Multi-wavelength square pulses are generated in the dissipative soliton resonance (DSR) regime by a Yb-doped fiber laser (YDFL) with a long cavity configuration. The spectral filter effect provided by a passive fiber with low-stress birefringence facilitates the establishment of multi-wavelength operation. Through appropriate control of the cavity parameters, a multi-wavelength DSR pulse can be generated in single- and dual-waveband regions. When the multi-wavelength DSR works in the 1038 nm waveband, the pulse duration can broaden from 2 ns to 37.7 ns. The maximum intra-cavity pulse energy is 152.7 nJ. When the DSR works in the 1038 nm and 1080 nm wavebands, the pulse duration can be tuned from 2.3 ns to 10.5 ns with rising pump power. The emergence of the 1080 nm waveband is attributed to the stimulated Raman scattering (SRS) effect. Our work might help a deeper insight to be gained into DSR pulses in all-normal-dispersion YDFLs.

We present experimental observations of soliton pulsations in the net normal-dispersion fiber laser using the dispersive Fourier transform (DFT) technique. According to the pulsating characteristics, the soliton pulsations are classified as visible and invisible soliton pulsation. The visible soliton pulsation converted from single- to dual-soliton pulsation with the common characteristics of energy oscillation and bandwidth breathing. The invisible soliton pulsation underwent periodic variation in the spectral profile and peak power but remained invariable in pulse energy. The reason for invisible soliton pulsation behavior was periodic oscillation of the pulse inside the soliton molecule. These results could be helpful for deepening our understanding of the soliton pulsation phenomena.

We investigate in detail the spectral pulsation of evolving soliton molecules in an anomalous dispersion fiber laser by utilizing the dispersive Fourier transform technique. The spectral pulsations of an evolving soliton molecule with different properties were achieved in our experiments, such as single-period and double-period pulsation. This type pulsation is characterized by the fact that the soliton molecules periodically experience oscillation in spectral profile and peak power due to the periodic vibration evolution of the internal pulses. However, the continuous weak energy exchange between the two solitons inside the soliton molecules results in an almost invariable pulse energy. In addition, analytical fitting models are given to further reveal the spectral pulsation phenomenon of the evolving soliton molecules. These experimental findings can shed some light on the dynamics of soliton molecules in fiber lasers.

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Recent progress in real-time measurement technology makes it possible to spectrally characterize multi-soliton complexes. Here, we report on real-time observation to the multi-pulse structures in a net-normal dispersion mode-locked fiber laser. With careful adjustment of cavity parameters, several kinds of multi-soliton complexes, such as the coexistence of soliton singlet and soliton molecule, as well as the coexistence of different types of soliton molecules, are realized in the experiment. Real-time spectral information of each pulse under multi-soliton states can be well resolved with the dispersive Fourier transformation technique, which is markedly different from the results measured by conventional measurement instruments. Note that this is the first reported the simultaneous generation of different types of soliton molecules in a fiber laser to our best knowledge. The findings help to understand the complicated dynamics of the coexisting pattern between different kinds of dissipative solitons.