Switchable femtosecond and picosecond spatiotemporal mode-locked fiber laser based on NALM and multimode interference filtering effects

We report, for the first time to the best of our knowledge, the switchable generation of versatile femtosecond and picosecond spatiotemporal mode-locked (STML) pulses in an all-few-mode fiber figure-8 laser at 1.55 μm. Both in the femtosecond and picosecond pulse regimes, the laser has pulse-pattern-tunable function, namely, the pulse patterns can be tuned from single-pulse, multiple pulses to soliton molecules. Particularly, the femtosecond soliton molecules operate at the loosely bound states while the picosecond soliton molecules can operate both at the loosely and tightly bound states. The theoretical analyses are carried out to give the further insight in the switchable generation of the femtosecond and picosecond pulses. It is found that the filtering effect of the nonlinear amplifying loop mirror (NALM) plays the major role in the generation of the femtosecond pulses while the multimode interference filtering effect induced by the active few-mode fiber-passive few-mode fiber structure plays the role in the generation of the picosecond pulses. The obtained results contribute to further exploring the characteristics of the STML fiber laser and such a laser with tunable output of versatile femtosecond and picosecond pulses has potential applications in optical communications, laser processing and measuring.