The problems with M2

M-2 is now widely used to characterize the quality of laser radiation. In the paraxial approach the inequality M-2 greater than or equal to holds, if M-2 is defined by the second moments. Nevertheless, in some publications M-2 <1 is presented, either theoretically or experimentally (Wang et al., Optik 1995:100(1):8; Lu et al., Optik 1995;100(2):91; Wang et al., Optics and Laser Technology 1999,31:151). In particular, it is stated that for a superposition of axially shifted Gaussian spherical beams, M-2 can become smaller than one (Wang et al., Optics and Laser Technology 1999,31:151). These problems with M-2 are briefly summarized. (C) 2000 Elsevier Science Ltd. All rights reserved.