Generation of period doubled solitons from a mode-locked fluoride fiber laser

Period doubling bifurcation (PDB), a universal phenomenon in nonlinear systems, provides a unique perspective for understanding the properties of nonlinear systems, possessing potential important applications. Although it has been intensively investigated in the near-infrared (NIR) spectral region, there are no relevant reports in the mid-infrared (MIR) spectral region. Here, by combined use of numerical analysis and experimental demonstration, the phenomenon of PDB from a fluoride fiber oscillator mode-locked by the nonlinear polarization evolution (NPE) technique is reported for the first time, to the best of our knowledge. In the numerical simulations, the phenomenon of PDB at 2.8 μm is unveiled and analyzed by solving the extended coupled nonlinear Schrödinger equations, in which the pump strength and polarization state are found to play a vital role. A soliton regime with a pulse duration of 309 fs, a repetition rate of 67.16 MHz and an average output power of 63 mW is experimentally achieved, presenting uniform pulse intensity. Based on the simulations, through improving the pump strength, a stable soliton pulse train with the period-doubled state is obtained. This work promotes the development of mid-infrared ultrafast fiber lasers, opening up new opportunities for the MIR optical frequency comb and weak signal detection.