Ultrashort rectangular optical pulse generation by nonlinear directional couplers

We numerically demonstrate the generation of femtosecond rectangular optical pulses by nonlinear directional couplers. Single and cascaded coupler structures are investigated and the conditions for generating a rectangular pulse are presented. It is shown that under realizable conditions, femtosecond rectangular-like pulse can be generated in a second-order cascaded NLDC with 5 mm half-beat length and picosecond ideal rectangular pulse can be achieved in a third-order cascaded NLDC. With respect to the short pulse propagation, the influence of intermodal and third-order dispersions are included in the numerical analysis.