Conservation laws, bound-state solitons and modulation instability for a variable-coefficient higher-order nonlinear Schrodinger equation in an optical fiber

Under investigation in this paper is a variable-coefficient sixth-order nonlinear Schrodinger equation, which describes the propagation of attosecond pulses in an optical fiber. Based on Lax pair, infinitely-many conservation laws are constructed. With the aid of auxiliary functions, bilinear forms are derived. In addition, the one- and two-soliton solutions are obtained via the Hirota method. The influences of variable coefficients for soliton velocity and profile are discussed. Particularly, the interaction periods and soliton separation factor of bound-state solitons are analyzed. Finally, modulation instability is investigated. The reported results could be used to understand related soliton molecule and optical instability phenomena in nonlinear optics.