Amplified noise nonstationarity in a mode-locked laser based on nonlinear polarization rotation

Beat note measurements between a mode-locked (ML) and a continuous-wave laser, as well as between two ML sources, were used to demonstrate that the sub-threshold, cavity filtered, amplified spontaneous emission is not stationary, even when a fast mode-locking mechanism, such as nonlinear polarization rotation, is used to generate short pulses. A relatively small gain modulation of a few percent created by high-intensity pulses can produce a significant modulation of the amplified noise once synchronously accumulated over several cavity round-trips, even if the repetition rate is faster than the gain dynamics.