Effect of lifting of mode degeneracy in unstable optical resonator under influence of active medium

The self-consistent problem of lasing from unstable resonator loaded with gain saturable medium is considered. A recently developed numerical method is expounded for solving such problem, viz., selective iterative method. It allows one to find efficiently both stable and unstable solutions. The calculations were performed for a strip confocal resonators whose equivalent Fresnel numbers were varied near the values corresponding to mode degenerecy points. It was founded that Rigrod medium stabilized one-mode lasing when small-signal gain was sufficiently large, and a two-mode regime occurred with lesser values of small-signal gain. An active medium of $\mathrm{CO}_2$-flow laser lifts a mode degeneracy as soon as threshould gain is exceeded.