Gabor小波分析太赫兹波时间-频率特性的研究

引入小波变换将太赫兹脉冲波形变换到二维的时间-频率平面,对太赫兹脉冲时间-频率特性作联合分析.Gabor小波的形状因子是控制小波时间-频率特性的重要参数,通过改变Gabor小波形状因子提高了小波分析太赫兹脉冲的时间分辨率和频率分辨率. 关键词： 太赫兹辐射 小波变换 频谱分析     

Terahertz波计算机辅助三维层析成像技术

介绍和讨论了Terahertz波计算机辅助层析成像技术(Terahertz-CT).与x射线计算机辅助层析成像技术(XCT)相比较，Terahertz-CT可获得更丰富的信息来处理图像.理论上，该技术可获得被测物在Terahertz波段的折射率和吸收率的三维分布.这意味着有可能利用Terahertz-CT进行暗箱识别探测，该技术的原理在安全检查和无损探测等方面的应用有着广阔的前景.     

Reconstruction algorithms applied to in-line Gabor digital holographic microscopy

This paper investigates the application of Fresnel based numerical algorithms for the reconstruction of Gabor in-line holograms. We focus on the two most widely used Fresnel approximation algorithms, the direct method and the angular spectrum method. Both algorithms involve calculating a Fresnel integral, but they accomplish it in fundamentally different ways. The algorithms perform differently for different physical parameters such as distance, CCD pixel size, and so on. We investigate the constraints for the algorithms when applied to in-line Gabor digital holographic microscopy. We show why the algorithms fail in some instances and how to alter them in order to obtain useful images of the microscopic specimen. We verify the altered algorithms using an optically captured digital hologram.     

Terahertz wide aperture reflection tomography

We describe a powerful imaging modality for terahertz (THz) radiation, THz wide aperture reflection tomography (WART). Edge maps of an object's cross section are reconstructed from a series of time-domain reflection measurements at different viewing angles. Each measurement corresponds to a parallel line projection of the object's cross section. The filtered backprojection algorithm is applied to recover the image from the projection data. To our knowledge, this is the first demonstration of a reflection computed tomography technique using electromagnetic waves. We demonstrate the capabilities of THz WART by imaging the cross sections of two test objects.     

Terahertz tomography using quantum-cascade lasers

The interfaces of a dielectric sample are resolved in reflection geometry using light from a frequency agile array of terahertz quantum-cascade lasers. The terahertz source is a 10-element linear array of third-order distributed-feedback QCLs emitting at discrete frequencies from 2.08 to 2.4 THz. Emission from the array is collimated and sent through a Michelson interferometer, with the sample placed in one of the arms. Interference signals collected at each frequency are used to reconstruct an interferogram and detect the interfaces in the sample. Because of the long coherence length of the source, the interferometer arms need not be adjusted to the zero-path delay. A depth resolution of 360 μm in the dielectric is achieved with further potential improvement through improved frequency coverage of the array. The entire experiment footprint is <1 m×1 m with the source operated in a compact, closed-cycle cryocooler.     

Terahertz two-pixel imaging based on complementary compressive sensing

A compact terahertz(THz) imaging system based on complementary compressive sensing has been proposed using two single-pixel detectors. By using a mechanical spatial light modulator, sampling in the transmission and reflection orientations was achieved simultaneously, which allows imaging with negative mask values. The improvement of THz image quality and anti-noise performance has been verified experimentally compared with the traditional reconstructed image, and is in good agreement with the numerical simulation. The demonstrated imaging system, with the advantages of high imaging quality and strong anti-noise property, opens up possibilities for new applications in the THz region.     

Image formation in holographic tomography

Tomography has been applied to holographic imaging systems recently to improve the 3D imaging performance. However, there are two distinct ways to achieve this: either by rotation of the object or by rotation of the illumination beam. We provide a transfer function analysis to distinguish between these two techniques and to predict the 3D imaging performance in holographic tomography when diffraction effects are considered. The results show that the configuration of rotating the illumination beam in one direction while fixing the sample leads to different 3D imaging performance as compared to the configuration of rotating the sample. The spatial frequency cutoff is nonisotropic in the case of rotating the illumination, and a curved line of singularity is observed. Rotating of the sample, on the contrary, has more symmetry in spatial frequency coverage but has a single point of singularity. The 3D transfer function derived can be used for 3D image reconstruction.     

Digital holographic tomography based on spectral interferometry

A digital holographic tomography system has been developed with the use of an inexpensive broadband light source and a fiber-based spectral interferometer. Multiple synthesized holograms (or object wave fields) of different wavelengths are obtained by transversely scanning a probe beam. The acquisition speed is improved compared with conventional wavelength-scanning digital holographic systems. The optical field of a volume around the object location is calculated by numerical diffraction from each synthesized hologram, and all such field volumes are numerically superposed to create the three-dimensional tomographic image. Experiments were performed to demonstrate the idea.     

Cell refractive index tomography by digital holographic microscopy

For what we believe to be the first time, digital holographic microscopy is applied to perform optical diffraction tomography of a pollen grain. Transmission phase images with nanometric axial accuracy are numerically reconstructed from holograms acquired for different orientations of the rotating sample; then the three-dimensional refractive index spatial distribution is computed by inverse radon transform. A precision of 0.01 for the refractive index estimation and a spatial resolution in the micrometer range are demonstrated.     

Digital holographic image fusion for a larger size object using compressive sensing

Digital holographic imaging fusion for a larger size object using compressive sensing is proposed. In this method, the high frequency component of the digital hologram under discrete wavelet transform is represented sparsely by using compressive sensing so that the data redundancy of digital holographic recording can be resolved validly, the low frequency component is retained totally to ensure the image quality, and multiple reconstructed images with different clear parts corresponding to a laser spot size are fused to realize the high quality reconstructed image of a larger size object. In addition, a filter combing high-pass and low-pass filters is designed to remove the zero-order term from a digital hologram effectively. The digital holographic experimental setup based on off-axis Fresnel digital holography was constructed. The feasible and comparative experiments were carried out. The fused image was evaluated by using the Tamura texture features. The experimental results demonstrated that the proposed method can improve the processing efficiency and visual characteristics of the fused image and enlarge the size of the measured object effectively.     

Terahertz interferometric synthetic aperture tomography for confocal imaging systems

Terahertz (THz) interferometric synthetic aperture tomography (TISAT) for confocal imaging within extended objects is demonstrated by combining attributes of synthetic aperture radar and optical coherence tomography. Algorithms recently devised for interferometric synthetic aperture microscopy are adapted to account for the diffraction-and defocusing-induced spatially varying THz beam width characteristic of narrow depth of focus, high-resolution confocal imaging. A frequency-swept two-dimensional TISAT confocal imaging instrument rapidly achieves in-focus, diffraction-limited resolution over a depth 12 times larger than the instrument's depth of focus in a manner that may be easily extended to three dimensions and greater depths.     

Acousto-optical coherence tomography with a digital holographic detection scheme

Acousto-optical coherence tomography (AOCT) consists in using random phase jumps on ultrasound and light to achieve a millimeter resolution when imaging thick scattering media. We combined this technique with heterodyne off-axis digital holography. Two-dimensional images of absorbing objects embedded in scattering phantoms are obtained with a good signal-to-noise ratio. We study the impact of the phase modulation characteristics on the amplitude of the acousto-optic signal and on the contrast and apparent size of the absorbing inclusion.     

三维温度场的激光全息与干涉层析

本文论述了多方向干涉层析再现三维温度场的原理,提出了多方向象面全息干涉法,并利用这种方法测量了三维温度场分布;进一步提出了一种更有实用价值的多方向法布里-珀罗干涉法,它具有实时性,能用于真实瞬态流场的测量.     

少量投影数字全息层析重建实验研究

提出了少量投影数字全息层析重建技术.以具有轴对称结构的光纤和非轴对称结构的石膏头像为实验样本,开展了基于单幅全息图和代数迭代重建算法的层析重建模拟分析及实验研究.模拟分析表明,代数迭代重建算法中的加权因子以及松弛因子对重建图像质量影响显著,因此对加权因子采用非线性加权计算方法,以及通过多次模拟重建结果的比较选择合适的松弛因子.层析重建实验结果表明,无论被测物体是轴对称结构或是非轴对称结构,少量投影数字全息层析重建是可行的,也是有效的.这为下一步针对生物样本内部多层折射率检测和基于单幅层析全息图实现三向实时数字全息层析重建技术研究提供了必要的基础.     

X光Gabor波带片编码成像技术实验研究

 基于星光II实验装置进行了Gabor波带片编码成像探索性实验研究。采用束匀滑光斑三倍频激光打靶，辐照专门设计的ＣＨ埋金属条靶（共两种），产生了有明确形状、边缘清晰的等离子体。研制了专门的Gabor波带片编码相机，并利用该相机采用单张胶片对不同的靶目标在不同的深度层次上曝光的技术，获得了具有深度层次信息的三维激光等离子体编码像。利用数值重建技术对实验所获得的图像解码，最终获得了在不同深度层次上的激光等离子体信息。     

X光Gabor波带片编码成像技术实验研究

基于星光II实验装置进行了Gabor波带片编码成像探索性实验研究。采用束匀滑光斑三倍频激光打靶,辐照专门设计的ＣＨ埋金属条靶（共两种）,产生了有明确形状、边缘清晰的等离子体。研制了专门的Gabor波带片编码相机,并利用该相机采用单张胶片对不同的靶目标在不同的深度层次上曝光的技术,获得了具有深度层次信息的三维激光等离子体编码像。利用数值重建技术对实验所获得的图像解码,最终获得了在不同深度层次上的激光等离子体信息。     

Terahertz tomography of a photo-induced carrier based on pump-probe spectroscopy using counterpropagation geometry

A novel technique for the terahertz (THz) tomography of a photo-induced carrier that is based on optical-pump THz-probe time-resolved reflection spectroscopy using counterpropagation geometry of the pump and probe pulses has been proposed. Transient reflection due to the photo-induced carrier provides information about the physical properties and spatial distribution separately. We have experimentally demonstrated this method using a silicon wafer. The obtained complex reflection can be reproduced by the exact solution of Maxwell's equations, assuming an exponential distribution of the photo-induced carrier density.     

Quantitative visualization of high-speed 3D turbulent flow structures using holographic interferometric tomography

Using holographic interferometry the three-dimensional structure of unsteady and large-scale motions within subsonic and transonic turbulent jet flows has been studied. The instantaneous 3D flow structure is obtained by tomographic reconstruction techniques from quantitative phase maps recorded using a rapid-switching, double reference beam, double pulse laser system. The reconstruction of the jets studied here reveal a three-dimensional nature of the flow. In particular an increasing complexity can be seen in the turbulence as the flow progresses from the jet nozzle. Furthermore, a coherent three-dimensional, possibly rotating, structure can be seen to exist within these jets. The type of flow features illustrated here are not just of fundamental importance for understanding the behavior of free jet flows, but are also common to a number of industrial applications, ranging from the combustion flow within an IC engine to the transonic flow through the stages of a gas turbine.