Generation of Solitons and Train of Soliton Pulses in Three Level Λ-Type Cold Atom Electromagnetically Induced Transparency System

We point out that a train of optical solitons can be generated in a cold-atom system by modulational instability (MI). By numerical simulation, we not only prove that the MI analysis of equations of motion for the atomic-state amplitudes and probe field can be replaced with that of one nonlinear Schrodinger equation, but also give the similarities and differences in the generation of the soliton wave train between the original Maxwell-Schrodinger equations and the nonlinear Schrodinger equation. Through the numerical simulation of the original equation of motion, we found that the generation of a single soliton or a couple of solitons can be achieved by adjusting the initial pulse amplitude, and propose a quantitative relationship. These results are different from those in previous reports, which mainly focus on the kind of nonlinear pulse that can propagate stably in an electromagnetically induced transparency (EIT) medium. The merits and shortages of our scheme are also shown by comparing what with a fiber system. By comparison, the cold-atom system can be a candidate for generating solitons and a train of soliton waves.