振幅掩膜紫外写入的长周期光纤光栅特性研究

振幅掩膜紫外写入的长周期光纤光栅的纤芯折射率分布函数为矩形波。以三层阶跃折射率光波导结构基础，用弱导标量近似和标量耦合模理论分析折射率调制类型为矩形波的长周期光纤光栅的特性。详细地给出耦合模方程近似处理的方法，并说明了其合理性。用数学软件Matlab进行了数值模拟计算，发现折射率调制类为矩形波的光栅传输谱不是由它的各次余弦光栅谱的线性叠加而成的。还研究了外部环境折射率、包层半径、栅占空比等光栅结构参量对矩形折射率调制的光栅传输谱的影响。同时指出了每阶包层模的双谐振峰位置随栅参量的变化规律。刊■刊『_型，刿j1

Research on the Characteristics of Long-Period Fiber Grating Ultraviolet-Written by Amplitude Mask

The refractive index distribution function of long period fiber grating (LPG), which is ultraviolet written by amplitude mask, is a rectangular wave in the core region. The theoretical method is presented to analyze the characteristics of LPG with the rectangular index modulation. Here, the weakly guiding scalar approximation and coupled mode theory based on the three layer step index optical fiber are used. The approximate numerical method to deal with the coupled mode equations is presented in detail and the rationality of the method is given. The transmission spectra of LPG are numerically calculated by using the mathematical software, Matlab. It's found that the transmission spectra of LPG with the rectangular index modulation are not the linear superposition of its cosine gratings' spectra. The effect of the structural parameters of gratings, such as ambient refraction index, the radii of fiber cladding and the duty cycle of fiber grating, on the transmission spectra of LPG with the rectangular index modulation is analyzed. Furthermore, the response of double resonant peak position of a single cladding mode to the parameters of gratings is studied.