Beam quality deterioration of lasers caused by intracavity beam distortions

It is well known that beam distortions in a laser cavity can deteriorate the laser's beam quality, but why and exactly how the impact of such distortions can strongly depend on details of the resonator setup has not been studied in detail. This article clarifies the issue with a simple resonant mode coupling model, explaining e.g. why strong beam quality deterioration is often observed near certain (but not all) resonator frequency degeneracies, while a resonator can be very tolerant to distortions in other cases. These findings lead to important conclusions, including practical guidelines for optimizing laser beam quality via cavity design.