Real-Time Transition Dynamics of Harmonically Mode-Locked Femtosecond Ultralong Ring Fiber Lasers

The transition dynamics between harmonic mode-locking states in Raman-assisted ultralong ring fiber lasers operating in the C-Band are experimentally studied in resonators of up to 5.4 km, capable of generating ultrashort pulses with energies of the order of 0.6 µJ and durations below 200 fs, with repetition rates as low as 38 kHz, which allow for real-time monitoring of pulse dynamics. Output signal is monitored through the time-stretched dispersive Fourier transform technique, confirming the expected solitonic behaviour associated to the quasi-lossless extended cavity, dynamic tunability and a variety of intermediate transient states. Transitions between stable mode-locked states from lower to higher harmonics are shown to be mediated by the formation of soliton-like mutually attractive pulse pairs that merge after a number of cycles. The absence of a direct path between some harmonic modes has been identified, which can have important implications in system optimization, and inform design for tunable operation.