光束控制系统热效应与球差对激光光束质量的影响

采用Strehl比、桶中功率、β参数和束宽描述远场光束质量，研究了光束控制系统热效应和球差对远场激光光束质量的影响.用四维仿真程序作了详细计算，给出了数值计算结果并进行了物理分析.研究表明，传输通道的热效应和正球差会使远场光强分布扩展，可聚焦能力下降，光束质量变差.有意义的是，当激光发射功率达到一定值时，合适选取负球差会在几何焦面得到比无球差时更高的峰值光强和更好的可聚焦能力.但在以前的工作中，由于焦移，类似的结论是在实际焦面上得出的.非线性热晕的存在是其物理原因，对此用数值计算例子作了说明. 关键词： 高功率激光 热效应 光束控制系统 球差     

宽频带光束非线性热像效应的实验研究

实验研究宽频带脉冲光束通过非线性介质时的非线性热像的形成过程,揭示了光束入射功率和脉冲宽度、介质厚度等因素对热像光强和位置的影响. 结果表明,与单纵模激光束一样,在宽频带脉冲光束情形下,非线性介质上游元件的模糊斑调制也可导致在下游共轭位置形成热像. 热像光强随入射光束功率的增加而增加;若脉冲能量一定,则热像光强随脉冲宽度的增加而减弱;若脉冲峰值功率一定,则热像光强随脉冲宽度的增加而增强. 关键词： 高功率激光 热像 宽带光束 小尺度自聚焦     

横向激发线偏振高斯光束的非傍轴传输

文章提出一种横向激发的任意线偏振高斯光束,该光束场量的模与传统的任意线偏振高斯光束具有相同的模,在源场区,其满足自由空间无源场的场方程。同时利用瑞利—索末菲衍射积分对该高斯光束的非傍轴传输特性进行了解析研究,给出了空间光强分布、束腰和远场发散角的解析通式,发现该类光束的远场光强呈一平顶空心光束。     

固体激光器光束合成特性分析

光束合成是研制高功率、高光束质量固体激光器的必由途径.建立板条介质固体激光器阵列光束合成的理论模型,从子束间距、整体相位差、偏振态、随机像差等方面,分析了合成光束的远场特性.通过数值模拟可以看到,子束间距决定了合成光束远场光斑的衍射旁瓣的大小和多少;子束间整体相位差决定着合成光束远场中心光斑是否分离,并影响着峰值光强和...     

划痕型缺陷对光场质量的影响

为了研究高功率激光系统中划痕型缺陷对光场质量的影响,采用分步傅里叶算法对非线性近轴波动方程进行求解,模拟分析了不同划痕参数(长度、宽度、深度)下,元件内部、光束近场、元件后传输光场以及光束远场的光强分布情况。数值模拟结果表明:随着划痕长度、宽度或深度的增加,元件内部以及元件后传输光场的峰值强度和对比度均会相应增强;光束近场的光强对比度也会略微增大;对于光束远场的强度分布,与划痕宽度方向所对应的频谱能量会不断增强。该项研究工作可以为划痕检验标准的制定提供一定的定量分析依据。     

大气湍流和热晕效应对列阵合成激光传输特性和光束质量影响的理论研究进展

介绍了大气湍流效应和热晕效应对列阵合成激光传输特性和光束质量影响的理论研究进展。主要介绍了合成激光在大气中传输的解析和数值模拟研究方法,大气湍流效应对列阵合成光束的光强分布、远场发散角、方向性、曲率半径和湍流距离的影响,以及大气热晕效应对列阵合成光束的光强分布、传输效率、重心偏移、热晕时间尺度和焦移的影响。研究结果表明,大气湍流效应和热晕效应对合成激光光束质量的影响与光束合成方式、合成光束参数以及大气参数密切相关。     

光纤激光相干阵列远场光强分布的影响分析

在光纤激光相干组束中,受到各种因素的影响,各个单元激光器出射激光的相位、振幅和偏振态不能保持一致,并随着时间而变化,均会降低相干组束后的光束质量、减弱光束中心的峰值光强和中心区域内的功率.针对光纤激光相干组束中每个单元光束的出射时的相位、振幅和偏振方向扰动对相干组束远场光强分布的影响进行了理论分析,并给出数值计算结果.结果表明远场光强分布的图形、峰值光强度和中心区域内的功率,受到各因素的影响,都会不同程度的变差或减弱.     

激光通道传输热特性对远场光束质量的影响

 通过仿真计算分析了激光在光束控制系统通道内传输所产生的热效应及其对远场光束质量的影响。激光传播由近轴波方程描述，用快速傅里叶变换技术求解；激光热效应引起的流场密度变化采用完全Navier-Stokes方程计算。计算给出了不同波长、不同吸收系数条件下的远场光斑情况。计算结果表明，在典型的工作条件和状态下，较高能量激光在光束控制系统通道内产生的热效应影响不容忽视，它会明显降低远场目标处的能量集中度，增大光斑的发散。     

一维激光阵列相干合束远场特性的研究

利用傅里叶光学方法,获得了相干合束远场光强的解析表达式,数值模拟和实验研究了一维锁相二极管激光阵列的相干合束远场特性。研究表明:随着参与合束的发光单元数目的增加,尖峰变锐;同相模的光能集中在更小的立体角内,使得相干合束的亮度增加;异相模的光束能量较为分散,其峰值光强有所降低。系统抖动引起远场峰值光强的降低,破坏了远场的对称性和光束质量。相干合束发光单元数目越多,系统抖动对远场的影响便可以忽略。     

折叠-组合腔不同曲率输出镜近场光束分析

多维轴对称折叠组合腔由一个或多个折叠腔和一个共轴球面腔组合而成,共用一个平凹输出镜产生并合光束。为了得到更稳定、更高功率的激光输出,基于矩阵理论建立了基模高斯光束斜入射通过凹面镜的理论模型,并对输出光束的近场光强分布和光斑大小做了非相干并合和相干并合的数值计算和模拟。结果表明随着输出镜曲率半径的减小,光强峰值和光斑大小有轻微减小,稳定性更高。     

Propagation offset characteristics of annular laser beams from confocal unstable resonators through the natural atmosphere

Based on the laser field from a positive confocal unstable resonator (ab initio), the propagation characteristics of the beam through atmosphere are investigated by means of fast Fourier transform (FFT), and it is assumed that the refractive index and absorption of atmosphere are only decided by the air molecules. Considering the thermal effects of the laser beam and wind velocity of air, we evaluate the beam offsets, beam spreading, and beam qualities of the laser field transmitting from the resonator through the natural atmosphere. Meanwhile, the far-field intensity distributions of the laser beam with various intensities in the atmosphere are obtained. It is shown that the far-field diffraction patterns are deformed due to absorption of air and wind velocity. β parameter and Strehl ratio, which are usually used for high-power lasers, are introduced to estimate the beam quality characteristics. With increasing the intensity of the beam, the far-field diagram patterns are outspreaded, the peak intensities reduced, the peak centers shifted, and the beam quality characteristics greatly degraded.     

激光窗口热透镜效应对光束质量的影响

 分析了高功率激光通过窗口时，引起的窗口热透镜效应及其对激光光束质量的影响。用冲量定理方法，推导了二维热传导方程的广义傅里叶级数解，然后按照平面热应力问题，运用有限元方法数值计算热变形。考虑热变形、折射率随温度变化，忽略光弹性效应，解出出光面的波前相差分布，讨论了远场光束质量。以空心环形光束为例，计算了熔融石英(SiO₂)和白宝石(Al₂O₃)两种常用窗口的温升分布、热变形、波前相差和光束质量，分析了冷却、辐照时间、光强分布的空间梯度和各种材料参数对光束质量的影响。     

Numerical investigation of beam quality in unstable optical resonators and the effects of practical imperfections

Field characteristics and beam quality of confocal positive branch unstable resonator under some practical imperfections has been investigated. Based on Fresnel-Kirchhoff integral, the near and far field intensity of the resonator was calculated numerically. Using the second order moment method, the beam quality factor, M² of the resonator was also calculated. Variety of practical distorting affects, mainly misalignment, off-axis, and thermal deflection of the mirrors, as well as gain non-uniformity were considered in these calculations. The results show that the beam quality factor is more sensitive to the off-axis and misalignment distortions while the non-uniform gain has significant effect only for unusual conditions. The effects of geometrical parameters, i.e. Fresnel number as well as magnification factor on the beam quality were also investigated.     

Optical resonator for high-power transverse flow CO2 lasers

A new design of the U-type resonator is described. In this way, a laser beam with symmetrical intensity profile (regarding to a symmetry plane) can be extracted from an active medium that exhibits gain asymmetry along one of the transverse directions. The whole area of the active medium cross-section can be used, and consequently the laser efficiency will be increased. This resonator structure was applied for efficiency power extraction (as a low order TEM modes laser beam) from a DC excited transverse flow CO₂ laser with cylindrical geometry. Although the cross-section area of the discharge was entirely used (including the cathode fall region), a symmetrical intensity profile of the laser beam (regarding to the two orthogonal symmetry planes) was obtained in the near field as well as in the far field; the gain asymmetry along the flow direction was compensated by the gas circulation fluidodynamical circuit with two counterflowing discharge channels. A double-U optical resonator was introduced in order to provide a laser beam with axial symmetry.For the practical construction of these two types of optical resonators we have developed two new types of 90° deflection elements: the first one, which does not reverse the image (and which has the properties of the pentaprism), and the second one, which rotates the image with 90° angle. Both elements exhibit good focusability if they are equipped with two concave mirrors.     

大口径超薄镜面对畸变波前的补偿

针对低频波前对光束质量的影响,优化设计了一个口径为300mm、半径与厚度比为20的薄型光学镜面,被用来对低频随机畸变波前进行补偿,并详细分析了补偿前后的光束质量变化。结果表明:无论是低频随机相位空间尺度参数变化,还是低频相位振幅参数发生改变,低频畸变经过补偿后,畸变波前明显变小,相位梯度在整个空间周期内都降低,远场强度的峰值增加,能量更集中,且能量集中度最高提高约50%。     

LDA侧面泵浦Nd:YAG激光器的热效应分析

 在高斯光强近似下对泵浦LD光强分布模型进行修正，建立了LDA侧面泵浦固体激光介质内热源分布的数值模型。用有限元法计算模拟了三角均匀分布侧面泵浦结构激光棒瞬态温升过程及稳态温度分布情况。讨论比较了泵浦源的高斯强度近似和均匀强度近似下激光棒内温度分布情况，并对激光棒的类热透镜的焦距进行实验测量。实验和数值计算说明了LDA泵浦结构和冷却场的非均匀分布使实际温度场偏离均匀泵浦时的二次曲线分布模型，激光晶体热效应产生的类透镜会聚作用的不对称导致了激光器输出光束质量在x，y方向上的不同。     

高功率激光器的光谱合束技术研究北大核心CSCD

为了获得高功率激光束,提出利用双色镜对典型波长2种不同类型(脉冲、连续)的高能激光进行合束,以实现高功率高能量激光输出。通过对双色镜的热效应和合束光斑远场激光参数进行仿真分析计算,热效应仿真结果表明,在单束激光10 kW、光斑直径15 mm条件下,双色镜面型热形变量均方根值为0.004λ(λ=632.8 nm),满足光学元件面型小于0.03λ精度要求。搭建了一套基于双色镜的光谱合束系统,并分别进行了高功率连续激光与高功率连续激光、高功率连续激光与高能量脉冲激光的合束试验,合束效率高于95%。试验结果表明,光谱合束可有效应用于高能激光领域。     

On the focusing limit of high-power femtosecond laser pulse propagation in air

Focused propagation of high-power femtosecond laser radiation in air is considered. Based on numerical solution of the nonlinear Schr?dinger equation for complex envelope of light wave electric field, evolution of the beam effective radius is studied. The dependence of the effective (rms) size of a focal spot and the maximally achievable intensity of laser radiation at focal waist on the initial pulse power is established. It is shown that focal spot of tightly focused intensive ultrashort laser radiation can change its size during the pulse passage through the beam waist. This is the consequence of pulse intensity clamping in region of beam focusing caused by gas photoionization and plasma producing. This may prevent laser intensity from its further growth in the focal region and arrest the transversal compression of the beam in the linear focus as a whole.     

强激光远场光束质量参数的测试

 提出漫射红外成像-多点标校测量方法,用于测量强激光远场光束质量参数。在激光远场距离处设置漫反射靶板,用成像探测器摄取经靶面漫射的脉冲强激光光斑图像;在靶面中心处挖小孔,孔后放置能量探测器实时测量激光脉宽和峰值功率。同时对整个激光光斑图像进行能量定标,进而得出远场脉冲强激光的实际空间能量/功率分布、总能量,以及相应的光束质量参数。应用该测量方法,对高能TEA CO₂激光进行测量研究,测得其远场光束截面半径为80.2 mm,发散角为1.55 mrad。