Q spoiling and directionality in deformed ring cavities.

A ray-optics model is developed to describe the spoiling of the high-Q (whispering gallery) modes of ring-shaped cavities as they are deformed from perfect circularity. A sharp threshold is found for the onset of Q spoiling as predicted by the Kolmogorov-Arnol'd-Moser (KAM) theorem of nonlinear dynamics. Beyond the critical deformation b(c), Q ~ (b - b(c))(-alpha), alpha asymptotically equal to 2.4-2.6. The escaping light emerges in certain specific directions, which may be predicted.