High-precision chirp-rate measurement of LFM microwave signals via photonic fractional Fourier transform with low-frequency detection

We propose and demonstrate a method for high-precision chirp-rate measurement of linear frequency-modulated (LFM) microwave signals. By leveraging tunable fractional Fourier transform within an optical frequency-shift loop, the system processes LFM microwave signals in the optical domain and extracts their chirp rates via low-frequency detection, avoiding the need for high-speed electronics. In the experiment, chirp-rate measurement across a range of -1400 MHz/µs to 800 MHz/µs is achieved, with errors below ±0.2 MHz/µs. This high measurement precision is maintained over a signal frequency range of 5 GHz to 40 GHz, even at a signal-to-noise ratio of -7.55 dB. Moreover, the method is capable of measuring the chirp rates of multiple spectrally overlapping LFM microwave signals. These results validate the proposed method as a robust and simplified chirp-rate measurement solution in advanced electronic reconnaissance applications.