Photonic-Assisted Filter-Free Microwave Doppler Frequency Shift Measurement Using a Fixed Low-Frequency Reference Signal

A filter-free photonic approach for measuring microwave Doppler frequency shift (DFS) based on two cascaded electro-optic modulators (EOMs) and a fixed low-frequency reference signal is proposed. An optical carrier is carrier-suppressed single-sideband modulated by the transmitted signal and the reference signal at EOM-I. The output from EOM-I is then single-sideband modulated by the echo signal at EOM-II. Subsequently, the output from EOM-II is injected into a photodetector to implement optical-to-electrical conversion. Both the value and direction of the DFS can be obtained by analyzing the frequency of the generated low-frequency electrical signal. The proposed approach has a compact structure and no optical filtering is needed. An experiment is performed to verify the proposed approach. For different microwave frequencies from 7 to 16 GHz, DFS from -100 to 100 kHz is measured, with a measurement error of less than ±0.05 Hz. The stability of the system is also verified. Within 30 minutes, the change of measurement error is less than ±0.05 Hz.