Route to L-band GHz nonlinear multimodal interference mode-locked fiber laser

Ultrafast fiber lasers over gigahertz (GHz) repetition rates have attracted substantial attention due to their wide range of applications. Harmonic mode-locking (HML) is an effective method to scale the repetition rate to GHz level. Our study experimentally confirms the formation process and evolution of GHz harmonic pulses in an anomalous dispersion laser operated in L-band by adopting the emerging multimode inference-based hybrid-fiber modulation device while optimizing intra-cavity dispersion, nonlinearity, and pulse polarization. Our findings reveal that the GHz pulse formation undergoes three phases in sequence, namely a multiple-pulse soliton bunch in burst mode, conversion to an HML pulse, and formation of the GHz pulse train with equal periodic interval distributions. The combined effect of the nonlinear multimode interference on intra-cavity pulses in temporal, frequency, and space domains, including coherent mode superposition, spectral filtering, and mode selection in composite fiber structures, may be associated with high-order pulse splitting. The maximum repetition rate of the laser is as high as 1.112 GHz, corresponding to the 103rd harmonic of the fundamental frequency. These results contribute to understanding the nonlinear characteristics of multimode interference equivalent mode-locked devices and provide valuable reference for promoting the development of the high-repetition-rate laser sources with high performance, low cost, and simple manufacture.