光纤的几何参数影响着光纤的光学传输和机械性能等，是衡量光纤质量的重要指标。近场光分布法是国标GB 15972.20-2008中推荐的几何参数测量方法。该方法在对光纤纤芯的测量中需对光纤通光照明，以区分纤芯和包层的边界。通光的纤芯端面是一个边缘并不清晰的发光亮斑，因而无法准确判断纤芯与包层的真实边缘。本文分析了光纤内光传播模场的分布，理论上光纤模场电磁矢量的解满足贝塞尔函数，但在近似情况下也可以用高斯函数代表光纤模场分布。因此本文利用高斯函数拟合光纤纤芯端面灰度分布，进而由拟合后的高斯函数得到纤芯与包层的真实边缘。本方法是对国标GB15972.20-2008的测量方法的进一步完善。实验测量结果表明，当光纤的切割效果不佳或成像质量较差时，模场灰度分布的高斯函数拟合法仍能保证测量的重复精度和测量数据的稳定性。

The geometry parameters of optical fiber affect the optical transmission and mechanical properties, which are the important indexes to measure the quality of fiber. Near-field light distribution method is recommended in GB15972.20-2008 for the measurement of geometry parameters. In order to distinguish the boundary between fiber core and cladding, the method needs to illuminate the fiber. The end face of the fiber core is a bright spot with unclear edge, so the true edge of the core and cladding cannot be accurately judged. In this paper, the distribution of mode field in optical fiber is analyzed. Theoretically, the solution of electromagnetic vector of mode field satisfies Bessel function, but Gaussian function can also be used under approximate conditions. Therefore, Gaussian function is used to fit the distribution of the fiber core in this paper, and the real edge of the fiber core and cladding can be obtained from the Gaussian function after fitting. This method is a further improvement on the measurement method of GB15972.20-2008. The experimental results show that when the cutting effect of the fiber is not good or the imaging quality is poor, the Gaussian function method fitting with mode distribution can still ensure the repeatability of the measurement and the stability of the measured data.