Dual-wavelength step-like pulses in an ultra-large negative-dispersion fiber laser.

We report on experimental observation of dual-wavelength step-like pulses delivered from an erbium-doped fiber laser operating in ultra-large negative-dispersion regime. The step-like pulses consist of two rectangular pulses with different energies, durations as well as optical spectra, and are distinct from the conventional multi-solitons or bound-state solitons in that each pulse holds the same property. We find the weaker (or stronger) rectangular pulse in step-like pulses is more sensitive to the backward (or forward) pump while is less sensitive to the forward (or backward) pump. Our results demonstrate that the dual-wavelength operation results from the combination of fiber dispersion, fiber birefringence, as well as cavity filtering effect, and the intensity difference between rectangular pulses can be attributed to different gain characteristics of the forward and backward pump.