Terahertz imaging technology for human security inspection
As growing violent and terrorist incidents are endangering people, security inspection in the public becomes increasingly important. The sparse linear array multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) based system, taking into consideration the high resolution of images and the low complexity of the system, is one of the new human body security inspection imaging technologies.
This paper mainly introduces the application of MIMO SAR in human body security inspection based on the investigation and analysis of current status and development of domestic and foreign studies on active millimeter-wave/terahertz-wave imaging technologies. Four frameworks are introduced, including single-input single-output (SISO) imaging systems, one dimensional MIMO-SAR with mirror imaging systems, one dimension MIMO-SAR with one dimension SAR imaging systems, and two dimension MIMO imaging systems. Then, the MIMO sparse line array is introduced. The main design idea of the system with MIMO line array is to combine a frequency modulated MIMO line array with an orthogonal synthetic aperture generated by the linear movement of the object under test, such as a conveyor. An array of transmitters illuminates the human with concealed weapon and an array of receivers records the back-scattered radiation coherently. Four kinds of MIMO sparse plane arrays are introduced, including tetragonum array, cross array, square array, and T array. The MIMO sparse plane arrays are designed with 16Tx and 16Rx, then, their performances are compared. Thirdly, signal processing basics and three efficient computational 3D imaging algorithms are presented, including back-projection (BP), fast-factorized back-projection (FFBP), and range migration algorithm (RMA). Those imaging algorithms can be implemented for parallel processing on a graphics processing unit for accelerating the image generation. The BP and FFBP kernels profit strongly from the parallelization since they perform the same computational operation for each voxel. Therefore, the volume reconstruction is mainly achieved through assigning each thread of the graphics processing unit to a voxel. The RMA algorithm also benefit from accelerated execution of the fast Fourier transform algorithm on the graphics processing unit without parallelization. In addition, three different imaging algorithms are compared in regard to their computational efficiency. Finally, this paper makes a prospect of development.
Three-dimensional terahertz imaging with MIMO-SAR
In recent years, the THz basic devices in China have made great progress and performances have been steadily improved, which has promoted the rapid development of THz radar system. Although the THz MIMO-SAR system structure is clear for the human body security inspection, but there are still some basic and engineering problems to be overcome, such as: radar chip with integrated array, faster imaging algorithm, a method to extract the echo characteristics and information of hidden objects from multiple view angles.
The THz near-field MIMO-SAR technology is still a hot spot in the THz radar and is important for the human security inspection and also need to be further in-depth study for promoting its important application in the field of military and civilian.
Liu Jie, An Jianfei, Zhou Ren, et al. Terahertz near-field MIMO-SAR technology for human security inspection[J]. Opto-Electronic Engineering, 2020, 47(5): 190682.