Abstract-Design optimizations of asymmetric directional couplers for the fundamental quasi-TE (TM) mode with the higher order quasi-TE (TM) modes (de)multiplexer including the ( ), , , and modes are studied by a full-vectorial H-field finite element method. Phase matching of non-identical nanowires are more critical and accuracy of the numerical method for the design is tested here. The effect of possible fabrication tolerance is also discussed thoroughly. These results show that a narrow separation of 100 nm or similar value should be employed for the quasi-TE mode splitters, but for the quasi-TM modes, a relatively wide separation of ~ 500 nm can be used. The fabrication of the single mode nanowire should be strictly accurate within +5 nm of the design values.Index Terms-Silicon photonics, mode (de)multiplexer, asymmetric directional coupler, finite element analysis.