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. 4 (Color online) Head-on interaction between the periodic-shaped dark-dark solitons via Solutions (13) with the same parameters as those in Fig. 3 except that $b(z)={5}/{2}+{5}/{2}\sin[({11}/{10})z]$.