Photonics enables real-time imaging radar with ultra-high resolution
Photonics-based radar with optical generation and de-chirp processing of broadband linear frequency modulation (LFM) signals enables real-time ultra-high resolution target detection and imaging.
Real-time radar detection and imaging is highly desired in applications such as pilotless automobiles, unmanned aerial vehicles and quick security checks [1, 2]. This requirement creates great challenges to the state-of-the-art electronics. On one hand, direct generation of linear frequency modulation (LFM) signals by means of direct digital synthesizers (DDS) is limited to a few gigahertz [3]. On the other hand, the precision of analog-to-digital converters (ADCs) in the receiver drops rapidly as the input bandwidth and sampling rate increase, which severely restricts the radar resolution as well as the processing speed. Microwave photonic technologies have been proposed as a promising solution to overcome the limitations of pure electronic systems [4]. Various schemes for photonic generation of linear frequency modulation (LFM) signals with a large bandwidth have been demonstrated [5, 6]. However, fast processing of such signals is still a difficult task in either electronic or photonics-based radar receivers. 
Figure 1. (a) Schematic diagram of the photonics-based radar. LD: laser diode; OC: optical coupler; DPMZM: dual-parallel Mach-Zehnder modulator; PD: photodetector; EA: electrical amplifier; PM: phase modulator; OBPF: optical band-pased filter; ELPF: electrical low-pass filter; ADC: analog-to-digital converter; DSP: digital signal processing. (b) Principle for de-chirping of LFM signals.
We have developed a photonics-based radar incorporating optical generation and de-chirp processing of broadband LFM signals. Figure 1(a) shows the schematic diagram of the photonics-based radar. A dual-parallel Mach-Zehnder modulator (DPMZM) is the key component for LFM signal generation, which is driven by an intermediate frequency (IF)-band LFM signal. The DPMZM works in a frequency quadrupling mode, i.e., only the frequency-swept ±2nd-order modulation sidebands are generated [7]. After optical-to-electrical conversion, an LFM signal is generated with both the center frequency and the bandwidth quadruped compared with the IF-LFM signal. With this method, high frequency and broadband LFM signal can be generated applying low-speed electronic devices. In the receiver, the collected radar echo is applied to drive an electro-optical phase modulator (PM), which modulates a reference optical signal from the DPMZM. The phase modulated signal contains a pair of optical sidebands, with one sideband temporally delayed compared to the other. When the two sidebands are selected out by an optical band-pass filter and sent to a photodetector for frequency mixing, de-chirping of the LFM signal can be implemented, as illustrated in Fig. 1(b). By properly setting the parameters of the LFM signal according to the detection range, the de-chirped signal can be controlled in a low frequency range, and it can be sampled by a low-speed ADC and quickly processed by a digital signal processing (DSP) unit. The achievable operation bandwidth of this photonics-based radar is mainly limited by the electro-optical devices, which can reach tens or even hundreds of gigahertz. Therefore, it is possible to realize real-time target detection and imaging with a very high resolution.
Figure 2. Spectrum of de-chirped signal when detecting (a) a signal target and (b) two targets separated by 2 cm
The established radar prototype works at K-band radar with a bandwidth of 8 GHz (18-26 GHz). The transmitter applies a 200 kHz continuous wave IF-LFM signal (4.5-6.5 GHz) as the input, and the receiver has a sampling rate of 100 MSa/s. When detecting a small trihedral corner reflector placed 2.1 meters away from the antenna, a single spectral peak is observed after performing fast Fourier transform (FFT) to the digital samples in a period of 5 us, as shown in Fig. 2(a). The 3-dB bandwidth of the spectrum peak is 223 kHz, indicating an effective range-resolution of 2.1 cm, which is close to the theoretical range-resolution (1.875 cm). When detecting two small corner reflectors that are separated by 2 cm along the range profile, two clearly separated spectral peaks are observed, as shown in Fig. 2(b), confirming the high range resolution of ~2 cm.
Figure 3. (a) Picture of three silver-paper-packed balls with a diameter of 2.5 cm fixed on a turntable, (b) imaging result of the three balls (the central spot is caused by the reflection from the metal area of the turntable axis).
Based on the radar prototype, fast inverse synthetic aperture imaging with a 2-D resolution as high as ~2cm x ~2cm can be achieved. Fig. 3 shows the imaging result of three silver-paper-packed balls with a diameter of 2.5 cm. The three balls are fixed on a turntable that is 2.65 m away from the radar antenna. As shown in Fig. 3(a), the three balls are placed at the three vertexes of an equilateral triangle with 20-cm length of each side. Fig. 3(b) shows the constructed image where the three balls are clearly separated with a distance of ~20 cm between each other (the central spot is caused by the reflection from the metal area of the turntable axis). Fig. 4 shows the imaging results of an electric fan having five blades. The length and width of each blade is 16 cm and 6 cm, respectively, as shown in Fig. 4(a). In the receiver, the digital samples in every 10 ms is processed as a frame. A fast imaging rate at 100 frame per second is achieved. Fig. 4 (b), (c) and (d) shows the image corresponding to the first frame, the second frame and the fifth frame, respectively. In these images, the five blades and the metal axis can be easily distinguished.
Fig. 4. (a) Photograph of the electric fan with its five blades packed with silver papers, (b) (c) and (d) is the image for the first, second and fifth frame, respectively.
In summary, we have designed a novel photonics-based radar applying photonic generation and de-chirp processing of broadband radar signals. The radar has great potential in real-time target detection and imaging with an ultra-high resolution.
Nanjing, China
Fangzheng Zhang received a BS degree from Huazhong University of Science and Technology (HUST), Wuhan, China, and PhD degree from Beijing University of Posts and Telecommunications (BUPT), Beijing, China, in 2008 and 2013, respectively. He is now an associate professor in the Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University of Aeronautics and Astronautics. His current work is focused on microwave photonics.
Shilong Pan received his PhD in electronic science and technology from Tsinghua University, China, in 2008. He is now a professor in the Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University of Aeronautics and Astronautics. His current work is focused on microwave photonics.
References:
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