【作者】 祁春超；

【导师】 程祖海； 左都罗；

【作者基本信息】 华中科技大学 ， 光电信息工程， 2010， 博士

【摘要】 迄今为止,光泵太赫兹(THz)脉冲激光技术仍是获取相干太赫兹辐射源的最有效的方式之一。该技术不仅可以获得窄线宽的激光支线,而且是除了自由电子激光之外的另外一种有望获得高能量太赫兹激光源的有效方式。该技术主要采用高能脉冲TEA(Transversely Excited Atmospheric) CO2激光器的适当支线与有机增益分子相互作用而获取太赫兹脉冲激光。本论文沿着太赫兹激光产生机理、光泵源的研究、输入输出耦合器的设计与特性分析、太赫兹激光器的实验研究这样一条主线从理论和实验两个方面逐一展开论述和讨论。从理论上首先对基于速率方程的连续太赫兹激光产生机理进行了简单的介绍,在分析了连续与脉冲太赫兹激光产生的差别后提出了基于速率方程的脉冲太赫兹激光产生模型；考虑到脉冲太赫兹激光的瞬态效应,然后又利用Maxwell-Bloch方程详细地分析了基于二能级模型的太赫兹激光建立过程以及三能级模型的太赫兹激光衰减过程。在讨论中,对过去理论中的一些问题点进行了完善,为建立一套完整的太赫兹激光理论奠定了坚实的基础。基于速率方程的脉冲太赫兹激光模型,解析求解了强泵浦场和强受激辐射太赫兹激光场在共振情况下与一个均匀加宽的三能级系统相互作用的问题。通过对含时与非含时方程的解析求解具体讨论了如吸收与增益系数、量子效率、泵浦强度范围等一系列问题。同时通过求解含时速率方程组,推导出了太赫兹增益分子吸收泵浦能量的表达式。为了验证理论的正确性,我们采用过去文献中的实验数据对推导出的结论给予了充分论证,结果表明理论推导与实验结果相当吻合。此外,对这些特性的研究有利于更加深入地了解和掌握太赫兹激光的工作机理。为了探讨太赫兹激光系统中光泵源的可行性、简易性问题,本论文从光放大和光注入锁定两个方面进行了实验研究。双程光放大和四程光放大的实验结果表明双程光放大可以部分有效地实现种子光的支线放大,而且可以获得近30倍的放大系数,然而四程光放大则完全达不到预期的效果。针对实验结果,我们从自激振荡的角度清晰地解释了产生这些现象的原因。与光放大的支线跳动、多模运转、输出激光峰值功率低的特点相比,注入锁定具有单支线、单纵模、输出激光峰值功率高的优点。在我们的光注入锁定实验中,这些优点得到很好体现。但由于注入激光频率与腔模式频率的失谐、较弱的注入光强、以及无光隔离措施等因素导致了单纵模的获取并不能达到100%的成功概率。通过和过去实验对比性分析,最终确定了上述因素是限制注入锁定实验成功的关键。综合上述不利因素后,本论文最终确定了利用可调谐TEA CO2激光直接泵浦产生太赫兹的实验方案。输入输出耦合器是腔式太赫兹激光器中重要的光学元件。本论文摒弃了过去传统的小孔输入输出耦合器,而采用基于厚衬底的电容性网栅输入输出耦合器,并且这些输入输出耦合器同时充当太赫兹激光器的封闭窗口。基于传输线理论模型,设计和制作了基于高阻硅衬底的镍金属电容性网栅输出耦合器以及基于ZnSe衬底的镍金属电容性网栅输入耦合器。输出耦合器的实验结果和理论预言在归一化共振频率ω0=0.85时比较吻合,而输入耦合器还有待改进。针对F-P双平衬底极易出现的etalon效应以及透射率对衬底厚度的依赖性,本论文提出了利用楔形衬底抑制etalon效应进而获得精确平坦透射率谱的想法,并且在理论上验证了其可行性。鉴于上述理论和实验研究,我们利用具有磁开关控制放电电路的TEA CO2激光器产生的短脉冲激光束与腔式F-P型太赫兹激光器中的NH3分子相互作用。并通过改变输出耦合器网栅参量、太赫兹增益物质工作气压、泵浦能量、泵浦支线等参量,最终获得了频率为3.33THz、最大光子转换效率为6.5%、最大能量为1.35mJ的太赫兹激光输出。同时对太赫兹工作特性进行了研究,这主要包括：(1)泵浦脉冲和太赫兹脉冲建立时刻的延时关系(2)泵浦能量与太赫兹辐射能量的关系(3)一定泵浦强度下,最佳太赫兹输出对NH3气压的依赖性(4)泵浦强度对最大太赫兹输出所对应的最佳工作气压的影响(5)最小泵浦强度和太赫兹激光阈值的确定(6)太赫兹大气传输。更多还原

【Abstract】 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.更多还原