模式是光纤光学中的基本概念，也是光纤激光器研究中最关注的问题之一。在一般的教材、文献中对模式的简并问题涉及较少，没有给出清晰、直观的物理图像。文中采用经典的电磁场理论，阐明了矢量模的简并度、标量模的简并度以及矢量模和标量模之间的简并关系。在弱导阶跃折射率光纤中，矢量模和标量模都是描述光场的正交完备基，矢量模[HE_1n^(o)、HE_1n^(e)]与标量模[LP_0n^(ye)、LP_0n^(xe)]描述的光场空间是2维的，矢量模[TE_0n、HE_2n^(o)、HE_2n^(e)、TM_0n]与标量模[LP_1n^(yo)、LP_1n^(ye)、LP_1n^(xo)、LP_1n^(xe)]描述的光场空间是4维的，矢量模[EH_{m-1, n}^(o)、EH_{m-1, n}^(e)、HE_{m+1, n}^(o)、HE_{m+1, n}^(e)]与标量模[LP_mn^(yo)、LP_mn^(ye)、LP_mn^(xo)、LP_mn^(xe)]描述的光场空间也是4维的。

Mode is the fundamental concept in fiber optics, which is one of the most concerned issues in the fiber laser research. However, the mode degeneracy issue has not been clarified in common literature. By the classical electromagnetic theory, the degeneracy of vector mode and linearly polarized modes was analyzed and the relation between vector mode and linearly polarized mode were discussed in this paper. In the weakly guiding step index fiber, the vector mode and the linearly polarized mode were both the orthogonal complete basis for describing the optical field. The optical field described by vector mode [HE_1n^(o)、HE_1n^(e)] and linearly polarized mode [LP_0n^(ye)、LP_0n^(xe)] were 2 dimensional; the optical field described by vector mode [TE_0n、HE_2n^(o)、HE_2n^(e)、TM_0n]] and linearly polarized mode [LP_1n^(yo)、LP_1n^(ye)、LP_1n^(xo)、LP_1n^(xe)] were 4 dimensional; the optical field described by vector mode [EH_m-1n^(o), EH_m-1n^(e), HE_m+1n^(o), HE_m+1n^(e)] and linearly polarized mode [LP_mn^(yo), LP_mn^(ye), LP_mn^(xo), LP_mn^(xe)] were 4 dimensional.