Pulse Response of Nonlinear Multimode Interference Couplers

The switching dynamics of short optical pulses in a multimode interference (MMI) coupler made of Kerr materials are numerically investigated using generalized nonlinear Schrödinger equations. These propagation equations contain four-wave mixing (FWM) as well as self-phase and cross-phase modulation and dispersion of different orders to properly describe the nonlinear propagation in the multimode waveguide section. The FWM effects are more dominant than those of self-phase and cross-phase modulation, which cause power exchange among the modes in the waveguide. In numerical modeling, a 2 × 2 MMI coupler is treated to operate as a nearly two-mode interference (TMI) coupler. For long TMI couplers, multiple switching takes place in a relatively narrow range of input powers. It is also found that the transmitted pulse becomes unstable because of the interplay between the modal dispersion (i.e., the group-delay difference) and the FWM effect when the duration of the input pulse is extremely short (typically sub-picoseconds). In general, the switching characteristics of the nonlinear TMI coupler are similar to those of a nonlinear directional coupler.