From coherence to incoherence in harmonic mode-locked lasers

Passive mode-locking is a well established technique to obtain short optical pulses with high repetition rates. In these devices, the combined dynamics of a gain and a saturable absorber promotes the pulsed emission over the continuous wave regime. Beyond the fundamental mode-locked (FML) solution, where a single pulse is present in the cavity, multi-pulse solutions are possible, the so-called harmonic mode-locked solutions (HML). In this contribution we want to answer a seemingly obvious question: Are the pulses in an harmonic state coherent?