Theory of nonlinear coupled-cavity mode locking

The mode-locking dynamics of coupled-cavity mode-locked (CCM) lasers with a Kerr medium in the external or the main cavity or both are analyzed without the approximation of weak external feedback; higher-order nonlinear and dispersive effects are taken into account. The pulse parameters and the stability regions of the steady-state solution relative to the cavity-length detuning and other laser parameters are calculated. The dependence of two distinct operating regimes of coupled-cavity mode locking on the pump rate and the cavity-length detuning is predicted. For quasi-matched cavity lengths stable CCM operation requires interferometrically exact cavity-length control. However, as a consequence of self-stabilization owing to self-frequency adjustments we found for sufficiently high pump rate, high feedback coefficients, and strongly detuned cavity lengths the existence of a stable self-sustained CCM operating regime, without the need for active cavity-length stabilization. Further, the operating principle of mode locking in a solid-state laser coupled to a linear external cavity with a moving mirror is described. Here the nonlinearity is provided exclusively by the Kerr effect in the laser host medium. A moving mirror is essential for stabilization against erratic jumping between coexisting multistable mode-locking states and simultaneously for startup of pulse formation.