All-Optical Switching in Nonlinear Multimode Interference Couplers

We numerically investigate all-optical switching in multimode interference (MMI) couplers made of self-focusing Kerr media using the beam propagation method. In numerical modeling, a $2\times 2$ MMI coupler is treated to operate as a nearly two-mode interference (TMI) coupler. Although this coupler operates as a conventional directional coupler in a linear case, it exhibits multiple switching in a nonlinear case unlike in the case of a nonlinear directional coupler, depending on the input power. In order to obtain guidelines for designing nonlinear TMI couplers and their application, the transmittance and switching properties are numerically analyzed for different coupler lengths and input powers. The field evolution in the nonlinear multimode waveguide constituting the TMI coupler is also presented for deeper understanding of the operation principle. The results obtained will be useful in developing novel all-optical devices.