The Fermi–Pasta–Ulam (FPU) problem is a paradox that was first analysed in the 1950s. Contrary to what one might intuitively expect, thermalization of coupled oscillators does not necessarily cause the initial modes to fade gradually. Instead it can yield quasi-periodic behaviour, known as FPU recurrence, which 'revives' the modes. 

The effect is highly relevant in the field of nonlinear optics. Arnaud Mussot and colleagues from Université de Lille in France and the Australian National University have now shown that under the influence of third-order dispersion, the FPU phenomenon can undergo multiple appearances and disappearances within an optical fibre. According to the authors, this behaviour is related to Čerenkov radiation. The team used two tunable diodes to generate the pump and signal beams.










These beams were coupled using a 50/50 fibre coupler, amplified by an erbium-doped fibre amplifier and then injected into a dispersion-shifted fibre. A pseudo-random encoding was applied to prevent stimulated Brillouin scattering. In experiments showing FPU recurrence in an optical fibre, multiple recurrences can occur when the optical frequency varies near the zero-dispersion wavelength of the fibre. The team found that by varying the pump wavelength between 1,561.6 nm and the zero-dispersion wavelength of 1,550.2 nm, the radiation loss could be made reversible or irreversible, resulting in reappearance and disappearance of the FPU phenomenon, respectively.


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