Thermodynamic approach of supercontinuum generation

This paper is aimed at providing an overview on recent theoretical and experimental works in which a thermodynamic description of the incoherent regime of supercontinuum generation has been formulated. On the basis of the wave turbulence theory, we show that this highly nonlinear and quasi-continuous-wave regime of supercontinuum generation is characterized by two different phenomena. (i) A process of optical wave thermalization ruled by the four-wave mixing effects: The spectral broadening inherent to supercontinuum generation is shown to result from the natural tendency of the optical field to reach its thermodynamic equilibrium state, i. e., the state of maximum nonequilibrium entropy. This approach also reveals the existence of a thermodynamic phase-matching. (ii) The generation of spectral incoherent solitons induced by the Raman effect in the low frequency part of the supercontinuum spectrum. Contrary to conventional solitons, spectral incoherent solitons do not exhibit a confinement in the space-time domain, but solely in the frequency domain. They owe their existence to the causality property underlying the Raman response function. Both phenomena of optical wave thermalization and spectral incoherent solitons are described in detail by the wave turbulence theory. In this way, we provide a unified nonequilibrium thermodynamic description of the incoherent regime of supercontinuum generation. (C) 2012 Elsevier Inc. All rights reserved.