Tricolor soliton molecules in synchronous mode-locked fiber lasers

Tricolor soliton molecules (TSMs), bound states of optical solitons with three distinct carrier frequencies, exhibit distinctive synchronization stability and intricate nonlinear interaction dynamics in synchronous mode-locked fiber lasers. In this work, we experimentally demonstrate the generation of stable synchronous TSMs and their multi-order harmonic mode-locking states in a passively synchronized mode-locked fiber laser system. Trapped via cross-phase modulation in a shared custom-fabricated single-walled carbon nanotube saturable absorber, the TSMs achieve group velocity synchronization by equalizing the cavity lengths of three independent optical paths using tunable time delay lines. By appropriately adjusting the pump powers and fine-tuning the intracavity polarization state, we realize multi-order harmonic mode-locking of synchronous tricolor pulses within the cavity. Using the dispersive Fourier transform technique, we unveil the real-time spectral-temporal evolution dynamics of these TSMs and their harmonic states. These findings enrich the fundamental investigations of multi-wavelength soliton molecular dynamics and harmonic mode-locking mechanisms, and provide a reliable experimental platform for nonlinear frequency conversion, ultrafast pump–probe spectroscopy, and coherent optical synthesis.