Open Access
Add to BibSonomy
Abstract
In recent years, metasurfaces have been widely employed in stealth technology, which brings great challenges for radar target detection. In order to address this issue, a novel detection approach for metasurface-stealth-target (MST) based on orbital angular momentum (OAM) vortex wave is proposed in this paper. Compared to a conventional plane wave detection system, the transmitting wave of the proposed approach is OAM-modulated, of which the wavefront is helical along the beam axis. Thus, the differentiated exciting source is introduced to different parts of MST. According to the established scattering model and full-wave simulation, the echo of MST has a strong correlation with OAM order l, which exhibits quite different scattering characteristics from the plane wave illumination. A chessboard metasurface (CM) is taken as an example to be irradiated by an OAM vortex wave of l=±2. The backscattering is significantly boosted compared to plane-wave detection, which is against the stealth ability of CM. This phenomenon is also verified by experiments. The results reveal that OAM detection is a promising approach for MST detection..
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As a two-dimensional equivalence of metamaterial, metasurface has drawn plenty of attention in recent years due to its excellent manipulation to electromagnetic waves. Compared to bulky metamaterial, metasurface is much more compact in structure so that it can be applied to more scenarios. In military field, metasurface is a significant stealth technology to reduce the radar cross section (RCS) of a target. Chessboard metasurface (CM) [1–5], phase gradient metasurface [6–9], diffuse metasurface [10,11], and coding metasurface [12–15] et al. are the typical representatives. These metasurfaces control the scattering of the incident wave by means of introducing phase discontinuity on the interface to guide the energy to the desired distribution. Accordingly, RCS in direction of threat can be reduced to a great extent, which brings huge challenge to radar detection system.
Quite recently, electromagnetic wave carrying orbital angular momentum (OAM) is becoming an attractive field. Its most distinctive feature is the helical wave front, of which the phase is l times of the azimuthal angle. Thus, it is usually called OAM vortex wave and plenty of reported literatures have focused on its generation methods [16–21]. OAM vortex wave has infinite eigenmodes denoted by OAM order l and arbitrary eigenmodes are orthogonal to each other, which has been proved by experiment [22]. Thus, OAM-based multiplexing system is expected to increase the channel capacity for communications [23]. Although conflicting opinions exist [24], OAM vortex wave is still promising in research and applications for its unusual electromagnetic characteristics. For instance, OAM vortex wave can be applied to radar detection field. It has the potential to acquire the cross-range profile and super resolution imaging ability of the radar target [25,26]. Thus, it matters as well to uncover the scattering characteristic of OAM vortex wave. According to the reported literatures, the scattering interaction between OAM vortex wave and some typical targets have been derived [27–31]. In [27], the RCS of a complex target of two transverse-deployed small metal balls was measured under OAM vortex wave illumination and different RCSs can be identified from the same incident angle. In [28], the RCS for perfect electric conductor (PEC) sphere and cone were calculated, respectively. The backward scattering features showed significant differences when OAM beams change their orders. Consequently, given a specific propagation direction, more information will be offered by the OAM beams for object detection and recognition. In [29], experiments were carried out in an anechoic chamber to measure the RCS of the targets. The results indicated that for the flat plate, RCS varied with OAM order. In [30] and [31], the scattering interaction of OAM vortex wave with perfect electromagnetic conductor (PEMC) sphere and metamaterial coated PEMC cylinder were analyzed, respectively. These researches reveal that the scattering characteristic of a target has strong correlation with the OAM order l, which is quite different from the conventional plane-wave radar detection. It is reasonable to speculate that the OAM-based radar system is also applicable to the challenge of metasurface-stealth-target (MST) detection. Nevertheless, to the best of our knowledge, the scattering problem of MST detected by OAM vortex wave has not been figured out yet.
In this paper, the scattering characteristic of MST with OAM vortex wave illumination is derived. A CM was exploited as an example to demonstrate the analysis. We use array method to establish the scattering model of this problem. Different OAM orders are taken into consideration to analyze the scattering energy distribution. The scattering pattern obtained by the model agrees well with the full-wave simulations conducted by commercial software HFSS. The results reveal that for OAM order l=±2, the backscattering of the designed CM is significantly boosted. That is to say, the detection probability of this stealth target is greatly increased. Prototypes of CM and OAM vortex wave antenna were fabricated and measured in anechoic chamber. The measured results validate the correctness of the simulations. To the authors’ best knowledge, this is the first time to uncover the scattering characteristic of MST using OAM vortex wave detection approach.
2. OAM detection modeling
The sketch map of OAM-based MST detection is shown in Fig. 1. A uniform circular array (UCA) is employed to transmit the OAM vortex wave along $- {\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over e} _z}$ direction. The MST composed of M × N unit cells is illuminated by the detection wave. According to array theory [32], the echo of the MST is the superposition of the scattering field of all the unit cells, which can be expressed as
In order to simplify the computation, scalar forms of these functions are utilized. For pattern function ${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over A} _{mn}}$, cosine model is used, which can be expressed as
In (5), ${\theta _{mn}}$, ${\varphi _{mn}}$, and ${R_{mn}}$ denote the elevation angle, azimuth angle, and distance between the mnth unit cell of CM and the center of UCA, respectively. We can investigate the scattering characteristic by using (5). It can be seen that the scattering echo is closely related with OAM order l.
3. Results and discussion
CM is a typical metasurface which is often employed to reduce the RCS of the target. A scattering scenario of CM with OAM vortex wave incidence is given in Fig. 2. The CM is composed of artificial magnetic conductor (AMC) and perfect electric conductor (PEC) etched on the film of F4B (εr = 2.65). The size of the AMC unit cell is 20mm × 20mm × 2 mm and the metallic patch on the surface is 16mm × 16 mm. There are 30 × 30 unit cells in total so that the whole size of CM is 600mm × 600mm × 2 mm. As a result, the phase cancellation scheme occurs at 4.8 GHz due to the 180deg reflection phase difference of AMC and PEC. The OAM vortex wave is generated by a UCA composed of 16 elements with the radius of 3λ and the distance between UCA and CM is 10λ, where λ represents the wavelength at 4.8 GHz.
Fig. 2. The scattering scenario. (a) CM illuminated by OAM vortex wave of UCA. (b) Top view of UCA and generated OAM vortex wave. (c) Top view of CM.
The illumination of OAM vortex wave results in the excitation of donut-like amplitude and helical phase distribution for the CM unit cells, which is the critical difference from the conventional plane wave detection. Fig. 3 depicts the excitation of CM simulated by Matlab using the established scattering model. In the simulation, q is set to be 1 and qe is 0.75. As a contrast, the full-wave simulation results obtained by HFSS are also shown in the figure. It can be seen that the excitation of each unit cell varies obviously with OAM order l, which will lead to different scattering characteristics. The full-wave simulation agrees well with the proposed scattering model.
Fig. 3. The excitation of CM unit cells illuminated by OAM vortex wave of UCA with different orders. (a) and (b) are the electric field amplitude distribution. (c), (d), (e), and (f) are the electric field phase distribution. (a), (c), and (e) are numerical results by the scattering model. (b), (d), and (f) are full-wave results by HFSS.
Given the excitation of CM, we can further investigate the far field distribution of the scattering energy. Fig. 4 shows the two-dimensional scattering pattern of yoz-plane at 4.8 GHz with different OAM order l. It is observed that the scattering pattern varies significantly with OAM order l. In general, the results predicted by the established scattering model are in good consistence with HFSS full-wave simulation results. The deviation mainly comes from the blockage effect and backward radiation of the UCA, which is not considered in scattering model.
Fig. 4. The two-dimensional scattering pattern of CM illuminated by OAM vortex wave with different order l. (a) l=±1, (b) l=±2, (c) l=±3, (d) l=±4.
It needs to be emphasized that the back scattering at θ=0° direction is a crucial indicator that affects the stealth performance of a CM. Due to the phase cancellation scheme, CM can reduce the RCS at its normal direction drastically. Nevertheless, a remarkable phenomenon is observed for the scattering pattern of l=±2. That is to say, an energy peak occurs at θ=0°, revealing that the RCS of the CM at normal direction is strongly increased. This is against the stealth scheme of CM. Fig. 5 gives the three-dimensional scattering patterns of CM with plane wave incidence and OAM vortex wave incidence of l=±2 at 4.8 GHz. It is obvious that different scattering patterns are observed. For plane wave incidence, the energy is deflected to the diagonal directions due to the phase cancellation scheme. While for OAM vortex wave of l=±2, a strong scattering peak occurs at the normal direction.
Fig. 5. The three-dimensional scattering patterns of CM illuminated by palne wave and OAM vortex wave of l=±2. (a) HFSS full-wave simulation. (b) Numerical simulation using the established scattering model.
In order to investigate the reason of such phenomenon, the phase distribution after superposition of OAM excitation phase and CM compensating phase for l=±2 is given in Fig. 6. It is seen that the superposition phase for l=±2 can be divided into four identical subarrays but with different orientations. This configuration gives rise to the scattering energy peak at normal direction. In other words, the interaction between OAM incident wave of l=±2 and CM phase distribution breaks down the phase cancellation scheme and thus, the backscattering energy is boosted significantly. It should be noted that it does not mean the same phenomenon will happen to other kinds of MST. The CM is exploited as an example to uncover the scattering characteristic of MST under OAM vortex wave illumination. The interaction between OAM vortex wave and other kinds of MST needs to be discussed case by case.
Fig. 6. The phase distribution after superposition of OAM excitation phase and CM compensating phase for (a) l = 2 and (b) l = -2.
The above results validate that the proposed approach is a good candidate for MST detection. Nevertheless, the CM is located in the near field of the UCA. In order to examine the far field detection effect, the distance between UCA and CM (DUC) is enlarged to satisfy the far field condition. For a UCA with the radius of 3λ, the far field range is at least 72λ. Hence, the scattering patterns with DUC of 72λ, 144λ, and 288λ are shown in Fig. 7, which are obtained by the scattering model for l=±2. The back scattering boost is always observed in the figure, which manifests that the proposed OAM detection approach is also effective for far field detection. However, it is worthing mentioning that the divergence of OAM vortex wave is still a problem for OAM radar detection just like other OAM-based wireless systems. Nevertheless, the OAM detection approach remains a promising solution for MST detection.
Fig. 7. The two-dimensional scattering pattern of CM illuminated by OAM vortex wave with different DUC for l=±2.
4. Experimental validation
For the sake of experimental validation, the prototypes of the proposed structures were fabricated using printed circuit board (PCB) technology as shown in Fig. 8. The UCA was fed by 16 coaxial cables with the same length. Each cable was connected to a power divider and a phase shifter and the phase shifters were connected to the input ports of the UCA. The neighbouring phase shifters can provide 45deg phase difference so that OAM vortex wave of l=±2 can be generated.
The prototypes were measured in the anechoic chamber. Subject to the hardware condition, the measurement was conducted only with DUC of 10λ and the measurement setup is depicted in Fig. 9(a). The UCA and CM were attached to a holding foam, which was put on the rotary table. A receiver was placed far away from the prototype to be tested. For comparison, the plane wave incidence case was also measured. The test frequency was 4.8 GHz and the normalized scattering pattern was shown in Fig. 9(b). It can be seen from the figure that the scattering peak of the CM for l=±2 occurred at the normal direction. To the contrary, the scattering energy was deflected to other directions for plane wave incidence case. It revealed that the backscattering of the CM was boosted by OAM detection approach. In general, the measurement result agreed well with the simulations, especially around the normal direction. The difference mainly came from the fabrication error, position calibration error, welding error, and the cables blockage error.
5. Conclusion
In summary, the scattering enhancement of MST based on OAM detection approach was proposed in this paper. A UCA was employed to generate the OAM vortex wave with different order l. The scattering model was established using array method and was applied to a CM to investigate its scattering characteristic. The numerical simulation results revealed that the scattering pattern had a strong correlation with OAM order l. For l=±2, a back scattering peak was observed at the normal direction, which broke the stealth scheme of the CM. All the results obtained using the established model were verified by full-wave simulations. Moreover, prototypes of UCA and CM were fabricated and measured in anechoic chamber. The measured results further confirmed the findings. Our research reveals that OAM radar system is expected to be capable of improving the detection ability for MST.
Funding
National Natural Science Foundation of China (62101592, 62201614, 62203465).
Disclosures
The authors declare no conflicts of interests.
Data availability
Data that support the findings of this study are available from the corresponding author upon reasonable request.
References
1. A. Murugesan, D. Natarajan, and K. T. Selvan, “Low-cost, wideband checkerboard metasurfaces for monostatic RCS reduction,” Antennas Wirel. Propag. Lett. 20(4), 493–497 (2021). [CrossRef]
2. S. Koziel and M. Abdullah, “Machine-learning-powered EM-based framework for efficient and reliable design of low scattering metasurfaces,” IEEE Trans. Microwave Theory Techn. 69(4), 2028–2041 (2021). [CrossRef]
3. J. Liu, J. Y. Li, and Z. N. Chen, “Broadband polarization conversion metasurface for antenna RCS reduction,” IEEE Trans. Antennas Propagat. 70(5), 3834–3839 (2022). [CrossRef]
4. S. J. Wang, H. X. Xu, M. Z. Wang, C. H. Wang, Y. Z. Wang, and X. H. Ling, “A low-RCS and high-gain planar circularly polarized Cassegrain meta-antenna,” IEEE Trans. Antennas Propagat. 70(7), 5278–5287 (2022). [CrossRef]
5. Y. Xi, W. Jiang, T. Hong, K. Wei, and S. X. Gong, “Wideband and wide-angle radar cross section reduction using a hybrid mechanism metasurface,” Opt. Express 29(14), 22427–22441 (2021). [CrossRef]
6. J. M. Zhang, L. Yang, L. P. Li, T. Zhang, H. H. Li, Q. M. Wang, Y. N. Hao, M. Lei, and K. Bi, “High-efficiency polarization conversion phase gradient metasurface for wideband anomalous reflection,” J. Appl. Phys. 122(1), 014501 (2017). [CrossRef]
7. M. C. Feng, Y. F. Li, Q. Q. Zheng, J. Q. Zhang, Y. J. Han, J. F. Wang, H. Y. Chen, S. Sui, H. Ma, and S. B. Qu, “Two-dimensional coding phase gradient metasurface for RCS reduction,” J. Phys. D: Appl. Phys. 51(37), 375103 (2018). [CrossRef]
8. Y. Li, J. Zhang, S. Qu, J. Wang, H. Chen, X. Zhuo, and A. Zhang, “Wideband radar cross section reduction using two-dimensional phase gradient metasurfaces,” Appl. Phys. Lett. 104(22), 221110 (2014). [CrossRef]
9. M. I. Mirzapour, A. Ghorbani, and F. A. Namin, “A wideband low RCS and high gain phase gradient metalens antenna,” Int. J. Electron. Commun. 138, 153887 (2021). [CrossRef]
10. Y. Zhuang, G. Wang, J. Liang, and Q. Zhang, “Dual-band low-scattering metasurface based on combination of diffusion and absorption,” Antennas Wirel. Propag. Lett. 16, 2606–2609 (2017). [CrossRef]
11. J. Su, Y. Lu, Z. Zheng, Z. Li, Y. Yang, Y. Che, and K. Qi, “Fast analysis and optimal design of metasurface for wideband monostatic and multistatic radar stealth,” J. Appl. Phys. 120(20), 205107 (2016). [CrossRef]
12. X. Liu, J. Gao, L. M. Xu, X. Y. Cao, Y. Zhao, and S. J. Li, “A coding diffuse metasurface for RCS reduction,” Antennas Wirel. Propag. Lett. 16, 724–727 (2017). [CrossRef]
13. L. R. Jidi, X. Y. Cao, Y. Tang, S. M. Wang, Y. Zhao, and X. W. Zhu, “A new coding metasurface for wideband RCS reduction,” Radio Engineering 27(2), 394–401 (2018). [CrossRef]
14. X. Yang, C. Liu, B. Hou, and X. Zhou, “Wideband radar cross section reduction based on absorptive coding metasurface with compound stealth mechanism,” Chinese Phys. B 30(10), 104102 (2021). [CrossRef]
15. C. Fu, L. Han, C. Liu, X. Lu, and Z. Sun, “Combining Pancharatnam-Berry phase and conformal coding metasurface for dual-band RCS reduction,” IEEE Trans. Antennas Propagat. 70(3), 2352–2357 (2022). [CrossRef]
16. D. Zhang, X. Y. Cao, H. H. Yang, J. Gao, and X. W. Zhu, “Multiple OAM vortex beams generation using 1-bit metasurface,” Opt. Express 26(19), 24804–24815 (2018). [CrossRef]
17. D. Zhang, X. Y. Cao, J. Gao, H. H. Yang, W. Q. Li, T. Li, and J. H. Tian, “A shared aperture 1-bit metasurface for orbital angular momentum multiplexing,” Antennas Wirel. Propag. Lett. 18(4), 566–570 (2019). [CrossRef]
18. A. Sawant, I. Lee, and E. Choi, “Amplitude non-uniformity of millimeter-wave vortex beams generated by transmissive structures,” IEEE Trans. Antennas Propagat. 70(6), 4623–4631 (2022). [CrossRef]
19. B. Y. Liu, S. W. Wong, K. W. Tam, X. Zhang, and Y. Li, “Multifunctional orbital angular momentum generator with high-gain low-profile broadband and programmable characteristics,” IEEE Trans. Antennas Propagat. 70(2), 1068–1076 (2022). [CrossRef]
20. R. Chen, W. X. Long, X. D. Wang, and J. D. Li, “Multi-Mode OAM radio waves: generation, angle of arrival estimation and reception with UCAs,” IEEE Trans. Wireless Commun. 19(10), 6932–6947 (2020). [CrossRef]
21. J. Hu, T. T. Qiu, W. Wu, R. Z. Zhao, and S. S. Qian, “A compact OAM antenna for generating CP radiation patterns of three modes by using half-mode SIWs,” Microw. Opt. Technol. Lett. 63(2), 612–619 (2021). [CrossRef]
22. F. Tamburini, E. Mari, A. Sponselli, B. Thidé, A. Bianchini, and F. Romanato, “Encoding many channels on the same frequency through radio vorticity: first experimental test,” New J. Phys. 14(3), 033001 (2012). [CrossRef]
23. R. Wang, M. Wang, Y. Zhang, D. Liao, and L. Jing, “Generation of orbital angular momentum multiplexing millimeter waves based on a circular traveling wave antenna,” Opt. Express 31(3), 5131–5139 (2023). [CrossRef]
24. M. Tamagnone, C. Craeye, and J. Perruisseau-Carrier, “Comment on ‘Encoding many channels on the same frequency through radio vorticity: first experimental test’,” New J. Phys. 14(11), 118001 (2012). [CrossRef]
25. K. Liu, X. Li, Y. Cheng, and Y. Gao, “OAM-based multitarget detection: from theory to experiment,” IEEE Microw. Wireless Compon. Lett. 27(8), 760–762 (2017). [CrossRef]
26. K. Liu, Y. Cheng, Y. Gao, X. Li, Y. Qin, and H. Wang, “Super-resolution radar imaging based on experimental OAM beams,” Appl. Phys. Lett. 110(16), 164102 (2017). [CrossRef]
27. C. Zhang, D. Chen, and X. F. Jiang, “RCS diversity of electromagnetic wave carrying orbital angular momentum,” Sci. Rep. 7(1), 15412 (2017). [CrossRef]
28. K. Liu, H. Y. Liu, W. E. I. Sha, Y. Q. Cheng, and H. Q. Wang, “Backward scattering of electrically large standard objects illuminated by OAM beams,” Antennas Wirel. Propag. Lett. 19(7), 1167–1171 (2020). [CrossRef]
29. X. X. Bu, Z. Zhang, X. D. Liang, Z. Zeng, L. Y. Chen, and H. B. Tang, “Scattering characteristics of vortex electromagnetic waves for typical targets,” in Proc. Asia-Pacific Microw. Conf. Kyoto648–650 (2018).
30. M. Arfan, A. Ghaffa, M. A. S. Alkanhal, M. Y. Naz, A. H. Alqahtani, and Y. Khan, “Orbital angular momentum wave scattering from perfect electromagnetic conductor (PEMC) sphere,” Optic 253, 168562 (2022). [CrossRef]
31. M. Arfan, A. Ghafar, M. Y. Naz, M. A. S. Alkanhal, A. H. Alqahtanim, and Y. Khan, “Laguerre–Gaussian beam interaction by a metamaterial coated perfect electromagnetic conductor (PEMC) cylinder,” Optical and Quantum Electronics 55(3), 252 (2023). [CrossRef]
32. P. Nayeri, A. Z. Elsherbeni, and F. Yang, “Radiation analysis approaches for reflectarray antennas,” IEEE Antennas Propag. Mag. 55(1), 127–134 (2013). [CrossRef]
