Characterizing the divergence properties of the laser diode beams propagation through collimator and aperture ABCD optical system

The propagation properties of Gaussian laser beams through a complete optical path including free space and the optics of transmitter and receiver containing a collimator, an aperture and a lens is studied. Based on the Collins integral and using the second order moment method, analytical formulas for intensity distribution and Power In Bucket (PIB) along the propagation path are derived. The effects of initial beam divergence, collimator–source separation distance and beam width deviation on laser beams properties are investigated. Obtained results are confirmed and illustrated with numerical examples and resulted graphs.     

位相畸变对强紫外激光在空气中长程传输的影响

 采用瞬态受激旋转拉曼散射（SRRS）模型,详细考虑了激光脉冲的位相变化、时间波形、泵浦光的衰减、斯托克斯光的非线性放大以及初始的随机斯托克斯散射场等诸多因素,研究了空间位相畸变对空气中长程传输过程中产生的SRRS效应的影响。研究结果表明：位相畸变对阈值条件和时间特性影响较小,而对光束质量影响较为明显;随着位相畸变的增大,光束的能量分布变得极不均匀,光束的光强对比度也明显变大,从而使激光光束质量变差,而且当传输距离达到阈值条件后,随着传输距离的增加,光束质量将进一步变差。     

Investigation of the spatial profile of stimulated Raman scattering beams in D<Subscript>2</Subscript> and H<Subscript>2</Subscript> gases using a pulsed Nd:YAG laser at 266 nm

The stimulated Raman scattering (SRS) beam spatial mode structure is studied in H₂ and D₂ gases, in a 60 cm-long high-pressure Raman cell, using a pulsed Nd:YAG laser at 266 nm. The conversion efficiencies of the first and second Stokes in H₂, D₂ and H₂/D₂ mixtures have been measured precisely in the 1–14 atm range. The SRS beam profiles were recorded for different gas pressures in the high-pressure Raman cell and interesting behaviour is revealed. Homogeneous SRS spatial profiles were obtained only for pressures higher than 7 atm. For lower pressures the profiles are either in the form of a ring or of a more complicated shape. The results are explained in terms of nonlinear self-focusing processes during the pump beam propagation in the Raman cell. Also, from our measurements, a more precise value of the nonlinear index of refraction for the H₂ and D₂ at 266 nm is proposed. PACS 42.65.Dr; 42.60.Jf; 42.60.Lh; 42.65.Jx     

The relay propagation of partially coherent cosh–Gaussian–Schell beams in turbulent atmosphere

We have studied the relay propagation of a partially coherent cosh–Gaussian–Schell beam in turbulent atmosphere. Analytical expressions for both the cross-spectral density at the relay system and average intensity at the target are derived. By using the analytical expressions some special cases are studied and some numerical simulation comparisons are made, especially the effects of the coherence of the beam, turbulence strength, aperture and its size on the relay propagation. Our study shows that the effects of diffraction and coherence of initial beam on the intensity profiles at the relay system are so small that they can be neglected when the effects due to turbulence are large enough. Even though the correction to the receiving beam at relay system is important, it is not necessary to improve the receiving beam when the effect of turbulence over the travel path is strong. A high peak intensity at target can be obtained by optimizing these factors, such as propagation distance, aperture and spatial correlation length.     

空气中激光等离子通道的演变和控制

为精确再现超强飞秒脉冲激光在大气中的传输特性,有效控制激光诱导等离子体通道的性能参数,基于扩展的非线性薛定谔方程,研究了空气中产生的等离子通道的演变过程。该模型在考虑衍射、色散和多光子效应的基础上,引入了拉曼散射、等离子体尾波场和相对论自聚焦等多种效应。讨论了电子密度和光强通量的空间分布特性,运用分步傅里叶法和有限差分法得到了电子密度和光强通量的分布,仿真结果显示,激光波长、单脉冲能量、脉宽和束腰半径等初始参数的变化将对等离子体通道的演变产生显著影响,为超短脉冲强激光在大气中成丝位置和形态控制提供了可能的途径。     

多艾里光束合成自聚焦光束的实验实现

多光束合成和单光束聚焦一直是提高激光束功率和功率密度的两个重要方法. 结合艾里光束在自由空间中沿弯曲路径传播的特性, 从数值模拟和实验两个方面, 研究了利用多个一维艾里光束合成自由空间自聚焦光束的方法, 并对所得到的模拟和实验结果进行了对比. 采用分步束传播法, 分别模拟了由四个和八个一维艾里光束合成的自聚焦光束在自由空间中的传播过程, 给出了自聚焦光束在传播过程中横向和纵向的光强分布和变化趋势. 采用计算全息和空间光调制器技术实验, 实现了多个一维艾里光束合成的自聚焦光束. 实验中分别测量了四个和八个一维艾里光束合成的自聚焦光束的横向光强分布. 实验结果和理论结果符合得较好. 另外, 为了进一步增大自聚焦光束的功率, 可以增加参与合成的一维艾里光束的数量. 同时, 自聚焦光束的焦距可以通过调整各个一维艾里光束的相对位置进行调节.     

Arbitrary laser beam propagation in free space

The propagation of arbitrary laser beams in free space is examined. For this purpose, starting with an incident field of arbitrary field distribution, the intensity at the receiver plane is formulated via Huygens Fresnel diffraction integral. Arbitrary source field profile is produced by decomposing the source into incremental areas (pixels). The received field through the propagation in free space is found by superposing the contributions from all source incremental areas. The proposed method enables us to evaluate the received intensity originating from any type of source field. Using the arbitrary beam excitation, intensity of various laser beams such as cos-Gaussian, cosh-Gaussian, general type beams are checked to be consistent with the already existing results in literature, and the received intensity distributions are obtained for some original arbitrary beam field profiles. Our received intensity formulation for the arbitrary source field profiles presented in this paper can find application in optics communication links, reflection from rough surfaces, optical cryptography and optical imaging systems.     

Intense femtosecond shaped laser beams for writing extended structures inside transparent dielectrics

We review recent results on the propagation and self-focusing of intense femtosecond laser pulses with shaped beam profiles in transparent dielectric media. At sufficiently high optical power, beam shaping seeds into the transverse modulation instability and results in the deterministic placement of intense laser filaments within the beam profile. Resulting spatial filament distributions may be utilized for writing complex extended structures inside transparent dielectrics. Specific examples of beam shapes we will discuss are Bessel beams, optical vortices, Bessel beams of higher order, and Airy beams.     

不同能量下环形高斯强激光光束在空气中的非线性传输

以线性传输模型为对比研究了不同初始能量的环形光束在空气中的非线性传输。结果显示,在传输初始阶段,非线性克尔效应减弱了线性空间啁啾导致的聚焦作用。线性聚焦使能量向光轴方向流动导致强光强核心,在光轴附近生成类高斯脉冲结构,增强了非线性自聚焦效应,导致光强急剧增加形成光丝。环形光束的初始能量的大小能够影响自聚焦焦距、光丝长度和光强通量。自聚焦焦距随着初始脉冲能量的增加而减小,但自聚焦焦距与初始功率的开方不成反比（这点与高斯光束不同）。光丝长度随着初始脉冲能量的增加而增加。轴上光强通量随初始脉冲能量的增加而增加。     

Polarization properties of polarized and partially coherent Electromagnetic Gaussian-Schell model pulse beams on slant path in turbulent atmosphere

This paper is based on the unified theory of coherence and polarization of stochastic electromagnetic beams and the extended Huygens–Fresnel principle, combined with the quadratic approximation of Rytov’s phase structure function and the generalized Stokes parameters. We have derived the novel expressions for the cross-spectral density matrix elements and the degree of cross-polarization of a class of elliptically polarized spatially and spectrally partially coherent Electromagnetic Gaussian-Schell model pulse (EGSMP) beams propagating through atmospheric turbulence along a slant path. Additionally, we calculate and analyze the effects of the turbulent intensity, the initial pulse duration, waist width of the beam, the spatial coherence length and temporal coherence length et al. on the polarization properties of fully polarized and partially coherent EGSMP beams. Finally, a comparison of the impact of those factors on the partially polarization beams is made. The results show that the influences of the turbulent intensity, the initial pulse duration, waist width of the beam, the spatial coherence length and temporal coherence length et al. on the polarization properties of fully polarized and partially coherent EGSMP beams are larger. While the effects of those parameters on the partially polarization and partially coherent EGSMP beams are smaller. It is noted that the results of this paper have established sound theoretical basis on the topic of improving performance of the laser system propagating through the atmospheric turbulence.     

衍射理论对局域空心光束及无衍射光束重建的描述

利用衍射理论导出了局域空心光束的传输表达式及光强分布,给出了局域空心光束的精细结构,详细分析了其演变过程。讨论了聚焦透镜的焦距f对径向暗域最大尺寸及轴上暗域长度的影响。结果表明,径向暗域最大尺寸及轴上暗域长度都随着f的增大而增大。通过轴棱锥-透镜系统获得局域空心光束,用体视显微镜和CCD照相机组成的系统拍摄光束强度分布。结果表明应用衍射理论可以较精确地描述局域空心光束的演变过程。找出了其应用中的不利因素,更清晰地展现无衍射光束的重建现象。这种描述方法弥补了几何理论和干涉理论的不足。     

Regularities of nonstationary self-focusing of profiled laser beams: Averaged description

Nonstationary self-focusing of powerful laser radiation whose transverse intensity profile is Gaussian, super-Gaussian, or annular is theoretically investigated in air in the single filamentation regime using the formalism of averaged parameters of the radiation. Analytical relations are obtained that make it possible to calculate the basic characteristics of nonstationary self-focusing of beams with non-Gaussian profiles. It is found that, at distances of up to the global nonlinear focus, the initial stages of the dependences of the effective radius of these beams on the generalized evolutional variable are qualitatively and quantitatively similar. After the focal waist, this similarity is not observed, and the spatial behavior of the effective beam radius depends on the initial radiation profile.     

小宽带光谱色散匀滑光束传输特性研究

利用激光聚变研究中心自主开发的小宽带传输放大软件对小宽带光谱色散匀滑(SSD)光束进行具有"时间、空间、光谱分辨"的传输模拟,研究了SSD光束通过空间滤波器和自由空间的传输特性.给出了时间、空间、光谱的对应分布图,对比分析了SSD光束与非SSD光束的传输效应.研究结果对于合理设计SSD光束的发散角与空间滤波器小孔之间的匹配关系、选择适宜的调制频率,以及选择束匀滑色循环数等都有重要意义,同时对于光路设计特别是细光束自由传输距离的设计具有指导意义.     

湍流大气中斜程传输光场的相位特性

运用功率谱反演法对斜程传输大气湍流扰动相位屏进行了数值模拟。通过建立激光束初始畸变相位模型,从波前峰谷值和波前功率谱密度函数两个方面,对激光束通过大气湍流后的相位特性进行了研究,重点分析了湍流传输距离和天顶角对激光束波前相位分布的影响。研究结果表明:激光束在湍流大气斜程传输后,其波前相位会发生明显畸变,且空间高频相位较低频相位所占比例明显增加;通过湍流后的波前相位与传输距离及天顶角密切相关,传输距离越长,天顶角越大,相位畸变程度越大,高频相位所占比例越多。     

同轴双无衍射光的理论与实验

提出同轴双无衍射光的概念。如果两束汇聚角略有不同的同轴无衍射光同时存在，则它们相互之间叠加干涉后产生一种纵向呈周期变化的光场结构，这是一种新的光束空间分布，这种光束在横向仍是一种同心环光斑，可用于精密对准。而在不同纵向位置，这些同心环的强度分布是不同的，且是周期性变化的。同轴双无衍射光的轴向周期性可用于沿光束传播方向的定位。文中提出了双环缝法、分段圆锥透镜法、正弦相位环光栅法等实现同轴双无衍射光的理论与实验方法，分析了同轴双无衍射光的横向与纵向传播的衍射光斑特性，理论计算与实验测量的结果基本相符合。     

超短脉冲激光光束被局域体全息光栅衍射的性质分析

利用二维耦合波理论,分析了超短脉冲激光光束被完全重叠型的局域体全息光栅衍射的时空变化性质,给出了衍射和透射脉冲激光光束沿光栅出射边界的强度时空分布。以LiNbO3晶体为例,数值研究了衍射光脉冲强度沿光栅出射边界的分布和脉冲波形的变化及光栅的总衍射效率受光栅二维尺寸、入射角度、光栅折射率调制度及入射脉冲的脉冲时域半峰全宽等条件的影响而变化的情况。与一维体全息光栅对超短脉冲激光光束衍射的性质,及此光栅对连续光衍射的性质作比较,给出了合理选择光栅参量及入射条件以在光栅出射边界上得到总衍射效率较大且分布较均匀的衍射光脉冲的方法。     

宽带泵浦激光的聚焦位置对SRS发生过程的影响

 结合SRS实验,重点研究了聚焦位置对Raman介质中的物理过程、Stokes光的输出特性的影响,比较了SRS阈值和转换效率的实验结果与理论预期。     

湍流大气中斜程传输部分相干光的光束扩展

运用维格纳分布函数与广义惠更斯菲涅耳原理,推导出部分相干光束在任意折射率起伏功率谱模型的大气湍流中斜程传输时,其束宽传输的一般解析表达式。在此基础上,以部分相干平顶(PCFT)光束为例,给出了PCFT光束在大气湍流中斜程传输的束宽扩展解析表达式。在合肥地区大气折射率结构常数高度分布模式下开展了数值计算和分析,并与水平传输时的结果进行了比较。研究结果表明,光束在斜程传输时的束宽不仅与光束阶数、空间相干度、束腰半径、湍流的强弱和传输距离有关,还与天顶角密切相关。光束的阶数越高,空间相干性越差,天顶角越小,则其束宽扩展受湍流的影响就越小。当天顶角小于60°时,大气湍流对斜程传输光束束宽的影响明显小于水平传输的情况。     

Spatiotemporal dynamics of the amplitude-phase characteristics of stimulated Raman scattering

A numerical model describing the processes of generation of stimulated Raman scattering (SRS) is developed; the calculations are performed and different scenarios of generation of Stokes radiation in compressed hydrogen are analyzed. The spatiotemporal profiles of intensities and phases of interacting waves, the radiation spectra, and the functions of spatial coherence are obtained by way of numerical simulation for different conditions of SRS generation. A substantial difference between these parameters in the cases of nonstationary and quasi-stationary regimes of SRS generation is revealed. The nonstationary regime of generation is characterized by more complicated spatiotemporal dependences than the quasi-stationary regime. However, in the quasi-stationary regime, the phase of the Stokes wave radiation varies over a larger interval during the pump pulse duration, which leads to a decrease in the degree of spatial coherence to lower values. For both regimes of generation, the value of the degree of spatial coherence decreases with an increase in the conversion factor to a certain threshold value and then stabilizes, which is in agreement with experimental data. The presence of a moving focus of SRS focusing of the Stokes beam is demonstrated. This effect is governed by the spatiotemporal shape of the pump beam intensity (the Gaussian profile), by a high SRS gain, and by the processes of diffraction. The results of the numerical simulation are in qualitative and quantitative agreement with experimental data obtained previously.     

Spatial and frequency manifestations of laser beam self-action effects in different schemes of inversionless amplification

Results of numerical simulation of coaxial propagation of an intense pump beam and a weak probe beam in a three-level medium with different types of energy diagrams in conditions of inversionless amplification are presented. The effect of the beam self-action and diffraction on the spatial and frequency characteristics of the probe wave were studied. These effects, in combination with linear absorption and refraction, give rise to complex spatial profiles of gain along the beam propagation path. The annular structure of the amplified beam and a small asymmetry of its frequency characteristics, resulting from the nontrivial gain profile over the beam cross section, were found and studied.