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 mu J 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.