Narrow bandwidth is a crucial factor in a high performance wavelength selective device(WSD). There are many different expressions to estimate bandwidths of many different WSDs. In this paper, we derive an important rule that the differential time delay between two coupling modes is the most important factor deciding the wavelength channel bandwidth for several different WSDs. This rule reveals that larger differential time delay between two coupling modes results in higher discrimination level of the operating wavelength. Based on the rule, a novel design of wavelength selective coupler that has a bandwidth narrower than normal couplers is presented. Narrow bandwidth is a crucial factor in a high performance wavelength selective device (WSD). There are many different expressions to estimate bandwidths of many different WSDs. In this paper, we derive an important rule that the differential time delay between two coupling modes is the based on the rule, a novel design of a wavelength selective coupler that has a bandwidth narrower than normal couplers is presented.