High-Resolution Real-Time Microwave Frequency Variation Detection Enabled by Photonic Delay Line

Real-time microwave frequency variation (MFV) measurement is crucial for a wide range of applications. However, current solutions often fall short in terms of resolution, speed, and range. We present a novel technique that leverages the sine–cosine encoder principle, enabled by a photonic delay line (PhDL), to detect rapid MFV in real-time with ultrahigh resolution. By incorporating PhDLs of 1 km or 50 m, our sine–cosine microwave frequency detector (MFD) achieved a resolution down to 50 Hz (more than four orders of magnitude better than microwave photonics-based frequency measurement techniques), a speed up to 63 THz/s, and a range of 7 GHz from 5.7 to 12.7 GHz, only constrained by the detection circuit noise, and the bandwidth and range of the microwave component used. The resolution can be further improved by using PhDL with longer fiber length, while the operation frequency can be extended up to 220 GHz with thin-film lithium niobate-based PhDL. This technology can accurately characterize all MFV parameters of a tunable signal source, including chirp rate, direction, waveform, range, tuning ripples, and nonlinearity. Practical demonstrations show that our sine–cosine MFD can precisely measure both minute Doppler frequency shifts from modulating a phase shifter and large chirp frequency sweeps from a Fourier domain mode-locked optoelectronic oscillator (FDML-OEO). This advancement paves the way for detecting MFV with unprecedented resolution and dynamic range and will be highly valuable in secure communications, passive Doppler radar, electronic warfare, metrology, and scientific research.