Solitons are self‑contained wave packets that can travel long distances without spreading because nonlinearity balances dispersion. Diffraction is the familiar spreading pattern that appears when a wave passes through a slit or around an edge. This paper reports laboratory experiments showing that deep‑water gravity‑wave solitons can undergo transverse diffraction and yet keep their core soliton behavior along the direction of travel. In other words, the packet’s sideways shape changes like an ordinary diffracting wave, while its forward motion retains the hallmarks of a soliton.

The team used a large wave basin 50 m long, 30 m wide and 5 m deep. Waves were made with 48 computer‑controlled flaps (each 0.62 m wide and hinged 2.8 m below the surface) at one end of the tank. The carrier frequency was f0 = 1.1 Hz, giving a carrier wavelength near 1.3 m. The researchers imposed a one‑dimensional soliton envelope along the tank and then introduced a controlled transverse degree of freedom by driving only a subset of the flaps (a slit) or by weighting flap amplitudes to make a smooth Gaussian profile (apodization). They varied the effective slit width D from 0.6 m up to the full basin width of 30 m and tested soliton envelope amplitudes of roughly 0.4–2 cm.

Surface elevation was recorded by an array of 45 resistive probes placed 20 m or 35 m from the wavemakers. The central probes had 0.5 m spacing for finer detail; others were spaced by 1 m. To interpret the data the authors compared experiments with numerical integrations of the hyperbolic nonlinear Schrödinger equation (HNLSE), which describes weakly nonlinear, paraxial deep‑water wave envelopes. They also used nonlinear spectral analysis based on the inverse scattering transform (IST) to identify and track the soliton content of the wave packets. IST gives a compact way to tell whether a coherent soliton component is present.