To explain the formation mechanism of extreme events such as ocean rogue waves and light pulse train excitation, under investigation in this paper is the (2+1)-dimensional Fokas system, which is considered as the propagation model of nonlinear waves in optical fibers. By applying the complex conjugate condition to the N-soliton solutions, the breather solutions are gained. Breathers can be transformed into nonlinear waves, which is known as state transition. By controlling the wave number ratio, five types of transformed waves are studied, including quasi-soliton, W-typed quasi-periodic wave, M-typed soliton, oscillation M-shaped soliton and multi-peak soliton. The Riemannian circle is introduced to present the gradient relationship of transformed nonlinear waves. When the phase shift produced by the elastic collision of two-soliton or two-breather is large but limited, the two V-shaped structures of two-soliton or two-breather have significantly separated, accompanied by the formation of a branch connecting the two V-shaped solitons. Two quasi-resonant interactions, namely weakly quasi-resonance and strongly quasi-resonance are investigated. Based on the asymptotic analysis method, the properties of the resonant branch are analyzed in detail, including the trajectory, amplitude and velocity. Moreover, the intermediate resonant branch is a new branch generated due to the increase of phase shift, which exhibits temporal invariance and spatial locality. By introducing the small parameter , the length of the intermediate resonant branch is studied. These results are not only foundational for understanding nonlinear wave dynamics in the Fokas system but also offer critical insights into extreme event formation across disciplines. They explain rogue wave generation in fluids, light pulse anomalies in optical fibers and wave behaviors in shallow water theory. The findings bridge mathematical integrability and physical phenomena, providing a universal framework for analyzing localized wave transitions and resonant interactions in high-dimensional nonlinear systems.
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