Some Effects of Spatial and Temporal Coherence in Holography

The following two problems have been investigated theoretically: systematic treatment of image formation in two-beam Fresnel holography from the viewpoint of spatial coherence, and influence of temporal coherence in two-beam Fraunhofer holography.     

High-Efficiency Light Coupling in a Submicrometric Silicon-on-Insulator Waveguide

Light coupling into a sub-micrometer-thick waveguide is usually done through a grating coupler. Coupling efficiency is strongly enhanced by addition of a mirror above the grating. This new kind of coupler can be designed to achieve efficiencies as great as 80%. Numerical calculations for a high-angular-spread Gaussian incident beam are compared with experimental results obtained for a standard silicon-on-insulator waveguide.     

Spatial Coherence and Index-profiling in Optical Fibres

A simple relation is proved to exist between the coherence properties of the light emerging from the end cross-section of multi-mode optical fibres, excited by means of spatially incoherent sources, and their refractive index distributions. Experimental results showing the possibility of visualizing the index profile are reported.     

Propagation of Short Light Pulses in Nonlinear Birefringent Fibre

Abstract

An analytical approach is developed to obtain the solution of coupled Schrödinger equations for polarization components of a solitary wave. Variational analysis allows us to describe optical pulse propagation in fibres with nonlinear birefringence. Conditions for polarization channel switching and locking are formulated.     

Study of Induced Temporal Coherence in Optical Parametric Down Conversion

Abstract

Temporal coherence of the idler photon beam induced by a coherent input signal was studied for non-degenerate optical parametric down conversion. When the optical path difference of the interferometer is much longer than the coherence length of the spontaneous idler radiation, the experimental results show that the first-order temporal coherence of the idler photon can be induced by a coherence input signal and the interference fringe visibility is determined by the intensity of the coherent input signal.     

Effects of Spatial Coherence on Fourier Imaging of a Periodic Object

The formation and the contrast of Fourier images of a periodic object, especially a sinusoidally transparent object, are investigated under partially coherent illumination. It becomes apparent that the spatial coherence of light affects seriously the contrast of Fourier images and the positions of the Fourier image planes.     

Effects of Output-coupling Apertures in a Resonator on Spatial Coherence of CO2 Laser

The degree of spatial coherence S n₁₂ (0) S has been measured for a CO₂ laser beam of radiation emerging from a central output-coupling hole, or four output-coupling holes in the centre-symmetric annular region, in a plane mirror of the Fabry-Perot resonator. The effects of these two kinds of output-coupling apertures on the spatial coherence are studied in association with mode structure in the resonator. The value of S n₁₂ (0) S is always above 0·9 for two points in any location in the beams of radiation emerging from the four output-coupling holes, while it varies with the time in the range 0·5 to 1·0 in a beam of radiation emerging from the central output-coupling hole. Some analytical considerations are given, according to which the value of S n₁₂ (0) S should be approximately unity over the cross-sectional area of a beam of radiation emerging from an output-coupling hole of 6 mm in diameter. This is partly in agreement with the results of the measurements. For the central coupling hole, the fact that S n₁₂ (0) S varies with time is explained qualitatively to be due to the intensity fluctuation in every operating mode, competing with each other in the active resonator. For the four coupling holes a particular mode may be made to survive due to relatively strong mode competition arising from the mode selective effects of the off-centred four coupling holes. On this account a high value of S n₁₂ (0) S may be obtained at two points in the beam separated by 25 mm.     

Spatial Coherence Measurements of XeCl Laser Radiation with a Super-Gaussian Unstable Cavity

Abstract

A Young's interferometer has been used for spatial coherence measurements of the 23 mm diameter laser beam provided by a XeCl laser fitted with a super-Gaussian resonator of magnification factor M = 7. It has been found that the fringe visibility reduces from 0·8 to 0·1 by increasing the slit distance from 4 mm to 15 mm, the transverse coherence degree of the laser beam being determined by the geometric constraints imposed by the unstable resonator.     

The Spatial Coherence Properties of Gaussian Schell-model Beams

The transverse and longitudinal spatial coherence properties of the light beams generated by planar gaussian Schell-model sources are discussed. It is found that for all gaussian Schell-model beams the ratio of the transverse coherence length to the beam width remains invariant upon propagation. An examination of the longitudinal coherence for both on-axis and off-axis pairs of points indicates that the longitudinal coherence will not, in general, die out as the separation between the points is increased. Rather, the degree of longitudinal coherence will approach a finite (non-zero) value as long as the source contains a finite coherence area, regardless of how small this area may be. Gaussian quasihomogeneous beams are studied as a limiting case. The relation of the present work to the analysis of speckle size is briefly discussed.     

Some Effects of Temporal Coherence on the First Order Statistics of Speckle

Moments and the first order probability density function of the intensity in a speckle pattern are considered as a function of the spectral bandwidth of the incident light. It is shown that the standard deviation of the intensity generally depends on the surface roughness of the diffuser, and on the position of measurement in the far field. Numerical results are given for a diffuser with a Gaussian distribution of surface height limited by a circular aperture. Exact and approximate forms of the probability density function are discussed. The analysis applies when the fluctuations in optical path are greater than the maximum wavelength incident.     

交通噪声时空因素分析的初探

噪声作为一种声音信号应该是时间和空间两种特性的组合。通过对道路交通噪声和铁路噪声两种声源信号的自相关函数（Autocorrelation Function）和双耳自相关函数（Interaural Cross-correlation Function）的分析,判定表征声音信号空间和时间的物理量是否能够更全面、准确地表明交通噪声的特性,并在此基础上进行主观评价实验,找出人的主观感受与这些物理量之间的关系。结果表明,相比传统的声压级测量自相关函数和双耳自相关函数接合时空特性可以更好地表征交通噪声的特性,初步认证其与主观反应之间的关系,具体的对应关系还需要进一步的研究。     

The Influence of Spatial Coherence on the Monochromatic Optical Transfer Function

It is widespread practice to measure the optical transfer function (OTF) by scanning the images of ‘simple’ objects. However, the illumination used in such systems is (generally) partially coherent, and not incoherent as is required by theory. The influence of spatial coherence is explicitly examined in order to obtain a general equation for the observed OTF for the case of one-dimensional test objects. The equation is applied to the case of a ‘perfect’ rotationally symmetric lens, using a slit object, and, on comparing the results with the true (incoherent) OTF of such a lens, significant differences have been found.     

High-Speed and High-Contrast Spatial Light Modulator that Uses Electroabsorption in a GaAs Single Crystal

A new spatial light modulator that uses electroabsorption induced by the Franz-Keldysh effect in a GaAs single crystal is proposed. The device has the same structure as the Pockels readout optical modulator, is driven by a high ac voltage, and is operated in the superprime mode. Its high-frame-rate (over 500-Hz) operation, high contrast ratio (350:1), and large gamma characteristics (2.47) were experimentally confirmed in the transmission mode, although its operating-wavelength (readout) region is restricted to a narrow band near the band gap. In the reflection mode, however, this device has a high contrast ratio and large gamma characteristic (approximately 5), offering the possibility of its use as a nonlinear device.     

Partially Coherent Imaging of a Straight-edged Object Associated with Periodic Structures of Acoustically Modified Spatial Coherence

Abstract

Spatial coherence of laser light can be modified by a progressive ultrasonic wave. The resultant optical mutual intensity has lateral and longitudinal periodicities in the near field behind the ultrasonic cell. Over the near field the longitudinal periodicity is lost in any plane parallel to the output plane of the cell but the lateral periodicity still remains. Such a plane serves as an object plane for partially coherent imaging. The imaging characteristics of an edge object, depending on various ultrasonic parameters, are described. Ringing suppression and contrast enhancement occur satisfactorily in the image if the ultrasonic parameters are properly controlled. Theoretical predictions are in good agreement with the experimental results.     

CCD实时图像采集处理系统测量光场相干性

本文描述了用CCD摄像机及DIP图像处理系统在双频光栅剪切干涉光路中测量光场相干性的原理和实测结果,以及讨论实验光路中逶镜非准直因素对测量结果的影响。     

视频编码中基于时空信息运动估计的研究

提出了一种基于视频图像的时间和空间相关性信息的运动矢量预测方法,由于相领两帧图像中的内容应具有相关性,图像中物体的运动轨迹也具有相似性,因此可以利用前一帧图像中宏块的运动矢量确定运动轨迹,进而估计当前宏块的运动矢量,但是这种运动估计是受限的,因为估计运动矢量时的宏块顺序是逐行的按先后顺充进行,在当前帧中,当前宏块预测时只能利用具有因果关系的相邻宏块,但其与相邻宏块并非都具有因果关系,因此给出了预测矢量的资格检验方法,用以判断和剔除伪预测矢量,在搜索过程中采用菱形搜索（Pyramid),相邻的两次搜索过程,要去除两个重叠区域,上述算法减少计算量,对Suzie序列测试结果,平均搜索次数是14－17次,并且解码图像的信噪比PSNR与FS搜索相比降级不多。