一种有效提高太赫兹时域光谱装置成像空间分辨率的方法

太赫兹波成像技术一个最显著的制约因素是其有限的空间分辨率。提出通过在样品前加小孔的方法来提高传统太赫兹时域光谱装置成像的空间分辨率。采用在样品前约2 mm处加直径为0.5 mm小孔的方法使成像的空间分辨率从1.276 mm提高到0.774 mm,提高0.502 mm,约39％。通过这个简单的方法在传统的太赫兹时域光谱成像装置上实现了空间分辨率从毫米量级到亚毫米量级的提高。聚乙烯板上直径为1 mm的小孔被作为成像的研究对象,分别采用传统的太赫兹时域光谱装置对样品进行直接成像和在样品前约2 mm处加直径为0.5 mm的小孔后对样品成像两种方式,并采用损失成像中信噪比较好的能量损失成像,对比两种方式得到的样品的太赫兹像,结果显示聚乙烯板上小孔的边界加小孔后成像比不加小孔直接成像明显清晰。证实了在样品前加小孔可以有效的提高太赫兹成像系统的空间分辨率。从理论上对通过在样品前加小孔提高系统空间分辨率的方法进行了分析,指出小孔尺寸越小,系统的空间相干长度越大,空间分辨率越高,但同时太赫兹信号的强度会相应减小。该方法可以简单有效的提高太赫兹时域光谱装置成像的空间分辨率,从而进一步拓展太赫兹谱成像技术的应用领域。     

基于迈克尔逊干涉仪的太赫兹光谱高速探测方法研究

太赫兹光谱技术作为获取物质在太赫兹频段信息的主要方法,已经被广泛应用于物质成分的测定,而其在成分分布成像方面则有着更广阔的应用前景,例如片剂药品的有效成分检测、行李安检的危险物品检测等。现有的太赫兹光谱探测方法时域光谱技术（THz-TDS）和频域光谱技术（THz-FDS）均不能很好地兼顾光谱分辨率与扫描时间;且获得物质光谱数据往往要花费数秒乃至数分钟时间（取决于光谱仪的结构）,这对多像素成像系统显得过于迟缓,更无法达到视频成像的速率需求,严重制约了太赫兹光谱成像的实际应用。目前的太赫兹波成像多为全频段波强度成像,只能反映样品的空间分布信息,并不能反映出样品的光谱即成分信息。因此,对太赫兹光谱探测速率的提升十分迫切,太赫兹光谱高速探测的实现不仅可以显著减少物质成谱的实验耗时,还为实现物质的太赫兹光谱成分分布成像提供了可能。提出了一种基于迈克尔逊干涉仪的太赫兹光谱高速探测方法,在设计了该方法装置结构的基础上,理论分析了其工作过程,同时进行了太赫兹光谱的计算。然后从数据采样、数据处理及参数选择这几个方面进行问题分析,计算得出该方法能够显著加快物质太赫兹光谱的扫描获取速率。最后,对该方法建模进行仿真研究,模拟实现其完整的探测过程。在仿真研究中,以太赫兹辐射源的频谱分布为例,将该方法的建模仿真结果与时域光谱技术（THz-TDS）测试结果进行了对比,结果表明时域光谱技术（THz-TDS）所测得的频谱曲线可以近似看作是该高速光谱探测法所得频谱曲线的包络线,两种不同方法所得频谱结果具有较强的一致性。总之,该方法能够进行样品的太赫兹光谱探测,且在保证分辨率相同的前提下,较时域光谱技术（THz-TDS）显著加快了成谱速率,为实用、高通量太赫兹光谱成像提供了一种可能。     

基于太赫兹时域光谱技术的伪色彩太赫兹成像的实验研究

提出了一种伪色彩太赫兹成像技术. 通过引入频域色彩区间积分, 建立了一套基于太赫兹时域光谱技术的伪色彩太赫兹成像系统, 实验分别研究了乳糖和对氨基苯甲酸两种不同白色化学粉末的伪色彩成像和灰度成像, 研究了不同颜色区间定义对伪色彩图像的影响, 讨论了利用不同频率信息成像系统所能达到的空间分辨率. 研究结果表明, 伪色彩成像技术可以将不同的物质信息同时成像在一张太赫兹图像中, 通过不同物质在太赫兹图像中呈现出的颜色差别来区分不同的物质及其分布. 克服了传统的太赫兹灰度成像技术中, 需要多张图像来区分不同的物质的问题, 提高了成像速度, 降低了筛选难度. 利用高频信息进行伪色彩成像, 可以将系统成像的空间分辨率提高到0.4 mm. 伪色彩成像方式可以更直观快捷地显示样品的基本属性, 对于实现太赫兹安检的初检和快速筛选具有重大的现实意义.     

太赫兹超分辨近场成像方法研究综述

受衍射极限的限制,传统太赫兹成像分辨率在毫米量级,无法满足目前前沿研究向微纳米尺度发展的主要趋势。高时空分辨率太赫兹成像技术成为当下太赫兹领域最重要的研究热点之一。近场太赫兹成像技术是实验中将太赫兹成像分辨率提升至微纳米量级的重要方法。介绍了近场太赫兹成像技术的基本原理,详述了多种近场太赫兹成像技术的发展历程与技术路线,从时空分辨能力、频谱分辨能力、成像质量、成像信噪比和适用场景等多个角度分析并总结了各种近场太赫兹成像技术的优势和不足。最后,讨论并展望了太赫兹超分辨成像未来的发展趋势。     

高空间分辨率高可见度的太赫兹光谱成像研究

太赫兹光谱成像,不但包括在二维图像空间的强度信息,同时可以得到太赫兹波段的光谱信息,构成了一个三维的数据矩阵。由于受到太赫兹成像系统内部硬件的限制和影响,太赫兹频域较高频段处信号存在能量弱、信噪比低的特点,导致所成的太赫兹图像普遍存在分辨率低、对比度低等问题。因此,利用三维数据矩阵,应用适合的算法,实现了提高太赫兹光谱成像空间分辨率、边缘细节可见度的目的。搭建了三维可移动式太赫兹时域光谱成像系统,实现了对标准高分辨率板的二维扫描。对该系统所采集到的信号分别进行时域、频域等多种方式成像对比,结合瑞利判据和分辨率标尺对成像系统的空间分辨率、景深进行标定,研究了提高太赫兹光谱成像的空间分辨率算法。然后,针对太赫兹频域高频区域信噪比低、对比度低、噪声原因复杂的特点,结合深度残差学习的图像去噪理论,提出了太赫兹图像深度去噪网络,在训练集中引入成像系统中真实的“太赫兹残差噪声”。最后,利用所训练出的模型对太赫兹频域高频区域图像进行盲去噪,并用重建图像分别与原始成像结果和传统太赫兹去噪算法结果进行比较,分别从主观和客观两个方面评价了不同算法对太赫兹频域高频图像的去噪效果。实验结果表明,通过该算法实现...     

太赫兹技术在爆炸物检测中的应用

研究了太赫兹成像技术在爆炸物探测中的应用,分析了太赫兹透射型时域光谱系统的实验装置,介绍了太赫兹时域光谱的测量步骤。确定了四种爆炸物样品(TNT,RDX,DNT,HMX)在太赫兹波段的吸收谱。结果表明,这四种爆炸物样品在0~2.5THz的频率范围内均存在特征吸收峰,这为太赫兹技术检测爆炸物提供了一种有效的途径。     

小波变换在太赫兹三维成像探测内部缺陷中的应用

采用Syn View Head 300对内部有胶和空气孔的样件进行了太赫兹二维扫描(xy轴方向),系统通过线性调频连续波技术得到样件内部的三维信息.检测薄层时,由于太赫兹源的波长在亚毫米量级,薄层的上下表面反射峰相距太近而难以辨别.为了提高太赫兹探测的纵向分辨率,采用小波变换对探测信号进行处理,对小波系数进行三维重构,获得的三维小波系数图像比原始三维探测信号更加精确.该方法有效提高了太赫兹成像的纵向检测精度,纵向分辨率可达1 mm.     

小波变换在太赫兹时域光谱分析中的应用

随着超快激光技术的发展及其人们对太赫兹(THz)电磁波波段与脉冲光源认识的进一步深入,太赫兹时域光谱(THz-TDS)技术作为一种新的、快速发展的光谱分析方法在许多领域备受关注。利用太赫兹时域光谱技术在空气环境下测量样品时,样品的太赫兹光谱会因空气中水蒸气的影响而出现振荡现象。文章利用太赫兹时域光谱分析技术分别在氮气和空气环境下测量了七种样品在0.2～1.9太赫兹波段的光谱,并以氮气环境下的太赫兹光谱为参考,利用小波变换对空气环境下测量的数据进行了处理,消除了太赫兹光谱中水蒸气吸收造成的影响,实验结果证明了该方法的可行性,在此基础上,还对其中四种样品做了成像和识别,并得到了较好的结果。     

基于太赫兹时域光谱技术的光学参数提取方法的研究进展

光学参数是宏观上表征材料光学性质的物理量,间接反映了材料的微观特性,对光学参数准确的提取可以研究材料的微观性质和机理。近年来,太赫兹时域光谱技术作为一种新兴的光谱分析手段已经成为研究的热点。由于太赫兹辐射能量低并且脉冲宽度窄(皮秒量级),太赫兹时域光谱技术在提取光学参数方面具有无损伤和高时间分辨率的特点。本文总结了基于太赫兹透射和反射时域光谱技术的光学参数提取方法的研究进展,着重阐述了几种经典方法,分析了每种方法的优缺点,并讨论了太赫兹时域光谱技术用于提取材料光学参数的挑战。研究结果表明,透射法适用于对太赫兹波吸收较弱的物质,而反射法则适用于对太赫兹波有强烈吸收的材料。     

基于BWO连续太赫兹波成像系统的无损检测

介绍了一种基于返波管（BWO）的太赫兹波成像系统。BWO连续太赫兹波成像是一种新的无损检测方法。实验过程中把样品放在X-Z二维电控平移台上进行扫描成像,透过样品的太赫兹波强度信息由热释电探测器接收,然后经过电脑成像。给出了应用0.71THz的连续太赫兹波对打孔铝板、公交卡、校园卡的内部结构和对隐藏在信封内硬币等物体和信封内纸片上的铅字迹的成像实验研究事例,并且测知该系统能够分辨出直径最小为1.5mm的小孔。这项工作揭示了使用BWO连续太赫兹波成像系统在无损检测和安全检查领域是有效的。     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.