Characterization of the ultrafast fiber laser mode-locked by nonlinear multimode interference

A passively mode-locked fiber laser with switchable output states is demonstrated by using an all-fiber saturable absorber (SA) based on a hybrid single mode fiber-graded index multimode fiber-single mode fiber-graded index multimode fiber-single mode fiber (SMSMS) incorporated into a ring cavity. We investigate the vectorial characteristics of the output soliton in order to analyze the influence of polarization dependence of the SMSMS structure on solitons generation. Due to the appearance of vector solitons, it is proved that nonlinear multimode interference (NL-MMI) plays a dominant role in solitons generation but not nonlinear polarization rotation (NPR). Meanwhile, abundant mode-locked states are achieved in the laser, such as fundamental soliton, higher-order harmonics, bound twin-pulse solitons, and quasi-harmonic bound states. This configuration provides a useful platform for investigating ultrafast soliton dynamics. •Reduced threshold of mode-locked fiber laser is realized by cascade graded index multimode fiber saturable absorber.•It is verified nonlinear multimode interference dominates solitons generation but not nonlinear polarization rotation.•Abundant states are observed such as fundamental soliton, higher-order harmonics, bound twin-pulse solitons, and so on.