模场直径是光纤波导基模的特征参量。单模光纤中的功率密度与模场直径成反比。随着光纤激光器单模输出功率的不断攀升，纤芯中的功率密度不断增加，过高的功率密度会使光纤波导产生光学损伤和热损伤。基于锥形光纤模场分布近似模型，研究发现不同参数的拉锥光纤模场直径最小值位置对应的归一化频点具有规律性。采用有限差分波束传播法(FD-BPM)对不同波长、不同数值孔径下纤芯直径和模场直径的对应关系进行模拟，结果表明：模场直径最小值位置在归一化频率1.8附近，与光纤参数和波长的选取没有直接关系。这个特征参数可为高功率激光在光纤中功率密度最大值位置的快速确定提供依据，也为光纤光学理论增加了新的内涵。

Mode-field diameter (MFD) is a characteristic parameter of fundamental mode in fiber-optics waveguide. Power density in single-mode fiber is inversely proportional to the MFD. As the single-mode output power of fiber laser increasing, the power density in the fiber increases intensely. Much higher power density can cause optical damage and thermal damage of fiber waveguide. Based on the approximate mode-field distribution model, a character was found that the minimum MFDs in tapered fibers with different original fiber parameters corresponded to an invariable fiber normalized frequency. The relations between the core diameters and MFDs were simulated by the finite difference beam propagation method (FD-BPM) in different wavelengths and in different core numerical apertures (NA). The results demonstrated the supposition presented above and indicated that the minimum MFDs corresponded to the normalized frequency 1.8 nearly, without direct relations with the wavelength and fiber para-meters. The results provide theoretical reference for the quick determination of the point of maximum power density in fiber, especially in high power case, as well as add new content to the fiber optics theories.