On flat-top response of a photonic crystal based optical filter designed for CWDM system

In this communication, an all-optic band pass filter, having nearly squared flat top spectral characteristic, is designed using 1D photonic crystal (PhC) with defects. Estimated defects are incorporated here into the PhC structure through proper extension in dielectric layers of specific unit cells. The additional optical path offered by the defect regions introduces additional phase shifts which in turn open up transmission windows with in the band gap of the PhC. Performance of the flat top filter is analyzed using transfer matrix method. The analysis reveals that the distribution of the introduced defects controls the transmission characteristics of the structure. To achieve flat top response, the condition regarding distribution of two defects within the PhC is derived. Silicon and silicon-dioxide were chosen as material pairs for simulation. The well mature silicon technology aids the practical realization of the structure. Width of the flat top pass band is further tailored to achieve exactly the ITU-T recommended maximum full width of 14 nm at half maxima to support shifts of signal wavelength in an allotted channel for a coarse wavelength division multiplexing (CWDM) system. The proposed model for flat top response is validated for practical implementation by simulating the structure using CST MW Studio. The results using CST MW Studio show close agreement with the MATLAB simulation.