It is shown, theoretically and experimentally, that there is a correspondence between the near field distribution of light from a ruby laser and its interferogram. Specifically, it is shown that, in general, filaments occur in ruby lasers but that these are not present for good quality crystals. An interferometric method for assessing crystal quality is described. The results of preliminary investigations of the spatial distribution of coherence in the near field of a good laser are given, and it is shown that there is a high degree of coherence between points separated by as much as 3.2 mm on the face of a ruby laser.
Michael Hercher, "Relationship between the Near Field Characteristics of a Ruby Laser and Its Optical Quality," Appl. Opt. 1, 665-670 (1962)