Revealing the Buildup Dynamics of Harmonic Mode‐Locking States in Ultrafast Lasers

TLDR

Harmonic mode‑locking (HML) enables high‑repetition‑rate ultrashort pulse generation in ultrafast lasers. The study reports the first experimental observation of the full buildup of a passive HML state in an ultrafast fiber laser using time‑stretch dispersive Fourier transform. The authors employed a time‑stretch dispersive Fourier transform to capture the ultrafast dynamics of the HML buildup in a fiber laser. The buildup proceeds through seven ultrafast phases—relaxation oscillation, spectral beating, giant pulse birth, self‑phase‑modulation instability, pulse splitting, repulsion and separation, and a stable HML state—revealing that single‑pulse splitting generates multiple HML pulses, early breathing, and that dispersive wave, gain dynamics, and acoustic effects govern the process, with acoustic resonance stabilizing the final state.

Abstract

Abstract Harmonic mode‐locking (HML) is an important technique enabling the generation of high‐repetition‐rate ultrashort pulses. Using an emerging time‐stretch dispersive Fourier transform technique, the experimental observation of the entire buildup process of the passive HML state in an ultrafast fiber laser is reported here. It is unveiled that the whole process of HML buildup successively undergoes seven different ultrafast phases: raised relaxation oscillation, spectral beating behavior, birth of a giant pulse, self‐phase‐modulation‐induced instability, pulse splitting, repulsion and separation of multiple pulses, and a stable HML state. It is observed that the multiple HML pulses originate from a single‐pulse splitting phenomenon and a remarkable breathing behavior occurs at an early stage of the HML buildup process. The numerical results confirm that the effects of dispersive wave, gain depletion and recovery, and acoustic wave play key roles in the earlier, middle, and later stages of this HML buildup process, respectively; as well, the acoustic resonance in the single‐mode fiber stabilizes the final HML state of lasers.