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We report on the first experimental observation, to the best of our knowledge, of soliton molecules in a spatiotemporal mode-locked multimode fiber (MMF) laser. By adjusting the waveplates inside the cavity, not only the spatiotemporal mode-locking state with a stable single pulse but also soliton molecules are observed. Various soliton molecules, including soliton pairs, soliton triplets, and soliton quartets with different pulse separations, are achieved. Transition of different operation states with pump power is given. The results would be beneficial for further understanding of the nonlinear dynamics in spatiotemporal mode-locked MMF lasers.

Dissipative nonlinear wave dynamics have been investigated extensively in mode-locked lasers with single transverse-mode, whereas there are few studies related to three-dimensional nonlinear dynamics within lasers. Until recently, multi-transverse-mode, or spatiotemporal, mode-locking (STML) was demonstrated in lasers with small modal (i.e., transverse-mode) dispersion, which is critical for achieving STML because the small dispersion can be easily balanced. Here, we demonstrate that STML can be achieved in lasers with much larger modal dispersion, and we find that the spatiotemporal selforganization of the multimode solitons is quite different from previous studies. Furthermore, we observe a new STML phenomenon of passive nonlinear auto-selection of single-mode mode-locking, resulting from the interaction between spatiotemporal saturable absorption and spatial gain competition. Our work significantly broadens the design possibilities for useful STML lasers thus making them much more accessible for applications, and extends the explorable parameter space of the novel dissipative spatiotemporal nonlinear dynamics that can be achieved in these lasers.

We demonstrate an experimental observation of soliton rains in an all normal dispersion Yb-fiber laser. The cavity consists of a narrow bandwidth filter and a birefringent plate (BP) filter. Soliton rain is obtained in the weak mode-locking regime, while multisolitons behaving as soliton bunches and harmonic mode-locking under strong mode-locking are also observed. Distinctive multisoliton interactions are observed via changing the pump power and adjustment of the waveplates as well as the BP. To the best of our knowledge, this is the first demonstration of soliton rains in normal dispersion fiber lasers.

Featured Application: High-capacity telecommunication, advanced time-resolved spectroscopy, self-organization and chaos in dissipative systems. Abstract:Benefiting from ultrafast temporal resolution, broadband spectral bandwidth, as well as high peak power, passively mode-locked fibre lasers have attracted growing interest and exhibited great potential from fundamental sciences to industrial and military applications. As a nonlinear system containing complex interactions from gain, loss, nonlinearity, dispersion, etc., ultrafast fibre lasers deliver not only conventional single soliton but also soliton bunching with different types. In analogy to molecules consisting of several atoms in chemistry, soliton molecules (in other words, bound solitons) in fibre lasers are of vital importance for in-depth understanding of the nonlinear interaction mechanism and further exploration for high-capacity fibre-optic communications. In this Review, we summarize the state-of-the-art advances on soliton molecules in ultrafast fibre lasers. A variety of soliton molecules with different numbers of soliton, phase-differences and pulse separations were experimentally observed owing to the flexibility of parameters such as mode-locking techniques and dispersion control. Numerical simulations clearly unravel how different nonlinear interactions contribute to formation of soliton molecules. Analysis of the stability and the underlying physical mechanisms of bound solitons bring important insights to this field. For a complete view of nonlinear optical phenomena in fibre lasers, other dissipative states such as vibrating soliton pairs, soliton rains, rogue waves and coexisting dissipative solitons are also discussed. With development of advanced real-time detection techniques, the internal motion of different pulsing states is anticipated to be characterized, rendering fibre lasers a versatile platform for nonlinear complex dynamics and various practical applications.

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We report on the experimental observation of stable single solitons and soliton molecules in a 2-μm thulium-holmium-doped fiber laser mode-locked through the nonlinear polarization evolution technique within an anomalously dispersive cavity. Single 0.65 nJ solitons feature a 7.3 nm spectral FWHM and 540 fs temporal duration, yielding a time-bandwidth product close to the Fourier-transform limitation. Under the same pumping power of 740 mW, stable out-of-phase twin-soliton molecules, featuring a temporal separation of 2.5 ps between the two ∼700  fs pulses, are generated in a deterministic way, while the central wavelength of the soliton molecules can be tuned from 1920 to 1940 nm. Finally, we present strong experimental evidence of vibrating soliton molecules.