Abstract
The combined influence of the resonator parameters and spatially nonuniform axially symmetric amplification distribution in a four-level active medium on the spatial structure of the fundamental mode of laser radiation is studied numerically by the Fox—Li method. The model of two beams interacting due to the spatial amplification nonuniformity is used to obtain simple analytic estimates. The transition of the spatial structure of the fundamental mode from the Gaussian one to the structure formed by a set of Laguerre—Gaussian beams is studied. It is shown that near the degenerate configurations of the resonator a decrease in the diameter of the pump beam leads to an increase in the number of Laguerre—Gaussian beams forming the fundamental mode. It is found that the range of detunings from the strong degeneracy within which substantial differences of the spatial structure of the fundamental mode from the Gaussian structure are realised, increases with increasing the gain.