The design of wavelength selective directional coupler made of W-index waveguide for WDM

An optical waveguides spectral filter constructed with a W-index waveguide, supporting a dominant mode with nonzero cutoff frequency and a Step-index waveguide is proposed and analyzed. The proposed filter offers a spectral width on the order of one nanometer, considerably narrower than that provided by a filter made of two Step-index waveguides having the same transmission wavelength. The narrow spectral width is achieved by exploiting a relatively large slope difference between the dispersion characteristics of the W-index and Step-index waveguides constituting the filter. For an example case, transmission characteristic of the proposed filter with peak transmission at 1.55 mu m is calculated and compared with that of a corresponding filter made of two Step-index waveguides.