Electromagnetic Vortex Imaging Based on OAM Multiplexing Beam With Orthogonal Polyphase Coding

The donut-shaped radiation energy pattern of orbital angular momentum (OAM) beams has been a longstanding challenge in electromagnetic (EM) vortex imaging. In this article, an effective EM vortex imaging scheme is developed based on the orthogonal polyphase-coded OAM multiplexing beam with mitigated beam energy divergence. Firstly, vortex EM waves are modulated with different orthogonal polyphase codes and transmitted simultaneously to illuminate the imaging scenario. The codes change the phase relationship between different transmitted signals, leading to a convergent energy distribution around the beam axis. Then, an OAM demultiplexing method is proposed to separate the radar echo of each OAM mode, which maintains the individual spiral phase fronts at the same time. Finally, the 2-D fast Fourier transform (FFT) is applied to the demodulated signals to reconstruct the target image. Simulation results show that the proposed imaging method has a higher azimuth resolution than the traditional method with small elevation angles and performs much better in low signal-to-noise ratio (SNR) environments. Furthermore, an experiment in an anechoic chamber validates the potential of the proposed method. This work can benefit the practical applications of OAM imaging technology.