Modulation instability (MI) is one of the most nonlinear instability of the focusing nonlinear Schrödinger equation that describes how a low amplitude noise on an input signal can be exponentially amplified to create high-intensity localised structures. Recently, MI has attracted renewed attention due to its potential link with the development of extreme events or rogue waves, and many theoretical, numerical and experimental studies have been reported in various physical systems including optics and hydrodynamics. A particular difficulty associated with experimental studies of MI in optics is the need for real-time measurement techniques. In the time domain, the time-lens approach is complex and constrains the measurement bandwidth and power. In the spectral domain, the dispersive Fourier transform is simpler but only typically allows for low dynamic range measurements and does not provide information about the associated temporal properties.
We report on our recent work on the use of machine learning to predict from real-time spectral data statistics for the maximum intensity of the localised temporal peaks in a fibre-optic chaotic MI field, peaks which are preferentially associated with extreme events. We subsequently train a neural network to correlate the spectral and temporal properties using data from numerical simulations and we use this model to predict the temporal probability distribution based on near 60 dB dynamic range real-time spectral data from experiments. These results open novel perspectives in all systems exhibiting chaos and instability where direct time-domain observations are difficult.
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