Modeling of multimode operation of a laser with an unstable resonator

A description is given of a new method for calculating the parameters of nonlinear multimode lasing and for obtaining a stability criterion of the solution representing multimode emission. The uniqueness of the stable solution is discussed. The results are given of investigations of the parameters of multimode radiation in a laser with an inertial saturable medium. The conditions for the appearance of multimode lasing are determined and the energy characteristics of multimode radiation are found as a function of the small-signal gain of the active medium, equivalent Fresnel number, and resonator magnification. It is shown that a change from single-mode to two-mode lasing occurs quasiperiodically when the equivalent Fresnel number is changed by unity and the small-signal gain is slightly higher than the threshold value. The optimal conditions for ensuring high brightness of multimode radiation are identified. The proposed numerical method can be used in calculations of the parameters of multimode operation of optically coupled lasers with unstable resonators, as well as lasers with three-mirror and stable resonators.