Enhanced Doppler frequency shift measurement and direction discrimination using photonic i/Q detection

We propose an enhanced approach to implement wideband Doppler frequency shift (DFS) measurement and direction discrimination based on photonic in-phase and quadrature (I/Q) coherent detection. Through the I/Q coherent detection by using an optical hybrid and two balanced photodetectors (BPDs), the microwave/millimeter-wave DFS of interest is estimated with clear direction discrimination, and in particular with a greatly improved resolution. In the proof-of-concept experiment, the DFSs from -90 to 90 kHz are successfully estimated for microwave signals at 14 and 18 GHz. The measurement errors are estimated to be less than ±5.8 Hz which is an order of magnitude lower than those (i.e., ±60 Hz) released before. Such results correspond to a higher resolution for radial velocity measurement as well.