Unambiguous Multidimensional Microwave Parameter Measurement for Multiple Targets Based on Photonic Fractional Fourier Transform

Microwave parameter measurement of electromagnetic targets has always been an important task in electronic reconnaissance and radar systems. Signal processing methods based on analog photonics technology have attracted widespread attention because they avoid the bandwidth bottlenecks and processing delays associated with analog-to-digital conversion and digital signal processing. However, most early photonics microwave parameter measurement methods were based on single-tone pilot signals and did not fully utilize the broadband advantages of photonic technology. In view of this, we first propose an unambiguous multidimensional microwave parameter measurement method for multiple targets based on photonic fractional Fourier transform (FrFT) in a wideband photonic radar system, which can simultaneously achieve Doppler frequency shift (DFS) and angle of arrival (AOA) measurements as well as high-precision 1-D distance imaging. The scheme fully exploits the analytical advantages of FrFT in handling linear frequency modulated (LFM) signals with large time-bandwidth products. By constructing an equivalent FrFT kernel to process broadband echo signals with multiple targets, blur-free parameter measurement is achieved. In addition, the 1-GHz upshifting of the acousto-optic frequency shifter (AOFS) effectively avoids FrFT self-interference between multitarget echo signals. Experimental results demonstrate that within the frequency band from 7 to 30 GHz, the system achieves accurate measurement of DFS values and directions, with a maximum measurement error of 0.15 Hz. Unambiguous AOA measurement is completed within the range of -77.16° to 77.16°, with the absolute error controlled within 2.73°. Meanwhile, the system realizes 1-D range imaging of targets, achieving a measured range resolution of 0.5 m, close to the theoretical value of 0.3 m. The research achieves highly integrated multiparameter measurement under an active detection framework, significantly enhancing the accuracy and efficiency of target detection, and providing new technical approaches and implementation schemes for future development of multifunctional integrated detection technologies.