Off-Axial mode selection in solid-state lasers due to nonuniformities

It is shown that the nonuniformity of excitation produced in circular cross-section solid-state lasers due to the focusing of pump power leads to a coupling of certain modes and reduction in their threshold of oscillation. These modes correspond in plane-wave representation to simple closed paths in the cavity. The off-axial radiation observed in the form of rings by Collins and Giordmaine and others is explained on the basis of the theory.