【作者】 秦应雄；

【导师】 李正佳； 唐霞辉；

【作者基本信息】 华中科技大学 ， 物理电子学， 2008， 博士

【摘要】 激光谐振腔是激光系统的核心部件,合理的谐振腔设计可以提高输出功率和改善光束质量。为了获得高功率和高光束质量的激光输出,论文对大增益区激光谐振腔进行了理论和实验研究。论文的主要内容包括:(1)建立了大菲涅尔系数激光谐振腔模式的数值计算模型,分析了不同有限元离散方法对模式计算精度和效率的影响,提出了一种等间距等面积的有限元离散方法。同时,提出了大菲涅尔系数激光谐振腔横模竞争占优模式判别方法,即光腔传输矩阵特征值γ的绝对值与激光光斑有效面积系数η0.05的乘积为模式鉴别因子σ,模式鉴别因子σ大的模式竞争占优势,并在多种激光谐振腔实验中验证了该方法的正确性。通过一个特定的光学聚焦系统,利用柯林斯公式建立了计算输出光束质量参数(包括聚焦光斑半径、光束远场发散角、M2因子等)的数值模型。(2)建立了一种基于二维衍射积分方程的非圆形光阑激光谐振腔的理论模型,方形光阑激光谐振腔的模式计算结果表明,除了存在分离变量方法获得规则分布模式外,还存在分离变量方法不能得到的旋转对称分布模式、十字架形分布模式和对角线对称分布模式。理论和实验研究了方形光阑、矩形光阑和三角形光阑激光谐振腔的光束特性,分别获得了TEM99模方形光斑、TEM0-11模矩形光斑和六边形光斑。对28mm×28mm方形光阑和直径为28mm圆形光阑进行了比较实验,在不增加发散角的情况下,方形光阑的激光输出功率高,在45#钢的激光相变淬火中获得了更加均匀的相变硬化区。(3)提出了一类全新的环形凹面镜激光谐振腔,系统地研究了不同光阑半径、不同曲率半径对输出光束的影响,并与平凹稳定腔进行比较。对于腔长为3m,曲率半径为15m的环形凹面镜激光谐振腔,当光阑半径为16mm时,菲涅尔系数高达8左右,获得了高光束质量的环形分布光束,其M2因子为1.1613,光束远场发散角(全角)为0.75mrad。环形凹面镜曲率半径变化对输出光束的光斑尺寸、远场发散角、聚焦特性,以及M2因子等影响不大。而当平凹稳定腔的反射镜曲率半径由10m变化到50m时,输出光束模式由TEM04模变化为TEM01模。(4)在2kW横流CO2激光器上进行了的相同光阑尺寸的环形凹面镜激光谐振腔、平凹稳定腔和平行平面腔的对比实验,在5A的放电电流激励下,分别获得了1820W的环形分布光束、1860W的TEM06模光斑和1000W的近似TEM03模光斑。环形分布光束以1m/min的扫描速度进行不锈钢的焊接,焊接熔深超过2.4mm。而平凹稳定腔和平行平面腔的输出光束不能进行深穿透焊接。(5)理论和实验研究了高功率横流CO2激光器折叠腔低阶模输出光束特性,提出了外桥内腔式折叠腔的动态补偿调整方法,解决了光腔失调问题,开发了高稳定性的N型三折激光谐振腔,成功应用于汽车安全气囊气体发生器三工位激光焊接系统中。更多还原

【Abstract】 Laser resonator is the core component of laser sysytem, and optimal design of laser resonator can improve laser power and beam quality. In this dissertation, in order to get laser beam with high power and high beam quality, theoretical and experimental studies on laser resonators with large gain zone are presented. The main contents are classified as follows.(1) The numerical model of high power laser resonator with big Fresnel Number is founded. The calculation precision and efficiency of different discrete finite element methods are analyzed, and a new discrete finite element method with characteristics of the same space and the same area to improve calculation efficiency is provided. A distinguishing method of dominant mode for transverse mode competition of laser resonator with big Fresnel Number is put forward, in which the mode distinguishing factorσis defined as the product of of absolute value of eigenvalueγof resonator transit matrix and the efficient area coefficientη0.05 of laser spot, the mode of(which have) the biggestσis dominant, the validity of which is tested in several experiments of laser resonators. By means of a optics focus system and Collins formula, model to calculate the beam quality parameter, including focus beam radius, far-field divergence angle, M2 factor etc, is established, which is benefit to evaluating beam quality.(2) Based on two- dimensional diffraction integral equation, the theoretical model of laser resonator with non-circle aperture is founded. The results of laser resonator with a square aperture show that there are some regular distribution mode, rotary symmetry distribution mode, cross distribution mode and diagonal symmetry distribution mode. The rotary symmetry distribution mode, cross distribution mode and diagonal symmetry distribution mode can not be abtained from Separation Variable Method. The experimentals on beam characteristics of laser resonators with square, rectangle and triangle aperture are studied. TEM99 mode square spot and TEM0-11 mode rectangle spot and hexagon spot are achieved respectively. The output beam characteristics of 28mm×28mm square aperture and circle aperture with diameter 28mm are compared, when a square aperture is used, laser output power is higher than circle aperture is used, but the divergence angle is same. The laser phase transformation hardening case on 45# steel caused by the square aperture beam is more uniform than circle aperture beam.(3) A new type of toric concave mirror laser resonator is presented. The effects of different aperture size and curvature radius on output beam property is discussed systematically. For a resonator with length of 3m, curvature radius of 15m, and aperture radius of 16mm, its Fresnel Number is about 8, and annular distribution mode with M2 factor of 1.1613 and far-field divergence angle (full Angle) of 0.75mrad is obtained. The change of curvature radius has no obvious effect on spot size of output beam, far-field divergence angle, focus spot size and M2 factor etc, whereas, when the curvature radius of concave mirror of plane-concave stable resonator is from 10m to 50m, the output beam mode of plane-concave stable resonator is from TEM04 to TEM01.(4) The experiments of toric concave mirror laser resonator, plane-concave stable resonator and parallel plane resonator with the same aperture size are done on HUST2000 high power transverse flow CO2 laser. Laser beam of 1820W with annular distribution, laser beam of 1860W with TEM06 distribution and laser beam of 1000W with approximate TEM03 distribution are got, respectively at discharge current of 5A. Scanning welding test on 316L stainless steel at 1m/imn velocity by annular distribution beam, the welding depth is more than 2.4mm, whereas the output beam of plane-concave stable resonator and parallel plane resonator can not be used for deep penetration laser welding.(5) Theoretical and experimental studies on property of lower-order mode laser beam of folded resonator are done. A dynamic adjustment method for folded resonator with structure of outside bridge and inside cavity is presented, so the problem of resonator malajustment is solved. A N-type three-folded resonator on transverse flow CO2 laser for industry is developed, which is applied to the laser welding system of gas generator body of automobile airbag.更多还原