在天文光纤光谱仪中,阶跃型圆柱多模光纤的出射场在有些情况下会变成环形分布的光斑而不是具有高斯分布的圆形光斑,这将对天文光谱测量造成重要影响,不仅引起伪光谱漂移导致光谱测量精度降低,而且还产生伪光谱结构使得原本复杂的天文光谱更加难以分析。利用光线理论对阶跃型圆柱多模光纤的光学传播性能进行分析,揭示了造成环形出射场形成的两大因素:一是入射偏移(包括中心偏移和角度偏移),另一个是圆柱光纤旋转不变性导致的空间光线传播方式。并通过建立入射中心偏移量与出射场环形分布的数学模型,研究了随着中心偏移量增大环形出射场的形成和演化过程,理论分析与实验现象吻合。 In astronomical fiber spectrometers, the exit field of a stepped cylindrical multimode fiber will in some cases become a ring-shaped spot instead of a circular spot with a Gaussian distribution, which will have a significant impact on astronomical spectroscopy measurements, not only causing false Spectral drift leads to lower spectral measurement accuracy, but also produces a pseudo-spectral structure that makes the original complex astronomy spectrum more difficult to analyze. The optical propagation of step-type cylindrical multimode fiber is analyzed by using light theory. Two major factors that cause the formation of circular exit field are revealed: one is the incident deflection (including the center offset and the angle offset), and the other is the cylinder Optical fiber rotation invariance caused by space light transmission. And by establishing the mathematical model of the center of the incident center and the annular distribution of the exit field, the formation and evolution of the annular exit field with the increase of the center offset are studied. The theoretical analysis agrees well with the experimental results.