Dark rectangular noise-like pulses in a figure-nine fiber laser based on a nonlinear amplifying loop mirror.

We report the experimental observation of dark rectangular noise-like pulses (NLPs) in a novel figure-nine fiber laser based on a nonlinear amplifying loop mirror for the first time, to the best of our knowledge. By strengthening the nonlinear and birefringent effects in the cavity, the fundamental and high-order harmonics of dark rectangular NLPs are achieved with net anomalous dispersion. The dynamics of dark rectangular NLP formation and the spectral evolution with pump power are experimentally investigated. The results demonstrated an interesting operation regime of the fiber laser, which will contribute to enrich the dynamics of mode-locked pulses.