Analysis and suppression of Q-switching instabilities in mode-locked lasers-a control systems approach

We investigate the origin of Q-switching instabilities in a mode-locked laser from a control systems viewpoint. The laser is represented as a linear control system, and we consider active laser stabilization either by gain or by loss control as well as passive stabilization via inverse saturable absorption. Gain control stabilization compensates for low-pass filtering in the gain relaxation branch of the system, while control of intracavity losses directly counteracts the destabilizing effect of the saturable absorber. For both stabilization approaches, design rules are derived for controller gain and bandwidth, and their applicability to different laser systems is investigated. Our discussion is concluded with a control systems analysis of passive laser stabilization via inverse saturable absorption. The control systems representation makes the established methods of control theory available to active and passive laser stabilization.