| In recent years,with the rapid development of terahertz(THz)technology,active THz imaging system has shown wide application prospects in many fields,such as materials science,physics,chemistry,biomedicine,nondestructive testing and so on.We build reliable and practical active imaging systems to study the applications of THz in different fields,such as nondestructive-testing(NDT),time domain spectrum analysis and human security inspection.The dual-band THzimaging system is a potential product for NDT using heterodyne detectors and continuous-wave(CW)sources.The operating frequencies of the system are 110.4 and 220.8 GHz.Multi-band fusion technology combines the advantages of the greater spatial resolution of the high-frequency band and the enhanced sensitivity of the low-frequency band to improve the detection ability of the system.Additionally,the interference cancellation technology is used to obtain a superior image quality.The spatial resolution of this system was approximately 3 mm.The results show that the system can be used for bonding quality and embedded defect detection in radomes and foam materials adhered to metal plates in aircrafts.Also,we emphatically studied on bifocal Gregory system that can be used for human security.Based on the theory of astigmatism and field curvature,we provide a scheme to realize the bifocal effect with a classical confocal system that can focus in a focal ring on an arbitrary plane parallel to the antenna aperture,which can be considered the target for the imaging system.The scheme can be applied to the Gregory and Cassegrain systems by simply changing the imaging distance while maintaining the classical system surface parameters,or we can obtain new surface parameters with these schemes by maintaining the imaging distance to generate a bifocal image effect.Compared with the bifocal system design method,which is based on ray tracing,our method is suitable for more facial designs,and the calculation is simple.As the design is not achieved by geometric ray tracing,it is impossible to further optimize the system by adding higher order terms to the surface parameters,and the spot quality is slightly worse than that of a system based on the ray tracing method.We built a Gregory scanning system to verify the ability of this method.When the operating frequency of the verification system is 220 GHz,the Field-of-View(FoV)at a specified distance of 8 m is approximately 50 cm×100 cm,and the imaging resolution throughout the entire FoV is better than 3 cm(the best resolution is about 2.5 cm).When the operating frequency of system is 650 GHz,the imaging resolution throughout the entire FoV is better than 2.5 cm(the best resolution is about 1.5 cm)while other parameters keep the same.Experimental results show that the imaging resolution of the classical dual reflector system can be optimized,and the proposed verification system can realize active THz imaging of the human body.The active THz imaging systems have achieved the expected performance targets for different application scenes,and proved the great potential of active THz imaging in NDT and human security inspection.At the end of this paper,we also propose some possible improvement ideas and methods for the further application of active THz imaging technology. |
