Research On The Key Techniques Of Fiber-based Terahertz Time-domain Spectroscopy And Imaging

In the past decade, terahertz (THz) technologies have been developed veryrapid and utilized in many fields, such as basic scientific research, industrialprocessing, military affairs, and so on. Among them, the terahertz time-domainspectroscopy (THz-TDS) with its unique technological advantages quickly attractedthe attention of the majority of research scholars and played a dominant role in manyfields. With more and more requirements in applications, developing a miniature,compact, and portable THz-TDS has become an important issue, which will have agreat influence on THz technology and product development in future.The main work of this thesis can be summarized into four sections, which canbe listed as follows:The first section (Chapter1and Chapter2) generally introduced thecharacteristics of THz radiation, the background of THz development andapplications. The working principle and structural composition of THz-TDS wasintroduced and analyzed. Furthermore, three important technological principles ofTHz-TDS have been discussed in details, including the principle of THz generation,the principle of THz detection, and extraction of optical parameter. Finally, focusingon the defect of typical THz-TDS system and its limitations, the motivation of thisthesis and the main research contents was pointed out.The second section (Chapter3and Chapter4) pointed out the key technologicalissues on developing the fiber-coupled THz-TDS system, which mainly include thehigh efficiency of laser-to-fiber coupling, the analysis of ultra-short laser pulsedelivering in fiber, the design of fiber-coupled photoconductive antenna, thesoftware development of motion control and data acquisition. In order to deal withthe problems, both the theoretical analysis and experimental research have beencarried out. Then, the process of developing the fiber-coupled THz-TDS and themajor components of system has been detailed discussed. Finally, an experimental setup of fiber-coupled THz-TDS has been constructed and the time-domain signalhas been obtained. This will provide an important technical support for developingminiature, compact, and portable THz-TDS system.The third section (Chapter5) summarized the research status of terahertzimaging, working principle of system, and the experimental setup. The majorresearch works can be divided into two parts: terahertz time-domain imaging andterahertz frequency-domain imaging. In experiments, the knife and high explosiveRDX were imaged in time region and frequency region, which show a great potentialapplication in security and identification.The fourth section (Chapter6) discussed the work on generating THz radiationby using laser-induced air-plasma. The experimental setup and working principle ofthe THz-TDS based on the air-plasma was investigated. In experiments, the terahertztime-domain signal could be extracted by using spectrum decoding and the singlepulse energy of THz radiation was measured. Ultrashort THz pulse with frequencyspectrum coverage area over0.1-5THz and single pulse energy about2.5μJ has beenobtained. These experimental results demonstrated that laser-induced air-plasma cangenerate ultra-wideband, narrow pulse width, high peak power of THz radiation.