天文探测中类地行星的探测需要进一步提高视向速度的测量精度。但圆形光纤扰模效果不佳所引起的入射信息变化时的谱线漂移，成为限制测量精度提高的一个主要因素。采用多边形光纤改善光纤传输系统的扰模特性。利用搭建的测试光纤远场和近场光斑质量的光学系统，通过对光纤出射场光斑质量进行评价，研究了圆形、长方形、正方形和八边形光纤在不同入射条件下的远场和近场分布及远场光斑的光强变化和近场光斑的质心偏移，得出了不同光纤的扰模增益系数。实验表明，在相同入射条件下，多边形光纤具有较好的扰模特性，其中八边形光纤的远场光强变化最小，近场光斑质心偏移最少，扰模特性最好。以八边形光纤代替圆形光纤，可有效减小入射偏差引起的谱线漂移，提高视向速度的测量精度。

The measurement precision of radial velocity needs improving for the detection of terrestrial planets. However, the spectral line shift during the change of coupling resulting from the insufficient scrambling of circular fibers limits the further improvement of the precision. The polygonal core fiber can provide better scrambling properties. Based the built experimental apparatus for measuring the near and far fields of the fiber output, the scrambling performance of the circular, rectangular, square and octagonal fibers under different coupling conditions is characterized via evaluating the near and far field patterns. The intensity variation in the far field and the barycenter shifting in the near field are investigated, and the scrambling gain is obtained for each fiber. The experimental results show that the polygonal core fiber has better scrambling performance. Especially, the octagon fiber has the minimum intensity variation in the far field, the minimum barycenter displacement in the near field, and the best scrambling performance. It is shown that replacing a circular fiber with an octagon one will effectively decrease the spectra line shift from coupling error and improve the measurement precision of the radial velocity.