长周期光纤光栅占空比对其光谱影响的理论分析

Theoretical analysis of the influence of duty cycle on the spectrum distribution of long period fiber gratings

为了探究长周期光纤光栅占空比对其光谱影响的规律,采用对长周期光纤光栅的矩形波折射率调制函数作傅里叶级数展开的方法,分析了占空比对各阶级数振幅的影响,计算了光纤前20个一阶包层模式有效折射率并分析了其耦合常数变化规律,分析了谐波光栅谐振波长随折射率调制量增加的漂移规律,最后给出制作无额外谐振光栅或传感用光栅对光栅占空比的设计要求.研究结果表明对于某些特定周期的光纤光栅,存在与其谐波光栅基本满足相位匹配条件的包层模式.

To investigate the influence of duty cycle on the spectrum distribution of long period fiber gratings and employing the Fourier series expansion of rectangle refractive index modulation function of the long period fiber gratings, the effect of the duty cycle on the Fourier series amplitude is analyzed; the effective indices of the first twenty modes are calculated and the changing rules of the coupled constants are also analyzed; the shifting rules of the harmonic gratings resonance wavelength are also analyzed as the index modulation intensity is increased; finally, the requirement to the duty cycle for the fabrication of fiber gratings with no harmonics or gratings used for sensors is given. It is found that for some gratings with special period there are cladding modes which are phase matching with its harmonic gratings.