Low-loss commercial optical fibers are known as perfect candidates to explore the richness of the dynamics of nonlinear physics. Indeed, the excellent knowledge of linear and nonlinear properties of these optical waveguides is a key ingredient to carry out experimental demonstration of the theoretical solutions of the nonlinear Schrödinger equation (NLSE). As soon as the early 80s, solitons were demonstrated in single-mode optical fibers with anomalous dispersion. More recently, taking advantage of the components of the telecommunication industry, more complex breather solutions have been experimentally generated. Such structures can also be detected in deep water and other nonlinear medium governed by the NLSE. Normally dispersive fibers have also stimulated recent experimental research, mainly driven by the interest in the study of dispersive shock waves. Most of the time, nonlinear dynamics observed in both anomalous and normal dispersion regimes of propagation are regarded as two completely different cases: one dominated by bright soliton-like structures and modulation-instability that provides an analogue to deep-water conditions; the other one ruled by dispersive shock waves (DSW) that satisfies the so-called nonlinear shallow water equations.
However, recent theoretical works have stressed that a shock wave may appear in the regime of focusing nonlinearity with weak dispersion, thus leading to the emergence of dispersive dam break flows in the NLSE box problem. A DSW-like nonlinear wave train regularizes an initial sharp transition between the uniform plane wave and the zero-intensity background. In particular, theoretical solutions essentially describe a modulated soliton train. This provides a new semi-classical interpretation of what has been previously described in the spatial domain as a nonlinear Fresnel diffraction. The box problem (i.e., an initial square profile) then gives rise to two counter-propagating modulation dynamics of opposite velocities, whose interaction may turn into a cluster of breathers.
In the present contribution, we confirm some of the above theoretical predictions by providing a detailed experimental observation of the regularization of sharp transitions from a super-Gaussian pulse in the presence of focusing nonlinearity and weak anomalous dispersion. Our results recorded in the temporal domain and based on an all-fibered test-based platform confirm the former qualitative behavior observed in the spatial domain as well as the strength of the space/time duality. After an initial shock induced by the overlap of the highly chirped and sharp wings of the pulse with the top region, strong temporal oscillations appear and nonlinearly reshape into a Peregrine-like structure at each maximum compression. This transient evolution is then marked by the breathing of the wave structures, both in the temporal and spectral domains. This transition may be followed by an asymptotic stage dominated by solitons. Finally, we also characterize the interaction event of the two counter-propagating dispersive dam break flows. Dispersive shock waves and Peregrine breathers have been shown to locally coexist, thus providing new insights into spontaneous pattern formations and novel possible interactions.