Polarization changes and beam profile deformation of light during acousto-optic interaction in isotropic media

A spatial Fourier transform approach is proposed to investigate the effects of polarization changes and beam profile deformation of light during acousto-optic (AO) interaction in isotropic media. The behaviour of the total scattered optical fields inside the AO cell can be properly described by a vector wave equation of which the permittivity is perturbed by an acoustic wave propagating inside the medium. In the Bragg regime, using a spatial Fourier transform approach, two coupled differential equations can be derived from the wave equation to depict AO interaction in the spatial frequency domain. Analytic solutions, which comprise the effects of changing polarization, beam deformation and propagating diffraction, can be found from the coupled equations. Detailed numerical simulations, including Fourier transforming the incident light profile to calculate the spectra of the scattered light beams and, hence, their profiles in space using the inverse transform, are presented.     

Experimental research on spatial filtering of deformed laser beam by transmitting volume Bragg grating

Using transmitting volume Bragg gratings (TVBG) as a basis,an experiment on one-dimensional spatial filtering of a deformed laser beam is designed.The deformed laser beam results from a He-Ne laser beam modulated by an amplitude modulation plate with a spatial frequency of 7.2 mm·1.Results show that when the central wave vector of the deformed beam satisfies the Bragg law of TVBG,the spatial profile of the -1st forward-diffracted order is similar to that of the undeformed He-Ne laser beam due to the TVBG with a spatial frequency selective bandwidth of less than 5.0 mm·1.The higher frequency components of the deformed beam are filtered out in the optical near field.Thus,the TVBG cleanup of the spatiallydeformed laser beam is realized experimentally.     

Modal correction for fiber-coupling efficiency in free-space optical communication systems through atmospheric turbulence

High-speed free-space optical communication systems have recently used fiber-optical components. The coupling efficiency with which the received laser beam can be coupled into a single-mode fiber is noticeably limited by atmospheric turbulence due to the degradation of its spatial coherence. Fortunately, adaptive optics (AO) can alleviate this limitation by partially correcting the turbulence-distorted wavefront. The coupling efficiency improvement provided by Zernike modal AO correction is numerically evaluated. It is found that the first 3-20 corrected polynomials can considerably improve the fiber-coupling efficiency. The improvement brought by AO is compared with that brought by a coherent fiber array. Finally, a hybrid technique that integrates AO and a coherent fiber array is proposed. Results show that the hybrid technique outperforms each of the two above-mentioned techniques.     

Experimental generation of a partially coherent Laguerre–Gaussian beam

We report experimental generation of a partially coherent Laguerre–Gaussian beam with mode orders 0 and l (i.e., LG_0l beam) using a rotating ground-glass plate and a spatial light modulator. Focusing properties of the generated beam are studied both theoretically and experimentally. It is found that the focused intensity of the LG_0l beam is shaped through varying the initial spatial coherence, and our experimental results agree well with the theoretical predictions. Our results will be useful in particle trapping and free-space optical communications.     

Design and performance analysis of beam steering birefringent mercurous chloride acousto-optic device

I.IntroductionThepcrformanccsofAcousto-optic(AO)dcviccareoftenlimitcdbythepropertiesofAOmcdiawhenitisusedinAOsigna1processingsystems.Thespectrumanalyzersandcorre1ationreceivers,forinstancc,areexpcctcdtoimprovcthereso1utionanddynamicrangcwhilereducingtheproccssorvolume.Rccent1y,ancwclassofmateria1s,themercurousha1ides,promiscstocontributeimportantsolutionstosuchnccdsandisrefincdtopractica1uti1izations.Themecurousha1ideweconsidcrinthispapcrismecurouschloride(Hg,C1,).Thesing1ecrystalofHg,Cl…     

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.     

Use of induced spatial incoherence for uniform illumination of laser fusion targets

We report a technique to achieve very uniform illumination of laser irradiated targets by inducing a controlled amount of transverse spatial incoherence in the output beam of a broadband laser. The spatial incoherence is created by imposing different optical delays upon different transverse sections of the beam, and choosing the delay increments to be larger than the laser coherence time t_c=1/δν. This technique may allow easy attainment of the pellet illumination uniformity required for laser fusion.     

Effects of a modified spectral model on the spatial coherence of a laser beam

The influence of a modified (bump) spectrum of refractive index fluctuations on the spatial coherence of an optical wave is studied here and compared with that based on a von Karman spectrum. Analytical expressions are derived for the mutual coherence function (MCF) and wave structure function (WSF) of a lowest-order Gaussian beam wave from which the beam spot size and degree of coherence are deduced. The qualitative behaviour of beam spreading and coherence length is basically the same for both spectral models. Also, when the radius of the Fresnel zone and initial beam radius are of comparable size, the presence of a spectral bump appears to have minimal effect on spatial coherence for all beams. However, the choice of spectral model is important for certain ranges of parameters. In particular, the implied spatial coherence length for a collimated beam based on the modified spectrum is significantly smaller than that based on the von Karman spectrum whenever the Fresnel zone is either much larger or much smaller than the initial beam radius, whereas for a focused beam the predicted coherence length based on the modified spectrum is slightly larger when the Fresnel zone size is much smaller than the initial beam radius.     

部分相干双曲余弦-高斯光束的传输特性和空间整形

 通过加入高斯项的空间相干度，引入部分相干双曲余弦-高斯（ChG）光束，推导出部分相干ChG光束通过近轴ABCD光学系统传输的解析表达式，并用以研究其传输特性和空间整形。结果表明，部分相干ChG光束的光强分布不能保持传输不变性，其光强分布不仅与传输距离和离心参数有关，而且还与空间相干参数有关。在传输距离固定时，适当改变部分相干ChG 光束的离心参数和空间相干参数，可实现光束的空间整形。光束质量因子随空间相干性的增大和离心参数的减小而减小。高斯-谢尔模型光束和完全相干ChG光束可作为部分相干ChG光束的特例处理。     

Coherence vortex properties of partially coherent flat-topped vortex beams

The analytical propagation expression of partially coherent flat-topped vortex beams through a paraxial optical ABCD system is derived, and it then is used to investigate the coherence vortex properties of partially coherent flat-topped vortex beams in the Fourier transform and fractional Fourier transform systems. It is shown that in the Fourier transform system the coherence vortex depends on the flat-topped beam order N, spatial coherence parameter α and position (x ₁,y ₁) of the reference point, whereas in the fractional Fourier transform system the flat-topped beam order N does not affect the spectral degree of coherence. Furthermore, in both transform systems, depending on the choice of the reference point, the zero value point of the spectral degree of coherence may be present or absent. In particular, if x ₁=y ₁=0 is selected, the phase at the zero value point of the spectral degree of coherence may be determinate; thus the coherence vortex does not exist.     

孪生光束干涉法测量光源的空间相干性

定量测定光源空间相干性在部分相干光成像, 非相干全息术及光信息处理领域具有重要的研究价值. 本文基于三角全息干涉光路提出了一种测量光源空间相干性的新方法. 利用三角干涉全息光路系统中分束镜产生的孪生光束进行干涉获得干涉图, 通过调整光源中心位置在写入平面内偏离光轴的量, 改变两孪生光束空间分离量的大小, 采集对应的一系列干涉图, 计算干涉图样的对比度, 从而对光源照明空间的波前上一系列不同距离的点对之间的空间复相干度进行测量. 实验系统光路配置较为简单且不需要使用特殊加工的光学元件. 针对一个准单色的扩展光源设计并进行实验, 结果表明利用文中提出的方法可以准确的测量光源的空间相干性, 实验结果相对于理论计算值的误差仅为3.8%. 关键词： 相干性 全息干涉 干涉仪 光学应用     

Measurement for Scattering Media by Synthesis of Optical Coherence Function with Super-Structure Grating Distributed Bragg Reflector Laser Diode

The application of the technique of synthesis of optical coherence function for detection in scattering media is investigated. By modulating the optical frequency, the technique synthesizes the coherence function into a delta-function-like peak at an arbitrary location, and thus can select interferometrically the information at that location. The location is adjustable by the modulation parameter or additional phase modulation. A multi-section super-structure grating distributed Bragg reflector laser diode (SSG-DBR-LD) of THz-order tunable range is employed to enhance the spatial resolution for suppressing the multiple scattering from locations other than that detected. In a preliminary experimental demonstration, a reflectometry of 550 μm spatial resolution was achieved and was used to detect scattering media.     

部分相干洛伦兹-高斯光束在湍流大气中的峭度参数

基于广义惠更斯-菲涅耳衍射积分公式和洛伦兹函数的厄米-高斯展开,导出了部分相干洛伦兹-高斯光束在湍流大气中经傍轴ABCD光学系统的峭度参数的解析表达式,对该峭度参数进行了数值计算。结果表明:空间相干长度和结构常数对峭度参数的影响与部分相干洛伦兹-高斯光束本身的光束参数有关;当空间相干长度小于光束参数时,其影响显著;当洛伦兹部分的光束参数较大或高斯部分的光束参数较小时,结构常数的影响较明显且随传输距离的增大而增强。     

Role of spatial coherence in polarization tomography

We analyze an experimental setup in which a quasi-monochromatic spatially coherent beam of light is used to probe a paraxial optical scatterer. We discuss the effect of the spatial coherence of the probe beam on the Mueller matrix representing the scatterer. We show that, according to the degree of spatial coherence of the beam, the same scattering medium can be represented by different Mueller matrices. This result should serve as a warning for experimentalists.     

Bragg scattering as a probe of atomic wave functions and quantum phase transitions in optical lattices

We have observed Bragg scattering of photons from quantum degenerate ^{87}Rb atoms in a three-dimensional optical lattice. Bragg scattered light directly probes the microscopic crystal structure and atomic wave function whose position and momentum width is Heisenberg limited. The spatial coherence of the wave function leads to revivals in the Bragg scattered light due to the atomic Talbot effect. The decay of revivals across the superfluid to Mott insulator transition indicates the loss of superfluid coherence.     

Distributed strain sensor based on dynamic grating in polarization-maintaining erbium-doped fiber

A novel distributed fiber-optic strain sensor based on dynamic grating in polarization-maintaining erbium-doped fiber (PM-EDF) is demonstrated experimentally. The dynamic grating is written with light beams polarized along one primary polarization axis of the PM-EDF, and read with a light beam polarized along the other primary polarization axis. When a strain is applied to the fiber, the detected Bragg reflection frequency of the dynamic grating shifts in proportion to the birefringence change, which is proportional to the strain. By employing the technique of synthesis of optical coherence function, the dynamic grating is localized in a region that can be shifted to sweep along the fiber to determine the location of the strain. In the experiment, a spatial resolution of 20 cm is demonstrated.     

Acoustooptic multiplication of the frequency shift of optical radiation on the basis of bragg polarization splitting

A technique for the acoustooptic multiplication of the frequency shift of an optical beam is proposed. The technique is based on the cascade diffraction of the beam by a single acoustic wave with the use of Bragg polarization splitting in a uniaxial crystal. The fundamental possibility of the practical realization of the technique is confirmed experimentally by using anisotropic acoustooptic diffraction in LiNbO₃.     

Statistical similarity and cross-spectral purity of stationary stochastic fields

In practical situations, one often generates a beam by superposition of two or more light beams. The beam generated by superposition displays, in general, different spectral properties than do the original beams. However, there are some optical beams, called cross-spectrally pure beams, which can generate a light beam of identical spectral distribution on superposition. The relationship between cross-spectral purity and spatial coherence has been the subject of investigations for some time. Recently, a concept of so-called statistical similarity has been introduced which provides a new way to elucidate complete spatial coherence. In this Letter, we discuss some implications of statistical similarity of an optical field on its cross-spectral purity.     

Effect of spatial coherence on the scintillation properties of a dark hollow beam in turbulent atmosphere

With the help of a tensor method, we derive an explicit expression for the on-axis scintillation index of a circular partially coherent dark hollow (DH) beam in weakly turbulent atmosphere. The derived formula can be applied to study the scintillation properties of a partially coherent Gaussian beam and a partially coherent flat-topped (FT) beam. The effect of spatial coherence on the scintillation properties of DH beam, FT beam and Gaussian beam is studied numerically and comparatively. Our results show that the advantage of a DH beam over a FT beam and a Gaussian beam for reducing turbulence-induced scintillation increases particularly at long propagation distances with the decrease of spatial coherence or the increase of the atmospheric turbulence, which will be useful for long-distance free-space optical communications.