Far-field time reversal subwavelength imaging of sources based on grating structure

Abstract

For far-field imaging applications, the imaging resolution of conventional lenses is limited by the diffraction limit because of the exponential decay of high spatial frequency waves. The key to realizing the subwavelength imaging lies in the collection of evanescent informations in far-field region. However, the collection of evanescent waves is not the only thing we need to do. The relation between target position and far-field information is also very important. In this paper, a far-field time reversal subwavelength imaging system is constructed with the help of an evanescent-to-propagating conversion plate, i. e., a grating plate. The designed grating plate is able to convert evanescent waves into propagating waves through the modulation in space-spectrum domain. In order to clearly understand the conversion, a focusing experiment is conducted with two sources and five time reversal mirror antennas. By recording the amplitudes of the time reversal signals in the two source positions, we can see that the amplitude of the refocusing signal at the original source position is much larger than that of the other signal. Through numerical simulation and experiment, the conversion of evanescent wave into propagative wave is proved finally. Then, according to the self-conjugation property of time reversal, the result of self-conjugation for channel response in complex environment is nearly the same as an impulse function. The image of source target can be reconstructed without exact prior knowledge of the expression of the spatial channel response. In order to exemplify the super resolution property of our designed system, experiments with simulation data and experimental data are executed with and without our designed grating plate, respectively. For imaging applications, we first record the forward signals received by the time reversal mirror antennas, and then record the refocusing field distribution on the imaging plane to obtain the image of the target. In the reconstruction process, another thing we need to notice is that the original sources should be removed. This is because in a real imaging application, we cannot know the exact position of target inadvance. The imaging results show that the resolution of our imaging system has overcome the diffraction limit. Compared with the imaging resolution of the imaging system without the grating plate, the imaging resolution of the system with our designed grating plate is improved obviously. Since this kind of method overcomes the intrinsical diffraction limit by transmitting evanescent information to far-field region in a way of converting them into propagative waves. This kind of method offers us a promising alternative to microwave far-field subwavelength imaging applications.