Hydrodynamic 2D Turbulence and Spatial Beam Condensation in Multimode Optical Fibers

Abstract

We show that Kerr beam self-cleaning results from parametric mode mixing instabilities, that generate a number of nonlinearly interacting modes with randomised phases -optical wave turbulence, followed by a direct and inverse cascade towards high mode numbers and condensation into the fundamental mode, respectively. This optical self-organization effect is analogue to wave condensation that is well-known in hydrodynamic 2D turbulence.

Wave condensation with weak disorder versus beam self-cleaning in multimode fibers

“…We have also considered the impact of strong disorder at the leading order linear regime, which revealed that random mode coupling among degenerate modes enforces the thermalization toward the Rayleigh-Jeans equilibrium distribution. We note in this respect that, according to the simulations reported in [42], strong disorder only weakly affects the evolutions of the modal components as compared to the significant impact of weak disorder discussed in this work. Nevertheless, it would be important to extend the theory developed for weak disorder to strong disorder in order to study the interplay of random mode coupling and nonlinearity, a feature that will be considered in future works.…”

“…Recent works indicate that this phenomenon of beam self-cleaning can be interpreted as a consequence of wave thermalization and condensation [41][42][43]. In particular, in Ref.…”

Random Mode Coupling Assists Kerr Beam Self-Cleaning in a Graded-Index Multimode Optical Fiber

“…The output beam for the considered model has a size that is close to that of the fundamental mode. It should be noted that the results of our numerical investigations are in complete agreement with our experimental data [14]. a) b) Figure 7: Dynamics of energy distribution by modes (a) and the output field (b) for the model with random linear coupling between modes with equal mode numbers in the nonlinear regime and for the shifted initial beam.…”

Nonlinear optics in multimode fibers

“…In the full multimode case, FWM generates a large number of nonlinearly interacting modes with randomized phases, or an optical turbulent state. This model permits to connect self-cleaning to 2D hydrodynamic turbulence and wave condensation [36].…”