A harmonically mode-locked dark soliton and bright–dark soliton pair ytterbium fiber laser

We report on an experimental study of a dark soliton and bright–dark soliton pair, harmonically mode-locked, all normal dispersion (ANDi) ytterbium fiber laser with a long cavity length. Mode-locked output up to the fourth harmonic with respect to the fundamental repetition rate has been realized. To the best of our knowledge, this the first such demonstration so far in ANDi mode-locked ytterbium fiber lasers with a birefringence filter as spectral modulation component. The experimentally recorded mode-locked spectrum shows that the generation of a dark soliton is always accompanied by strong continuous-wave emission. Furthermore, by changing the pump power, the fundamental bright–dark soliton pair mode-locked operation can be evolved into the state of the second order bright soliton coexisting with the fundamental dark soliton. Additionally, bright–dark soliton pairs, which are symmetric relative to the vertical coordinate, can be interconverted by rotating waveplates in a fixed maximum pump power condition. The generation of the dark pulse is probably due to the large normal dispersion introduced in the ring cavity except for the nonlinearity.