Four-wave mixing (FWM), a nonlinear optical effect, is an important phenomenon for applications requiring parametric amplification and frequency conversion. Now, scientists in Europe have shown that wave-breaking effects can dramatically affect nonlinear frequency-conversion processes. Specifically, they report that dispersive shock waves, which mimic undular bores in hydrodynamics, can spontaneously form during FWM. Julien Fatome and co-workers from France, Germany and Italy studied the formation and collision of such optical undular bores in standard telecom fibres using nearly periodic optical pulses. The scientists used a 6-km-long nonzero dispersion-shifted fibre with a low normal dispersion of −2.5 ps nm−1 km−1. Optical pulses from an external cavity laser with a wavelength of 1,555 nm were modulated by a LiNbO3 intensity modulator at 28 GHz and then coupled to the optical fibre. The power of the input pulses was about 5 W. The temporal waveform of the pulses was observed at the output using an optical sampling oscilloscope. The scientists found that the initial modulation underwent a hydrodynamic type of instability that greatly affected the FWM process. They observed the generation and interaction of undular bores under different FWM configurations. These results provide a better understanding of several applications based on FWM in the high-efficiency regime.
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Horiuchi, N. Shock waves. Nature Photon 8, 499 (2014). https://doi.org/10.1038/nphoton.2014.159