Bessel–Thompson Filter Using Double-Series-Coupled Microring Resonator

We analyzed the dispersion characteristics of double-series-coupled microring resonator filter and found that the coupling efficiency between resonators for the zero third-order dispersion (Bessel-Thompson) filter is one-third of that of a flat passband (Butterworth) filter. By controlling the coupling efficiencies between waveguides, the flat-top group delay response (Bessel-Thompson filter response) was realized. However, under the symmetric arrangement of coupling efficiency, the Bessel-Thompson filter lost transmittance by 1.25 dB. Thus we found the optimum condition of coupling efficiencies for achieving zero third-order dispersion (flat-group delay) with unity transmittance is an asymmetric arrangement of coupling efficiencies.