Role of spontaneous emission in the dynamics of mode locking by synchronous pumping

Numerical simulations tracing the pulse evolution in the case of mode locking by synchronous pumping are presented. It is argued that strictly steady-state pulses are impossible in principle because the wings of the pulses are embedded in background radiation generated by spontaneous emission. The simulations demonstrate that the perturbations introduced by the stochastic background can be severe, and that these can apparently be minimized only at the expense of the overall quality of the mode-locked pulse envelopes.