A geometric analysis of the probe volume in a dual beam laser Doppler velocimeter

The probe volume of the dual beam laser Doppler velocimeter is analyzed from a geometrical standpoint. The volume is graphically reconstructed and the existence of the characteristic dual beam laser Doppler velocimeter fringes is demonstrated. The fringes are analyzed and are shown to be composed of moving tubes or pockets of coherent radiation. The composition of the light is analyzed and the extent of the coherence of the laser radiation within the light pockets is considered. The scattered light from the tubes is analyzed and the coherence enhancement of the fringe structure is shown. This information is used in an attempt to resolve the controversy as to whether the dual beam laser Doppler velocimeter is governed by a fringe counting phenomena or by a beating phenomena of light from two sources of coherent radiation of different frequencies.     

Evaluation of the decay in temporal coherence of a laser beam propagating in a slowly fluctuating medium

A simple method is described for evaluating the change in temporal coherence which occurs when a monochromatic light wave traverses a medium the properties of which fluctuate randomly. The degree of second-order coherence is formulated in terms of the amplitude and phase autocorrelation functions which are derived from the photocurrent produced in a light beating experiment. This method makes it possible to evaluate even a small deterioration in temporal coherence which is closely related to the frequency fluctuations in the light wave. The change in temporal coherence of a laser beam propagating through a slowly fluctuating gaseous path has been measured.     

Saturation resonance of atoms optically pumped by a monomode laser beam

The existence of two types of phenomena (the population effect and the Zeeman coherence effect) in saturation resonances by magnetic mode crossing have been predicted theoretically. A direct experimental proof is given here, by the observation of the Π component of the fluorescence light emitted by neon atoms optically pumped by a monomode 6328 Å laser beam.     

Reduction of the coherence time of an intense laser pulse propagating through a plasma

We present direct measurements of the coherence time of a laser beam after propagation through an underdense plasma. At an intensity of 10(14) W/cm(2), a large decrease of the coherence time is observed, from 300 ps to a few picoseconds. This decrease is larger as the plasma density is increased or as the light is scattered at larger angles. The amount of temporal decorrelation as well as the effect of the plasma density, laser intensity, and scattering angle all coincide with trends observed in recent numerical simulations.     

Coherence of a squeezed sodium atom laser generated from Raman output coupling

The coherence of a squeezed sodium atom laser generated from a Raman output coupler, in which the sodium atoms in Bose-Einstein condensate （BEC） interact with two light beams consisting of a weaker squeezed coherent probe light and a stronger classical coupling light, is investigated. The results show that in the case of a large mean number of BEC atoms and a weaker probe light field, the atom laser is antibunching, and this atom laser is second-order coherent if the number of BEC atoms in traps is large enough.     

光束的空间相干性与空间质量评价

光束空间质量评价的标准应该能够反映光束的空间相干性和强度(功率或能量)分布,并主要由空间相干性的优劣来说明.本文对模式纯度(1-δ)以及空间质量评价因子-衍射极限倍数因子M²做了讨论,并对此因子的表达形式提出了我们的改进建议.     

Slow light beam splitter

We demonstrate a slow light beam splitter using rapid coherence transport in a wall-coated atomic vapor cell. We show that particles undergoing random and undirected classical motion can mediate coherent interactions between two or more optical modes. Coherence, written into atoms via electromagnetically induced transparency using an input optical signal at one transverse position, spreads out via ballistic atomic motion, is preserved by an antirelaxation wall coating, and is then retrieved in outgoing slow light signals in both the input channel and a spatially-separated second channel. The splitting ratio between the two output channels can be tuned by adjusting the laser power. The slow light beam splitter may improve quantum repeater performance and be useful as an all-optical dynamically reconfigurable router.     

Generation of light carrying orbital angular momentum via induced coherence grating in cold atoms

We report on the generation of light carrying orbital angular momentum through Bragg diffraction into an electromagnetically induced coherence grating in a degenerate two-level system of cold cesium atoms. The induced Zeeman coherence grating is shown to contain the spatial phase structure of the incident beams. The exchange of phase information between a light beam with orbital angular momentum and a long-lived atomic coherence opens up the way to process quantum information encoded in a multidimensional state space.     

冷原子穿越激光束的量子隧穿时间

原子通过激光冷却技术能够被制备在低温状态，这时冷原子云会展现出量子力学的波动性.研究了一束冷原子入射到一个蓝失谐的激光束上所表现出的量子力学隧穿效应.蓝失谐的激光束相对于冷原子而言等效于一个量子力学势垒.根据二能级模型，在理论上分析了具有内部结构的原子矢量物质波穿过激光束的量子力学反射与透射，特别是对原子穿越激光束所需的时间——量子隧穿时间进行了详细的研究.量子力学波动性使得冷原子穿越一个激光束时明显地展现出与经典粒子(热原子)不同的结果. 关键词： 冷原子 原子光学 量子隧穿     

Enhanced spatiotemporal laser-beam smoothing in gas-jet plasmas

Spatiotemporal smoothing of large-scale laser intensity fluctuations is observed for a laser beam focused into underdense helium plasmas. This smoothing is found to be severely enhanced when focusing the laser beam into a helium gas jet. In contrast to other experiments with preformed plasmas, the average and the peak laser intensities are well below the threshold for ponderomotive self-focusing. The coherence characteristics of the transmitted light are measured for various electron densities, and the smoothing effect is explained by multiple scattering of laser light on self-induced density perturbations.     

倾斜离轴高斯-谢尔模型光束在大气湍流中通过猫眼光学镜头反射光的光强特性

本文研究了大气湍流和探测光空间相干性对倾斜离轴激光束通过猫眼光学 镜头反射光光强特性的影响. 研究表明: 由于光阑效应以及光束倾斜或离轴, 反射光光强出现振荡和非对称分布现象. 大气湍流不会改变其非对称性, 但湍流中其光强不再振荡. 当猫眼光学镜头存在微弱正离焦δ_max情况下, 轴上光强可达到最大值. 猫眼光学镜头焦距越大, 所需δ_max越大. 但是, 大气湍流和探测光相干性好坏都不会改变所需δ_max值. 在大气湍流中传输距离足够远时, 反射光强会成为离轴类高斯分布. 随着传输距离的增大, 相干性越差的探测光的反射光束扩展可以更小, 这与部分相干光自由空间传输规律完全不同. 探测光相干性越好, 其反射光强受湍流的影响越大, 但差异不大. 本文所得结果对激光主动探测的应用具有意义. 关键词： 猫眼光学镜头 大气湍流 部分相干光 光强     

宽带低相干光单次时间相干性测量研究

传统测量光束时间相干性的方法是通过机械扫描的方式实现的,这种方法不能够实现单次测量,而且对于相干时间较短的宽带光测量误差较大。本文提出了一种单次时间相干性测量的新方法,通过给迈克尔逊干涉仪的反射镜引入楔角,使光束波前产生随位置变化的延迟差,可从单次测量的一幅干涉图中计算提取出光场完整的时间相干性信息。实验中测量了不同宽带入射光的时间相干性,均与理论结果吻合较好。单次时间相干性测量的方法将为高功率宽带激光装置提供更为方便的时间相干性测量手段,提高实验测量效率。     

亚微米局域空心光束的产生及其在单原子囚禁与冷却中的应用理论研究

提出了一种采用单模光纤、环形二元相位板和微透镜组成的光束整形系统产生亚微米局域空心光束的方案. 根据瑞利-索莫菲衍射积分公式, 数值计算了微透镜焦平面附近的场分布, 详细研究了空心光束的暗斑尺寸与单模光纤模场半径和微透镜焦距的关系. 数值计算结果表明: 在微透镜焦平面附近光场分布近似对称, 在焦点处场强近似为零, 周围场强逐渐增大, 形成半径约为0.4 μm的三维封闭的球形空心光场区域, 即亚微米局域空心光束. 当局域空心光束为蓝失谐时, 光场中的原子将被囚禁在光场最弱处. 若加上抽运光, 原子将受到蓝失谐局域空心光束与抽运光共同激发的强度梯度Sisyphus冷却. 本文利用该方案产生的亚微米局域空心光束构建单原子的囚禁与冷却器件, 并以单个⁸⁷Rb原子为例, 利用Mont-Carlo方法研究亚微米局域空心光束中单原子囚禁与强度梯度冷却的动力学过程, 结果表明利用该器件可以获得温度在5.8 μK量级的超冷单原子.     

新型部分相干光束的产生及其相干特性

利用相干理论,研究了新型部分相干光束的相干特性.用激光光束透过旋转的特殊毛玻璃,产生一种相干性分布特殊的新型部分相干光束.实验上经双孔干涉,记录了该光束经不同小孔间距双孔产生的干涉条纹,计算得出所对应的相干度,并获得了该光束在不同传输距离下的相干度分布情况,发现该光束的相干度分布与高斯-谢尔模型光束的相干度分布不相同,其相干度随着传输距离的增大而变强.     

Propagating a partially coherent radially polarized beam through aligned and misaligned ABCD optical systems

In the present work the propagation of a partially coherent radially polarized beam through aligned and misaligned ABCD optical systems is discussed. The elements of the beam coherence-polarization matrix for a partially coherent radially polarized beam propagating through these systems are derived for the first time. Using the derived analytical results the intensity distribution of the focused partially coherent radially polarized beam after a low-NA lens is investigated. It is shown that the coherence of the light source affects the beam intensity profile at the focal plane significantly, while for the given coherence of the light source the focal length of the used lens does not affect the beam intensity profile at the focal plane. This is consistent with the case of a partially coherent Hermite-Gaussian beam, as expected.     

Experimental realization of an optical one-way barrier for neutral atoms

We demonstrate an asymmetric optical potential barrier for ultracold 87Rb atoms using laser light tuned near the D2 optical transition. Such a one-way barrier, where atoms incident on one side are transmitted but reflected from the other, is a realization of Maxwell's demon and has important implications for cooling atoms and molecules not amenable to standard laser-cooling techniques. In our experiment, atoms are confined to a far-detuned dipole trap consisting of a single focused Gaussian beam, which is divided near the focus by the barrier. The one-way barrier consists of two focused laser beams oriented almost normal to the dipole-trap axis. The first beam is tuned to present either a potential well or barrier, depending on the state of the incident atoms. On the reflecting side of the barrier, the second beam optically pumps the atoms to the reflecting (barrier) state, thus producing the asymmetry.     

激光聚变驱动器的光束相干性及其控制:回顾与展望

高功率强激光与物质相互作用蕴藏着丰富的非线性效应,激光聚变驱动器的光束具有高度相干性,它在光束传输过程中极大地凸显了这种效应,并不可避免地制约着激光功率的提升和激光能量的有效利用。回顾激光聚变驱动器的发展史,在提升激光输出能力的主线外,还存在一条与光束相干性做斗争的暗线贯穿其中。以激光与物质相互作用为牵引,从高功率强激光传输中非线性效应抑制和激光等离子体相互作用的抑制两方面回顾了激光聚变驱动器光束相干性的控制现状,并针对潜在需求,展望了未来高功率激光发展的创新技术。     

激光束散角复合控制技术

为了降低部分相干光光学系统设计的复杂度及成本,增加部分相干光应用的便捷性,提出了一种液晶空间光调制器的激光相干度及束散角复合控制方法。首先介绍了对激光光束进行相干度和束散角复合控制的基本理论和方法;然后分别设置了相干度和束散角检测实验,检测了本方法所调制激光光束的相干度和束散角的准确性。实验结果表明,采用液晶空间光调制器生成相干度为0.9 mm、束散角为7.5 mrad,以及相干度为1.5 mm、束散角为3.8 mrad的部分相干光束,其相干度与理论值相比误差在5%以内,其相干度均方根误差分别为0.027386和0.031314,峰谷值分别为0.084658和0.089103;其束散角与理论值相比误差在5%以内,其束散角均方根误差分别为0.022478和0.023186,峰谷值分别为0.081201和0.092130。可见,该方法可以实现高精度的相干度及束散角复合控制。     

激光减速原子束频标的建议

本文提出一个激光减速的碱金属原子束频标的方案。用共振激光束对原子束同时进行减速和选态,使原子速率降到10m/s以下,而光抽运作用使原子自动集中到基态超精细结构中具有最大磁量子数的塞曼子能级上。为避免重力场中束轨迹下垂,用偏转磁铁或多束激光使束由水平转成垂直向上,然后用级联磁共振使原子过渡到频标所需的m_F=0能级。利用原子上升和重力场中自由下落两次通过单个微波谐振腔而取得Ramsey共振信号,线宽约为¹Hz。信号用另一束激光检测。予期这种频标的稳定度和准确度可比现有束型频标提高一个数量级以上。文中详细讨论了激光减速和选态的方法,克服横向加热效应的措施,实现级联磁共振的办法,以及获得垂直束装置的设计等。     

Experimental and theoretical investigation of loss of coherence on scattering of a beam with helical wavefront

Theoretical and experimental investigations of the second order intensity coherence for a beam with helical wavefront i.e. an optical vortex scattered through a rotating ground glass have been reported. The results have been compared with scattering of the Gaussian beam of a He-Ne laser. It shows that the second order intensity coherence or intensity correlation curve for the vortex decreases much faster than that of the Gaussian beam of the He-Ne laser. We attribute this behaviour to the helicity of the wavefront.