DARKER-THAN-BLACK SOLITONS - DARK SOLITONS WITH TOTAL PHASE-SHIFT GREATER-THAN PI

We discuss the stationary- and collisional-phase properties of dark op tical solitons in a saturable nonlinear medium. Both the special case of an analytically solvable model of a saturable nonlinearity and othe r similar nonlinearities are considered. It is shown analytically that for a saturable nonlinearity the total phase shift across a single so liton may be much larger than pi which is the maximum value in a Kerr medium and corresponds to a black soliton. Furthermore, for a given ph ase shift larger than pi we find analytically that there exists two so liton solutions with different contrast. It is demonstrated numericall y that these dark solitons which have an extraordinarily large total p hase shift, propagate stably, and survive after collisions, maintainin g almost the same large value of the phase shift. Additionally, we sho w that the stability criterion introduced earlier for multiple dark so litons is not sufficiently general and is effective only for some type s of nonlinearity.