Generation of high-order rotational lines in hydrogen by four-waveRaman mixing in the femtosecond regime

More than 40 rotational Raman lines are generated using an 800-fs Ti:sapphire laser. The spectral region extends from the near-infrared to the far-ultraviolet with a considerably flat intensity distribution. The effects of laser polarization, pulsewidth, hydrogen pressure, and focusing conditions on the efficiencies of stimulated Raman scattering, four-wave Raman mixing, self-phase modulation, self-focusing, and harmonic generation are investigated. A white light continuum, generated by self-phase modulation, acts as a seed beam for the generation of high-order rotational lines through four-wave Raman mixing. Strong self-phase modulation, however, suppresses the generation of the Raman emission, due to a line broadening of the pump beam. Thus, optimization of experimental conditions is necessary for the efficient generation of high-order rotational lines     

水平大气信道对圆偏振光偏振特性的影响

基于Mie散射理论利用偏振相关的Monte Carlo仿真方法对圆偏振光在水平大气中传输进行建模仿真。仿真结果表明:在大气能见度为5 km、传输距离为11.16 km条件下,接收光斑中心处保偏性能很好,且优于接收光斑边缘处保偏性能。进行了11.16 km水平大气链路左旋圆偏振光退偏实验,实验结果表明:在大气能见度为5~10 km,大气折射率结构常数Cn2=1.82510-13 m-2/3条件下,左旋圆偏振光经过长距离水平大气信道传输后偏振旋向不发生改变;接收光斑中心处保偏性能良好,优于接收光斑边缘处,和仿真结果趋势一致。     

部分相干圆偏振反常涡旋光束的紧聚焦及辐射力研究

根据矢量衍射理论和部分相干理论,推导了部分相干圆偏振反常涡旋光束经过高数值孔径聚焦透镜后的光强表达式.详细讨论了入射光束参数以及聚焦透镜的数值孔径大小对光束紧聚焦特性的影响.研究结果表明:自旋方向,拓扑荷数以及数值孔径大小对光强分布有影响,相干长度以及光束阶数仅改变光强值,光束经过紧聚焦后在轴向方向上自旋角动量可以转换...     

Stimulated Raman scattering in the visible with a multipass cell

In experimental studies of stimulated Raman scattering in a multipass cell (MPC) with a frequency-doubled single-mode Nd:YAG laser at 532 nm, it is found that only forward Stokes generation is observed and that the growth and saturation of the Stokes beam are in agreement with a simple plane wave, transient theory scaled to account for the focusing, and the multiple passes. The absence of second Stokes generation, backward Stokes generation, and anti-Stoke generation is discussed in terms of the low gain per pass in the MPC. It is concluded that the MPC provides an ideal system for studying stimulated Stokes scattering with visible lasers without the four-wave processes usually seen in a single-focus geometry     

Direct Measurement of Raman Scattering Tensor of Orientation‐Fixed Single Iodine Molecules

Raman spectroscopy is the most widely used noninvasive analytical technique. Apart from the fingerprint Raman frequency for identifying vibrational mode of certain functional groups, the Raman scattering tensor can also be used to determine the corresponding vibrational symmetry as well as the orientation of this functional group with respect to the rest of the molecule. For gaseous single molecules, only limited structural information can be obtained from Raman spectroscopy owing to their freely rotating and randomly oriented nature. Here, a method, for the first time, is developed to directly determine the Raman scattering tensor on orientation‐fixed single iodine molecules, which are confined inside the nano‐sized channels of zeolite AlPO₄–11 (AEL) single crystal. The experimental results are in good agreement with theoretical predictions based on a density functional theory. The optical transparency and appreciable size of the crystal facilitate the Raman exploration and the 3D manipulation. It is also demonstrated that iodine molecules' orientations are randomly distributed inside the nano‐channels of AlPO₄–5 (AFI) crystal, which indicates that by carefully choosing the relevant zeolite crystal, the big family of zeolites can be utilized as directing template database for orienting a large number of guest molecules to estimate their structures by polarized Raman spectroscopy.     

Stimulated Raman forward scattering with a divergent or convergent pump beam

Stimulated Raman forward scattering is investigated for a divergent or convergent pump beam. The rate equations for this case are derived and analytically solved, including pump depletion. The solutions are compared to an experiment where the generation of a Stokes pulse from noise is investigated with a pump beam of variable divergence. Theoretical and experimental results are in good agreement. Stimulated Raman forward scattering thus may be controlled by the beam geometry, e.g., in Raman pulse compressors. An example is discussed in the Appendix.     

Stimulated rotational Raman scattering in the atmosphere

Theoretical and experimental information in the literature is used to calculate the gain coefficient for stimulated rotational Raman scattering by atmospheric N2and O2. The dependence on laser wavelength and polarization, as well as on the pressure and altitude is discussed. It is pointed out that because of pressure broadening, the gain coefficient is independent of altitude up to an altitude of 40-50 km, where the Raman transition becomes Doppler broadened. The 1 percent conversion threshold for vertical propagation from the ground up is calculated for various transitions and laser beam characteristics. The highest-gain transition, theS(8) transition of N2, is shown to have a 1 percent conversion intensity threshold of ∼ 1.2 MW/cm2for linearly polarized light at a wavelength of 400 nm.     

Knots, symmetry, and scattering

We investigate the physics underlying the scattering of circularly polarized plane waves from a toroidal knot and related unknot. We find that backscattering along the axis of rotational symmetry from trefoil knots is cross polarized; copolarized backscatter is in the numerical noise. In contrast, untrefoils give appreciable backscattering cross sections for both polarizations. We also study an intermediate class of structures: morphs, which provides a geometrical bridge between trefoil and untrefoil. We pursue physical insight for the depolarization associated with asymmetric objects, tracing the connection from the induced charge distribution, to the induced moments, to the polarization of the backscattered field     

通过圆偏振光提高飞秒激光诱导击穿光谱的发射强度

激光诱导击穿光谱(LIBS)是一种快速、实时的元素成分分析技术。为了提高LIBS的灵敏度,人们已经提出多种方法来提高LIBS的光谱强度。本文采用飞秒脉冲激光烧蚀黄铜产生LIBS,对比了圆偏振和线偏振下LIBS光谱的强度,结果发现圆偏振下的光谱强度比线偏振下的强,光谱强度大约提高了15%。采用飞秒激光照射金属时,金属内部的自由电子吸收光子的能量。在线偏振飞秒激光场中,电子在脉冲的每个光学周期中经历交替的加速和减速;而圆偏振飞秒激光可以连续加速电子,因此电子可以获得更高的能量,这使得圆偏振飞秒激光产生的光谱强度不同于线偏振飞秒激光产生的光谱强度,圆偏振激光有助于改善飞秒LIBS信号的强度。     

小型化低雷达散射截面圆极化微带天线研究

提出了一种改进的小型化低雷达散射截面（Radar CrossSection,RCS）圆极化微带天线．基于天线的辐射接收理论,研究天线散射的基本理论,提出了该圆极化天线散射的分析与控制方法．对典型的圆极化微带天线,通过在天线地板上添加圆形和矩形结构,令参考天线在保持电性能指标不变的前提下,实现了天线的小型化和低RCS特性．实验结果表明：天线的RCS被明显降低,具有大于50%的小型化效果．     

Traveling-wave tube analysis of the Adler tube

The cyclotron waves on an electron beam in the pump field of the Adler tube are studied with respect to their circular polarization. It is found that positive and negative circularly polarized beam waves are coupled through the pump electric field. As a consequence of the coupling, the small signal traveling-wave analysis leads to the conclusion that the positive circularly polarized beam wave is comprised of the fast signal wave and all idlers of frequenciesn(omega pm omega_{p})where n is an even integer, while the negative circularly polarized wave is comprised of all idlers with odd integral values of n. This information appears to be new, and can perhaps be used to design input and output couplers which discriminate against unwanted idler waves.     

基于主动式圆偏振光照射的探测建模与分析

圆偏振光有良好的“偏振记忆”效应,为了验证在 强散射介质中通过圆偏振光的旋性 差异可以有效抑制散射光的影响,实现复杂背景下的目标探测功能。从经典的米氏散射 理论和菲涅尔反射理论出发,结合偏振蒙特卡罗方法,构建了主动式圆偏振光在散射介质中 探测目标的模型,通过追踪每个光子的偏振态变化,统计分析了圆偏振光经过介质散射和目 标反射后的Stokes矢量信息。仿真结果表明,文中建模方法可以明显区分出散射介质中是否 存在被探测目标;不同的介质环境和不同的探测距离都会对目标的偏振成像造成不同程度的 衰减,而圆偏振差分成像及圆偏振度成像均可实现偏振成像的增强处理。文中建模方法可以 为全偏振探测的理论研究及实际应用提供借鉴。     

泵浦光椭圆度对铷原子极化率的影响

在核磁共振陀螺系统中,需要用圆偏振光来泵浦碱金属（例如铷）原子,使其产生极化。然而实际应用中的泵浦光不是理想的圆偏振光,为了研究泵浦光椭圆度与铷原子极化率的变化关系,推算了泵浦光经过四分之一波片后的表达式,并由此得出泵浦光照射铷原子气室前的椭圆度,建立了数值仿真模型,研究了椭圆度对铷原子极化率的影响。同时通过实验得到了铷原子极化率与椭圆度的关系曲线。仿真和实验结果表明,泵浦光椭圆度越大,铷原子极化率越大,随着椭圆度的进一步增大,铷原子极化率增速变慢逐渐饱和;此外,相同的椭圆度下,铷原子极化率随着泵浦光功率的增大而增大。     

Complex signal received through an anisotropic panel intercepting acircularly polarized beam

An inclined planar panel, which is generally anisotropic, intercepting a circularly polarized beam introduces loss and phase shift, and depolarizes the received signal. Orthogonal linearly polarized waves aligned to the symmetry planes are often used in the measurement of such panels. Using these data, convenient graphs are presented to illustrate the loss and phase shift of a circularly polarized beam, as well as the amount of depolarization into an orthogonal circularly polarized channel, without explicitly determining the intermediate axial ratio or polarization ellipse tilt angle of the emerging wave     

毫米波圆极化单脉冲阵列天线的研究

在腔模理论基础上分析了单阵元圆极化微带贴片天线,研究了圆极化微带阵列天线的设计方法,采用相位旋转法设计了中心频率为35.75GHz的64元圆极化微带阵列天线与平面和差结构,使天线和和差馈电处于一个平面.利用Ansoft公司的HFSS软件进行了仿真优化.经过对天线实物的测试,此天线提供了21dB的增益,具有良好的轴比和驻波比,差波束零值深度大于20dB.     

圆偏振光偏振复用激光通讯系统

提出了一种采用圆偏振光的偏振复用通讯系统。其基本设想是信号光在空间中以左 旋和右旋圆偏振光的形式进行传播,在接收端将这两种偏振光区分并分别接收。本文详细论证了在两个相对运动点之间采用圆偏振光实现偏振复用激光通讯的可行性。     

受激旋转喇曼散射对强紫外光光束质量的影响

在惯性约束核聚变中,为了获得最佳的燃料压缩,到达靶上的光束能量必须均匀,要求有非常好的光束质量。采用瞬态受激旋转喇曼散射(SRRS)模型,对强紫外激光束在空气中长程传输过程产生的SRRS效应进行了研究。讨论了产生SRRS效应的阈值条件,以及衍射效应和光强热斑对强紫外激光光束质量的影响。研究结果表明,衍射效应对阈值条件影响较小,但却使激光光束质量下降,光强热斑则不仅使SRRS效应的阈值降低,而且还会导致激光光束质量明显变差。     

双光栅纯转动拉曼测温激光雷达单色仪的光学设计

纯转动拉曼测温激光雷达中采用双光栅单色仪结构的优点是能提供优于10-7抑制比,能够很好地抑制瑞利-米氏散射,得到纯度很高的转动拉曼谱,并且能够提高光的透射率,具有长期稳定性,但双光栅单色仪的光路结构复杂一直是系统中的难点。首先对两种光栅光路结构进行讨论,得出光纤对称式光栅光路结构对于光栅效率的利用要优于光纤直线排列式光路结构,并通过对光栅公式和转动拉曼光谱公式结合,推导出光栅闪耀级次和衍射角关系方程。当光栅常数为600line/mm时,其第5级为最佳闪耀级次。     

倾斜波束圆极化天线的CSRR-FS结构设计研究

为了拓展倾斜波束圆极化天线(Circularly Polarized Antenna with Tilted Beam,CPA-TB)的带宽,在分析了该天线结构及其特征的基础上,提出了一种花纹式螺旋臂(Flower-Spiral,FS)协同互补开口谐振环(complementary split ring resontor,CSRR)的CSRR-FS结构.该结构通过FS增加了天线表面电流路径,实现了天线驻波比(voltage standing wave ratio,VSWR)的拓展;利用CSRR改变了低频点反射板的电流分布,实现反射板小型化的同时,减小了天线的交叉极化分量,增强了天线的辐射性能,拓展了天线的轴比带宽.仿真和实测结果表明:与普通的平面反射板相比,在保证圆极化、波束倾角30°的条件下,基于CSRR-FS结构的CPA-TB相对带宽为24%(5.5-7GHz),天线工作带宽拓展了1.8倍.CSRR-FS结构的提出为CPA-TB带宽的拓展提供了新的途径.     

反共振空芯光纤中氘气受激拉曼散射实验研究

报道了基于反共振空芯光纤的氘气单程和级联受激拉曼散射实验研究,详细分析了反共振空芯光纤中氘气受激拉曼散射的过程,研究了输出光谱和拉曼谱线功率随氘气压强和泵浦激光功率的变化规律,指出降低气压、采用峰值功率相对较低的泵浦脉冲可以有效抑制转动受激拉曼散射,提高振动受激拉曼散射效率。此外,通过进一步设计、拉制传输带位置合适、带宽较窄的反共振空芯光纤,利用1064 nm脉冲激光泵浦,可以实现高效的氘气一阶振动斯托克斯光(1561 nm)和二阶级联振动斯托克斯光(2925 nm)输出。