Study On Continuous-wave Terahertz Imaging

The terahertz wave (THz) refers to the frequency range between 0.1 to 10 THz. It can be used as imaging light source,just like visible light and X radiation. THz wave has many unique qualities that it can be strongly absorbed by water and can pass through many materials like clothes and papers which are opaque to visible light. All of those make THz imaging technology own great value in medical diagnosis, security inspection and so on. With the development of semiconductor technologies, continuous-wave(CW) THz imaging system has many advantages such as low cost, simple structure and high speed, showing its'practicability in many applied fields.A transmission CW THz imaging system was introduced. Applications of THz imaging were verified and key factors affecting imaging quality were discussed from theoretical and experimental aspects. The main work and innovations are listed as follows:1. Hardware was appropriately chosen by considering the performance of CW THz imaging system, such as the use of CO2 laser-pumped far-infrared laser source and pyroelectric detector. The imaging system was set up and optimized, so the imaging system has merits of low cost, high power and high responsivity.2. The control program of a two-dimensional translation stage and data capture card was compiled by Visual Basic software, and point scan imaging system without pause during scanning process was implemented to speed up the pace.3. THz imaging principle was theoretically studyed, while a variety of materials such as leaves, lamb, metal plates, envelopes and pencil characters were imaged by THz imaging system, good imaging results were obtained, which are very essential to further research. The feasibility of the imaging system and its important value were verified by analyzing the experimental results,which provides a reference value for the practical applications.4. Currently CW THz imaging technology is relatively mature, but the image quality is poor, so how to improve the image quality of system becomes the most concern. Several key factors affecting the image quality were discussed, such as the spot size on the surface of the object, chopper frequency, step length, light intensity, image processing and so on. Through comparing the imaging results of the different parameters, eventually a high-quality THz imaging with the resolution of 0.4mm was achieved by finding the suitable imaging parameters.