Topological beam stirring in a multicore fiber

TL;DR

The paper investigates a new nonlinear beam-stirring phenomenon in a hexagonal 7-core multicore fiber, observed at kW-scale peak powers where energy redistributes among weakly coupled cores. By combining sub-nanosecond experiments with a seven-core Kerr-nonlinearity model solved via split-step methods, the authors show that core powers approach equi-partition with fluctuations below $5\%$, accompanied by a bell-shaped far-field profile. The mechanism relies on large nonlinear phase shifts along the pulse, causing oscillations among cores that statistically average over the pulse length, yielding robust equilibration over about $10~\mathrm{m}$. This work suggests a route to power-equalized amplification and improved beam quality in multicore fibers, with potential extensions to other topologies and pulse regimes, and invites deeper exploration of phase dynamics of partial beams for coherent vs. incoherent regimes.

Abstract

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.

Figures (5)

• Figure 1: Experimental setup: $\text{L}_1$ – lens, $\text{AL}_{1,2}$ – aspheric lenses for beam collimation at the output of the 7CF, the image of the fiber output beam is captured with CMOS camera
• Figure 2: a) The output power fraction in individual cores (C0-C6, C0 is central) as a function of the input peak power of the pulse propagating in a 11-m 7CF coiled into a ring with diameter of 6 cm; b) the relative difference between the maximum and minimum output power (core fractions) as a function of the input peak power (blue line corresponds to experimental data, orange line – to numerical modelling, yellow line – to analytical approximation).
• Figure 3: The relative difference between the maximum and minimum power (core fractions) versus fiber length for three input peak power values.
• Figure 4: NF images of the 7CF output captured at 1064 nm (a-c) demonstrate the spatial beam stirring, namely the formation of 7 beams with equal intensities when the $P_\text{in}$ increases. FF images (d-f) demonstrate the coherent combining of the beams with the establishment of bell-shaped transverse profile (g-i). Fiber length - 11 m.
• Figure 5: The power distribution along the fiber in different cores normalized to input power $P_\text{in} =6.32\,$kW and corresponding output pulses at MCF parameters: a, d) $\delta n_\text{max}=3\cdot 10^{-5}$, $J=9\cdot10^{-6}$; b, e) $\delta n_\text{max}=3\cdot 10^{-6}$, $J=9\cdot10^{-7}$, c, f) $\delta n_\text{max}=3\cdot 10^{-6}$, $J=10^{-7}$.