Autofocusing of digital holographic microscopy based on off-axis illuminations

An auto-focusing method for digital holographic microscopy has been proposed by employing two off-axis illumination beams. When specimens are illuminated by two plane waves in different directions, it is found that the farther the reconstruction plane is from the image plane, the wider the two reconstructed images are separated from each other. Thus, the image plane can be determinated by finding the minimum of the variation between the two reconstructed object waves on both the amplitude and phase distributions. The feasibility of the proposed method is demonstrated by the corresponding simulation and experiment.     

菲涅耳望远镜合成孔径激光成像雷达实验室验证

对菲涅耳望远镜合成孔径激光成像雷达进行了实验室尺度条件下的原理验证实验。实验中利用不同曲率半径、垂直正交偏振的两个球面波通过二维(2D)扫描方式照明远距离处的目标,接收望远镜接收到的目标回波经过偏振分光镜分成两束作为信号光和本振光进入2×4 90°桥接器,桥接器输出的四路光信号被两个平衡探测器接收,平衡探测器输出电信号经模数转换后经过复数化、两维相位二次项匹配滤波算法处理后可以重构出目标图像。对4.3m处点目标和2D面目标进行了成像实验,取得了具有良好成像分辨率和对比度且带有散斑效应的预期成像结果,证明了该合成成像激光雷达概念的正确性。     

Parallel phase-shifting dual-illumination phase unwrapping

We propose a single-shot phase unwrapping technique using a single wavelength and parallel phase-shifting interferometry. In the proposed technique, an object is illuminated by two laser beams, which are emitted from the same laser, and have different illumination angles and polarizations. Two types of object waves generated by the two beams are separately and simultaneously recorded by a polarization-imaging camera. In the path of the reference wave, an array of phase retarders is placed to implement the parallel phase-shifting technique. A highly accurate three-dimensional shape is reconstructed from a single hologram. We numerically simulated the proposed technique and conducted a preliminary experiment to verify its effectiveness. It was confirmed that millimeter-order height, which was several thousand times the wavelength of the laser, can be reconstructed by the proposed technique without wrapping.     

菲涅耳非相干数字全息大视场研究

菲涅尔非相干相关全息术(Fresnel incoherent correlation holography, FINCH)通过空间光调制器(spatial light modulator, SLM)将来自物点的光波分解为曲率半径不同的两束自相干光,干涉条纹由CCD记录.由于受限于SLM与CCD的像素数目及像素尺寸, FINCH技术与光学全息术相比记录视场要小得多.本文通过对FINCH系统的记录过程进行理论分析,给出了SLM所能记录的视场角,说明通过调控加载在SLM上的双透镜光轴中心,能够扩大SLM的有效直径从而将SLM的有效记录范围增大2.77倍,有效扩大了系统的记录视场.搭建了非相干光反射式数字全息记录系统并对理论分析进行了实验验证,结果表明:在SLM上依次加载不同光轴中心位置的双透镜掩模进行FINCH记录及再现,将得到的各子图像拼接融合可以得到高分辨率大视场图像,为菲涅尔非相干全息术在高分辨大视场显微成像的进一步应用提供了有力支撑.     

Multiple-exposure high-sensitivity holographic interferometry for studying moving objects

A method for increasing the sensitivity of measurements through aberration compensation upon reconstruction of interferograms from two multiple-exposure holograms is proposed. At the early stage of object investigation, the holographic structures recorded at certain time instants are rerecorded by two coherent beams on new image carriers. In this case, the interference moiré method is employed to monitor the equality of the vectors of the holographic structures rerecorded. At the final stage, the new nonlinear holograms are processed in an optical analyzer of conjugate holograms with the use of incoherent light. The hologram thus reconstructed offers a high sensitivity of measurements and is free of aberrations. The method is tested by visualizing the temperature-field variations in a glass substrate with a conducting coating.     

Parallel two-step phase-shifting microscopic interferometry based on a cube beamsplitter

Parallel two-step phase-shifting interferometry for microscopy is presented, and the recording condition for generalized two-step phase-shifting interferometry is discussed. A 45° tilted cube beamsplitter enables to replicate the orthogonally linear polarized object and reference waves into two parallel beams, respectively. As a consequence, two interferograms with quadrature phase shift are obtained along the two beams, and phase reconstructed with an improved algorithm. To reconstruct the phase distribution from the two-step phase-shifting interferograms, a certain recording condition should be satisfied. However, the recording condition has not ever been discussed before. In this paper, the recording condition for the two-step phase-shifting interferometry is derived and that is: the intensity of reference wave should be no less than two times object wave intensity.     

Review of Fresnel incoherent correlation holography with linear and non-linear correlations

Fresnel incoherent correlation holography(FINCH) is a well-established incoherent imaging technique. In FINCH, three selfinterference holograms are recorded with calculated phase differences between the two interfering, differently modulated object waves and projected into a complex hologram. The object is reconstructed without the twin image and bias terms by a numerical Fresnel back propagation of the complex hologram. A modified approach to implement FINCH by a single camera shot by pre-calibrating the system involving recording of the point spread function library and reconstruction by a nonlinear cross correlation has been introduced recently. The expression of the imaging characteristics from the modulation functions in original FINCH and the modified approach by pre-calibration in spatial and polarization multiplexing schemes are reviewed. The study reveals that a reconstructing function completely independent of the function of the phase mask is required for the faithful expression of the characteristics of the modulating function in image reconstruction. In the polarization multiplexing method by non-linear cross correlation, a partial expression was observed, while in the spatial multiplexing method by non-linear cross correlation, the imaging characteristics converged towards a uniform behavior.     

New multiplexed system for synchronous measurement of out-of-plane deformation and two orthogonal slopes

We propose a novel system for synchronous measurement of out-of-plane deformation and two orthogonal slopes using a single camera. The linearly polarized reference beam introduced by an optical fiber interferes with the unpolarized object beam to measure the out-of-plane deformation. A modified Mach—Zehnder interferometer is used to measure the two orthogonal slopes of the out-of-plane deformation. One of the object beams of the Mach—Zehnder interferometer is an unpolarized beam, and the other object beam is split into two orthogonal linearly polarized object beams by a polarizing prism. The two beams are orthogonally polarized. Hence, they will not interfere with each other. The two polarized beams respectively interfere with the unpolarized beam to simultaneously measure the two orthogonal slopes of the out-of-plane deformation. In addition, the imaging lens and apertures are respectively placed in three optical paths to independently control the carrier frequencies and shearing amounts. The effectiveness of this method can be proved by measuring two pressure-loaded circular plates.     

Scattered ultrasound for incoherent insonification in transmission imaging

Spatially incoherent ultrasound is produced by the scattering of coherent ultrasonic waves at statistically moved particles. Incoherent insonification is much more uniform at near-field distances than coherent ultrasound. The lateral resolution is improved and the coherent ‘ringing’ in a sharply imaged object disappears.     

双光子光折变介质中非相干耦合亮-暗屏蔽光伏孤子对

对有外加电场的双光子光伏光折变晶体中两束偏振方向和波长都相同的互不相干光束的耦合进行研究,给出产生亮-暗双光子光折变屏蔽光伏孤子对需满足的条件.以Cu:KNSBN晶体作为研究对象,选取α=117.3,β=83.79,η=1.5×10-4,σ=104,δ=0.005,r=10时,给出双光子光折变晶体中的非相干耦合亮-暗屏蔽光伏孤子对2个孤子分量光强的空间分布,证明有外加电场的双光子光伏光折变晶体中存在非相干耦合亮-暗屏蔽光伏孤子对,指出孤子对是由偏振态和波长都相同的两束互不相干光形成的,当外加电场方向和晶体中光伏电场的方向与晶体光轴方向相同时,双光子光折变晶体中可支持亮孤子峰值光强稍大于暗孤子最大光强的非相干耦合亮-暗孤子对,当外加电场方向和晶体中光伏电场的方向与晶体光轴方向相反时,双光子光折变晶体中可支持亮孤子峰值光强稍小于暗孤子最大光强的非相干耦合亮-暗孤子对.     

On the properties of a ‘finite beam’, reflection-type volume hologram

The properties of a volume-phase hologram of the reflection type are studied in the case when the reference and object waves are beams of finite width. Numerical results are obtained by considering a sufficiently large number of terms in a previously derived infinite series solution. The results reproduce Kogelnik's one-dimensional theory [1] when the thickness of the hologram is small in comparison with the widths of the beams but otherwise the finiteness of the recording beams leads to significant variations in the amplitude of the reconstructed beam.     

Design of the spatial filter window for digital holographic convolution reconstruction of object beam field

In the convolution reconstruction process of digital holographic object beam field, the object beam fields with different magnifications can be obtained when the reconstructing beams are spherical waves with different wave curvature radii. This paper presents a theoretical and experimental discussion on the useful spatial frequency spectrum of the hologram numerically illuminated by the spherical wave. The result shows that there would be an image in the spatial frequency spectrum of digital hologram, which is completely the same as the object, if the wave surface radius of the spherical wave is equal to the distance from the object to the CCD sensor. Taking this image as a reference and designing a filter, the position of the reconstructed images with different magnifications can be predicted correctly in the reconstructed plane. Additionally, since the spectrum distribution of zero-order diffraction can be forecasted accurately in theory, its contribution can be effectively eliminated through changing the spatial filter shape; then a reconstructed object field containing more high-frequency information can be obtained.     

Entangled-photon imaging of a pure phase object

We demonstrate experimentally and theoretically that a coherent image of a pure phase object [implemented by a microelectromechanical system (MEMS) micromirror array] may be obtained by use of a spatially incoherent illumination beam. This is accomplished by employing a two-beam source of entangled photons generated by spontaneous parametric down-conversion. One of the beams probes the phase object while the other is scanned. Though each of the beams is, in and of itself, spatially incoherent, the pair of beams exhibits higher-order interbeam coherence.     

利用相位模板实现数字全息超分辨成像

为了简化数字全息超分辨记录系统,分别在其物光和参考光部分引入一块相位模板,以获得垂直和倾斜方向照明物体的光束和具有不同载波频率的参考光束.当这些具有不同照射方向的光透过物体后,可以使CCD在位置固定的情况下记录到携带低频和高频信息的物体衍射场,不同载波频率的参考光则保证了高频和低频信息在复合全息图的频谱面上能够相互分离.实验结果证明,通过将记录到的物体高频和低频信息合成,可以获得超出系统衍射极限分辨率的再现像.     

Simultaneous Diffraction of Two Waves at a Transmission Volume Hologram

On the basis of the theory of coupled waves the simultaneous diffraction of two waves at a plane dielelectric transmission volume hologram is described. The intensities of the output beams strongly depend on the phase shift between the incident waves and the phase shift of the holographic grating with respect to the interference pattern of the read-out waves. This gives the possibility to transfer the energy from one beam into the other and can be used for the modulation and deflection of the reconstructed beams. The possibility of a separate determination of the different phase shifts by intensity or phase measurements is discussed.     

Image formation by inertialess dynamic holograms recorded in a medium with quadratic nonlinearity at different frequencies of the object and reference waves

It is shown theoretically and experimentally that the hologram recorded in a quadratically nonlinear medium is capable of forming the image in the case when the wavelengths of the object and reference beams are essentially different. The relations are derived that determine the wavelength of the reconstructed image, the direction of propagation of the restored beam, and the longitudinal shift of the reconstructed image depending on the wavelength of the beam scattered by the object. The experiment is carried out in a noncollinear arrange-ment of interaction of the object and reference waves in the KTP crystal. As a light source, a YAG:Nd laser (λ=1.064 μm) with a pulsewidth of 300 ps was used. The wavelength of the object wave was tuned by the Ba(NO₃)₂ crystal, which generated from one to three Stokes SRS components with the frequency shifts Δν, 2Δν, and 3Δν (Δν=1047 cm^?1). These components, upon interaction with the reference wave of the fundamental frequency formed the image at the wavelengths 0.532, 0.563, and 0.6 μm with an almost diffraction-limited quality. The experimental values of the angles of propagation of the beams that formed reconstructed images at different frequency shifts of the object wave corresponded to those predicted theoretically. We have confirmed the presence and direction of longitudinal shifts of the reconstructed images upon variation of the wavelength of the light scattered by the object. Thus, we have demonstrated the feasibility of the multicolor (multiwave) holographic generation of high-quality images separated in space. The inertialess nature of the beam coupling makes it possible to use the holograms of this type for ultrafast commutation of the information fluxes in optical computers and communication lines.     

Transformation of images upon dual-frequency recording and reading at sum frequencies of dynamic χ<Superscript>(2)</Superscript> holograms using spherical reference waves

The transformation properties of images reconstructed by dynamic χ⁽²⁾ holograms recorded with radiation of the sum frequency with the use of spherical reference waves are studied theoretically and experimentally. Notions of fields and zones of locrises as regions of localization of holographic images formed by reference and object beams of different frequencies at arbitrary frequency ratios of these beams and distances from the reference point source to the hologram are defined. The domains of the parameters of the reference source ensuring the formation of real and imaginary images are determined. The reconstruction of real images by dynamic χ⁽²⁾ holograms is demonstrated.     

Simultaneous diffraction of two waves at a reflection volume hologram

Based on the theory of coupled waves the simultaneous diffraction of two waves at a plane dielectric reflection volume hologram is described. By analogy to the transmission volume grating the intensities of the output beams strongly depend on the phase shift between the incident waves and the phase shift of the grating with respect to the interference structure of the read-out waves. This gives the possibility of transferring the incident energy from one wave into the other and can be used for the modulation or deflection of the reconstructed beams. The separate determination of the different kinds of phase shifts by intensity or phase measurements is discussed.     

基于空间光调制器的非相干数字全息单次曝光研究

菲涅耳非相干相关全息术(Fresnel incoherent correlation holography,FINCH)利用在空间光调制器(spatial light modulator,SLM)上加载双透镜模式对同一物点光分束自相干,并通过改变加载的相位因子得到不同的相移全息图.本系统利用SLM可分区编码调制特性,将FINCH成像中SLM上分三次加载的0°,120°,240°相位双透镜掩模各提取1/3组成一幅复合相移模式加载,并研究了三种相位分布方式对FINCH成像质量的影响.结果表明:三个相位在SLM上分布间隔越大,再现像越清晰.在此基础上,提出了一种新的掩模加载方式,在SLM加载透镜阵列,每一个相位因子对应一个双透镜,具有一个光轴.实验表明,通过这种加载方式,通过SLM后形成的三个相移图能够一次在电荷耦合器上记录,并且三个相移图不重叠,然后通过MATLAB编程计算将不同相移角度的全息图分别提取出来,通过三步相移计算合成一幅包含有物光波的复值全息图,最后通过数值再现算法重建待测样品.此系统可用于对光源相干性较低的实时成像系统,也为微小形变测量、动态物体的观测提供了新方法,为非相干数字全息术的发展提供了新思路.     

Photorefractive two-wave mixing in the presence of an incoherent beam

We present the numerical results of the photorefractive two-wave mixing in the presence of an incoherent beam by considering the influence of the ratios of the intensities and the light-excitation cross sections of the incoherent beam to the coherent beams on the space-charge field and the effective coupling coefficient. The results show that the incoherent beam can control the intensity of the coherent beams effectively. The experimental results agree with the theory well. One application is demonstrated to obtain the positive and negative coherent replicas of an incoherent image. The coherent images have a resolution of 80.6 line pairs/mm. Received: 2 March 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999