Photonic approach for simultaneous and unambiguous measurement of Doppler-frequency-shift and angle-of-arrival based on cascaded dual-parallel Mach-Zehnder modulators

A single-channel photonic scheme using cascaded dual-parallel Mach-Zehnder modulator (DPMZM) architecture for measuring Doppler frequency shift (DFS) and angle of arrival (AOA) is presented and experimentally demonstrated. The scheme features simultaneous and unambiguous measurements of DFS and AOA. The transmitted and a low-frequency reference signals are fed into the first modulator (DPMZM-1) to modulate the optical carrier. The DPMZM-1 operating in carrier suppressed single sideband (CS-SSB) mode generates two +1-order optical sidebands. The two sideband signals are then launched into the second modulator (DPMZM-2) and modulated by the echo signal. The main modulator and two sub-modulators of the DPMZM-2 operate at the maximum and quadrature points, respectively. The photodetector converts the optical signal output from DPMZM-2 into a down-converted low-frequency (DCLF) signal. The DFS is derived by comparing the frequency of the DCLF signal with that of the reference signal. The value of AOA can be obtained based on AOA-to-power mapping. Experimentally, the error in DFS measurement is maintained within ±0.1 Hz over a bandwidth of 4–18 GHz. The spurious suppression ratio exceeds 36 dB. The AOA error is less than ±1.4° from −62.7° to 62.7°.