Wideband Microwave Doppler Frequency Shift Measurement and Direction Discrimination Using Photonic I/Q Detection

Abstract

An enhanced approach to realizing wideband microwave Doppler frequency shift (DFS) measurement and direction discrimination based on photonic in-phase and quadrature (I/Q) coherent detection is proposed and demonstrated experimentally. In the proposed approach, the DFS between the transmitted microwave signal and the received echo signal is converted into two quadrature low-frequency electrical signals through the coherent detection by using an optical hybrid and two balanced photodetectors (BPDs). The microwave DFS of interest can be estimated with an unambiguous direction, and in particular with a greatly improved resolution. Meanwhile, photonic coherent and balanced detection effectively eliminates the optical signal to signal beating interferences. In the proof-of-concept experiment, the DFSs from -90 to +90 kHz are successfully estimated for microwave signals at 10, 14, 18, and 38 GHz. The measurement errors are estimated to be less than 5.8 Hz which are an order of magnitude lower than those (i.e., 60 Hz) released before. Such results provide a high resolution for radial velocity measurement as well. In addition, the performance of the proposed approach in term of the signal-to-noise ratio and stability is discussed.Index Terms-Doppler frequency shift measurement, microwave photonics, I/Q coherent detection, optical microwave processing. 0733-8724 (c)

Photonics for microwave measurements

“…In a state‐of‐the‐art microwave spectrum analyzer such as Rohde&Schwarz FSW85, the upper limit of the frequency range is 85 GHz. Also, the photonic DFS measurement approaches are totally independent of the frequency of the microwave carrier, providing a frequency coverage or a frequency tuning range greater than 100 GHz in theory and an experimentally validated measurement range from 10 to 38 GHz. Table summaries the performance specifications of a few selected photonic microwave measurement systems.…”

“…In , an enhanced photonic approach based on optical vector mixing was proposed in which an IQ coherent detection was employed. As shown in Fig.…”

“…Meanwhile, a DFS of 926 Hz was estimated for detecting a target moving at 5 km h –1 . Compared with electronic Doppler radars, the operation of the photonic‐assisted systems is independent of the frequency of the microwave carrier in theory. It was also experimentally demonstrated that these systems can provide a wider frequency coverage from 10 to 38 GHz.…”

“…The DFS measurement plays a critical role in many applications, such as mobile communications, metrology, medical imaging, electronic warfare, and radar systems . Recently, several photonic approaches based on optical mixing or optical vector mixing were proposed to perform DFS measurement , with a wider frequency coverage or frequency tuning range, as compared with conventional electronic solutions.…”

All-Optical Pulsed Signal Doppler Frequency Shift Measurement System

“…Furthermore, they require electrical components that limit the system operating frequency [3], or an additional microwave reference source that increases the system cost [13]. Many reported techniques rely on measuring the phase difference of two output signals via an oscilloscope to obtain the sign of a DFS in order to determine a target moving direction [3], [6]- [9], [11], [14]. The problem of these techniques is that they can only be used in continuous wave (CW) radar systems.…”