding mode coupling coefficient. The results show that the change of grating tilt does not affect the coupling mode resonance wavelength but affects the spectral transmittance. The spectral response of TLPFG with first order fifth order cladding mode coupling at θ <80 ° was investigated. The influence of grating structure parameter (L, Λ, θ) and film parameters (h3, n3 ) On the sensitivity of the refractive index of the film. The results show that the response sensitivity can be increased by an order of magnitude compared with the non-inclined LPFG, and the resolution of the film can reach 10-9.