Calorimetry of photon gases in nonlinear multimode optical fibers

TL;DR

This paper demonstrates that nonlinear multimode optical fibers can be used to perform optical calorimetry on photon gases, confirming that their thermodynamic behavior obeys the second law, and offering new light management techniques.

Contribution

It introduces a novel optical calorimetric method to measure thermodynamic variables of photon gases in nonlinear fibers, clarifying their physical meaning and thermodynamic behavior.

Findings

Heat flows from hot to cold photon subsystems.

Photon gases obey the second law of thermodynamics.

New approach for light-by-light management of laser beams.

Abstract

Because of their massless nature, photons do not interact in linear optical media. However, light beam propagation in nonlinear media permits to break this paradigm, and makes it possible to observe photon-photon interactions. Based on this principle, a beam of light propagating in a nonlinear multimode optical system can be described as a gas of interacting particles. As a consequence, the spatio-temporal evolution of this photon gas is expressed in terms of macroscopic thermodynamic variables, e.g., temperature and chemical potential. Moreover, the gas evolution is subject to experiencing typical thermodynamic phenomena, such as thermalization. The meaning of thermodynamic variables associated with the photon gas must not be confused with their classical counterparts, e.g., the gas temperature cannot be measured by means of standard thermometers. Although the thermodynamic parameters of a multimode photon gas result from a rigorous mathematical derivation, their physical meaning is still unclear. In this work, we report on optical calorimetric measurements, which exploit nonlinear beam propagation in multimode optical fibers. Our results show that, indeed, heat only flows from a hot to a cold photon gas subsystem. This provides an unequivocal demonstration that nonlinear multimode wave propagation phenomena are governed by the second law of thermodynamics. In addition to be fundamental, our findings provide a new approach to light-by-light activated management of laser beams.