Lee B H, Nishii J
Osaka National Research Institute, AIST, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
Appl Opt. 1999 Jun 1;38(16):3450-9. doi: 10.1364/ao.38.003450.
The spectral spacing of the interference fringes formed by a pair of long-period fiber gratings was investigated. The variation of the fringe spacing was measured while the separation between the gratings was changed from 22 to 500 mm. When the grating separation was much longer than the length of the individual grating, the inverse of the fringe spacing became linearly proportional to the grating separation and to the differential effective group index of the fiber. In the third stop band of the grating pair, made along a dispersion-shifted fiber centered at 1.55 microm, the differential effective group index was calculated to be approximately 6.4 x 10(-3), which is approximately twice the differential effective index of the fiber. The discrepancy between the two indices was observed to decrease with the band order, a phenomenon that is explained by the first-order dispersion of the fiber. The measured interference fringes were not regularly spaced in the frequency domain, but regular spacing is required in wavelength-division multiplexing communication systems. Analysis of the second-order dispersion of the fiber and the grating-induced nonlinear phase shift within grating regions as the factors that induce chirping on the fringe spacing is presented.
研究了由一对长周期光纤光栅形成的干涉条纹的光谱间距。在光栅间距从22毫米变化到500毫米的过程中，测量了条纹间距的变化。当光栅间距远大于单个光栅的长度时，条纹间距的倒数与光栅间距以及光纤的微分有效群折射率呈线性比例关系。在沿中心波长为1.55微米的色散位移光纤制作的光栅对的第三阻带中，计算得出微分有效群折射率约为6.4×10⁻³，约为光纤微分有效折射率的两倍。观察到这两个折射率之间的差异随带阶数减小，这种现象由光纤的一阶色散来解释。在频域中测得的干涉条纹间距并不规则，但波分复用通信系统需要规则的间距。本文分析了光纤的二阶色散以及光栅区域内光栅引起的非线性相移作为导致条纹间距啁啾的因素。
