Wideband Microwave Doppler Frequency Shift Measurement and Direction Discrimination Using Photonic I/Q Detection

An enhanced approach to realizing wideband microwave Doppler frequency shift (DFS) measurement and direction discrimination based on photonic in-phase and quadrature coherent detection is proposed and demonstrated experimentally. In the proposed approach, the DFS between the transmitted microwave signal and the received echo signal is converted into two quadrature low-frequency electrical signals through the coherent detection by using an optical hybrid and two balanced photodetectors. The microwave DFS of interest can be estimated with an unambiguous direction, and in particular with a greatly improved resolution. Meanwhile, photonic coherent and balanced detection effectively eliminates the optical signal to signal beating interferences. In the proof-of-concept experiment, the DFSs from -90 to +90 kHz are successfully estimated for microwave signals at 10, 14, 18, and 38 GHz. The measurement errors are estimated to be less than ±5.8 Hz which are an order of magnitude lower than those (i.e., ±60 Hz) released before. Such results provide a high resolution for radial velocity measurement as well. In addition, the performance of the proposed approach in term of the signal-to-noise ratio and stability is discussed.