Photonic Microwave Frequency Measurement With High Accuracy and Sub-MHz Resolution

A dynamic photonic instantaneous microwave frequency measurement approach is reported based on frequency-to-time mapping and filtering using an integrated ring resonator, a local oscillator for heterodyne detection, and an electrical bandpass filter. The unknown signal is time-swept through the narrow linewidth of the ring resonator, realized on a silicon nitride platform. A shifted local oscillator at a fixed frequency difference from the center of the ring resonator defines an intermediate frequency for heterodyne detection. After conversion to the electrical domain using a photodiode, this intermediate frequency is filtered by a narrowband electrical bandpass filter. The proposed system demonstrates a resolution of less than 1 MHz and a measurement accuracy of $\pm$ 0.4 MHz over a frequency range of 10 GHz, only limited by the used experimental components. To the best of our knowledge, this is the highest accuracy and resolution shown so far for photonic microwave frequency measurement systems. Since the method can be easily integrated on any photonic platform, it might become an integral part of future wireless devices.