Instabilities leading to self-pulsing, hysteresis, and chaos in a multimode laser

Abstract

We consider the emergence of instabilities in a laser regarded as a nonlinear self-oscillating system with spatially separated elements. Studies of different models of the interaction of the active medium with radiation (atomic polarization following the electric field or having inertia and inversion with or without the amplitude-phase grating created by counterpropagating coherent waves) reveal three mechanisms leading to laser instabilities. The first mechanism is the lack of uniformity in the temporal intensity variations of the opposite radiation fluxes. The second mechanism is the development of nutation oscillations. The third mechanism is connected with the interaction between the counterpropagating waves that is due to the dispersion of radiation by the periodic structure of the inversion. The main cause of instability in specific situations and for particular values of the laser parameters is explained. Possible successions of bifurcations are identified, which switch the system from cw to periodic, quasiperiodic, and irregular pulsation regimes in the laser. Three types of irregular pulsation have also been determined: with constant or irregular field phase and with irregular microsecond pulsing displaying regular fine structure on a scale of the order of the round-trip cavity time. Hysteresis of the pump-dependent pulsation period is considered with parameters corresponding to two excited natural frequencies.