DARK OPTICAL SOLITONS WITH REVERSE-SIGN AMPLITUDE

We prove the existence of anti-dark-solitons, i.e., dark solitons of the reverse sign of the amplitude, near the zero-group-dispersion wavelength of normally dispersive optical fibers using direct numerical simulations of the nonlinear Schrodinger equation which includes the third-order dispersion. These solitons were predicted earlier in the small-amplitude limit [Yu. S. Kivshar, Phys. Rev. A 43, 1677 (1991)] and they may exist only for a certain propagation direction, so that the interaction of dark and anti-dark-solitons propagating in opposite directions is possible; this is probably the only possibility to observe direct collisions of dark and bright solitons. We investigate the collision numerically and demonstrate that it looks elastic at least for small-amplitude solitons.