程成 

 

 

论文题目：气体激光动力学研究及大功率激光器优化设计     

 

作者简介: 程成，男，1953年11月出生，1999年07月师从浙江大学何赛灵教授，于2002年03月获博士学位。

                                       

                           

                                       

摘  要

           

l          气体激光(二氧化碳激光、铜蒸汽激光等)是激光大家族中应用最广、种类最多、发展较快的一种激光器。大功率二氧化碳激光器已经大量应用于汽车工业的焊接、切割、表面处理等;铜蒸汽激光已被成功应用于铀同位素分离、激光扫雷、纺织机高速摄影，等等。尽管如此，由于激光动力学过程的复杂性或其它原因，激光器的设计目前仍停留在传统的方法上，即局部的、待设计参量逐个逐个确定的、理论和实验数据相互补充的这样一种半经验的方法和水平上。由于激光器的应用极其广泛，全世界的气体激光器市场每年的份额达数百亿美元，由此带来的隐形损失是巨大和无法估量的。

本论文首次提出了一种全新的激光器设计概念，并由此发展了一套优化设计的方法和技术。其基本思路是：根据气体激光动力学理论和谐振腔理论，建立气体激光动力学方程和谐振腔方程，详细研究激光等离子体中激光能级粒子数密度、电子温度、电子密度、气体温度以及激光光子密度等时间空间行为，详细研究它们与宏观参量（例如放电电压、电流、气体压强等等）以及它们自己相互之间的关系。引入近年来发展较快的一种全局优化方法---遗传算法，以激光功率等为目标函数，应用反演的方法，即通过使激光功率的最大化来同时确定影响激光输出功率的多个参量的最优组合，从而避免了通常由局域化的“经验设计”而带来的巨大的隐性损失，极大地提高激光器件的性能和输出功率等。根据这个方法,先后对铜蒸汽激光（CVL）的LC放电电路、大口径CVL以及CVL系统（包括放电系统和光学系统）分别进行了优化; 对CO₂激光器谐振腔、三成份工作气体（CO₂、N₂、He）以及五成份工作气体（CO₂、N₂、He、Xe、H₂）参量分别进行了全局优化。通过实验，验证了优化设计的数据，获得了满意的结果。

技术关键于2000年12月申请了中国发明专利，并已在中国专利公报上公开。五成份气体参量的关键数据于2001年8月申请了另一个中国发明专利，亦已受理和公开。

 

l         论文主要包含三个部分。

第一部分为铜蒸汽激光器(CVL)。 概述了铜蒸汽激光的基本工作原理，简述了铜蒸汽激光辐射原子跃迁，论述了铜蒸汽激光研究的发展与现状。详细研究了激光的动力学过程，讨论和发展了一套激光动力学速率方程。通过周期性时间空间自洽的数值解法，系统研究了铜蒸汽激光脉冲过程以及弛豫过程中的激光行为。首次对两个方面的机理：充氢机理和“黑心“机理进行了深入研究，给出了最佳充氢量，提出了可消减激光“黑心”的措施。最后，阐述了铜蒸汽激光器的应用与展望。

第二部分为二氧化碳激光器。首先，讨论了二氧化碳激光粒子数的反转机理，包括电子碰撞激励、激发态N₂分子等共振转换激励过程，激光上下能级CO₂(00⁰1，10⁰0)以及最低能级CO₂(01¹0)的弛豫过程等几个方面。然后，对典型的封离型二氧化碳激光系统，讨论了小信号增益，饱和增益以及激光的输出功率。详细讨论了二氧化碳激光动力学过程，给出了激光振动-转动能级跃迁及其速率方程、激光等离子体电子密度和电子温度、五（或三）成份混合气体的气体温度。通过数值求解激光能级粒子数速率方程、等离子体电子密度、电子温度、混合气体温度方程和谐振腔方程等，研究了这些参量对激光输出功率的影响。

第三部分为优化设计与实验。在这个部分中，简单论述了遗传算法的基本原理，讨论了应用遗传算法来优化激光器的思想和方法，给出了优化大功率激光器的计算机程序框图，具体讨论了其中某些重要的设计程序。对铜蒸汽激光器LC放电电路、大口径激光器和激光器系统分别进行了优化; 对二氧化碳激光器谐振腔、三成份工作气体和五成份工作气体气压参量分别进行了优化,讨论了优化结果，介绍了相应的实验以及实验所获得的结果。

 

l         国外，对于激光器的优化，据2003年1月在美国召开的SPIE国际光学工程会议、每两周一次收到的Laser Focus World、每期的Optics and Laser Technology杂志的论文以及在网上的搜索，没有见到相关报道，近年来发展较快的一些全局优化技术（例如遗传算法）还没有完全进入激光器研究领域。据国际杂志（例如J. Physics D: Applied Physic、Optics and Laser Technology等）对我们的论文评语以及其他专家的评审意见，认为创新之处是：

1.       “遗传算法应用于激光器的优化是一项崭新的工作，其他人从未做过，这是一个突破。预料相当宽广的激光研究领域会很快应用这个方法，该项工作将会被许多研究者引用……”。

2.       “对于大功率气体激光器，从机理上和设计原理上去深入探讨，抓住了要害，是一个相当困难而又富有挑战性的重要课题。由此使人们更深入地认识了CVL和CO₂激光的一些动力学过程，例如：CO分子对CO₂激光能级有重要的激励作用、是电子密度而不是电子温度对CO₂激光光强有较灵敏的影响等等，从而从本质上提高气体激光器的转换效率、输出功率以及光束品质等”。

3.       “将理论分析的结果，应用于相当成熟的CO₂激光器改进，获得了很好的实验结果，证明上述理论与设计方法是正确的和有价值的”。

 

l     本论文提出和发展的方法和技术，可方便地推广应用到其它光器件上，例如：半导体激光、固体激光、射频激励CO₂激光等， 也可应用到光放大器（例如掺铒（铥、镱）光纤放大器）器件或系统上去，适应更大功率、密集波分复用、激光测量以及其它需求。由于激光器谐振腔的相似性，估计对谐振腔的优化将可以很快地获得结果。但对于其它参量的优化，将会费时多日。鉴于激光器应用的广泛性以及它在工业生产和科研部门中的重要性，由优化带来的潜在的市场前景和巨大的经济效益，难以估量。

l      

Abstract

 

l          Gas lasers, such as CO₂ lasers and copper vapor lasers (CVLs), have wide applications and have developed rapidly over the past years. High-power CO₂ lasers have been used substantively into welding, incising and surface treatment in e.g. automobile industries. CVLs have been successfully applied to separate isotopes, detect mines and photograph high-speed objects. However, due to the complexity of the gas laser kinetics and other factors, the design of these lasers still remains in a conventional technology, or in a semi-empirical way, in which the design parameters are determined locally, in a one-by-one fashion through comparing experiments and theories. The world market for gas lasers is tens of billion dollars per year, and the lack of good designs could cause tremendous losses.

In this PhD thesis, a new concept of designs for lasers is presented, and the method of optimal designs is developed. First, the gas laser kinetics equations are established according to the laser kinetics theory and the resonator theory. The spacetime behaviors of the laser plasma kinetics are studied, including the populations of the laser levels, the electron temperature, the electron density, the gas particle energy and the laser photon density. The relations between the kinetics parameters and the macro parameters (e.g., discharge voltages, currents, gas pressures, etc.) are investigated in details. The genetic algorithm, a global optimization method that has been developed quickly in recent years, is introduced into the studies. Taking the laser power as an objective function, the maximum laser power can be obtained by utilizing an inverse method, i.e. by searching the optimal kinetics/macro parameters that give a high laser power. Therefore, the “empirical design” can be avoided, and the laser performance can be improved significantly. Using this method, the LC circuit of CVLs, the large bore CVLs and the CVL systems (including the discharge system and the optical system) are optimized. The resonator of the CO₂ lasers, the gases of three elements (CO₂, N₂ and He) and five elements (CO₂, N₂, He, Xe and H₂) are also globally optimized. Finally, the optimal designs are validated by experiments, and the results are satisfactory.    

The key technologies were applied for Chinese invention patent on Dec. 2000, and have been published in Chinese Patent Bulletin. Another invention patent on five gas elements was applied on Aug. 2001 and published subsequently.

 

l         The PhD thesis consists of three parts.

        The first part is about copper vapor lasers (CVLs). We begin with the discussion on the atomic radiation transitions in the CVLs, followed by the review of the progresses in the CVLs in recent years. We then study in details the lasing kinetics mechanisms, the additive hydrogen into the CVLs and the "black center" of the CVLs with a large bore. Applications and developments of the CVLs in the future are summarized.

        The second part is about CO₂ lasers. We first discuss the mechanisms of the population inversion, including excitations and relaxations (electron collision excitation, resonance transition excitation impacted by excited N₂ molecules, the relaxation of both the lower-upper laser levels and the lowest level of CO₂). Some important parameters, such as the small signal gain, the saturation gain and the laser output power, are described for a typical sealed-off CO₂ laser. We then discuss the vibrational-rotational level transitions and the population rate equations for the laser. In addition, electron temperature, electron density and the mixtured gas temperature in the laser plasma are investigated in details.

       The third part is on the optimal designs and experiments of gas lasers. The principles of the genetic algorithm (GA) are described, and a method for applying GA to optimize the lasers is presented. A program is presented for obtaining the maximum laser power. The method is used to optimize the CVL and the CO₂ laser, including LC circuit parameters of the CVL, the CVL system, the lasing resonator and the gas components mixed in the CO₂ laser tube. A CO₂ laser with the optimized resonator was made and verified by experiments.

 

l          For the global optimization of lasers, there is no available reports in the SPIE International Conference on lasers (e.g. in San Jose, USA, Jan. 2003), Laser Focus World received every two-weeks, Optics and Laser Technology received every month and the Internet. In other words, the optimization technology of the genetic algorithm developed quickly in recent years has not been applied in the area of laser researches considered in the present PhD thesis. According to the review reports for our manuscripts submitted to the international journals (J. Physics D: Applied Physic, Optics and Laser Technology), the novelty of our works can be concluded as follows:

1.           “The application of the genetic algorithm to optimization represents a new method of optimization of lasers. I have not seen this method applied in this area before. It is anticipated that this method will be applied by other researchers in this area of laser research, and quite possibly to a wider range of laser analyses. …the results are useful to other researchers and may well be referenced by researchers in future work”.

2.           “It is a very difficult, however, challenging subject probed deeply from mechanisms and design principles. By it some of kinetics mechanisms, such as CO molecules having an important effect on CO₂ laser levels; the electron density in spite of the electron temperature having a crucial effect on the laser intensity, are realized more clearly. Therefore, the laser efficiency, the output power and the beam quality can be increased in essence”. 

3.           “Applying the results of theoretical analysis to the improvement on the CO₂ lasers obtains available experimental results, which proves that the theory described above and the design method are correct and valuable”.

 

l                    The method and technique presented in this PhD thesis can be extended and applied to other optoelectronic devices, such as semiconductor lasers, solid lasers and radio-frequency pumping CO₂ lasers, and also optical amplifiers (such as erbium-doped fiber amplifiers). For the resonators of other lasers, the optimal results can be easily obtained with the present method due to the similarities of these resonators. However, optimization of the other parameters may take more time. The market potentialities and benefits that the laser optimization can bring are huge, owing to their wide applications and important roles in industries, technologies and sciences.

 

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