Experimental study on harmonic mode-locking in soliton fiber lasers by bidirectional adjustment of pump power

Harmonic mode-locking (HML) in soliton fiber lasers is crucial for generating high-repetition-rate pulse trains beyond the fundamental cavity frequency, enabling advanced applications in, for example, optical communication and precision sensing. However, achieving HML in experiments is challenging, owing to its inherent instability and high sensitivity to laser parameters, resulting in complex and iterative adjustments. In this paper, a novel HML technique utilizing bidirectional adjustment of pump power is proposed, and it is experimentally demonstrated in an all-fiber hybrid mode-locked soliton laser. By first increasing the pump power to generate a soliton bunch with a certain number of pulses and then gradually decreasing it, HML can be achieved at an order corresponding to the number of pulses in the soliton bunch. Experimental results on the evolution of temporal pulse trains during bidirectional adjustment of the pump power enable a relationship to be established between pump power and soliton bunching with increasing pump power, and reveal the collapse of the soliton bunch and subsequent gradual uniform distribution of solitons into an HML state with decreasing pump power. Second- to sixth-order HML is successfully generated using the proposed technique, and an analysis of the results provides a deeper understanding of the observed pulse dynamics.