Spectral shaping to improve the point spread function in optical coherence tomography

We demonstrate inhibition of the sidelobes of the axial point spread function in optical coherence tomography by shaping the power spectrum of the light source with a remaining power of 4.54 mW. A broadband amplified spontaneous emission source radiating at 1565 +/- 40 nm is employed in a free-space optical coherence tomography system. The axial point spread functions before and after optical spectral shaping are presented. Results show that spectral shaping of the source can inhibit sidelobes of the point spread function up to 12.9 dB, with an associated small increase of 2.2 dB in noise floor in the far field. The effect of spectral shaping on axial resolution is demonstrated according to three metrics. Image quality improvement is also illustrated with optical coherence tomography images of an onion before and after spectral shaping.     

Partially coherent Fresnel diffraction by a slit aperture. IV. Effect of the phase term in the coherence function

A formula was derived in paper 1 of the this series for investigating the Fresnel diffraction field of a slit aperture when the mutual coherence function of the illumination contains a quadratic phase term. That formula is applied to study the intensity distribution in the Fresnel diffraction field of a slit aperture illuminated by a quasi-monochromatic incoherent slit source. The phase term has a big effect on the features of Fresnel diffraction.     

Imaging theory of an aplanatic system with a stratified medium based on the method for a vector coherent transfer function

A simple formalism relating image fields to object fields, similar to that of the scalar and paraxial case, is presented for an aplanatic system obeying the sine condition, which shows that the vector plane-wave spectrum of image fields is equal to the product of the vector coherent transfer function due to the x- and y-polarized point electric field source and the scalar spectrum of the corresponding transverse object fields. Utilizing this formula and dyadic Green's function, a rigorous imaging theory of an aplanatic system for the point electric current source through a stratified medium is readily developed.     

Effects of coherence on anisotropic electromagnetic Gaussian-Schell model beams on propagation

An analytical formula for the cross-spectral density matrix of the electric field of anisotropic electromagnetic Gaussian-Schell model beams propagating in free space is derived by using a tensor method. The effects of coherence on those beams are studied. It is shown that two anisotropic stochastic electromagnetic beams that propagate from the source plane z=0 into the half-space z>0 may have different beam shapes (i.e., spectral density) and states of polarization in the half-space, even though they have the same beam shape and states of polarization in the source plane. This fact is due to a difference in the coherence properties of the field in the source plane.     

Tensor theory of electromagnetic radiometry

A theory of electromagnetic radiometry is built on the premise that the electromagnetic generalised radiance has a tensor structure, represented by the electric, magnetic and mixed generalised radiance tensors as fundamental quantities. They allow overcoming the limitations due to the scalar generalised radiances, proposed for characterizing stationary random electromagnetic sources. Furthermore, they provide a unified framework for completely describing the energy flux and the states of spatial coherence and polarization of random electromagnetic fields. So, the fundamental quantities of both the scalar generalised radiometry and the classical radiometry or photometry are deduced as particular cases of the tensor theory. A new procedure of analysis of (second-order) correlations, subject to the accomplishment of conservation laws, is also introduced. It reveals that (1) the primary sources of the measurable radiometric quantities associated to the random electromagnetic fields in any states of spatial coherence and polarization are the individual radiators of the radiant source (the correlations of the electric and magnetic field vectors only modulate the contributions given by those radiators) and (2) there are two physical mechanisms for the transport of measurable radiometric quantities by the electromagnetic field, i.e. the propagation of the contributions from individual radiators and their redistribution over each wavefront on propagation. The term redistribution refers to the transfer of portions of the measurable quantity over the wavefronts on propagation, without change its total value over each wavefront. In this context, a physical meaning is given to the negative values of the generalised radiance, which gives new insight about the Poynting’s theory of energy transport.     

部份相干光照明的随机孔屏衍射特性

本文从部份相干理论出发，导出了随机孔屏的衍射强度期望分布及涨落方差表达式；对随机孔屏的衍射特性作了较深入的分析；讨论了光源的空间特性对衍射图形的影响；研究表明：衍射强度的光场分布不仅与孔和屏的几何特性有关，还与光源的形状和大小密切相关，文中还指出：在特定明情况下，形状一定的小孔无论在数目还是尺寸上发生随机变化，都不会改变衍射场的强度期望分布和涨落方差分布。     

Geometric optics approach to optical fibre excitation by partially coherent sources

A general partially coherent source field on the end of an optical fibre has been represented by a characteristic angular spectrum of independent plane waves. The launching of both trapped and tunnelling (leaky) power is investigated for both step and graded-index fibres by considering the effect of one component of the angular spectrum, using geometric optics acceptance theory. The effect of the degree of coherence on the pulse response of the fibres is briefly discussed.     

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.     

Beam shaping by spatial coherence modulation based on spatial coherence wavelets

A novel technique for designing specific shapes of power distributions is presented. It bases on the modulation of the spatial coherence of the optical field in amplitude and phase. The modulating functions are synthesized from the phase-space diagram of the field, provided by the marginal power spectrum. Numerical results for the phase modulation of the complex degree of spatial coherence are presented. This beam shaping technique is suitable in applications that require specific shapes and sizes.     

扰动大气中部分相干光束的光谱特性

对扰动大气中传输的部分相干光束的光谱演化特性进行了研究。利用推广的惠更斯一菲涅耳原理．得到了扰动大气中部分相干光束的交叉光谱密度,据此分析了在大气扰动影响下部分相干光束的频移效应以及空间相干度的变化规律。结果表明：由于大气扰动的影响,在光束传输过程中,轴上光谱先是出现蓝移,蓝移量先增大后减小,传输足够远距离之后将出现红移,并且光束的相干性越差,就需要传播越远的距离才会发生红移;而光束的空间相干度随传输距离增大到一定程度之后开始减小,最终将趋于零。通过与自由空间中部分相干光的传输特性比较,发现当光束的传输距离比较小时．大气扰动效应对光谱特性的影响很小,只有在传输距离足够远时,大气扰动对光谱特性的影响才会明显地表现出来。     

Beam radiated from quasi-homogeneous uniformly polarized electromagnetic source scattering on quasi-homogeneous media

We study the coherence properties of the field generated by beam radiated from quasi-homogeneous (QH) electromagnetic source scattering on QH media. Formulas for the spectral density and spectral degree of coherence of the three dimensional scattered field are derived. The results show under assumption that the diagonal correlation coefficients of the source are proportional to each other, the far field of the scattered light satisfy two reciprocity relations analogous to that in the scalar case, that, the spectral density is proportional to the convolution of the spectral density of the source and the spatial Fourier transform of the correlation coefficient of the scattering potential; the spectral degree of coherence is proportional to the convolution of the diagonal correlation coefficients and the strength of the scattering potential.     

Measurement of hard X-ray coherence in the presence of a rotating random-phase-screen diffuser

A suitably large coherence area is important in coherent X-ray optics, when using techniques such as interferometry or phase contrast imaging (PCI). The work done by Suzuki using a prism interferometer to measure X-ray coherence at 12.4 keV [1] is here extended to consider the use of a diffuser at the bio-medical imaging energy of 25 keV. In order to achieve a broader, more even X-ray field and eliminate speckle, a spinning piece of paper may be used as a rotating random-phase screen to diffuse the hard X-ray beam, but this will concomitantly decrease the magnitude of the complex degree of second-order coherence. We also study the effect of source size and source-to-sample distance on coherence, where imaging area and required flux must be considered. Coherence measurements at the 20XU beamline at the SPring-8 synchrotron are compared to results from wave-optical computer modelling. These show that while the diffuser will decrease the magnitude of the complex degree of coherence, further free-space propagation will lessen this effect. In the design of an experiment, the collimating slit size and use of a diffuser must therefore be balanced with distance from the source, in order to maximise coherence while maintaining the desired field of view and exposure time.     

Coherence and polarization properties of a three-dimensional, primary, quasi-homogeneous, and isotropic source and its far field

We study the spatial coherence properties of a three-dimensional, primary, quasi-homogeneous, and isotropic source and the far field that it generates. We assess the spectrum, degree of polarization, electromagnetic degree of coherence, and effective degree of coherence of the source and of the far field. We also demonstrate the formalism for a spherical Gaussian Schell-model current source. The polarization properties of light are fully accounted for in the analysis, and it is found that certain results pertaining to scalar coherence theory are reproduced also within the electromagnetic treatment.     

多π关联对π发射源相干性的分析

对高能重离子碰撞中不同相于性程度π源产生的多π关联事件进行了蒙特卡罗模拟,比较了不同多重数子事件中的多π关联对π源相于性程度依赖的敏感性.对Bevalac流光室2.1A GeV Ne+Pb和1.2A GeV Ar+KCl中心碰撞产生的π源的相干性程度进行了分析,并与以往的2π干涉学的结果进行了比较.     

空间相干与入射光子能量对临床类同轴X射线相衬成像影响

在临床实际应用中,X射线源是一个具有有限尺寸的多色光源.基于Fresnel-Kirchhoff衍射理论,考虑空间相干的影响,新的类同轴X射线相衬成像公式被导出.分别代表吸收效应与相位效应的吸收衬度传递函数和相位衬度传递函数从新公式得到.吸收衬度传递函数和相位衬度传递函数的曲线由Matlab给出,不同源半径的最佳成像位置也被计算.理论分析结果如下:1）当光源半径足够大时空间相干效应须被考虑,减小源半径可以增强相位效应|2）源半径与最佳成像位置间有确定关系,并在本文中给出|3）源尺寸效应对相位效应影响不是无限的|4）减小入射X射线光子能量将会显著增强相位效应,但不会对最佳成像位置有影响,因此,上述结论可以被拓展到多色光情况.为了验证理论结果,本文给出了对一个表面破损的光学玻璃微聚焦相衬成像的实验结果.其中一些实验结果与理论分析符合很好,但也有一些实验未达到预期,相关的解释文中也已给出.     

空间相干与入射光子能量对临床类同轴X射线相衬成像影响

在临床实际应用中,X射线源是一个具有有限尺寸的多色光源.基于Fresnel-Kirchhoff衍射理论,考虑空间相干的影响,新的类同轴X射线相衬成像公式被导出.分别代表吸收效应与相位效应的吸收衬度传递函数和相位衬度传递函数从新公式得到.吸收衬度传递函数和相位衬度传递函数的曲线由Matlab给出,不同源半径的最佳成像位置...     

Effects of Spatial Coherence and Dispersive Diffraction on a Change in Optical Spectrum

This paper describes a change in optical spectrum on propagation in free space for a particular class of spatially partially coherent fields emanating from a polychromatic secondary source. A rectangular opening aperture in an opaque screen, illuminated with an extended incoherent polychromatic primary source, works as the secondary source. The spectral change is explored in detail theoretically and experimentally in association with spatial coherence in the secondary source as well as dispersive diffraction by the secondary source. The peak shift of the spectrum is associated with the coherence area for characterizing the secondary source. The peak shift becomes maximum if the secondary source is spatially coherent, but no shift occurs if the secondary source is spatially incoherent.     

Synchrotron beam coherence: a spatially resolved measurement

We report a precise and spatially resolved measurement of the complex degree of coherence of a one-dimensional 1.5-keV beam produced by a third-generation synchrotron source. The method of phase-space tomography is used, which requires only measurements of the x-ray intensity. We find that the field is statistically stationary to within experimental error, the correlations are very well approximated by a Gaussian distribution, and the measured coherence length is in excellent agreement with expectations.     

Sound source in the presence of wall shear layers

Numerical solution techniques for evaluating the acoustic field generated by a single line source located inside or outside a wall shear layer of an infinitely long lined rectangular duct are presented. A formula for calculating wave attenuation due to an acoustic lining is given.