In this paper, we present a method for a 3D-simulation of the population inversion and photon density in a solid state laser. The method approximates the electrical field distribution of the beam by a finite number of eigenmodes. These modes can be computed by a Gauss mode analysis. The population inversion is approximated by a finite volume discretization. Using the classical rate equations, we derive suitable rate equations for the discretized population inversion and the eigenmodes of a laser. The simulation results show the influence of the pump configuration to the distribution of the eigenmodes. Therefore, the new simulation method can be used to optimize the beam quality of a solid state laser.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.