从三层平板波导麦克斯韦方程的模场解出发，根据光斑尺寸的定义，讨论了波导模式，波导尺寸、折射率差、工作波长以及波导非对称性对光斑尺寸的影响，给出了最小光斑的波导条件。分析和模拟结果表明，对称三层平板波导TE模的最小光斑尺寸(MMSS)与波导芯/包层介电系数差的平方根成反比，TM模最小光斑尺寸则依赖于波导芯层和包层介电系数的具体大小，相同波导结构条件下TM模的等效光斑尺寸较TE模更大, 二者皆与工作波长成正比；非对称波导中光斑分布是非对称的，其光斑尺寸介于分别以两个包层为包层的对应对称三层平板波导之间，其最小光斑尺寸随对称因子的增大先减小后增大。

The minimum mode spot size of a three-layer dielectric optical waveguide was studied. Based on the mode-field solutions expression of Maxwell function, The influences of mode polarization, waveguide size, dielectric coefficient difference, wavelength and asymmetry on mode spot size, according to its specified definition are investigated. The analysis and simulation results indicated that there existed a minimum mode spot size (MMSS) in a three-layer dielectric optical waveguide. As to the symmetric waveguide, the MMSS of TE mode is inversely proportional to the square root of dielectric coefficient difference. However the MMSS of TM mode is dependent on the specified dielectric coefficients of waveguide and larger than that of TE mode. Both of them are proportional to the wavelength of guided wave. As to the asymmetric waveguide, the shape of mode spot is also asymmetric. The MMSS has the same dependences on dielectric coefficient difference as symmetric one. Besides, it decreases at first and then increases with the increasing of waveguide symmetry factor.