Performance Studies On Pulsed THz Laser Pumped By Tunable TEA CO₂ Laser

Recently, with wide application of THz technology, more and more research institutes have dedicated to the technology which has great potential applications. The research on the most important part of THz technology-THz radiation sources has become increasingly important. However, new technologies of THz sources, such as various of tunable THz wave coherent radiation based on non-linear optical elements, quantum cascade lasers and free electron laser and so on, are difficult widely popular, because of weak output power, such expensive equipment or other shortcomings. Therefore, the traditional optically pumped THz lasers are still the more practical THz radiation sources, as the technology becomes more mature and highly reliable. So, it is not only the focus in this field and but also the purpose of this research subject, which is how to improve the pumping efficiency to obtain high-energy THz laser in order to meet the THz technology.In this thesis, optical pump source, THz oscillator and detection ate are studied theoretically and experimentally, which purposes are to increase pumping efficiency and detect accurately THz output laser parameters through improving pump laser power density, reforming THz laser cavity and exploring a variety of detection methods. Details are as follows:(1) Based on density matrix equation of three-level system in optically pumped THz laser, the interaction of strong pump field, strong THz laser field in resonance and the three-level system is analysed. The gain coefficient, quantum efficiency and other issues are discussed specificly.(2) In the research of optical pump source, the lines of the TEA CO2 laser are tuned by a grating, and two TEA CO2 lasers as the optical pump source, namely low-energy TEA CO2 laser and high-energy TEA CO2 laser, are studied experimental and analysed. The power density, pulse width, laser waveform are discussed with the different gas mixtures, as well as some laser parameters such as laser structure, stimulating energy, gas temperature effect the output characteristics of the TEA CO2 laser. Finally, the pump laser with high power density and narrow pulse width is obtained by using reasonable gas composition and optimizing laser parameters.(3) In this thesis, the design and structure of THz cavity are studied theoretically and experimentally. THz resonance cavity is composed of quartz tube, GaAs crystal as the input window, the quartz crystal as the output window and a sealed system. It has some advantages, for example simple design and using flexible, which can increase pump efficiency.(4) Upon completion of the THz cavity devices, using two tunable TEA CO2 laser as the pump source, respectively, focused on the work of pumping different substances (methanol gas and ammonia) generate THz laser. The THz output energy, characteristics and the energy conversion efficiency for different pump lines, pump energy, working gas pressure, cavity structure are experimented and analysed. The high THz laser energy is detected, using three methods:attenuation, calibration, and aperture. The THz laser energies are obtained over 100 mJ. The THz laser energy pumped by 10P(32) line is 300mJ.(5) Following the successful acquisition of high-energy THz laser, atmospheric transmission of THz laser and imaging applications are studied in this thesis. In the atmosphere, THz laser can be transported in short-range. In imaging, THz imaging is detected on metal objects with and without package, and the images are relatively clear. It proves the detecting effect is favorable on THz imaging on the metal objects.In the thesis, the pulsed THz laser pumped by the tunable TEA CO2 laser is developed independently. And the first high-energy THz laser are generated successfully in China, optimizing the pump light source, designing the reasonable THz cavity and using selectively different THz gain materials. Then, the imaging of metal objects by THz laser has a good detection result.