By manipulating an axicon inside a thin-disk laser resonator and accounting for thermal loading effects on the active medium, we analyze the diffraction loss of the system’s eigenmode (specifically the Bessel-Gauss mode) using both numerical and theoretical approaches. Furthermore, we examine the self-consistency condition via paraxial ray analysis to determine the stability of periodic ray trajectories after one or two complete round trips. A theoretical description of the diffraction loss is provided and compared with numerical results. Our findings demonstrate that the small radius of curvature of the thin-disk/output-coupler mirror can effectively control diffraction loss. To validate the analytical results, we employ the well-known Fox-Li method to numerically compute the diffraction loss, revealing remarkable agreement between the analytical and numerical outcomes
