Abstract
We propose a simple design concept to improve the fabrication tolerance and to optimize the extinction ratio in a directional coupler by controlling the waveguide width and the maximum coupling length. The device characteristics were analyzed based on a 2-dimensional structure. For the extremum coupling length, the characteristics of the optimized design show stable behavior even in the presence of small variations of waveguide width. The design concept of a maximum coupling length not only improves the fabrication tolerance, larger than 0.2 $\mu$m, but also the extinction ratio, larger than 18 dB.