Propagation of a partially coherent beam from an unstable resonator through turbulent atmosphere

The influence of atmospheric turbulence on the propagation of a partially coherent beam from an unstable resonator was studied numerically. The resonant mode of the unstable resonator is obtained by iterative calculation using the Huygens–Fresnel formula. Also, using the extended Huygens–Fresnel integral, the intensity distribution of a propagating laser beam is calculated for different conditions. The influence of turbulence on the profile of partially coherent beams of an unstable resonator is studied. The effects of geometrical parameters of the resonator on the far-field beam profile are investigated. The results show that an unstable resonator with higher magnification has a superior far-field beam profile under partial coherency and turbulence conditions.     

Experimental estimation of the quality of a laser beam

A technique for measurement of the integral parameter of real laser beams is proposed. The technique makes it possible to arrive at a quantitative estimation of the deviation of the beam’s envelope of a surface from the ideal envelope of the surface of a Hermitian-Gaussian beam. The values of this parameter are found experimentally by the Rayleigh method, taking into account (or ignoring) the particular forming optical system. Translated from Izmeritel’naya Tekhnika, No. 3, pp. 27–29, March, 2009.     

All-plastic organic dye laser with distributed feedback resonator structure

We demonstrated an all-plastic waveguide organic dye laser with distributed feedback (DFB) resonator. We fabricated DFB structure on a surface of SU-8 2002 photoresist polymer using the interference of two beams of a frequency-tripled Nd:YAG pulse laser at 355 nm. The typical grating pitch of fabricated DFB structure was 190, 380 and 570 nm, corresponding to the number of mode m, and each corresponding grating amplitude was ca. 1, 4-5, and 25 nm, respectively. Shallow amplitude of 1 or 4-5 nm is ascribed to the wide incidence angle of the interference beams. Threshold of lasing for m = 3 is lower than that for m = 1 or m = 2 depending on the emission wavelength. DFB structure with the smaller amplitude of gratings at m = 1 and 2 is required for the higher threshold of laser emission. By controlling the grating pitch with nanometer scale, we can tune the wavelength of laser emission with 40 to 60 nm tunability. Effective energy transfer via nonradiative Förster transfer mechanism assists in lowering the threshold of laser emission.     

Influence of laser array performance on spectrally combined beam

Abstract

Incoherent spectral beam combining (SBC) of multiple laser beams is accomplished along the emitters’ arraying direction. Considering that the output beams from a laser array (LA) usually have deflection angles, positional displacements and divergence angles even after being collimated, a propagation model of SBC systems based on multilayer dielectric gratings has been built up. On the basis, properties of the spectrally combined beam affected by parameters of the LA have been discussed in detail. Simulation results show that with the increase in the deflection angle, both the power and the beam quality of the combined beam degrade dramatically. The positional displacement has little impact on the intensity distribution and the beam quality of combined beam but change the wavelength composition of the combined beam. The divergence angle strongly affects the intensity distribution and the beam quality of the combined beam. Additionally, the effect of the deflection angle on the output beam quality is more obvious and may shift the beam spot when comparing with that of the divergence angle.     

圆形金属腔旋流二氧化碳激光器

现有的工业ＣＯ２激光器,主要为横流,纵流和扩散冷却三种,章提出一种新型激光器,即圆形金属腔ＣＯ２激光器,该激光器的腔体由多个等距,同轴安装的空腔单元组成,每个腔单元有多个流道,放电在流道与腔体接口处产生,气流将放电及被其激活的粒子吹入腔内,形成增益,输电激光,章研究了电极结构,金属腔壁对工作气体的冷却作用和胺内的增益分布,研究结果表明,多通道放电吹入和腔内旋流,有利于腔内的均匀放电,金属腔壁有利于腔内对流冷却,降低对气速度的要求,从而缩小激光器的体积,相邻的腔单元相对转动适当角度,可以提高腔内增益分布的圆周均匀性,从而获得光束质量好的激光束。     

Gaussian laser beam transformation into an optical vortex beam by helical lens

In this article, we investigate the Fresnel diffraction characteristics of the hybrid optical element which is a combination of a spiral phase plate (SPP) with topological charge p and a thin lens with focal length f, named the helical lens (HL). As incident a Gaussian laser beam is treated, having its waist a distance ζ from the HL plane and its axis passing through the centre of the HL. It is shown that the SPP introduces a phase singularity of pth order to the incident beam, while the lens transforms the beam characteristic parameters. The output light beam is analyzed in detail: its characteristic parameters and focusing properties, amplitude and intensity distributions and the vortex rings profiles, and radii, at any z distance behind the HL plane, as well as in the near and far field.     

Target-based coherent beam combining of an optical phased array fed by a broadband laser source

The target-based phasing of an optical phased array (OPA) fed by a broadband master oscillator laser source is investigated. The specific scenario examined here considers an OPA phasing through atmospheric turbulence on a rough curved object. An analytical expression for the detected or received intensity is derived. Gleaned from this expression are the conditions under which target-based phasing is possible. A detailed OPA wave optics simulation is performed to validate the theoretical findings. Key aspects of the simulation set-up as well as the results are thoroughly discussed.     

不同强度分布激光水平传输稳态热晕效应的数值模拟

采用数值模拟的方法,以高斯光束、平顶光束、有中心遮拦的平顶光束为例,研究了激光在均匀大气传输过程中产生的稳态热晕效应。详细分析了发射功率、传输距离、光束直径、横向风速四种参数对热晕效应的影响,得到了以上三种光束的斯特列尔比和峰值偏移量随广义热畸变参数N的变化关系。数值仿真结果表明,在其他参数一致的情况下,发射功率越大、传输距离越长,热晕效应越强;而光束直径和横向风速的增加会减小热晕效应;不同激光强度分布对热晕效应的影响不同。在同样广义热畸变参数N的条件下,高斯光束的热晕效应最严重,平顶光束次之,空心光束的热晕效应最小。

     

熔锥光纤与球微腔耦合系统的理论模拟

利用H. J. Shaw提出的单模光纤定向耦合器的分析方法并结合微球腔的特性,对熔锥光纤与球微腔系统的耦合特性进行了理论分析和数值模拟。研究表明,由于原本简并的球腔模式受腔体偏心率的影响而解除,系统吸收峰间距减小,Q值( 106 ~107 )比将腔体视为理想球时提高了三个数量级。这与已报道的实验结果相吻合。     

LDA光纤耦合高效率高亮度激光输出研究

为了获得LDA(激光二级管阵列)光纤耦合高效率高亮度激光输出,光纤的芯径要尽可能小,并且耦合光束的光参数积必须小于耦合光纤的光参数积,因此必须对耦合光束进行整形。通过比较,选用反射整形法将慢轴的光参数积压缩到了原来的1/4。将LDA光束与芯径400靘,数值孔径0.37的光纤进行耦合试验,得到的耦合效率约为48%,亮度达到108量级。     

Numerical simulation of the generation of multiple laser filaments by an axicon array

The implementation of an axicon array is proposed for generation of regularized multiple laser filaments. The results of numerical simulations demonstrate that the transverse patterns of multiple filaments are determined by the arrangement of the axicon array. Parallel filaments are generated by each axicon unit. Meanwhile, it is shown how the the length of filaments can be increased.     

Unstable Cr:LiSAF laser resonator with a variable reflectivity output coupler

The performance of a flash-lamp-pumped Cr:LiSAF unstable laser resonator utilizing a fourth-order super-Gaussian variable reflectivity mirror as an output coupler is described. The super-Gaussian mirror results in a smooth, flattop transverse beam profile in the near field that is advantageous for nonlinear frequency-conversion applications. Long-pulse and Q-switched operation of the Cr:LiSAF unstable laser resonator are described and compared with stable resonator operation. We obtained tunable ultraviolet radiation extending from 267 to 290 nm by frequency mixing theQ-switched Cr:LiSAF laser output with lithium triborate and beta-barium borate nonlinear crystals.     

椭圆柱透镜组准直半导体激光束的优化设计

采用光线矢量法设计了两个相互垂直的椭圆截面柱透镜组来准直半导体激光束。利用多目标优化中的加权和法进行优化计算,并提出自适应加权系数优化法。此方法能根据计算结果自动调整加权系数,从而达到最佳的优化结果。计算结果表明,该准直系统可达到45μrad左右的发散角,远远优于其它截面柱透镜组（毫弧度数量级）的准直效果。同时两柱透镜间距取适当值,还可在远场形成圆形光宽。     

Algorithm for calculating the refraction patterns of a planar laser beam in an optically inhomogeneous medium

An algorithm for simulation of the distribution of a planar laser beam in a three-dimensional, optically inhomogeneous medium is proposed. A technique for numerical calculation of the trajectories of the beam in a geometrically optical approximation is described. A calculation of refraction patterns in probing of a thermal boundary layer near a cylindrically shaped heated body that takes into account edge effects is performed. Translated from Izmeritel’naya Tekhnika, No. 5, pp. 36–39, May, 2008.     

环形斑激光束在非线性克尔介质中的光束分裂效应

通过数值求解非线性Schrodinger方程,详细讨论了环形斑激光束在非线性克尔介质中的传输特性.当入射光超过一定功率时,光束产生多级自聚焦现象;传输过程中光束出现分裂现象,光束分裂的方式、分裂程度与入射光强度有关,并且光束分裂与自聚焦焦点处强度相联系.     

磁流体薄片中聚焦激光束对交流磁场的相位独立响应

报导了磁流体薄片中聚焦激光束在探测50Hz交流磁场时产生与磁场相位无关响应的效应。聚焦于并通过磁流体薄片的633nm激光束的偏转角仅与交流磁场的幅度相关。为了探寻其机理，还研究了磁流体中聚焦激光束对开／关磁场或激光的瞬态响应。提出了基于磁熵守恒的磁流体中磁性粒子密度再分布正反馈模型来解释这一效应。这一模型也可以预 磁流体中一种磁－热失稳现象。     

Mathematical simulation of the transformation of radiation formed by a plane resonator and an optical system in a paraxial approximation

A mathematical model of the transformation of radiation produced by a flat optical resonator, single lens, and a two-lens optical system with the use of the Kirchhoff scalar theory of diffraction is constructed and investigated. __________ Translated from Izmeritel’naya Tekhnika, No. 11, pp. 20–23, November, 2007.     

Measurement of the quality of laser beams by methods of fourier optics

A new method of determining the magnitude of M², the coefficient of exceedance of the diffraction limit of the divergence of a laser beam, based on the principles of Fourier optics is proposed. By means of the new method, it is possible to substantially simplify the design of the measuring transducer and expand the types of devices that may be used to measure the parameters of laser beams. An experimental comparison of the proposed method with the method regulated by ISO Standard 11146 is conducted. __________ Translated from Izmeritel’naya Tekhnika, No. 10, pp. 42–45, October, 2006.     

Optimization of the arrangement of fiber laser array in beam coherent combining

Based on the expanding of the fundamental mode of a step-index fiber in terms of Laguerre–Gaussian modes, the accurate expression for the beam propagation of a fiber laser in free space is obtained. Thereby, the coherent combining beam of three fiber laser arrays including circular arrangement, square arrangement, and diamond arrangement are numerically analyzed. The study shows that all the beams gradually concentrate centrally on the propagation axis and the highest far-field peak intensity can be obtained by using the circular arrangement. Meanwhile, the far-field intensity of the circular arrangement by using the Laguerre–Gaussian approximation is also compared with that by using the pure Gaussian approximation, which indicates that the pure Gaussian approximation will induce much error in the far-field intensity. Finally, the influence of the radius of a circular fiber laser array on the far-field intensity is studied, of which the result shows that the far-field intensity decreases with increasing radius. Therefore, the fiber laser elements are suggested to be placed as close as possible.