Dynamics of coupled dark and bright optical solitons

We theoretically examine the interaction between two solitons in a double-mode optical fiber. The bound state between two solitons of different modes is investigated, including both dark and bright solitons. It is shown that interaction between dark solitons as well as between bright solitons is always attractive, but the interaction between bright and dark solitons may be repulsive. Analytical results are in agreement with numerical ones.