Near-zero temperature beam expansion in nonlinear optical fibers

Multimode nonlinear systems offer a fertile platform for optical functionalities that are fundamentally inaccessible in linear or single-mode regimes, a potential revitalized in recent years by the framework of optical thermodynamics. In this work, we reveal a previously unexplored class of wave phenomena in optical fibers driven by the interplay between beam expansion and nonlinear mixing. We predict that, at high powers, tightly focused beams can undergo an irreversible expansion, which self-organizes their intensity and phase into the fundamental mode of an arbitrary guiding landscape, with conversion efficiencies exceeding 90%. Viewed through the optical thermodynamic framework, these states correspond to near-zero-temperature thermodynamic equilibria, formed through an optical analog of Joule–Thomson expansion. We show that expansion-driven effects can exceed fundamental limits of linear-optics in graded-index and step-index fibers, revealing a new approach to autonomous beam shaping and beam capture without switching or adaptive control.