An integrated optical beamforming network for two-dimensional phased array radar
Introduction
Traditional phased array radar implements phase-shifting network by using phase shifters, thus the bandwidth is greatly limited due to beam squint [1]. With the development of microwave photonics, phased array radar combined with photonic technology comes into being [2]. By using the optical true time delay network (OTTDN), the advantages of microwave photonic link such as broad operation bandwidth, low power consumption, and anti-electromagnetic interference can be integrated into phased array radar [3], [4], [5]. For optically controlled phased array radar, the number of radiation units determines beamwidth, hence increasing the radiation units can enhance the radar’s angular resolution. However, the increase of radiation units means an increase in the complexity of OTTDN. Nowadays, most phased array radar systems based on OTTDN are composed of discrete components, so they are plagued by large size and high power consumption. To exert the superiorities of optically controlled phased array radar, the integration of OTTDN based on microwave photonic system is of great importance, and integrated photonics has attracted great interest from researchers in the past decades [6], [7].
Extensive effort has been devoted to developing diverse methods for OTTDN. For the non-integrated schemes, the general system uses components such as single-mode fiber (SMF) [8], dispersion compensated fiber (DCF) [9], and fiber Bragg grating (FBG) [10]. For the integrated system, photonic crystal waveguide [11], waveguide grating [12], [13], micro-ring resonator [14], [15], [16], [17] and switchable waveguide delay line [18], [19] are usually integrated to realize different delays. At present the non-integrated solution is gradually transforming into the integrated one due to the limitations of volume and consumption. The disadvantage of the most integrated scheme is that it has only single-channel delay, and cannot simultaneously satisfy large delay amount and high delay accuracy. With the development of optoelectronic integration technology, multi-channel delay required by delay network can be realized on a single chip. The switchable waveguide delay line has become one of the best candidates because of its large bandwidth, high delay accuracy, easy integration and so on.
In this paper, an integrated OTTDN based on switchable waveguide delay line is proposed, and then applied for an 8 × 8 two-dimensional phased array radar system. This OTTDN takes into account both the high delay resolution and the compact volume. The delay measurements show that the maximum delay mean error is −0.55 ps, and the maximum root mean square error (RMSE) is only 0.80 ps. Using these results, the two-dimensional phased array radar can have a very high beam steering accuracy. A special design is also used to reduce the power consumption of this integrated switchable waveguide delay line. When the four delay lines work at the same time, the power consumption of the four delay lines is only 4.32 W.
Section snippets
Design of OTTDN
For phased array radar system, the number of required delay units is proportional to the number of array elements, therefore, the design difficulty of large-scale array delay network increases significantly. In our two-dimensional OTTDN design, two aspects are considered to reduce the complexity of the delay network.
Firstly, we use two different angles () to describe the beam pointing angle, and then to calculate the delay difference between the neighboring row and column. is the angle
Measurement of optical switch
Optical switch is an important component in OTTDL, its performance is characterized firstly. The periodic square wave voltage signal is applied on waveguide thermo-electrode as the switching signal, as shown in Fig. 6(a). The output port of the switch is connected to oscilloscope after conversion by photodetector, and Fig. 6(b) presents the measured optical response. It can be seen that the rise and fall time of the optical signal are and . While the response time of the thermo-optic
Conclusion
In this paper, we propose an OTTDN used for two-dimensional phased array radar, and design the corresponding delay unit. The delay unit is realized by an integrated on-chip method, and it has the advantages of low power consumption, small footprint, and high precision. The size of the four-channel and 7-bit optical delay chip is only 5.7 × 15.8 mm2, the maximum and minimum delay of a single channel are 0677 ps and 097 ps, respectively, which can obtain the scanning beam angle of 45135° in
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is supported by the National Key Research and Development Program of China (Grants no. 2018YFB2201700).
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