Demonstration of photonic-assisted microwave frequency measurement using a notch filter on silicon chip

We propose and demonstrate a notch filter on a silicon chip and apply it to a photonic microwave frequency measurement system with high precision and adjustable wide measurement range. The on-chip notch filter is implemented by embedding a serially coupled double microring resonator in a Fabry-Perot cavity consisting of two circular Bragg gratings with high reflectivity. The notch filtering with high quality factor and high rejection ratio is obtained by reconstructing the power transmission spectrum of the double microring resonator through the coherent interference within the Fabry-Perot cavity. The proposed notch filter is fabricated on a silicon-on-insulator wafer and the rejection ratio of the notch filter is measured to be 33.54 dB and the quality factor is 3.93×104. Based on the notch filter, a photonic microwave frequency measurement system is presented where the unknown radio frequency signal is detected by the variation of the output optical power from the notch filter through sweeping reference frequency. The measurement bandwidth of the system can be flexibly adjusted by tuning the output wavelength of the laser source. Broad measurement range of 0-26.62 GHz with a low error of ±0.25 GHz are obtained in a proof-of-concept experiment.