Switchable different pulse state in 2.8μm femtosecond mode locked fluoride fiber laser

We experimentally demonstrate, for the first time, a 2.8μm femtosecond mode-locked fluoride fiber laser (MLFFL) with switchable pulsed-state including monopulse, dipulse, harmonic mode-locking and soliton molecules. Specifically, through adjustments to the cavity parameters, the mode-locking operation from monopulse to dipulse within a specific pump power range can be realized, with the maximum output power of 165mW and 175mW, respectively. Further increasing the pump power, the operating regime of the oscillator switches to second-order harmonic mode-locking, where the pulse fundamental repetition rate is ~155MHz with the output power of 330mW. In this case, bound soliton molecule can also be acquired by appropriately optimizing the orientation of the waveplates while keeping the pump power fixed. In particular, with increased pump power, multiple states of bound soliton molecule pairs can be achieved as well. The laser system is simple in structure, self-starting and good stability. Our experimental results may provide solutions for practical applications which requiring different pulse state switchable.