Photonic Approach to Wide-Frequency-Range High-Resolution Microwave/Millimeter-Wave Doppler Frequency Shift Estimation

High-resolution Doppler frequency shift (DFS) estimation in a wide frequency range is essential for radar, microwave/millimeter-wave, and communication systems. In this paper, a photonic approach to DFS estimation is proposed and experimentally demonstrated, providing a high-resolution and frequency-independent solution. In the proposed approach, the DFS between the transmitted microwave signal and the received echo signal is mapped into a doubled frequency spacing between two target optical sidebands by using two cascaded electrooptic modulators. Subsequently, the DFS is then estimated through the spectrum analysis of a low-frequency electrical signal generated from the frequency beating of the two target sidebands with an improved resolution by a factor of 2. In the experiments, DFSs from -90 to 90 kHz are successfully estimated for microwave/millimeter-wave signals at 10, 15, and 30 GHz, where the estimation errors are lower than ±5 ×10^-10 Hz. For radial velocity measurement, these results reveal a range from 0 to 900 m/s and a resolution of 1 ×10^-11 m/s at 15-GHz frequency band, or a range from 0 to 450 m/s and a resolution of 5 ×10^-12 m/s at 30-GHz band. To eliminate the estimation ambiguity, a reference branch is introduced for generating an indicator frequency to discriminate the sign of DFS and the direction of radial velocity for approaching or receding motion. In addition, extended discussions on the signal-to-noise ratio, the minimum measurable DFS, and other detection features of the proposed approach are presented.