Analysis of relaxation oscillations and gain switching of unidirectional erbium-doped waveguide ring lasers

Relaxation oscillations and gain switching of erbium-doped waveguide ring lasers (EDWRLs) are studied using numerical simulations based on time-dependent rate-propagation equations. The counter-directional wave suppression is analyzed for different waveguide ring cavity configurations and pumping schemes. It is shown that the counter-directional wave suppression in unidirectional EDWRLs undergoes relaxation oscillations synchronously with oscillating power. It is also shown that the suppression in the first spike is maximal, so the gain switching technique provides the most favorable conditions for unidirectional lasing. Furthermore, for the one-end-pumped gain-switched EDWRL, highly unidirectional operation is possible with no intracavity elements included. In this case the counter-directional wave suppression considerably exceeds its steady-state value. The gain-switched suppression caused by intracavity elements is close to the steady-state value.