Generation and breakage of noise-like square pulses in a Tm-doped figure-8 fiber laser

We report the generation of tunable double and multiple noise-like square pulses (NLSPs) in a Tm-doped fiber laser, passively mode-locked by a hybrid scheme, found for the first time in the 2 μm band. The dynamical characteristics of double NLSPs are investigated by tuning polarization and pump. The results show that the evolution of the double NLSPs differs in three pumping ways of double pumps, enabling the multi-pulse pattern to be more controllable. The tri- and quad-pulse patterns are also found to appear from a single pulse in a wave-breaking way. A followed simulation successfully yields multi-pulse solutions and unveils the walk-off mechanism and the wave-breaking origins, which should be attributed to excessive chirping or mode-locking jump induced by polarization transfer.