Stabilization of passive harmonic mode-locking by amplitude modulation

We demonstrate a fiber laser that is passively mode locked by a semiconductor saturable absorber and stabilized by the amplitude modulation. The synchronization of a dropout-free harmonic mode locking to an external clock was obtained with the pulse characteristics solely determined by the saturable absorber action, and not by the applied amplitude modulation. The results show that significant suppression of undesired harmonics can be obtained with small perturbations provided by the amplitude modulation.