Studies On Generating Mechanism Of Optically Pumped Pulsed Terahertz Laser And Properties Of Key Components

So far, the technique of optically pumped terahertz (THz) is still one of the most powerful ways to generate coherent terahertz radiation. The technique not only can obtain narrow laser line, but also can gain higher THz output energy besides free-electron laser. The technique is realized by using high-energy TEA CO2 laser line interacting with organic molecule to obtain THz pulse laser.In the dissertation, the theories and the experiments related with THz laser have been gradually discussed from the mechanism of THz radiation, investigations on optically pumped source, the design and the properties analysis of the input and the output couplers, to the experimental investigations on THz laser.The generation mechanism of continuous-wave (CW) THz laser has been firstly theoretically analysed based on the rate equations. After analyzing the difference between CW and pulsed THz laser, a theory model of pulsed THz laser based on the rate equations has been proposed. Then, in order to study the transients in a pulsed THz laser, the Maxwell-Bloch equations based density matrix have been solved to investigate the THz laser setup in a two-level system and the THz decay in a three-level system. In the process of deduction, many errors in the past documents have been corrected, thus it helps to establish a completed THz laser theory.A theoretical model of pulsed THz laser based on the rate equations dealing with strong pump laser field and intense THz radiation field on resonance interacting with a homogeneously broadened three-level molecular system has been solved analytically. A discussion is presented on the properties of the solutions for time-independent and time-dependent absorption coefficients, gain coefficients, quantum efficiency, and pump intensity range and so on. On the other hand, the energy absorbed from pump light has been deduced by solving time-dependent rate equations. To demonstrate the rationality of the theory model, the data from the past documents have been used in the equations. It is demonstrated that these theoretical results agree well with the experimental data. In short, it is helpful to understand deeply the work mechanism of the THz laser by investigating the properties.To investigate the feasibility and simplicity of optical pumping source in THz laser system, optical amplification and optical injection-locking have been studied experimentally in the article. The results of two-pass and four-pass light amplification show that two-pass light amplification not only can be more effective to realize the amplification of the seed light but also can obtain an amplification factor of 30, however it is impossible to achieve expected result for the four-pass light amplification. With respect to the experimental results, the phenomena can be explained according to parasitic oscillation. Compared with the disadvantages of varied laser line, multi-mode operation and low peak power, injection-locking has great merit in single laser line, single longitudinal mode and high peak power. In our experiment, these advantages have been observed. However, the single longitudinal mode can not be achieved in a successful probability of 100% due to the frequency detuning between injected light and cavity mode, weak injection intensity and no optical isolation device. Compared with the experiments in the past documents, it is demonstrate that the factors mentioned above are the key to limit injection-locking to be successful. Considering the disadvantageous factors of the two experiment schemes, a feasible and simple scheme using tunable TEA CO2 laser as optical pumping source is determined.The input and output couplers are the important optical components in cavity THz laser. Unlike the past design schemes such as hole couplers, a novel coupler with capacitive mesh on thick substrate has been proposed. At the same time, the coupler acts as sealed window. Using the transmission line model, a nickel capacitive mesh output coupler with high-resistivity silicon substrate and a nickel capacitive mesh input coupler with ZnSe substrate have been designed and fabricated. The transmittance spectrum of output coupler agrees well with numerical result as a resonance frequency ofω0=0.85 is selected. However, it needs to be improved with respect to the input coupler. Considering the etalon effect and the thick-dependence of the transmittance occurring frequently in the coupler with the F-P shaped substrate, a novel output coupler with wedged substrate has been proposed to suppress the etalon effect and obtain a flat transmittance spectrum. The design concept has been approved to be feasible after numerical analysis.Combining the theoretical and experimental researches mentioned above, a home-made TEA CO2 laser with magnetic switch controlled discharge circuit has been utilized to interact with ammonia gas molecules in an F-P THz cavity. Finally, a THz laser with a frequency of 3.33THz, a photon conversion efficiency of 6.5% and an output energy of 1.35mJ has been developed successfully by changing some parameters such as mesh period and gap of the output coupler, operating gas pressure of THz laser, pump energy, and pump laser line. On the other hand some THz laser properties have been investigated, including:(1) the delay between start of pump pulse and start of THz pulse (2) the relations between pump energy and THz emitting energy (3) the dependence of optimum THz output on NH3 gas pressure under a particular pump intensity (4) the influence of the pump intensity on the optimum NH3 gas pressure suitable for the optimum THz output (5) the minimum pump intensity and the THz lasing threshold (6) THz atmospheric transmission.