Bose-Einstein condensation of an optical thermodynamic system into a solitonic state

TL;DR

This paper demonstrates that beam self-cleaning in multimode fibers can be understood as a Bose-Einstein condensation process, with experimental evidence showing the transition to a solitonic state analogous to BEC, involving thermalization and dissipative effects.

Contribution

It provides the first full analogy between beam self-cleaning in fibers and Bose-Einstein condensation, highlighting the role of dissipative processes and the Townes soliton in this phenomenon.

Findings

Efficient beam self-cleaning occurs after thermalization.

Transition to a Townes soliton profile at threshold intensity.

Dissipative processes are crucial for self-cleaning efficiency.

Abstract

Recent years have seen a resurgence of interest in multimode fibers due to their intriguing physics and applications, with spatial beam self-cleaning (BSC) having received special attention. In BSC light condenses into the fundamental fiber mode at elevated intensities. Despite extensive efforts utilizing optical thermodynamics to explain such counterintuitive beam reshaping process, several challenges still remain in fully understanding underlying physics. Here we provide compelling experimental evidence that BSC in a dissipative dual-core fiber can be understood in full analogy to Bose-Einstein condensation (BEC) in dilute gases. Being ruled by the identical Gross-Pitaevskii Equation, both systems feature a Townes soliton solution, for which we find further evidence by modal decomposition of our experimental data. Specifically, we observe that efficient BSC only sets in after an initial thermalization phase, causing converge towards a Townes beam profile once a threshold intensity has been surpassed. This process is akin to a transition from classical to quantum-mechanical thermodynamics in BEC. Furthermore, our analysis also identifies dissipative processes as a crucial, yet previously unidentified component for efficient BSC in multimode fiber. This discovery paves the way for unprecedented applications of multimode-fiber based systems in ultrafast lasers, communications, and fiber-based delivery of high-power laser beams.