Abstract

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? Similarly, one may ask whether the HML solution is uniquely defined. The answers to these questions are actually far from trivial if one considers a unidirectional ring laser or a Vertical External-Cavity Surface-Emitting Laser; in these two configurations, the optical pulses never cross each other within the active material: How could they then exchange any information regarding their relative phase? We base our analysis upon the well accepted Haus master equation [1] that was recently generalized to allow studying multi-pulse regimes [2,3]. However, we reached identical conclusions using a first principle model model [4] which is based upon time-delayed equations.

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