Drift instabilities in the pulses from cw mode-locked lasers

The phenomenon of drift instability was recently identified in optics and hydrodynamics by several authors. Here we show that in optics also the temporal analogon exists. It can be found in all types of mode-locked lasers that may be described as two coupled clocks, i.e., actively mode locked, synchronously pumped, and certain passively (additive-pulse) mode-locked lasers. The relevant parameter is a timing mismatch as obtained, e.g., by cavity length misadjustment. Locking over a finite interval of mismatch is also observed, and turns out to be crucially important for the generation of stable pulses. Based on dedicated experiments and numerical simulations we explain the analogy, and the threshold behavior of locking.