Inverse scattering for frequency-scanned full-field optical coherence tomography

Full-field optical coherence tomography (OCT) is able to image an entire en face plane of scatterers simultaneously, but typically the focus is scanned through the volume to acquire three-dimensional structure. By solving the inverse scattering problem for full-field OCT, we show it is possible to computationally reconstruct a three-dimensional volume while the focus is fixed at one plane inside the sample. While a low-numerical-aperture (NA) OCT system can tolerate defocus because the depth of field is large, for high NA it is critical to correct for defocus. By deriving a solution to the inverse scattering problem for full-field OCT, we propose and simulate an algorithm that recovers object structure both inside and outside the depth of field, so that even for high NA the focus can be fixed at a particular plane within the sample without compromising resolution away from the focal plane.     

Super-resolution technique of microzooming in electro-optical imaging systems

Abstract

The main resolution limit for an electro-optical imaging system comprising a focal plane array (FPA) sensor obeys Nyquist theorem which states that an object must be sampled at more than twice the cut-off frequency to avoid aliasing. A novel super-resolution technique, microzooming (MZ), is presented. MZ provides electro-optical imagery with an extended unaliased bandwidth using multiple aliased images at different optical magnifications. A number of simulation and experiment results demonstrate that MZ is a practical advance for reducing aliasing and enhancing resolution as well as helping to suppress white noise.     

Self-pumped phase-conjugate interferometer with a photorefractive iron-doped lithium-niobate crystal

A single object wave is amplitude divided by a beam splitter into two waves of equal intensity that are made to interfere at the back surface of an iron-doped lithium-niobate crystal so that the normal to the back surface is the angular bisector of the input waves. The interference results in the formation of a phase grating (Bragg grating) in the volume of the crystal. These waves are diffracted at the Bragg grating on both the front focal plane and the back focal plane of the crystal. The wave diffracted in the back focal plane from the Bragg grating and counterpropagating to the incident wave is observed to be the phase conjugate of the input object wave. The wave diffracted in the front focal plane of the Bragg grating is incorporated into the design of an interferometer to measure a specific in-plane displacement of the object wave. It is theoretically evaluated and experimentally demonstrated that interferometers such as those that incorporate conjugate-wave pairs are highly sensitive.     

复谱频域OCT系统高分辨率图像重建

为了提高复杂多层样品层析图像的分辨率,构建了复谱频域光学相干层析成像(CSOCT)系统.由于其在低亮度和高速成像方面相对于时域OCT具有更高的灵敏度,因此在光学相干层析成像系统中具有重要的作用.本文对测试样品二层盖波片进行成像实验,基于光学相干层析基本理论,采用五帧相移算法,最终获得测试样品的复谱频域OCT图像.实验结果表明,该系统可以消除谱频域OCT图像中的寄生项和镜像,改善和提高层析图像的分辨率.     

Monte Carlo modeling of optical coherence tomography imaging through turbid media

We combine a Monte Carlo technique with Mie theory to develop a method for simulating optical coherence tomography (OCT) imaging through homogeneous turbid media. In our model the propagating light is represented by a plane wavelet; its line propagation direction and path length in the turbid medium are determined by the Monte Carlo technique, and the process of scattering by small particles is computed according to Mie theory. Incorporated into the model is the numerical phase function obtained with Mie theory. The effect of phase function on simulation is also illustrated. Based on this improved Monte Carlo technique, OCT imaging is directly simulated and phase information is recorded. Speckles, resolution, and coherence gating are discussed. The simulation results show that axial and transversal resolutions decrease as probing depth increases. Adapting a light source with a low coherence improves the resolution. The selection of an appropriate coherence length involves a trade-off between intensity and resolution.     

傅里叶变换同轴数字全息记录余弦变换谱系数

余弦变换广泛应用于图像和视频的压缩编码以及模式识别之中。余弦变换为实偶对称的傅里叶变换,把实偶对称物体传送到位于傅里叶变换透镜前焦面的空间光调制器上,采用单色均匀平面激光光波照射,则在后焦面上为光波复振幅分布为实偶对称物体的傅里叶变换,即为物体的余弦变换。由于余弦变换谱系数既有正实数又有负实数,提出了采用傅里叶变换同轴数字全息方法记录余弦变换谱系数,通过把数字全息图减去参考光光强和物光光强而得到余弦变换系数。理论推导和实验结果表明了该方法的可行性,该方法可应用于图像的快速余弦变换。     

Application of the correlation coefficient method for determination of the focal plane to digital particle holography

The correlation coefficient (CC) method, which was proposed by our research group, is applied to digital particle holography to locate the focal plane of particles. It uses the fact that the CC is maximum at the focal plane. The factors influencing this method are discussed with a numerical simulation of holograms. For real holograms, the Wiener filter was proposed to process both recorded holograms and reconstructed images. The application results using the dot array target showed that the Wiener filter is a very effective tool for processing holography-related images. The effects of the dot size and the object distance on the errors in the determination of the focal plane by the CC method were investigated by using the calibration target.     

Use of confocal microscopes in conoscopy and ellipsometry. 1. Electromagnetic theory

An optical system consisting of two objective lenses in a confocal arrangement is examined. It is shown that a simple algebraic relation exists between the electric field in the back focal plane of the first objective lens, which focuses the incident light, and the Fourier transform of the electric field in the focal plane of the same lens. The relation holds for high angles. If a thin object is placed in the focal plane it is possible to write the electric field by use of a Fourier transform relation at the exit aperture of the second lens. The theory is generalized for objects that are positioned at oblique angles with respect to the optical axis of the system. This configuration is clearly identical to the setup of a spatially resolving ellipsometer.     

太赫兹光场数据采集与数字重聚焦实验研究

本文对太赫兹光场数据采集与数字重聚焦成像进行实验研究。太赫兹成像因其穿透性、无损性等优点,近年来备受国内外研究者关注。太赫兹波段的光场成像技术有望增强图像质量、改善应用效果。本文在分析光场成像基本原理、系统结构、重建方法的基础上,应用太赫兹焦平面阵列相机进行太赫兹光场数据采集和数字重聚焦实验。首先采集太赫兹光场原始数据,然后通过数字重聚焦进行计算成像,最后对重构图像做增强处理,得到了深度、角度及目标物轮廓分辨力强的太赫兹图像。实验证明了太赫兹光场成像技术的可行性及其改善图像质量、丰富复现效果的能力。     

Wave-front sensing from subdivision of the focal plane with a lenslet array

A wave-front sensing scheme based on placing a lenslet array at the focal plane of the telescope with each lenslet reimaging the aperture is analyzed. This wave-front sensing arrangement is the dual of the Shack-Hartmann sensor, with the wave front partitioned in the focal plane rather than in the aperture plane. This arrangement can be viewed as the generalization of the pyramid sensor and allows direct comparisons of this sensor with the Shack-Hartmann sensor. We show that, as with the Shack-Hartmann sensor, when subdividing in the focal plane, the quality of the wave-front estimate is a trade-off between the quality of the slope measurements over each region in the aperture and the resolution to which the slope measurements are obtained. Open-loop simulation results demonstrate that the performance of the lenslet array at the focal plane is equivalent to that of the Shack-Hartmann sensor when no modulation is applied to the lenslet array. However, when the array is modulated in a manner akin to that of the pyramid sensor, subdivision at the focal plane provides advantages when compared with the Shack-Hartmann sensor.     

Computer-generated holograms of three-dimensional objects synthesized from their multiple angular viewpoints

Synthesizing computer-generated holograms (CGHs) of a general three-dimensional (3D) object is usually a heavy computational task. We propose and demonstrate a new algorithm for computing CGHs of 3D objects. In our scheme, many different angular projections of computer-designed 3D objects are numerically processed to yield a single two-dimensional complex matrix. This matrix is equivalent to the complex amplitude of a wave front on the rear focal plane of a spherical lens when the object is located near the front focal point and illuminated by a plane wave. Therefore the computed matrix can be used as a CGH after it is encoded to a real positive-valued transparency. When such CGH is illuminated by a plane wave, a 3D real image of the objects is constructed. The number of computer operations are equivalent to those of a two-dimensional Fourier CGH. Computer and optical constructions of 3D objects, both of which show the feasibility of the proposed approach, are described.     

用二次傅里叶变换实现CCD的精密定焦

根据傅里叶变换系统中物面平移其频谱面上光强分布不变的特点,提出一种利用二次傅里叶变换实现CCD精密定焦的方法。将CCD和其透镜作为逆变换系统,输入面为两个狭缝,与一个附加透镜组成一个正变换系统。通过镜组移动改变正、负傅里叶变换系统的距离,用软件比较CCD上两个波峰的间隔是否变化来对CCD进行定焦。根据傅里叶变换原理和几何光学成像原理对CCD正焦和离焦时的波峰特点进行了理论分析,证明了离焦时波峰间隔变化量与镜组移动量、离焦量、狭缝间隔成正比,与两透镜的焦距成反比,由此关系即可实现CCD定焦。实验表明,该方法的相对定焦误差为0.2%。     

Three-dimensional image sensing by chromatic confocal microscopy

In the image of a confocal microscope, only those parts of an object appear bright that are located in the focal plane of the objective. Because of an axial chromatic aberration deliberately introduced into the microscope objective, the location of the focal plane depends on the wavelength used. By using a white-light source and examining an object with a depth variation less than the axial range of the chromatic focus, we find that all parts of the object appear sharp and bright in the image, but according to its height they appear in different colors. A camera with black-and-white film sequentially combines, with three selected chromatic filters, intensity and tone of color of each object point. For each tone of color one can assign a height by using a calibration curve. This assignment could be made unequivocal by the selection of filters with adequate chromatic transmission.     

A Method for Evaluation of Two-point Resolution of Axially Symmetrical Filters

The rapid convergence of the recently developed series representation of the Fresnel diffraction pattern [1] enables us to evaluate the central intensity maximum of the focal patterns using only the first three terms. This fact leads to a simple graphical method of determining the two-point resolution achieved in the Fraunhofer focal plane by the filters with real pupil functions. The method is tested for the Fresnel and Gabor zone plates, the Wilkins filter and the central obstruction.     

A Formal Representation of Abbe's Theory of Microscopic Image Formation

The theory of image generation for systems with coherent wave propagation is formally represented using the mathematical methods of Fourier optics. Abbe's theory of constructing the secondary image (the image conjugate to the object) by coherent superposition of the waves originating from the primary image (at the back focal plane of the system for plane wave illumination) is shown to be included within the general Fourier optical treatment of optical transfer systems.     

The application of an image intensifier to atmospheric Cherenkov gamma ray astronomy

A design is presented for a focal plane instrument which will produce images of the atmospheric Cherenkov radiation produced by TeV cosmic gamma rays with resolution of ≈ 0.1 arcmin. The instrument stores the image as photoelectrons in flight for a 30 ns interval while a readout pulse can be generated to cause cascade intensification of the photoelectrons from the appropriate 5 ns interval containing the Cherenkov radiation image. For Cherenkov astronomy, the instrument could be placed in the focal plane of a ≈ 2 m mirror, or it could be used with a ≈ 12 m mirror if a size-reducing image-intensifier preceded it. The sensitivity of both configurations to TeV gamma radiation from a point source is calculated and compared to other Cherenkov facilities.     

Grazing-incidence telescope-spectrograph for space solar-imaging spectroscopy

The design of a stigmatic grazing-incidence instrument for space applications to solar-imaging spectroscopy is presented. It consists of a double telescope and a spectrograph: Telescope I consists of a single cylindrical mirror with parabolic section, focusing the radiation on the entrance slit of the spectrograph in the spectral dispersion plane; telescope II consists of two cylindrical mirrors with aspherical section in a Wolter configuration, focusing the radiation on the spectrograph focal plane in the direction perpendicular to the spectral dispersion plane. The spectrograph consists of a grazing-incidence spherical variable-line-spaced grating with flat-field properties. Telescope II is crossed with respect to the grating and telescope I; i.e., it is mounted with its tangential planes coincident with the grating equatorial plane. The spectrum is acquired by a detector mounted at near-normal incidence with respect to the direction of the exit beam. The spectral resolution is also preserved for off-axis angles. The effective collecting area of the instrument can be preserved by adoption of a nested configuration for telescope II without degradation of the spectral resolution.     

感光材料分辨力测定实验研究

使用ＰＩ－Ｃ感光材料分辨力测定仪,测定了感光材料分辨力。测定试验不仅证明感光材料分辨力测定仪器操作稳定可靠、重复性好,而且测定实验加深了对感光材料的感光影象特性的研究。实验表明,最佳焦面实际集中在各种胶片乳剂层表层。     

Finite conjugate embedded relay lens hyperspectral imaging system (ERL-HIS)

We present a novel embedded relay lens hyperspectral imaging system (ERL-HIS) with high spectral resolution (nominal spectral resolution of 2.8 nm) and spatial resolution (30 μm×8 μm) that transfers the scanning plane to an additional imaging plane through the internal relay lens so as to alleviate all outside moving parts for the scanning mechanism used in the traditional HIS, where image scanning is achieved by the relative movement between the object and hyperspectrometer. The ERL-HIS also enables high-speed scanning and can attach to a variety of optical modules for versatile applications. Here, we also demonstrate an application of the proposed ERL-HIS attached to a microscopic system for observing autofluorescent images of sliced cancer tissue samples.     

High-resolution coregistered intravascular imaging with integrated ultrasound and optical coherence tomography probe

We report an integrated ultrasound (US) and optical coherence tomography (OCT) probe and system for intravascular imaging. The dual-function probe is based on a 50 MHz focused ring US transducer, with a centric hole for mounting OCT probe. The coaxial US and light beams are steered by a 45° mirror to enable coregistered US∕OCT imaging simultaneously. Lateral resolution of US is improved due to focused ultrasonic beam. Mirror effects on US were investigated and invitro imaging of a rabbit aorta has been carried out. The combined US-OCT system demonstrated high resolution in visualizing superficial arterial structures while retaining deep penetration of ultrasonic imaging.