Tunable single passband microwave photonic filter using a cascaded pair of microring resonators

We propose a novel integration based technique employing a cascaded pair of microring resonators on silicon-on-insulator (SOI) platform for achieving a tunable single passband microwave photonic filter (SPMPF). A double microring structure is used to provide an all-integration approach to eliminate the usual selectivity dependence of the radio frequency (RF) response on the shape of a single ring resonator. Instead, the selectivity of the SPMPF is enhanced by controlling the relative difference between the bandwidths of the two optical ring resonators which directly maps to the 3dB-bandwidth of the SPMPF. Experiment shows the realization of a tunable SPMPF with about 20 dB out-of-band suppression ratio. The SPMPF is demonstrated with a tuning capability from 6 GHz to 17 GHz without altering the optical carrier frequency, while maintaining the filter bandwidth below 1.9 GHz. Moreover, the shape factor was also kept within 1.78 with a variation of only +0.05 throughout the entire tuning range, thus successfully accomplishing shape invariant tuning of the single passband without compromising the filter rejection bandwidth.