Statistics of modal condensation in nonlinear multimode fibers

TL;DR

This paper investigates how linear disorder and nonlinearity influence modal power distribution in multimode fibers, revealing energy condensation phenomena and extending thermodynamic models to complex optical states.

Contribution

It introduces a new 3D mode decomposition method, a novel model for linear disorder, and a weighted Bose-Einstein law to describe steady-state modal distributions in nonlinear multimode fibers.

Findings

Energy condensation occurs in higher modes before the fundamental mode in the soliton regime.

The weighted Bose-Einstein law accurately describes modal distributions across regimes.

Local condensation in higher modes precedes global condensation in the fiber.

Abstract

Optical pulses propagating in multimode optical fibers are affected by linear disorder and nonlinearity, and experience chaotic exchange of power among modes. On the other hand, complex systems can attain steady states characterized by energy condensation into single as well multiple sub-systems. In this work, we study beam propagation in multimode optical fibers in the presence of linear random mode coupling and Kerr nonlinearity; both effects lead to a mode power redistribution at the fiber output. We use a new 3D mode decomposition method to obtain, with unprecedented accuracy, measurements of the modal distribution from long spans of graded-index fiber; we perform numerical simulations using a new model for the linear disorder; we introduce a weighted Bose-Einstein law and show that it is suitable for describing steady-state modal power distributions both in the linear and nonlinear regimes. We show that, at power levels intermediate between the linear and the soliton regimes, energy condensation is attained locally by the second, third and fourth modal groups, before global condensation to the fundamental mode is reached in the soliton regime. Our results extend the thermodynamic approach to multimode fibers to unexplored optical states, which acquire the characteristics of optical glass.