Lugiato–Lefever equation

The numerical models of lasers and the most of nonlinear optical systems stem from Maxwell–Bloch equations (MBE). This full set of Partial Differential Equations includes Maxwell equations for electromagnetic field and semiclassical equations of the two-level (or multilevel) atoms. For this reason the simplified theoretical approaches were developed for numerical simulation of laser beams formation and their propagation since the early years of laser era. The Slowly varying envelope approximation of MBE follows from the standard nonlinear wave equation with nonlinear polarization ${\displaystyle \mathbf {P} ^{\text{NL}}}$ as a source:

${\displaystyle \nabla ^{2}{\cal {E({\vec {r}},t)}}-{\frac {n^{2}}{c^{2}}}{\frac {\partial ^{2}}{\partial t^{2}}}{\cal {E({\vec {r}},t)}}={\frac {1}{\varepsilon _{0}c^{2}}}{\frac {\partial ^{2}}{\partial t^{2}}}\mathbf {P} ^{\text{NL}},}$ where :${\displaystyle {\cal {E}}({\vec {r}},t)\propto E({\vec {r}}_{\perp },z,t)e^{i(nk_{0})(z-ct)}+{\text{c.c.}}}$

resulting in the standard "parabolic" wave equation:

${\displaystyle {\frac {\partial E}{\partial z}}+{\frac {\;\omega _{0}\ }{k_{0}c^{2}}}{\frac {\partial E}{\partial t}}-{\tfrac {1}{2k_{0}}}\ i\ \nabla _{\perp }^{2}E={\frac {1}{\varepsilon _{0}k_{0}c^{2}}}{\frac {\partial ^{2}}{\partial t^{2}}}\mathbf {P} ^{\text{NL}}~}$, under conditions :

${\displaystyle \displaystyle \left|\ \nabla ^{2}E\ \right|\ll \left|\ k_{0}\nabla E\ \right|}$   and   ${\displaystyle \displaystyle \left|\ {\frac {\partial ^{2}E}{\partial t^{2}}}\ \right|\ll \left|\ \omega _{0}\,{\frac {\partial E}{\partial t}}\ \right|\ }$ .

The averaging over longitudinal coordinate ${\displaystyle z}$ results in "mean-field" Suchkov-Letokhov equation (SLE) describing the nonstationary evolution of the transverse mode pattern.

The model usually designated as Lugiato–Lefever equation (LLE) was formulated in 1987 by Luigi Lugiato and René Lefever as a paradigm for spontaneous pattern formation in nonlinear optical systems. The patterns originate from the interaction of a coherent field, that is injected into a resonant optical cavity, with a Kerr medium that fills the cavity.

The same equation governs two types of patterns: stationary patterns that arise in the planes orthogonal with respect to the direction of propagation of light (transverse patterns) and patterns that form in the longitudinal direction (longitudinal patterns), travel along the cavity with the velocity of light in the medium and give rise to a sequence of pulses in the output of the cavity.

The case of longitudinal patterns is intrinsically linked to the phenomenon of “Kerr frequency combs” in microresonators, discovered in 2007 by Tobias Kippenberg and collaborators, that has raised a very lively interest, especially because of the applicative avenue it has opened.