MMI-based all-optical multi-input XOR and XNOR logic gates using nonlinear directional coupler

We propose an all-optical 2-input XOR logic gate based on multimode interference and employ it as a basic structure in order to develop 3- and 4-input XOR logic gates. A 3 dB MMI coupler has been designed to be used as 2-input XOR logic gate. The integration of optical 2-input XOR gates encounters with the phase matching issues. Therefore, a nonlinear directional coupler is associated to produce a relative phase shift of pi between the coupled and uncoupled branches. Such phase shift is cancelled with launching a simultaneous high intensity control field. Also, by assigning logic 1 to one of inputs in n-input XOR structure, the (n - 1)-input XNOR logic gate has been achieved. Two dimensional beam propagation method has been used to simulate and verify the performance of proposed all-optical 2-, 3- and 4-input XOR logic gates. Simulation results show that the proposed structure is almost insensitive to fabrication process tolerances.