Variational principle for the Kaup-Newell system

The Kaup-Newell system describes the pulse propagation in optical fibers. This paper applies the semi-inverse method to construct the system’s variational formulations, two trial-variational formulations with an unknown function are established, and a detailed derivation is given to determine the unknown function. Finally, the Kaup-Newell system with two variables is converted to a partial differential equation with only one variable.