Coexistence of rectangular and Gaussian-shape noise-like pulses in a figure-eight fiber laser

We experimentally report the coexistence of the rectangular noise-like pulse (NLP) and the Gaussian-shape NLP in a figure-eight fiber laser. Benefiting from the strengthened nonlinear effect of a segment of highly nonlinear fiber (HNLF) in the cavity, the coexistent NLPs with various patterns, i.e. one rectangular pulse with one Gaussian-shape pulse, one rectangular pulse with two Gaussian-shape pulses and two rectangular pulses with one Gaussian-shape pulse, are formed depending on the cavity parameters setting. In particular, the evolution of these coexistent NLPs properties with pump power is investigated. It is found that the duration of the rectangular pulse always increases, while the Gaussian-shape pulse has almost no changes with the increasing pump power. The achieved results demonstrated for the first time, to the best of our knowledge, the pulses with different shapes can coexist under the NLP regime, which contributes to further understanding the fundamental characteristics of the NLPs and multiple pulses.