Measurements of the Doppler-frequency-shift (DFS) and angle-of-arrival (AOA) of microwave signals are used to determine the radial velocity and position of objects, which has been extensively applied in the fields of radar, electronic warfare, and satellite communication [[1], [2], [3]]. However, conventional electronic solutions are limited in bandwidth, size, and electromagnetic interference. Fortunately, microwave photonics technology, with its advantages of ultra-wideband, small size, immunity to electromagnetic interference, and lightweight, offers a feasible way to overcome the bottlenecks faced by electronic methods [4]. In the past few years, many schemes based on microwave photonics technology to measure DFS or AOA have been presented. Typically, methods for measuring DFS include using in-phase and quadrature (I/Q) detection [[5], [6], [7], [8], [9]], introducing a reference signal [[10], [11], [12], [13]], adding a frequency shift such as using Serrodyne modulation [[14], [15], [16]], or an acousto-optic modulator [17,18]. Meanwhile, AOA is commonly acquired by measuring relative delay time [19,20], optical power [21,22], DC voltage [23,24], and electrical power [25,26]. However, these methods cannot simultaneously obtain AOA and DFS information. Therefore, measuring these two parameters in a single photonic system is highly desirable and meaningful.

Recent researches have unveiled some novel photon-based methodologies that facilitate the simultaneous measurement of DFS and AOA. In Ref. [27], DFS and AOA are simultaneously acquired using a dual polarization modulator. In Refs. [28,29], a wavelength-division multiplexing structure or an optical filter is applied following the modulator to get the required optical sideband signal, allowing for simultaneous DFS and AOA information. In Ref. [30], two push-pull Mach-Zehnder modulators placed in parallel divide the optical signal into two independent arms. The DFS is extracted by the intermediate frequency information. The AOA is calculated through the analysis of phase differences in the electrical waveforms of the two arms. In Ref. [31], the measurements of DFS and AOA are implemented by inputting a reference signal to a dual-parallel Mach-Zehnder modulator (DPMZM). Nevertheless, the applied reference source with high-frequency needs to be dynamically adjusted to align with variations in the frequency of the transmitting source. In addition, the aforementioned schemes of measuring AOA are directional ambiguity, and the measurement range is within 0°–90°. To achieve unambiguous AOA measurement, a dual-channel solution utilizing dual-polarization dual-drive MZM is adopted [32,33]. The polarization beam splitter divides the optical signal from the modulator into two channels with orthogonal polarization states. Unambiguous DFS and AOA information is obtained from two photodetectors (PD). In Refs. [32,33], the measurement error is within ±2.2° for AOA ranges of −70.8° to 70.8° and −66.44° to 66.44°, respectively. However, the dual-channel configuration also requires a high-frequency reference source that changes depending on the transmitted signal. The use of two PDs in combination with a polarization control device increases system complexity and measurement stability.

In this paper, a single-channel photonic solution based on cascaded DPMZMs for simultaneous measurement of DFS and AOA is proposed. The DPMZM-1 is operated in the carrier suppressed single sideband (CS-SSB) mode. By feeding the transmitted signal (TS) and low-frequency reference signal (RS) into DPMZM-1, two +1st-order optical sidebands are produced. The role of low-frequency RS is to identify the direction of the DFS and enable frequency down-conversion of the echo signal (ES). Two +1st-order sideband signals are output from the DPMZM-1. The two sidebands are then launched into the DPMZM-2 as optical carriers and modulated by the received ES. The main MZM and two sub-MZMs in DPMZM-2 operate at the maximum and quadrature points, respectively. The PD converts the optical output from DPMZM-2 into a down-converted low-frequency (DCLF) electrical signal. The DFS is derived by comparing the frequency difference between the DCLF signal and RS. The AOA information is obtained simultaneously by AOA-to-power mapping methodology. In the experiment, the measured errors of DFS remain within ±0.1 Hz over a bandwidth of 4–18 GHz. The spurious suppression ratio exceeds 36 dB. The detection sensitivity of echo power reaches −45 dBm. Furthermore, the measured AOA error is less than ±1.4° from −62.7° to 62.7°. The proposed solution does not involve any optical filters and polarization control devices, which improves the robustness of measurement system.