Analysis of soliton switching in dispersion-decreasing fiber couplers

We present an analytical and numerical investigation of the propagation and the switching of fundamental solitons in a two-core nonlinear fiber coupler constructed with dispersion decreasing fiber (DDF) with a variational method using the Lagrangian density formulation. The analytical solutions were directly obtained from the coupled nonlinear Schrodinger equations. Our simulations considering six different profiles, linear, hyperbolic, exponential, logarithm, constant and Gaussian. The transmission characteristics, the critical energy and the compression factor of first order solitons obtained by the analytical procedure agree well with the results from numerical analysis. Twin core nonlinear directional couplers that include loss are also examined. The inclusion of loss results in the increase of the critical power of the coupler and broadening of the switched pulses, while the switching characteristics deteriorate. To compensate for this worse behavior, associated to the loss, we have investigated the effect of the DDF profile on the performance of the coupler. We show that appropriate shaping of the DDF profile is quite effective to recover, almost completely, the original switching behavior associated to the lossless situation. It is concluded that the hyperbolic dispersion profile is nearly optimum to the recovery of the lossless switching behavior.