在采用86.5%功率通量法定义光束束宽和远场发散角的前提下，对由此定义的M2参数M2PC的最小可能取值及具有最小M2PC参数的光束特性等问题进行了讨论。结果表明：共焦腔的自再现模应具有最小的M2参数M2PC；当采用86.5%功率通量法定义光束束宽和远场发散角时，在近轴近似条件下，由具有同一中心的两基模高斯光束轴向叠加所形成的自再现光束，M2PC大于等于1。

When the real-beam width and far-field divergence angle are defined by 86.5% power content values, the minimum of the defined M2 factor M2PC and the characteristics of the beam with the smallest M2PC are discussed initially in this paper. The results indicate that the self-reproduction modes in confocal cavity possess the minimum of the M2 factor M2PC. When the real-beam width and far-field divergence angle are defined by 86.5% power content values, in paraxial approximation, among the self-reproductive beams formed by axial superpositon of two fundamental mode Gaussian beams with the same center, the M2PC is not less than 1.