Measuring spatial coherence by using a mask with multiple apertures

A simple method to measure the complex degree of spatial coherence of a partially coherent quasi-monochromatic light field is presented. The Fourier spectrum of the far-field interferogram generated by a mask with multiple apertures (small circular holes) is analyzed in terms of classes of aperture pairs. A class of aperture pairs is defined as the set of aperture pairs with the same separation vector. The height of the peaks in the magnitude spectrum determines the modulus of the complex degree of spatial coherence and the corresponding value in the phase spectrum determines the phase of the complex degree of spatial coherence. The method is illustrated with experimental results.     

Experimental study of coherence vortices: local properties of phase singularities in a spatial coherence function

By controlling the irradiance of an extended quasimonochromatic, spatially incoherent source, an optical field is generated that exhibits spatial coherence with phase singularities, called coherence vortices. A simple optical geometry for direct visualization of coherence vortices is proposed, and the local properties and the spatial evolution of coherence vortex are experimentally investigated. To our knowledge, this is the first direct and quantitative experimental measurement of a generic coherence vortex.     

Surface plasmons modulate the spatial coherence of light in Young's interference experiment

It is shown how surface plasmons that travel between the slits in Young's interference experiment can change the state of spatial coherence of the field that is radiated by the two apertures. Surprisingly, the coherence can both be increased and decreased, depending on the slit separation distance. This results in a modulation of the visibility of the interference fringes. Since many properties of a light field-such as its spectrum, polarization, and directionality - may change on propagation and are dependent on the spatial coherence of the source, our results suggest that the use of surface plasmons provides a new way to alter or even tailor the statistical properties of a light field.     

Longitudinal purely spatial coherence of a light field

The effects of longitudinal purely spatial coherence of light and the results of observation of these effects in an interference experiment are considered under the condition that the length of temporal coherence l _c is considerably smaller than the length of longitudinal spatial coherence ρ_‖ of the field. It is shown that, for l _c ? ρ_‖, the longitudinal purely spatial coherence of the light field in fact governs the coherence of the wave train in the process of its propagation. The length and the time of coherent (“free”) path of the wave train are considered as new spatial and temporal scales of a partially coherent light field.     

空间相位调制对强非局域空间光孤子的影响

利用空间相位调制技术和数值分析的方法，讨论了高斯光束在强非局域非线性平板波导中的传输问题，发现只要介质的响应具有对称性,光束就会有相同的演化规律，可控的空间相位调制参数不仅能让光束自偏转还能实现分束，其全新特性在全光开关及分光器上有潜在的应用价值. 关键词： 强非局域空间光孤子 空间相位调制 光偏转 光开关     

Retrieving the complex degree of spatial coherence of electron beams

The possibility to characterize the coherence properties of an electron source is presented. The method, based on the determination of centered-reduced moments of the beam spot, allows the evaluation of both amplitude and phase of the complex degree of spatial coherence. The experimental results are in agreement with a different approach based on the Fourier analysis and with calculations according to the Van Cittert—Zernike theorem.     

Effects of spatial coherence on near-field spectra

Expressions are derived for the spectrum of the field generated by a planar, homogeneous, secondary source of any spectral distribution and of any state of spatial coherence. It is shown that the state of coherence affects the contributions of the homogeneous as well as the evanescent waves of the emitted field. The near-field spectra are studied in detail. The analysis is illustrated by examples.     

Measurements of complex degree of spatial coherence at the end face of an optical fiber

The phase and modulus of the complex degree of spatial coherence at the exit end face of an optical fiber are measured with the help of a real-time data processing computer. Interference fringes produced by superimposing wavefronts of the image of the end cross-section are scanned by a linear photodiode array to calculate the one dimensional value of spatial coherence. Evolution of phase measured at the different positions in the fringe pattern reveals that the phase is intimately related to modal structures of the propagating field in an optical fiber supporting a small number of guided modes.     

The coherence of reverberant sound fields

A new method of measuring spatial correlation functions in reverberant sound fields is presented. It is shown that coherence functions determined with appropriate spectral resolution contain the same information as the corresponding correlation functions, and that measuring such coherence functions is a far more efficient way of obtaining this information. The technique is then used to verify theoretical predictions of the spatial correlation between various components of the particle velocity in a diffuse sound field. Other possible applications of the technique are discussed and illustrated with experimental results obtained in an ordinary room.     

Phase control in an open Λ-type system with spontaneously generated coherence

This paper investigates the control role of the relative phase between the probe and driving fields on the gain, dispersion and populations in an open A system with spontaneously generated coherence （SGC）. It shows that by adjusting the value of the relative phase, a change from lasing with inversion to lasing without inversion can be realized; the values and frequency spectrum regions of the inversionless gain and dispersion can be obviously varied; high refractive index with zero absorption and electromagnetically induced transparency can be achieved. It is also found that when the driving field is resonant, the shapes of the dispersion and the gain curves versus the probe detuning are very similar if the relative phase of the dispersion lags π/2 than that of the gain, however for the off-resonant driving field the similarity will disappear; the gain, dispersion and populations are periodical functions of the relative phase, the modulation period is always 2π; the contribution of SGC to the inversionless gain and dispersion is much larger than that of the dynamically induced coherence.     

Relationship between spectral properties and spatial coherence properties in one-dimensional free fields

Some consequences of a recently derived relationship between the spectral properties and the spatial coherence properties of a stationary optical field are studied for the case when the field is one-dimensional. It is found that the spectrum may be determined from the knowledge of the spatial coherence of the field at all pairs of points that lie within an interval whose length is at least as great as the coherence length of the light.     

A study on spatial coherence using quadratic radially distributed apertures (application to confocal imaging)

We study the spatial coherence problem using an amplitude modulation applied to confocal imaging systems. This type of modulation assumes a quadratic radial distribution. The mutual coherence intensity or the coherence factor is calculated and compared with the results obtained for clear circular apertures.     

Emergence of X-shaped spatiotemporal coherence in optical waves

Considering the problem of parametric nonlinear interaction, we report the experimental observation of electromagnetic waves characterized by an X-shaped spatiotemporal coherence; i.e., coherence is neither spatial nor temporal, but skewed along specific spatiotemporal trajectories. The application of the usual, purely spatial or temporal, measures of coherence would erroneously lead to the conclusion that the field is fully incoherent. Such hidden coherence has been identified owing to an innovative diagnostic technique based on simultaneous analysis of both the spatial and temporal spectra.     

Optimisation using measured Green's function for improving spatial coherence in acoustic measurements

Aberrating materials can degrade acoustic measurements by distorting the acoustic wavefront and causing acoustic speckle (as opposed to speckle noise which is a manifestation of coherent backscatter). The amplitude and phase fluctuations associated with acoustic speckle can introduce considerable measurement uncertainty which is difficult to deal with. This paper demonstrates a new technique which optimises the spatial distribution of the generation of the ultrasound to compensate for the aberration. This technique uses experimentally measured Green's functions to allow the calculation of the field resulting from the generation wavefront during optimisation. The technique is used to improve the accuracy of velocity measurements in a steel sample using 82 MHz SAW waves. This is achieved by optimising for improved spatial coherence in the measurement region which suppresses the speckle noise. Experimental evidence of acoustic aberration arising from grain structure is shown for steel and aluminium and the measured Green's function optimisation technique is shown to overcome the resulting acoustic speckle. The technique was performed using the Adaptive Optical Scanning Acoustic Microscope (AOSAM) at Nottingham University, UK.     

The effects of longitudinal spatial coherence of light in interference experiments

The physical conditions needed to observe the effects of longitudinal spatial coherence of emission of an extended thermal source are considered. Results of experiments on observation of interference pulses of longitudinal spatial coherence, performed using a scanning longitudinal shearing Michelson interferometer, are presented. Distinctions between the effects of temporal and longitudinal spatial coherence of light are established. It is shown that the presence of a noncompensated optical layer in one of the arms of the interferometer makes the interference pulses of the temporal and spatial coherence of light shift relative to each other (“recession”).     

First direct measurement of the spatial coherence of sunlight

Direct sunlight is often deemed incoherent, hence unsuitable for antenna power conversion. However, all radiation exhibits spatial coherence when detected on a sufficiently small scale. We report the first direct measurement of the spatial coherence of solar beam radiation, achieved with a customized tabletop cyclic-shearing interferometer. Good agreement is found between experiment and theory, with promising ramifications for solar aperture antennas.     

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     

激光空间相干性对照明均匀性的影响

 研究了激光相干性对照明均匀性的影响，为照明激光器的选择提供了理论参考。其中，部分相干高斯光束分解为模式间相互独立的厄米-高斯光束的迭加。采用相位屏的近似处理方法对激光通过大气湍流的传输进行计算模拟。数值模拟的结果表明：当照明光束的空间相干性降低时，其照明均匀度逐步提高。因此对于照明激光器而言，选择空间相干性较差的激光器对其照明均匀度更加有益。     

Point sources and rays in the phase-space representation of random electromagnetic fields

Random stationary electromagnetic fields can be modeled in terms of point sources disposed onto two distinct layers, named the radiant and the virtual layers. It allows describing the propagation of the power and the states of spatial coherence and polarization of the field by means of separate phase-space ray-maps. Spatial coherence of the field, correlation of the polarization angles and achievement of the Fresnel-Arago laws within the structured spatial coherence supports are the necessary conditions that determine the electromagnetic point sources in the virtual layer. The contributions of these sources produce two types of modulations on the electromagnetic field at the observation plane, one of them of scalar type is the interferometric power modulation, and the other one of vector type is de modulation of the polarization state provided by the radiant point sources, according to the Stokes' parameters. Some examples are presented.