Terahertz Time-domain Spectroscopy Imaging Study

Terahertz (THz) radiation is gradually known to have an ability to bring the new opportunity into the scientific research and the technology application. Terahertz radiation itself has a huge research value, which attracted many researchers'attention from different fields. The ultrafast laser technology get a development quickly as long as ten or more years. It provides the stable and credible excitation source for the THz pulse generation, which brings a booming development of the mechanism research, the measuring technology and the technology application of THz radiation. THz imaging is one of the most promising applications in the THz science and technology. It is expected that the whole world will get a significant impact.This thesis mainly investigates THz time-domain spectral imaging. Firstly, we compared many aspects between the pulse THz imaging and the continuous-wave THz imaging. Secondly, we investigate the method of image processing of THz time-domain spectral imaging. We analyzed that, under both of time-domain mode and the frequency-domain mode, the imaging features by the point-by-point scanning of THz imaging. An identification method based on the THz multi-spectral imaging is presented. Finally, we carried out the imaging analysis to four basic shapes of copper holes with the different sizes, and quantitatively assessed the identification ability of THz imaging to four small structures.The works of this thesis can be summarized as follows:(1) By a series of experiments, we compared many aspects between the pulse imaging and the continuous-wave imaging, such as resolution, noise, imaging speed, and the amount of information, price, complexity, portability and their applications. These two systems have their own features and also have a good complement each other.(2) We investigate the method of image processing of terahertz time-domain spectral imaging, in the time-domain mode and the frequency-domain mode, respectively. We do the THz imaging experiment by scanning four explosives. Under the time-domain mode, we mainly investigated the amplitude and phase imaging. While under the frequency-domain mode, we can obtain the THz imaging by different parameters at each frequency, such as amplitude, absorption, refractive index, energy, power and so on. Especially, the absorption imaging can avoid the effect of the systemic noise and the absorption of water vapour. (3) By using of THz-TDS energy imaging, the energy distribution of terahertz wave passing through the different small structures, such as linear fibers, dots, cycle block, square block, is obtained. The experimental results show that the resolution of THz-TDS imaging can reach the level less than 1 mm. In addition, an approach to measuring the size of unknown objects by THz imaging has been introduced and applied in this research.In summery, the detailed investigation and analysis to THz time-domain spectral imaging technology is presented. The theoretical research, the experimental setup, the method of data processing, imaging ability and so on, are studied. The research results have an important significance to improve the resolution of THz imaging and THz recognition technology.