Solitons, single- and synchronous dual-wavelength noise-like pulses and hybrid regimes from a thulium-doped fiber laser

In a passively mode-locked thulium-doped fiber laser within the anomalous dispersion regime, we experimentally demonstrate conventional soliton (CS), hybrid (noise-like pulse (NLP) and solitons), as well as switchable single-wavelength and synchronous dual-wavelength NLP regimes. By properly adjusting the PCs, we can switch between CS to hybrid and single- or dual-wavelength NLP regimes. With sufficient pumping power and through manual polarization adjustments, self-starting mode locking at 1996 nm can be observed. At the pump power of 2.5 W, stable mode locking with an output power of 86 mW is obtained. When generating the CS state, Kelly sidebands due to the spectral interference of dispersive waves are observed. Keeping the pump power at 2.5 W but with a modification of the polarization adjustment we can switch to five different regimes involving NLPs with squared envelope. The noise-like nature of the squared pulses was confirmed in each case through an autocorrelation measurement. By additional adjusting of the PCs, single- and synchronous dual-wavelength squared NLPs can be generated, at either one of two wavelengths in the former case, and at both simultaneously in the latter. The pulse trains from the single-wavelength operations display a small difference in their repetition frequencies inherently determined by intracavity dispersion, whereas in dual-wavelength operation the pulse train displays a single repetition rate whose value is intermediate between the single-wavelength frequencies. The obtained results are valuable to deepen our understanding on the formation of CS and dual-wavelength pulses with noise-like squared profiles in fiber lasers.