An optically controlled phased array antenna ... - nuaa.edu.cn

An optically controlled phased array antenna based on single sideband polarization

modulation Yamei Zhang, Huan Wu, Dan Zhu, and Shilong Pan*

Key Laboratory of Radar Imaging and Microwave Photonics (Nanjing Univ. Aeronaut. Astronaut.), Ministry of Education, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

*[email protected]

Abstract: A novel optically controlled phased array antenna consisting a simple optical beamforming network and an N element linear patch antenna array is proposed and demonstrated. The optical beamforming network is realized by N independent phase shifters using a shared optical single sideband (OSSB) polarization modulator together with N polarization controllers (PCs), N polarization beam splitters (PBSs) and N photodetectors (PDs). An experiment is carried out. A 4-element linear patch antenna array operating at 14 GHz and a 1 × 4 optical beamforming network (OBFN) is employed to realize the phased array antenna. The radiation patterns of the phased array antenna at −30°, 0° and 30° are achieved.

©2014 Optical Society of America

OCIS codes: (050.5080) Phase shift; (280.5110) Phased-array radar; (060.5625) Radio frequency photonics.

References and links 1. R. Tang and R. W. Burns, “Array technology,” Proc. IEEE 80(1), 173–182 (1992). 2. N. A. Riza, Selected Papers on Photonic Control Systems for Phased Array Antennas (SPIE, 1997). 3. J. P. Yao, “Microwave photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). 4. D. Dolfi, F. Michel-Gabriel, S. Bann, and J. P. Huignard, “Two-dimensional optical architecture for time-delay

beam forming in a phased-array antenna,” Opt. Lett. 16(4), 255–257 (1991). 5. N. A. Riza, “Liquid crystal-based optical control of phased array antennas,” J. Lightwave Technol. 10(12), 1974–

1984 (1992). 6. N. A. Riza, “Acousto-optic liquid-crystal analog beam former for phased-array antennas,” Appl. Opt. 33(17),

3712–3724 (1994). 7. W. Ng, A. A. Walston, G. L. Tangonan, J. J. Lee, I. L. Ncwberg, and N. Bernstein, “The first demonstration of

an optically steered microwave phased array antenna using true-time-delay,” J. Lightwave Technol. 9(9), 1124–1131 (1991).

8. A. Meijerink, C. G. H. Roeloffzen, R. Meijerink, L. M. Zhuang, D. A. I. Marpaung, M. J. Bentum, M. Burla, J. Verpoorte, P. Jorna, A. Hulzinga, and V. Etten, “Novel ring resonator-based integrated photonic beamfomer for broadband phased array receive antennas—part I: design and performance analysis,” J. Lightwave Technol. 28(1), 3–18 (2010).

9. J. L. Corral, J. Mart, S. Regidor, J. M. Foster, R. Laming, and M. J. Cole, “Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber gratings,” IEEE Trans. Microwave Theory Technol. 45(8), 1531–1536 (1997).

10. N. A. Riza, “Analog vector modulation-based widely tunable frequency photonic beamformer for phased array antennas,” IEEE Trans. Microwave Theory Technol. 45(8), 1508–1512 (1997).

11. L. A. Bui, A. Mitchell, K. Ghorbani, T. Chio, S. Mansoori, and E. R. Lopez, “Wide-band photonically phased array antenna using vector sum phase shifting approach,” IEEE Trans. Antennas Propag. 53(11), 3589–3596 (2005).

12. N. A. Riza, S. A. Khan, and M. A. Arain, “Flexible beamforming for optically controlled phased array antennas,” Opt. Commun. 227(4-6), 301–310 (2003).

13. X. K. Yi, T. X. H. Huang, and R. A. Minasian, “Photonic beamforming based on programmable phase shifters with amplitude and phase control,” IEEE Photonics Technol. Lett. 23(18), 1286–1288 (2011).

14. S. L. Pan and Y. M. Zhang, “Tunable and wideband microwave photonic phase shifter based on a single-sideband polarization modulator and a polarizer,” Opt. Lett. 37(21), 4483–4485 (2012).

#203399 - $15.00 USD Received 19 Dec 2013; revised 31 Jan 2014; accepted 1 Feb 2014; published 10 Feb 2014(C) 2014 OSA 24 February 2014 | Vol. 22, No. 4 | DOI:10.1364/OE.22.003761 | OPTICS EXPRESS 3761

15. E. Carpentieri, U. F. D’Elia, E. De Stefano, L. Di Guida, and R. Vitiello, “Millimeter-wave phased-array antennas,” in IEEE Radar Conference (RADAR'08) (2008), pp. 1–5.

16. Y. M. Zhang and S. L. Pan, “Generation of phase-coded microwave signals using a polarization-modulator-based photonic microwave phase shifter,” Opt. Lett. 38(5), 766–768 (2013).

1. Introduction

Phased array antenna plays an important role in multifunctional radars and high-throughput wireless communications [1]. In the phased array antenna, beamforming network is one of the key components. Thanks to the inherent advantages brought by the photonic technology, such as small size, low weight, large bandwidth, low transmission loss, high tunability, and immunity to electromagnetic interference [2,3], it is of great interests to implement the beamforming network in the optical domain. Previously, a lot of efforts have been devoted to the development of the optical beamforming network (OBFN) [4–13]. Although optical true-time delay can realize squint-free beamformer [7–9], beamforming networks based on phase shifters are still widely used if the fractional bandwidth is not large. In 1997 and 2005, Riza [10] and Bui et al. [11] proposed OBFNs based on vector-sum phase shifting, respectively. In [11], the optical RF signal to be phase shifted is split into two paths with different fiber lengths by a variable directional coupler. By controlling the coupling ratio of the variable directional coupler, the combined signal after photodetection has different phase shifts. The key limitation associated with this approach is that N independent sets of laser source, modulator and vector-sum module are required for N antenna elements, which is complex and costly. Besides, flexible beamforming technologies based on programmable photonic processor comprising a 2-D array of liquid crystal on silicon (LCoS) pixels are also proposed by Riza [12] and Yi et al. [13], respectively. The methods are flexible since the photonic processor can manipulate the amplitude and phase of different optical spectral components independently. However, the state-of-the-art photonic processor is still complex, lossy and costly. In addition, the number of the output ports of the photonic processor is small (typically ≤4), so the antenna elements in the phased array antenna are limited.

In this paper, a novel phased array antenna based on a simple OBFN is proposed and demonstrated. A polarization modulator (PolM) incorporated with an optical bandpass filter (OBPF) is used to perform optical single sideband (OSSB) polarization modulation. A 1 × N optical coupler is followed to split the OSSB polarization-modulated signal into N paths. In each path, a polarization controller (PC) and a polarization beam splitter (PBS) are inserted to realize a phase shifter [14], in which the phase shift is adjusted by the PC. The signals are then converted back to the electrical domain by photodetectors (PDs). A 1 × N OBFN is thus achieved. By incorporated the OBFN with an N-element linear patch antenna array, a phased array antenna is implemented. An experiment is carried out. A phased array antenna based on the OBFN is experimentally realized. The radiation patterns of the phased array antenna are measured.

2. Principle

Figure 1 illustrates the schematic diagram of the principle of a general beamforming network using phase shifters. As can be seen, N antennas incorporated with N phase shifters forms a phased array antenna. In the phased array antenna, the adjacent antennas are separated by a distance of d. When the beam direction is steered to an angle of θ, we have

sinR d θΔ = ⋅ (1)

where ΔR is the minimum distance from the wave front to the antenna elements. As the signals radiated from the N antennas have the same phase shifts at the wave front, the phase difference between the adjacent antennas is expressed as

2 /Rφ π λΔ = ⋅ Δ (2)


where λ is the wavelength of the radiated microwave signal. Substituting (2) to (1), we obtain

1 1sin sin2

Rd d

λ φθπ

− −Δ ⋅ Δ= =⋅

(3)

As can be seen from (3), the beam pointing angle θ is a function of the phase difference Δφ which can be adjust by the phase shifters.

Fig. 1. The schematic diagram of beam forming using phase shifters.

Fig. 2. The schematic diagram of the proposed phased array antenna. LD: laser diode; PC: polarization controller; OBPF: optical bandpass filter; PolM: polarization modulator; PBS: polarization beam splitter; PD: photodetector.

Figure 2 shows the schematic diagram of the proposed phased array antenna. The key components for the phased array antenna are the N OSSB-polarization-modulation-based microwave photonic phase shifters, which are implemented by a shared laser source, a shared PolM, a shared OBPF, a 1 × N optical coupler, N PCs, N PBSs and N PDs. When a microwave signal is applied to the PolM, two double-sideband (DSB) modulated signals with complementary phase modulations along the two orthogonal axes are generated. The DC bias of the PolM is controlled to introduce a phase difference of 90° between the two principal axes of the PolM. The OBPF is followed by the PolM to remove the sidebands in one side (for example, the left sidebands), so an OSSB polarization-modulated signal is generated. For this OSSB polarization-modulated signal, assume that the optical carrier along one principal axis has a phase of 0°, the phases of the optical carrier along the other principal axis and the right sidebands along the two principal axes are 90°, 90°, 0°, respectively, i.e. the combined optical carrier and sideband are circularly polarized with opposite handedness. This OSSB polarization-modulated signal is split into N paths by the 1 × N coupler and each path includes one PC, one PBS and one PD. The PC together with the PBS is served as a polarizer. When the polarization direction of the polarizer is aligned with one principal axis of the PolM, the phase of the generated microwave signal at the PD is −90°, and if the polarizer is rotated to select the optical signal along the other polarization axis, this phase is changed to 90°. For other polarization directions, the phase shift is 2α + 90° [14], where α is the angle between the polarization direction of the polarizer and one principal axis of the PolM. As a result, the


phase shift can be independently and continuously tuned from −180 ° to 180° by adjusting the setting of the PC before the PBS in each path. Because both the combined optical carrier and sideband are circularly polarized, the optical carrier and the sideband after the polarizer have unchanged magnitudes, resulting in an invariable amplitude of the microwave signal when tuning the phase. The generated N microwave signals with independently-controlled phase shifts are then radiated by an N-element linear patch antenna array. By carefully adjusting the PC in each path, the far-field radiation with desired pattern can be achieved. Because the laser source, PolM and OBPF are shared by all phase shifters, the system is simple and compact. The flat power response of the phase shifters makes the system free of phase-amplitude coupling effect which is a serious problem in conventional electrical beamforming networks. In addition, the bandwidth of the phase shifter is broad, limited only by the bandwidth of the PolM and the PDs [14], so the proposed phased array antenna is suitable for frequency-agile system. The fiber-based PC and PBS array in the proposed scheme can also be replaced by 2-D spatial light modulators and free-space PBSs [2]. Therefore, the proposed system can be potentially altered to have large number of antennas. This kind of optically controlled phased array antenna would be attractive for communications, automotive systems, tracking radar and satellite links at millimeter-wave band [15], in which the frequency is too high for the electronic technology to achieve high performance and the fractional bandwidth is relatively small.

Fig. 3. Photos of the proposed phased array antenna in an anechoic chamber.

3. Experimental demonstration

A proof-of-concept experiment is carried out based on the configuration shown in Fig. 2. Photos of the experiment setup are illustrated in Fig. 3. The key parameters of the devices used in the experiment are as follows. The wavelength of the LD is 1561.9 nm and its power is 10 dBm. The 3-dB bandwidth and the half-wave voltage of the PolM (Versawave Inc.) are 40 GHz and 3.5 V, respectively. The OBPF has a 3-dB bandwidth of 50 GHz. The OSSB polarization-modulated signal is split by a 1 × 4 coupler. The PDs have a bandwidth of 20 GHz and a responsivity of 0.8 A/W. The 4-element linear patch antenna array has a 10-dB bandwidth of 500 MHz and a center frequency of 14 GHz. The antenna elements are separated by a distance of 100 mm, which is close to half of the operating RF signal wavelength. The RF signal introduced to the PolM has a frequency of 14 GHz and a power of 15 dBm. The radiation patterns are measured in an anechoic chamber.

From [14], the phase shifter, comprising of a LD, a PolM, an OBPF, a PC, a PBS and a PD, has flat phase responses over a large working bandwidth of 10-40 GHz. By controlling the polarization states of the PC, the phase of the RF signal can be tuned from −180° to 180°, which indicates that the OBFN based on the phase shifters can operate in a wide frequency range. To investigate the feasibility of the proposed phased array antenna, radiation patterns


with different pointing angles are measured. As can be calculated from (3), phase shifts of [0°, 84°, 168°, −106°], [0°, 0°, 0°, 0°] and [0°, −84°, −168°, 106°] are required to obtain the radiation patterns with pointing angles of −30°, 0°, and 30°, respectively. These phase shifts are achieved by carefully adjusting the PCs with the assistance of a vector network analyzer (VNA, Aglient N5230A). Figure 4 shows the simulated and the experimentally measured radiation patterns of the phased array antenna based on the OBFN. As can be seen, all the main lobes of the measured radiation patterns are steered to the desired angles, which agree very well with the simulated radiation patterns. The 3-dB bandwidth of the main lobes at −30°, 0°, and 30° are 30° (−45° to −15°), 26° (−13° to 13°) and 30° (15° to 45°), respectively. The small difference of the side lobes is caused by the uneven powers emitted from the 4 antennas. For practical system, the PCs should be replaced by electronically-controlled PCs. In that case, the radiation pattern of the phased array antenna can be switched at a high speed as the PC can be replaced by a PolM [16].

Fig. 4. The simulated and experimentally measured radiation patterns of the phased array antenna based on the OBFN when the angle of the beam is (a) −30°, (b) 0°, and (c) 30°. The black dashed and the red solid curves are the simulated and experimental results, respectively.

4. Conclusion

A novel optically controlled phased array antenna using OSSB polarization modulation based OBFN was proposed and demonstrated. Experimental results demonstrated that the OBFN can be used to steer the main lobe of the radiation pattern of the phased array antenna to any desired directions by simply adjusting the PC in each path. Good agreement between the theoretical results and the experimental results are confirmed. The proposed phased array antenna is suitable for the frequency-agile system and can be possibly swept at a high speed, which may find potential applications in radars and satellite communications.

Acknowledgments

This work was supported in part by the National Natural Science Foundation of China (61107063, 61201048), the Jiangsu Provincial Funds for Distinguished Young Scientists (BK2012031, BK2012381), the Fundamental Research Funds for the Central Universities, the Foundation of Graduate Innovation Center in Nanjing University of Aeronautics and Astronautics (kfjj130113), the China Postdoctoral Science Special Foundation (2013T60533), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.


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