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在长周期光纤光栅外镀折射率随环境变化而改变的敏感薄膜,其谐振波长随薄膜折射率的变化而改变。对无吸收薄膜,光栅耦合强度为π/2时,其传输谱中谐振波长对应的损耗峰峰值为零。镀复折射率薄膜的长周期光纤光栅,耦合强度为π/2时,损耗峰不为零,在谐振波长对薄膜折射率变化响应灵敏的区域,包层模传输常数的虚部较大,损耗峰幅值减小,甚至消失,不利于谐振波长的测量。通过求解镀复折射率薄膜长周期光纤光栅的耦合模方程,得到了损耗峰峰值的解析解。找到了损耗峰为零的条件。在此条件下设计光栅,可以有效地避免损耗峰减小带来的不利因素。
In the long-period fiber gratings, the refractive index changes with the ambient refractive index changes sensitive film, the resonant wavelength changes with the refractive index of the film changes. For non-absorbing films, grating coupling strength of π / 2, the transmission spectrum corresponding to the resonant wavelength peak loss of zero. Long-period fiber gratings coated with complex refractive index films show that when the coupling strength is π / 2, the peak of loss is not zero. In the region where the resonant wavelength is sensitive to the change of refractive index of the film, the imaginary part of the cladding mode transfer constant is large, Peak amplitude decreases or even disappears, is not conducive to the resonant wavelength measurement. By solving the coupling mode equation of the long period fiber grating with the cladding index film, an analytical solution of the peak to peak loss is obtained. Find the peak loss of zero conditions. Design grating under this condition, can effectively avoid the loss factor to reduce the negative factors.