摘要
随着TEACO_2激光器技术的日益成熟,它已经在工业加工、军事、环境保护和科学研究等许多领域中被广泛应用。最近国际上的热门研究领域激光推进,TEACO_2激光器被认为是最可选激光器之一。光束质量是影响这种远距离传输应用的关键因素。相位共轭自适应谐振腔被认为是改善TEACO_2激光器光束质量的最佳方案。为获得高光束质量的高功率TEACO_2激光,本文对相位共轭自适应谐振腔的基本原理以及这种谐振腔必需的高光束质量的泵浦光源的产生方法进行了理论和实验探索。
本文首先介绍了相位共轭基本原理和关键技术,并重点介绍了四波混频产生相位共轭的基本原理和相位共轭谐振腔的特性。实现相位共轭首先必须得到高光束质量的泵浦光。而猫眼谐振腔因其良好的逆向反射特性,被认为是得到高光束质量泵浦光的最有效的方法之一。因此,本文随后从理论上分析研究了猫眼谐振腔的特性,同时还根据实验条件进行了理论计算。
通过比较分析各种光束质量评价方法后,确定采用衍射极限倍数β作为实验中光束质量评价标准,并采用聚焦光束测量法,利用分束采样红外CCD成像的方法测量TEA CO_2激光器猫眼谐振腔的光束质量。光束质量数据远优于平面镜-圆锥镜谐振腔、平凹腔的结果。对低输出能量的原因也进行了探讨,结论是大气衰减损耗对实验的影响基本可以忽略,衍射损耗也不足以引起低能量输出,但是平面反射镜的不稳定性引起的损耗则相当可观,应该尽量减小。
最后,利用光子牵引探头测量了猫眼谐振腔的激光脉冲时间波形。通过和平凹腔对比,发现猫眼谐振腔因其腔长长、损耗大,激光脉冲开始时刻与放电结束时刻的间隔比平凹腔的要长,同时实验中还通过傅立叶变换证实了“拍频”现象。
With the improvement of its performance, TEA CO2 laser has been widely applied in many areas such as processing, defense, environment protection and scientific research. It is considered as the most favorable laser for the application of laser propulsion which is increasingly studied recently. Beam quality is a key factor that has significant influence on this kind of energy transportation application, and it is recognized that phase conjugation cavity would improve the beam quality of TEA CO2 laser effectively. For the purpose of high quality high power TEA CO2 laser, the principle of phase conjugation cavity and the method to obtain high quality pump beams which are required by this kind of cavity are both theoretically and experimentally studied.
Firstly, the principles and key technologies of phase conjugation are introduced. Then, the principles of four wave mixing technology and the characteristics of phase conjugation cavity are discussed. The prerequisite of realizing phase conjugation beam is to obtain pump beams with high beam quality and appropriate energy. It is recognized that cat's-eye cavity could be one of the most effective methods to obtain appropriate pump beams due to its excellent retro-reflectivity. Thus, the characteristics of cat's-eye cavity are discussed, and theoretical simulations are also made with the specific conditions of our experiments.
Several methods of beam quality valuation are compared, and the times of diffraction limitβis chosen as the criterion applied in the experiments. Theβfactor of the cat's-eye cavity of TEA CO2 laser is measured by infrared CCD using focused beam measurement method. The b factors are much better than those of plano-conic cavity and plano-concave cavity. The reasons for low pulse energy are also discussed. It is concluded that the influence of atmosphere attenuation could be ignored in the experiment and the diffraction loss would not be the reason of low energy also. However, the influence of maladjustments of the plano-plano output coupler should be minimized as small as possible.
At last, the temporal waveform of laser pulse is measured by a photon drag detector. The interval between the end of discharge and the beginning of the laser pulse of cat's-eye cavity is longer than that of the plano-concave cavity, which may be the results of high loss of the cat’s-eye cavity. Besides, the phenomenon of“beat frequency”is also confirmed by the experiments.
引文
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