从理论和实验两方面研究了涡旋光束经菱形光阑后远场的衍射图样。结果发现，夫琅禾费衍射图样中暗条纹的数目正好等于入射涡旋光束的拓扑电荷数的数值，而每个暗条纹的中心都是一个相位奇点。基于这个衍射特性，提供了一种测量光学涡旋轨道角动量的简易方法。

The far-field diffraction pattern of a vortex beam by a rhombus aperture is investigated theoretically and experimentally. It is found that the number of the dark stripes in the Fraunhofer diffraction intensity distribution is just equal to the topological charge value of the measured optical vortex, and the centre of each dark stripe is just a phase singularity point. Based on this property, it provides us a simple way to detect the orbital angular momentum (OAM) of an optical vortex beam.