Real‐Time Observation of Soliton Buildup and Self‐Organization in a GHz Harmonic Mode Locked Fiber Laser

Ultrafast fiber lasers operating at GHz repetition rates are critical for applications requiring high‐speed optical pulse sources. To achieve such rates, passive harmonic mode locking (HML) constitutes a logical pathway, whose practical implementation is hampered by the insufficient understanding of the complex multi‐scale buildup processes leading to stationary HML. Here we experimentally capture and analyze the full GHz‐HML buildup in a soliton fiber laser hosting over a hundred intracavity solitons, employing real‐time spectral, and temporal measurements via dispersive Fourier‐transform and high‐bandwidth detection. We reveal two prototypical stages: an initial rapid soliton generation phase driven by nonlinear effects including relaxation oscillations, soliton pulsation, and dispersive wave interactions, followed by a prolonged self‐organization phase where repulsive interactions from gain depletion and recovery lead to uniform soliton spacing. These insights advance the understanding of dense soliton interactions and the nonlinear dynamics of HML, facilitating the development of robust, high‐repetition‐rate ultrafast fiber lasers for diverse photonic applications.