部分相干厄米高斯光束在斜程大气湍流中的扩展

基于广义惠更斯-菲涅耳原理,推导出部分相干厄米高斯（H-G）光束通过斜程大气湍流传输的均方根束宽和角扩展的解析表达式,并用以研究了传输路径和光束阶数对部分相干H-G光束传输的影响。引入相对束宽和相对角扩展来定量描述光束抗拒大气湍流的能力。结果表明:在相同条件下,部分相干H-G光束斜程传输受大气湍流的影响要小于水平传输,与水平传输相比斜程传输更有利于部分相干光束在大气湍流中的传输,光束阶数越大部分相干H-G光束受大气湍流的影响越小。     

The temperature dependence of intensity and the function of the transition dipole moment for the induced optical absorption bands of oxygen in the region of the Herzberg photodissociation continuum (200–230 nm)

Application of coherent interaction of laser light with a focused ultrasonic wave to the technique of acoustooptic visualization in multiple-scattering media is discussed. By analyzing spatial distribution of the optical radiation modulated by ultrasound (the photocurrent at ultrasonic frequency), images of large-sized inhomogeneities embedded into the scattering medium have been obtained. A light-absorbing half-plane and a square with sides of 5 mm were used as the inhomogeneities. The visualization was performed under optimal conditions for measuring the alternating photocurrent calculated for the proposed model of coherent interaction between the laser and acoustic beams (the Raman-Nath diffraction). The alternating current at the ultrasonic frequency was obtained as a result of mixing the waves of the diffraction fields on the detector’s photocathode. All experimental values were obtained from a single measurement without averaging the alternating photocurrent at the ultrasonic frequency of 3 MHz, with the scattering parameter varying up to μL≤37.5, where μ is the extinction coefficient and L is the thickness of the scattering medium along the laser beam axis. The measured quantities varied in the course of the measurements by more than 10 orders of magnitude.     

Influence of the illumination coherency and illumination aperture on the ptychographic iterative microscopy

While ptychography is an algorithm based on coherent illumination,satisfactory reconstructions can still be generated in most experiments,even though the radiation sources that are used are not ideally coherent.The underlying physics of this phenomenon is that the diffraction patterns of partially coherent illumination can be treated as those of purely coherent illumination by altering the intensities of the diffracted beams relative to their real values.On the other hand,due to the inconsistency in the altering interference among all the diffraction beams,noise/distortion is always involved in the reconstructed images.Furthermore,for a weak object,the noise/distortion in the reconstruction can be mostly reduced by using a highly curved beam for illumination in the data recording and forcing the dark field diffraction to be zero in the reconstruction.     

Effect of diffraction on stimulated Raman scattering of laser radiation in the middle atmosphere

The theoretical aspects of propagation of laser radiation along slanted paths in the atmosphere at high altitudes under conditions of stimulated Raman scattering (SRS) by molecular nitrogen are considered. The SRS power conversion coefficient, the transverse intensity distribution, and the effective size of laser beams with different initial spatial profiles are numerically simulated along the propagation path. The behavior of these parameters is studied for different ratios between the optical path length and the length of free diffraction of the initial laser beam. It is shown that, at a given level of the increment of the SRS amplification, the diffraction can both increase and decrease the efficiency of the Raman interaction of waves owing to the intensity redistribution in the transverse beam profile. This effect is the most pronounced for beams with non-Gaussian initial spatial intensity profiles.     

Concept of formation length in radiation theory

The features of electromagnetic processes are considered which connected with finite size of space region in which final particles (photon, electron–positron pair) are formed. The longitudinal dimension of the region is known as the formation length. If some external agent is acting on an electron while traveling this distance the emission process can be disrupted. There are different agents: multiple scattering of projectile, polarization of a medium, action of external fields, etc. The theory of radiation under influence of the multiple scattering, the Landau–Pomeranchuk–Migdal (LPM) effect, is presented. The probability of radiation is calculated with an accuracy up to “next to leading logarithm” and with the Coulomb corrections taken into account. The integral characteristics of bremsstrahlung are given, it is shown that the effective radiation length increases due to the LPM effect at high energy. The LPM effect for pair creation is also presented. The multiple scattering influences also on radiative corrections in a medium (and an external field too) including the anomalous magnetic moment of an electron and the polarization tensor as well as coherent scattering of a photon in a Coulomb field. The polarization of a medium alters the radiation probability in soft part of spectrum. Specific features of radiation from a target of finite thickness include: the boundary photon emission, interference effects for thin target, multi-photon radiation. The theory predictions are compared with experimental data obtained at SLAC and CERN SPS. For electron–positron colliding beams following items are discussed: the separation of coherent and incoherent mechanisms of radiation, the beam-size effect in bremsstrahlung, coherent radiation and mechanisms of electron–positron creation.     

高重复频率相干衍射辐射超宽谱太赫兹源

研究了一种新型相干衍射辐射（CDR）太赫兹源,该太赫兹源基于中国工程物理研究院自由电子激光相干强太赫兹源（FEL-THz）的加速器电子束束流,具有MHz量级重复频率。理论分析、数值计算和PIC模拟证明了此太赫兹源时间长度可达到ps量级,中心频率在100~400 GHz可调,截止频率位于1~2 THz,脉冲峰值功率可达10 kW量级,平均功率可达到W量级,并且功率随束流流强成平方正比关系。     

Calculation of light fields of concentrated sources in turbid media with strongly anisotropic scattering

An efficient approach to solving the equation of radiation transfer in a turbid medium with strongly anisotropic scattering is proposed based on the small-angle approximation of the theory of radiation transfer with allowance for the dispersion of the radiation propagation path length, as well as with allowance for polarization. The approach is applied to the solution of the problem of the field of a pointlike isotropic radiation source in a two-dimensional medium.     

Two-dimensional vectorial photonic crystals formed in azo-dye-doped liquid crystals

A 2D, square lattice, vectorial photonic crystal is formed by vectorial holography using an azo-dye-doped liquid-crystal material. Four coherent beams are interfered to generate a highly stable, switchable polarization holography structure. The formation of the vectorial lattice by the liquid-crystal molecular orientation is confirmed by diffraction pattern and polarization microscopy analysis. Simulations of the alignment and diffraction pattern are in good agreement with the experimental results. Polarization sensitive diffraction behavior is also consistent with the Kakichashvili model.     

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.     

Influences on the beam propagation ratio M

A beam is described by its transverse power density distribution, its wavefront, and its transverse distribution of coherence. The beam propagation ratio, M², is expressed as the square root of the sum of each of the three beam attributes. The shape of the power density distribution governs the diffraction of the beam. The stronger it is modulated, the higher the M². In comparison to an non-aberrated beam, the M² of a beam with an aberrated wavefront is increased. Similarly, the M² of a partially coherent beam is higher than the one of a fully coherent one. A concise formula is derived. The components of M² of a mixture of Hermite–Gaussian beams and of a spherically aberrated Gaussian beam are given as examples.     

用于实现散射介质中时间反演的数字相位共轭的相干性

抑制散射介质对光的散射,调控光在散射介质中的传输,是光通信、生物光子学、光镊等领域的重要课题.设计并实现了基于宽谱光源和数字相位共轭的可调控光在散射介质中传输的时间反演实验系统.实验获取了不同相干长度下物光和参考光束之间的光程差与干涉图样、相位图及时间反演信号之间的关系,分析了光源相干性对调控光在散射介质中传输的影响.实验结果表明,基于宽谱光源的相干特性和数字相位共轭技术,通过调节光程差能选择性获取同一散射角度及相同传输路径的光束的相对相位,再利用空间光调制器对参考光束进行调控,实现光束的反向传播,从而选择性实现对同一散射角度及相同传输路径的光的时间反演.     

空-频域中自成像分析和解释

基于魏格纳变换和魏格纳分布函数,分析讨论一维物体在空域-频域空间的自成像及其形成过程。从成像过程中各衍射频谱分量的光程差出发,给出Talbot效应和Montgomery效应的统一解释。对于周期物的Talbot效应,得到了用杨氏双缝干涉解释自成像现象的理论依据。周期物的自成像是物平面上间距为2倍周期,光程差为波长的整数平方倍的各衍射频谱分量同相相干迭加的结果。Montgomery效应是物平面上间距为抛物线关系,光程差为波长整数倍的各衍射频谱分量同相相干迭加的结果。     

Quantum fluctuations in second-harmonic light generation

Second-harmonic light generation (SHLG) is analyzed from the viewpoint of the photon statistics of the fundamental and generated beams versus the path traversed by the two waves in the medium. The calculations lead to an anti-bunching effect for coherent incident light.     

非傍轴双曲余弦-高斯脉冲电磁光束远场特性

利用矢量瑞利-索末菲衍射积分公式,推导出了非傍轴部分空间相干部分光谱相干双曲余弦-高斯(ChG)脉冲电磁光束在自由空间传输时交叉谱密度矩阵的远场解析公式,并用来表示脉冲电磁光束的光谱密度(光强)和偏振度。结果表明,对非傍轴远场部分空间相干部分光谱相干ChG脉冲电磁光束,其非傍轴性主要由参数f, f决定,而离心参数、脉冲宽度和时间相干长度影响其非傍轴行为。非傍轴部分空间相干部分光谱相干高斯-谢尔模型脉冲电磁光束的远场传输可作为特例处理。     

Propagation of single-cycle terahertz pulses in random media

We describe what are to our knowledge the first measurements of the propagation of coherent, single-cycle pulses of terahertz radiation in a scattering medium. By measuring the transmission as a function of the length L of the medium, we extract the scattering mean free path l(s)(omega) over a broad bandwidth. We observe variations in l(s) ranging over nearly 2 orders of magnitude and covering the entire thin sample regime from L/l(s)<1 to L/l(s)~10 . We also observe scattering-induced dispersive effects, which can be attributed to the additional path traveled by photons scattered at small angles.     

Optical resonant processes in thin excited films

It is shown that coherent processes of elastic scattering of resonant radiation form a “buffer” electromagnetic field near the boundary of excited media. This part of radiation is not governed by the standard refractive index, but precisely this part of radiation forms the beams reflected and refracted by the excited medium. The presence of the “buffer” field causes the suppression of stimulated emission near the boundary and leads to the appearance of a frequency angular broadening of the beam transmitted through a thin film of excited atoms at an oblique angle.     

Transmittance of partially coherent cosh-Gaussian, cos-Gaussian and annular beams in turbulence

Average relative power transmittance is evaluated, by incorporating atmospheric turbulence, for partially coherent cosh-Gaussian, cos-Gaussian, Gaussian and annular beams. For all the collimated versions of these beams, against the increasing propagation length, there is a typical trend of the decrease in the relative average power transmittance with incremental drop being much less for partially coherent cos-Gaussian beams. The change in the transmittance versus the propagation length will be similar to the corresponding collimated cases, when these beams are focused at a certain focal length. Also partially coherent beams are less sensitive to propagation length changes, except for cos-Gaussian case. Partially coherent cosh-Gaussian beams exhibit a drop in the transmittance as the displacement parameter of the beam is made larger, whereas this trend is just the opposite for partially coherent cos-Gaussian beams. When examined versus the source size, for all the four types of beams, the transmittance has a similar behavior, i.e., it becomes high at small source sizes, falling with increasing source size, and following a dip, it starts to rise, eventually approaching the plane wave limit of unity. The occurrence of the dip coincides with the smallest source size for cosh-Gaussian, with the largest for cos-Gaussian, and about the same source size for Gaussian and annular beams. In general, the average relative power transmittance of coherent beam is affected much more than the partially coherent beams against the variations in source properties.     

自成像局域空心光束产生的新方法及粒子俘获

本文提出了产生自成像局域空心光束(self-imaged bottle beams)的一种光学元件------液体轴棱锥.从衍射理论结合几何光学对经过轴棱锥后的光场进行了分析, 得出注入液体折射率小于轴棱锥材料折射率时可产生自成像局域空心光束.并通过软件MathCAD模拟, 得到一个完整周期光束的变换过程和局域空心光束的演变过程.研究发现液体轴棱锥产生的自成像局域 空心光束具有周期及相干长度可调的特点.分析了如何利用自成像局域空心光束对粒子进行俘获, 讨论了用自成像局域空心光束进行多层面粒子俘获的优势.     

Influence of atmospheric turbulence on the propagation of superimposed partially coherent Hermite-Gaussian beams

The influence of atmospheric turbulence on the propagation of superimposed partially coherent Hermite-Gaussian (H-G) beams is studied in detail. The closed-form propagation equation of superimposed partially coherent H-G beams through atmospheric turbulence is derived. It is shown that the turbulence accelerates the evolution of three stages which superimposed partially coherent H-G beams undergo. The turbulence results in a beam spreading and a decrease of the maximum intensity. However, the larger the beam number M, the beam order m, the separate distance x_d, and the smaller the beam correlation length σ₀ are, the less the power focusability of superimposed partially coherent H-G beams is affected by the turbulence. Specially, superimposed partially coherent H-G beams are less sensitive to turbulence than superimposed fully ones, and than partially coherent H-G beams if the beam power focusability and the maximum intensity are taken as beam criterions. However, the maximum intensity of superimposed partially coherent H-G beams is less sensitive or more sensitive to turbulence than that of superimposed Gaussian Schell-model (GSM) beams depending on σ₀.     

Nonparaxial propagation of spatially and spectrally partially coherent electromagnetic Cosh-Gaussian pulsed beams

Based on the generalized Rayleigh–Sommerfeld diffraction integral, the analytical expression for 3×3 cross-spectral density matrix of nonparaxial spatially and spectrally partially coherent electromagnetic Cosh-Gaussian (ChG) pulsed beams propagating in free space is derived, and used to formulate the spectral density and spectral degree of polarization of electromagnetic pulsed beams at the z-plane. It is found that the parameters f and f_αα are the key parameters in determining the nonparaxiality of spatially and spectrally partially coherent electromagnetic ChG pulsed beams. And the decentered parameter, pulse duration and temporal coherence length can change the nonparaxial behavior of the electromagnetic ChG pulsed beams. The effect of decentered parameter, pulse duration and temporal coherence length on the spectral density and spectral degree of polarization of electromagnetic ChG pulsed beams is illustrated through numerical calculations. Propagation of nonparaxial spatially and spectrally partially coherent electromagnetic Gaussian Schell-model pulsed beams can be treated as a special case when the decentered parameter of electromagnetic ChG pulsed beams approaches to zero.