Bilinear Forms and Dark-Dark Solitons for the Coupled Cubic-Quintic Nonlinear SchrÕdinger Equations with Variable Coefficients in a Twin-Core Optical Fiber or Non-Kerr Medium*

Mei-Xia Chu,Bo Tian,Yu-Qiang Yuan,Ze Zhang,and He-Yuan Tian

Fig. 6 (Color online) Overtaking interaction between the periodic-shaped two dark-dark soltions via Solutions (13) with the same parameters as those in Fig. 5 except that $b(z)=1+\sin({z}/{2})$.