  Today, wide-open, high-resolution Doppler frequency shift (DFS) estimation is
essential for radar, microwave/millimeter-wave, and communication systems.
Using photonics technology, an effective approach is proposed and
experimentally demonstrated, providing a high-resolution and
frequency-independent solution. In the approach consisting of two cascaded
opto-electronic modulators, DFS between the transmitted microwave/
millimeter-wave signal and the received echo signal is mapped into a doubled
spacing between two target optical sidebands. Subsequently, the DFS is then
estimated through the spectrum analysis of a generated low-frequency electrical
signal, with an improved resolution by a factor of 2. In experiments, DFSs from
-90 to 90 KHz are successfully estimated for microwave/millimeter-wave signals
at 10, 15, and 30 GHz, where estimation errors keep lower than +/- 5e-10 Hz.
For radial velocity measurement, these results reveal a range from 0 to 900 m/s
(0 to 450 m/s) and a resolution of 1e-11 m/s (5e-12 m/s) at 15-GHz (30-GHz)
frequency band.
