Effect of atmospheric turbulence upon the spatial coherence of laser radiation

The present article is an analysis of the effect of atmospheric turbulence upon the spatial coherence of laser radiation. Young's experiment was used for an experimental study of the coherence of radiation traversing path lengths of 100 and 200 m in the atmosphere.The experimental measurements of the relation between the coherence function of the radiation and the temperature gradient are in strong agreement with the theoretical values. The relation between the coherence function and the distance between the field points considered disagrees with the theoretical values. The discrepancy can be explained by the limitations of the approximation of geometrical optics.     

Spatial coherence in the transition radiation spectrum

The formal expression of the spectral distribution of the transition radiation intensity will be here derived in the case of a relativistic three-dimensional charged beam. Charged beams with a particle density such as is typically encountered in a particle accelerator will be considered. In particular, a sufficiently high particle density will be supposed so that a continuous spatial distribution function can be reliably attributed to the charged bunch. The formula of the spectral distribution of the transition radiation intensity originated by a relativistic three-dimensional charged beam - already presented in a previous work - will be here submitted to a formal check and interpreted in the physical consequences. The present work contains an additional mathematical derivation of the radiation energy spectrum consisting in a different method to implement the continuous limit in the distribution function of the particle coordinates. In the former derivation of the formula, the average operation with respect to the continuous distribution function of the particle coordinates was applied to the radiation intensity of a N electron bunch. In the present one, it is applied to the radiation electric field of a N electron bunch. The comparison of the two alternative but in any case equivalent formal routes to the spectral distribution of the transition radiation intensity will offer the possibility to directly cross-check the mathematical self-consistency of the presented results within the limits of applicability of the continuous limit approximation. According to such results, both the flux and the angular distribution of the photons emitted at a given wavelength - even shorter than the longitudinal length of the bunch - are expected to undergo a modification as the beam transverse size is varied with respect to the observed wavelength. As a function of the beam transverse size the spatial coherence degree of the transition radiation source is thus expected to change. The physical consistency of such an effect occurring in the transition radiation emission by a charged beam can be argued on the basis of a compatibility criterion with other similar relativistic electromagnetic radiative phenomena and interpreted in the framework of the temporal causality and the Huygens-Fresnel principles. Finally, the aspect of the applicability of the continuous limit approximation to the case of a charged beam in a particle accelerator is treated in terms of a practical quantitative criterion.     

Effect of spatial coherence on radiation forces acting on a Rayleigh dielectric sphere

We show that the radiation forces (RFs) on a Rayleigh dielectric sphere induced by a partially coherent light beam are greatly affected by the spatial coherence. We find that the magnitude of the RFs greatly decreases as the spatial coherence decreases and derive an inequality for the required correlation width sigma(0) (i.e., the spatial coherence of the beam) to stably trap the particles.     

The influence of the radiation spectrum bandwidth on the resolution of an image up-converter

The influence of the infra-red spectrum bandwidth on the transverse resolution of an image up-converter is investigated. It is shown that by suitable placing of the infra-red image plane, the deterioration of the resolution due to the infra-red spectrum bandwidth does not occur.     

The effect of spatial coherence of sound field on subspace-based high-resolution spatial spectrum estimation

When the signals received by any two array elements are not fully correlated, the performance of DOA (Direction Of Arrival) estimation methods might degrade if the plane-wave model assumption is still used. For narrowband uniform linear array, by utilizing the property of Toeplitz matrix, it is proved that the received signals can be expressed as the sum of several plane-wave signals, which can be viewed as the "splitting" parts of the true sources. The "splitting" of sources makes it difficult to estimate the true number of sources. As a result, spurious peaks might occur in the spatial spectrum when subspace-based high-resolution spatial spectrum estimation technique is used.     

微扫描对空间分辨率的影响

在详细分析使用微扫描技术减少混淆效应的基础上，给出一种利用微扫描技术进行空间分辨率影响分析的像素传递函数（PTF）方法，并基于2×2和4×4两种微扫描形式对该函数进行分析计算和讨论。计算结果表明，在焦平面阵列给定的情况下，微扫描技术能适当提高焦平面空间的分辨率，有效减小图像混淆，明显改善成像质量。指出在没有高空间分辨率成像器件的情况下，利用微扫描技术和低分辨率成像器件来获得较高空间分辨率的图像是完全可能的。     

上海同步辐射装置波荡器光源空间相干性的研究

利用高斯-谢尔光源模型理论与交叉谱密度函数在自由空间传输的规律, 研究了上海同步辐射装置波荡器光源(BL15U)的空间相干性; 讨论了预聚焦镜、单色器对光束空间相干性的影响;实验测量了单色光狭缝S2处光束的横向相干长度(Z方向). 理论计算表明, S2处光束的横向相干长度为66.5 μm,但实验测量结果为27 μm. 理论与实验相差较大的原因是由于S2上游光学元件预聚焦镜或单色器周期性高频振动导致了光束空间相干性的退化.实验结果表明,上海同步辐射装置波荡器光源已有较强的空间相干性, 可以满足微米尺度的硬X射线相干性实验.     

Influence of the spatial coherence of X-ray radiation on mirror reflection from multilayered mirrors

The influence of the degrees of spatial coherence of random modulated X-ray radiation on the intensity of the diffractive reflection of such radiation from a multilayered mirror, and on the spatial coherence function of the reflected field (depending on the statistical characteristics of the incident radiation and the parameters of the multilayered periodic structure), is considered.     

Acousto-optic Bragg diffraction in paratellurite by the sidelobes of the spatial radiation spectrum of an acoustic transducer

Acousto-optic Bragg diffraction in paratellurite is investigated within the two first diffraction orders for the case of diffraction by the sidelobes of the spatial radiation spectrum of an acoustic transducer. One of the diffraction orders is due to anisotropic diffraction, and the other, to isotropic diffraction. Such a diffraction regime is achieved when the diffraction plane is inclined toward the optical axis of the crystal. For light with a wavelength of 0.63 × 10^–4 cm diffracted by a “slow” sound wave with a frequency of 26 MHz, the effect manifests itself when the angle between the acousto-optic diffraction plane and the optical axis of paratellurite is ~3°. The effect is experimentally verified. The diffraction efficiency is 20% for each of the diffraction orders for a microwave signal of 8 V at the transducer.     

激光器阵列的部分相干性对相干合成远场输出特性的影响

对于相干合成的光纤激光器阵列来说,如何定量的分析阵列的相干性对于评价阵列的性能具有极其重要的意义.文章推导了光纤激光器阵列部分相干合成远场强度分布的解析表达式,详细探讨了激光器阵列的部分相干性对相干合成远场图样分布、中心主瓣宽度、斯特列尔比、桶中功率的影响.最后采用主振荡并联放大器方案对三路掺镱光纤放大器进行实时相位探测与校正,实验给出了三路光纤放大器相干合成的结果. 关键词： 光纤激光器阵列 相干合成 相干度     

Second-harmonic conversion of neodymium glass laser radiation with controlled spatial coherence

The experimental results on the second optical harmonic conversion of neodymium glass laser radiation with controlled spatial coherence in a nonlinear KDP crystal are presented. The conversion efficiency is studied depending on the radiation power density on the crystal and the crystal rotation angle in the principal optical plane with respect to the radiation propagation direction. The experiments were performed in the radiation power density range I = 0.2–3 GW/cm² for the number of cavity transverse modes N = 100–1000, radiation divergence 2ч = 0.5–3.5 mrad, spectrum width of 26 Å, and pulse duration of 2.5 ns.     

激光器阵列的部分相干性对相干合成远场输出特性的影响

对于相干合成的光纤激光器阵列来说，如何定量的分析阵列的相干性对于评价阵列的性能具有极其重要的意义.文章推导了光纤激光器阵列部分相干合成远场强度分布的解析表达式，详细探讨了激光器阵列的部分相干性对相干合成远场图样分布、中心主瓣宽度、斯特列尔比、桶中功率的影响.最后采用主振荡并联放大器方案对三路掺镱光纤放大器进行实时相位探测与校正，实验给出了三路光纤放大器相干合成的结果.     

Measurement of the spatial coherence function of undulator radiation using a phase mask

A measurement of the horizontal coherence function of 7.9 keV radiation from an undulator beam line at the Advanced Photon Source is reported. X-ray diffraction from a phase-shifting mask was used, and the coherence function was measured as a function of the width of beam-conditioning slits in the beam line. The coherence distribution is found to be best described by a Lorentzian function.     

Measurement of the spatial coherence of hard synchrotron radiation using a pencil beam

We present a simple method to measure the spatial coherence of hard x-ray beams.Based on the convolution of Gaussian functions,we analyze the diffraction patterns of a grating irradiated by partially coherent hard x rays with a constrained beam diameter.The spatial coherence properties of an x-ray beam are obtained from the width of the diffraction peaks with high accuracy.The results of experiments conducted by combining a pinhole with a grating show a good agreement with our calculation using the Gaussian–Schell model.     

Effect of coherence and polarization on resolution of optical imaging system

The effect of coherence properties of illumination on image resolution, well known in a scalar case, is studied for the case of vector electromagnetic illumination. Using an example of vector Gaussian Schell-model illumination, we analyze the dependence of optical system resolution on the transverse correlation lengths of the orthogonal field components and on the ratio of the powers of these components, each taken separately.     

Frequency resolution and amplitude accuracy of the coherent optical spectrum analyzer with a swept local oscillator

We present a model for coherent optical spectrum analysis that relates the key parameters of frequency resolution, power uncertainty, and sweep speed. The theoretical model is compared with experimental results obtained from spectral measurements of an external-cavity laser.