The beam quality of annular lasers and related problems

A detailed study of the beam quality of annular lasers, for example, lasers with or without phase distortions emitted from unstable resonators, is mad e, where the power in the bucket (PIB) and beta -parameter are chosen as cr iteria for characterizing the beam quality in the far field. It is shown th at better focusability is achieved at the real focal plane rather than at t he geometrical focal plane due to the focal shift, and the negative quartic phase distortions may improve the beam quality measured in terms of the PI E and /or beta -parameter. From a practical point of view the PIE and beta can be chosen as beam quality characteristic parameters for such lasers. Ho wever, some related problems remain, which are discussed briefly.