采用一种基于三电场分量的全矢量有限元法，其中引入了完善匹配层(PML)吸收边界条件，分析由梯形截面硅基垂直多槽纳米线构成的平行定向耦合器.考虑了波导侧壁倾角、耦合波导间距、槽宽及槽折射率的变化对定向耦合器性能的影响.给出了准TE与准TM偶、奇模有效折射率、耦合长度及模场分布，揭示了其模式的混合特性及模场分布特点.分析结果表明，恰当选择结构参数及材料参数，可实现两偏振态下相同耦合长度，定向耦合器在偏振无关条件下工作.

A directional coupler composed of two vertical multiple-slotted silicon wires with trapezoidal cross-section is characterized using a full-vectorial finite element method in terms of the electric fields, where the perfectly matched layer absorbing boundary conditions are adopted. The effects of the structural or material parameters including the sidewall angle, the gap between the coupled waveguides, and the width and index profiles in the slot regions on the behaviors of the directional coupler are taken into account. The effective indexes of the even and odd modes and the corresponding coupling length, both in quasi-TE and quasi-TM modes, are presented. The strongly-hybrid nature of the guided-modes is effectively demonstrated. The results show that polarization-independent directional couplers can be realized by properly choosing the structural and material parameters.