Tunable asymmetric mode conversion using the dark-mode of three-mode waveguide system

A design scheme for low-reflection asymmetric mode conversion structure in three-mode waveguide system is proposed. By using a darkmode of three-mode system, which can be interpreted in terms of destructive interference of transition amplitudes, the transmission characteristics for forward and backward directions can be designed separately. After explanation of the proposed design scheme, we demonstrate an example of asymmetric mode converter that consists of two gratings. The proposed scheme may be useful for the design of tunable asymmetric transmission devices due to its design flexibility and efficient design process.