讨论与M2参数有关的一些问题。当采用光束二阶强度矩方法定义光束光斑大小和远场发散角时,M2参数满足M2≥1。但当改变其定义后,例如,采用86.5%(或其它百分比)功率通量法定义光束光斑大小和远场发散角时,由此定义的M2参数(M2pc)的最小值是多少,什么样的光束具有最小的M2参数(M2pc)。本文在采用86.5%(或其它百分比)功率通量法定义光束光斑大小和远场发散角的前提下,对这些问题进行了讨论。结果表明:共焦腔的输出光束应具有最小的M2参数(M2pc);当采用86.5%功率通量法定义光束光斑大小和远场发散角时,M2参数(M2pc)仍满足:M2pc≥1;但当采用其它百分比定义时,M2参数(M2pc)有可能小于1,M2pc的大小取决于所定义的光斑半径内包含的光功率的百分数。

The problems concerning the M2 factor are discussed. When the real-beam spot size and far-field divergence angle are defined by the second moments,it is pointed out that M2≥1 holds. But when the premise of the definitions by the second moments is changed,what will happen? For example,when the real-beam spot size and far-field divergence are defined by 86.5% (or other percentage) power content values,here what is the minimum of the defined M2 factor (M2pc)? What kind of beam possesses the smallest M2 factor (M2pc)? In this paper,these problems have been discussed,when the real-beam spot size and far-field divergence are defined by 86.5% (or other percentage) power content values. The results indicate that the output beam of the confocal cavity possesses the smallest M2 factor (M2pc). When the real-beam spot size and far-field divergence are defined by 86.5% power content values,it is pointed out that M2pc≥1 holds all the same. But when the definition with other percentage of the beam power is introduced. The M2 factor defined by the power content can be smaller than one,depending on the percentage of the enclosed power.