Topological beam stirring in a multicore fiber
Multicore fibers (MCF) are perspective media for telecommunications, sensing, imaging and laser technologies. Here, the effect of beam stirring between weakly coupled cores is observed for sub-nanosecond transform-limited pulses of several kW peak power propagating in ~10 m long 7-core fiber, for the first time to our knowledge. In contrast to low-power domain where the output power distribution in the cores is random with large fluctuations sensitive to fiber disturbances, at high power of the input pulse injected in the central core the output power becomes equalized between the cores with fluctuations reduced to <5% being insensitive to disturbances. Similar behavior is observed in cut-back experiments showing that equi-partition is approached at a distance of ~5 m. The performed modeling describes well the experimental results and clarifies mechanisms of the new effect reasoned by a large nonlinear phase shift changing along the pulse and thereby resulting in statistical averaging over the pulse length of multidirectional power transfer processes between cores, thus leading to the robust equilibrium (equi-partitition for hexagonal MCF topology). At the same time, the combined output beam measured in a far field takes a stable bell-shaped profile instead of speckled beam at low powers, similar to the beam self-cleaning effect in multimode fibers.