Cubic�quartic optical solitons having quadratic�cubic nonlinearity by sine�Gordon equation approach

. We reveal cubic–quartic optical solitons with quadratic–cubic nonlinearity for the first time. Both polarization-preserving and birefringent optical fibres are considered. The study is also extended to include the perturbation terms of Hamiltonian type. The integration algorithm adopted in the present work is the method of sine-Gordon equation.