Generation and optimization of 2-μm square-wave noise-like pulses in a modified figure-eight fiber laser

We present the generation and optimization of square-wave noise-like pulses (NLPs) in a mode-locked Tm-doped fiber laser. Mode-locking operation around the 2-μm band is achieved by a nonlinear amplifying loop mirror. To optimize the output performance, the figure-eight cavity is modified by employing a polarization-dependent isolator in a unidirectional loop, and the cavity length is only 17.2 m. First, by employing a cavity with pure anomalous dispersion, a conventional soliton can evolve into a square-wave NLP by properly setting the pump power and polarization controllers. The pulse energy of the fundamental-frequency operation can be varied from 2.29 to 3.4 nJ. Using an ultrahigh-numerical-aperture fiber to reduce the net dispersion to −1.033  ps2, the 3-dB bandwidth of the spectrum is broadened to 14.78 nm, and the duration of the autocorrelation spike is only 421 fs. The maximum single-pulse energy can increase up to 4.97 nJ. Due to dispersion management mechanism, the threshold and output power are also significantly improved.