CPT-symmetric coupler with intermodal dispersion

A dual-core waveguide with balanced gain and loss in different arms and with intermodal coupling is considered. The system is not invariant under the conventional PT symmetry, but obeys CPT symmetry where an additional spatial inversion C corresponds to swapping the coupler arms. We show that second-order dispersion of coupling allows for unbroken CPT symmetry and supports propagation of stable vector solitons along the coupler. Small-amplitude solitons are found in explicit form. The combined effect of gain-and-loss and dispersive coupling results in several interesting features which include a separation between the components in different arms, nontrivial dependence of stability of a soliton on its velocity, and the existence of more complex stationary two-hump solutions. Unusual decay dynamics of unstable solitons are also discussed.