本文从严格的场分布方程出发,导出了四包层光纤中LP模的特征方程,精确计算了几个低次模的归一化截止频率,给出了它们与结构参数的一般变化曲线.结果表明:四包层光纤的截止特性与单包层和W型光纤有较大不同;在所需波长要得到单模传输,第二包层的高度和宽度不宜太大. Based on the strict field distribution equation, this paper derives the eigenvalue equation of LP mode in quadruplexed fiber and the normalized cut-off frequencies of several low-order modes are precisely calculated, and the general curves of them with the structural parameters are given. The results show that the cut-off characteristic of the four-cladding fiber is quite different from that of the single-clad fiber and the W-type fiber. To obtain the single-mode transmission at the desired wavelength, the height and width of the second cladding should not be too large.