Wideband microwave Doppler frequency shift measurement based on acousto-optic frequency shift and DP-QPSK receiver

Microwave Doppler frequency shift (DFS) measurement is one of the backbone parts in modern radar systems. In this paper, a simple and novel optical approach to implement broadband microwave DFS measurement is proposed. Two carrier-suppressed single-sideband modulation (CS-SSB) modules, which are constituted by two Mach Zehnder modulators (MZMs) and two optical band pass filters (OBPFs) are employed for modulating both transmitted signal and echo signal. Due to the frequency shift by an acousto-optic modulator (AOM), the proposed method converts the DFS signals to the intermediate-frequency (IF) domain for simultaneous estimation of DFS value and direction, which helps to avoid low-frequency interference. Moreover, an integrated dual-polarization quadrature phase shift keying (DP-QPSK) receiver is used to obtain the in-phase and quadrature (I/Q) DFS terms and improve the system stability. In the experiment, the microwave DFS is estimated with a clear direction and a maximum measurement error of 4.9 Hz over an ultra-wide operation frequency from 10 to 40 GHz. The micro-Doppler frequency shift (mDFS) measurement is also carried out to demonstrate the advantage of low-frequency DFS measurement. Excellent system stability is also observed in the experiment, where the corresponding I/Q amplitude and phase imbalance are maintained lower than 2.5 dB and 4.8 degrees during 60 min.