Photonic-Enabled Doppler Frequency Shift Measurement for Weak Echo Signals Based on Optical Single-Sideband Mixing Using a Fixed Low-Frequency Reference

A simplified photonic-assisted Doppler frequency shift (DFS) measurement approach is proposed based on a single dual-polarization binary phase-shift keying (DP-BPSK) modulator with the help of a fixed low-frequency reference. The low-frequency reference and the transmitted signal are combined and then single-sideband mixed in the upper dual-drive Mach-Zehnder modulator (DD-MZM) in the DP-BPSK modulator to generate an optical sideband of the transmitted signal and a mixed optical sideband of the two signals, which is combined with the optical sideband of the echo signal from the other DD-MZM. The combined optical signal is detected in a low-speed photodetector. Both the direction and the value of the DFS can be measured by analyzing the frequency of the generated photocurrent. An experiment is performed. DFS from -100 to 100 kHz is measured for microwave signals from 6.9 to 16.1 GHz with a measurement error of less than 0.04 Hz. The stability of the system is also studied, and the change of the measurement error is less than 0.04 Hz in 30 minutes.