An integrated photonic microwave bandpass filter has been demonstrated incorporating a coherently coupled microring resonator in low-loss polymers. The resonator was designed to have an extremely small bandwidth so that it could be used to selectively pass the optical signal carrying the microwave signal to attain efficient bandpass filtering. The demonstrated device may feature compact size, simple structure, stable operation, possible tuning via the thermooptic effect, and furthermore flexible integration with other electrical/optical devices.
We made and tested two resonators with a single ring and double rings, and performed a theoretical fitting of their measured transfer characteristics in order to precisely predict the performance of the resultant microwave filters. For the microwave filter, the bandwidth becomes smaller, the roll-off sharper and the band rejection higher, as the number of the rings increases. Our filter exhibited the center frequency of 10 GHz, the 3-dB bandwidth of 1.0 GHz, the corresponding quality (Q) factor of 10, and the rejection outside of the passband of ~25 dB.