戊唑醇和己唑醇的太赫兹（THz）光谱研究

以飞秒激光为基础的太赫兹时域光谱(THz-TDS)技术是一种新型的相干远红外光谱测量技术.文章运用THz-TDS研究了杀菌剂戊唑醇和己唑醇在THz波段的光学特性.在室温氮气环境中,得到了样品在0.2～2.2 THz波段的吸收谱和折射率谱.结果表明戊唑醇和己唑醇在此波段的吸收谱存在显著差异,其吸收峰分别位于2.05THz(戊唑醇)和1.34THz,1.75THz(己唑醇),两种样品的平均折射率分别为1.37和1.42;THz-TDS技术可以分辨化合物结构上的微小差异,可以应用于物质检测与分析,这为农药分子鉴别及更宽有效光谱区的实验测试研究提供了新的实验方法.     

苏丹红I号的太赫兹光谱研究

采用太赫兹时域光谱(THz-TDS)技术测量了苏丹红Ⅰ号在0.5～2.0THz波段的光谱特性,得到了样品在室温氮气环境下的吸收谱和折射率谱,同时利用密度泛函理论计算了该分子在0.5～2.0THz的振动吸收谱.研究发现,苏丹红Ⅰ号在该波段范围存在THz光谱响应,且实验谱和计算谱之间能较好地进行相互印证,说明THz-TDS技术可以用来探测该样品分子的结构和振动情况.本文还给出了与光谱特征吸收对应的分子振动模型,用以认识分子对THz波的响应机制.该研究结果为将THz-TDS技术应用于食品质量安全检测提供了依据.     

基于太赫兹时域光谱技术的煤岩界面识别

为了研究基于太赫兹时域光谱的煤岩界面识别可行性,首先应用太赫兹时域光谱技术,以气煤和砂岩粉末压片作为实验样本进行了相关实验,得到了太赫兹脉冲穿过不同样本时所产生的时间延迟和衰减幅度。然后采用Hilbert-Huang变换对煤岩样的太赫兹光谱进行了时频域分析,并与由传统快速傅里叶变换方法获得的样品频谱进行了对比,结果表明,在时频域内太赫兹光谱信号具有更高的分辨率,可以更好、更直观地分析太赫兹脉冲在任意时刻的频率、能量成分,研究任意频率在不同时刻对吸收谱线的作用。最后介绍了基于太赫兹时域光谱技术的煤岩界面识别过程,通过实验验证了采用Hilbert谱和边际谱提取出的光谱特征值可以很好地区分煤岩介质。     

太赫兹时域光谱技术在涂层检测中的研究进展

涂层结构具有美观、隔热和耐腐蚀等优点,被广泛应用于航空航天、汽车船舶制造和制药等领域。但在涂层制备和服役过程中会不可避免地出现气泡和裂纹等缺陷,且涂层厚度和均匀性会直接影响涂层寿命,因此对涂层结构的检测显得十分重要。与常规无损检测技术相比,太赫兹时域光谱(THz-TDS)技术非接触性好、抗干扰性强,适用于涂层结构快速无损检测。首先,简要地介绍了太赫兹波、太赫兹时域光谱技术和典型的太赫兹时域光谱系统;然后,详细地阐述了太赫兹时域光谱技术在光学参量、厚度和微结构检测方面的国内外研究进展,特别是太赫兹波在汽车船舶、航空航天和生物制药方向的厚度检测与成像;最后,分析了涂层结构无损检测的现状,对现有的太赫兹波检测涂层存在的问题进行了总结,并对其未来的发展进行了讨论和展望。     

光纤耦合太赫兹时域光谱采样技术

主要对基于光电导天线的典型光纤耦合太赫兹时域光谱采样技术及典型光纤耦合太赫兹时域光谱系统进行了介绍.光电导天线是太赫兹时域脉冲信号产生和探测的关键部件,目前基于光电导天线的光纤耦合太赫兹时域光谱采样技术主要有3种,分别为等效时间采样法、异步采样法和腔长调谐光学采样法.对3种不同采样方法的技术原理进行了分析,并对基于不同采样原理的光纤耦合太赫兹时域光谱系统结构进行了介绍,给出了基于不同采样原理的光纤耦合太赫兹时域光谱系统典型产品说明和关键技术指标,说明了基于不同采样方法的系统特点.可为不同的应用需求提供技术参考,为自主搭建基于不同采样技术的光线耦合太赫兹时域光谱系统提供技术支持.     

MWNTs和MWNTs/云母复合物的太赫兹光谱研究

文章采用THZ-TDS技术分别研究了室温大气环境下多壁碳纳米管(MWNTs)和MWNTs/云母复合物在0.2～1.75THz波段的吸收谱、折射率谱、复介电常数和屏蔽效能。结果表明两种样品在此波段有明显的特征吸收峰,且吸收谱存在显著差异,两种样品的平均折射率分别为4.27(MWNTs)和1.81(MWNTs/云母复合物)。随着频率的增加,MWNTs的折射率先增大后减小,而MWNTs/云母复合物的折射率减小,呈现反常色散现象。MWNTs在0.25～1.5THz的范围内屏蔽效能达到30dB以上,而MWNTs/云母复合物在0.2～1.75THz的范围内屏蔽效能小于30dB。     

太赫兹成像技术的应用研究

太赫兹辐射(THz)是指频率范围在0.1～10 THz的电磁波,波段位于毫米波与红外波段之间,其应用主要集中在太赫兹时域光谱技术和太赫兹成像技术两方面,其中太赫兹成像技术在实际应用中更加广泛.介绍了太赫兹辐射及其性质,简述了太赫兹成像技术及其发展过程.分析了太赫兹成像技术在安全检查、癌症检测和无损检测等领域的发展过程,结果表明使用太赫兹成像技术可以对违禁药物和危险隐藏物等进行安全检查,对人体癌变组织进行成像检测,以及对航空航天材料中的缺陷进行无损检测.随着太赫兹成像技术的不断发展,其在安检、医疗和无损检测等领域有着重要的应用前景和研究价值.     

宽频太赫兹时域光谱在药物鉴别中的初步研究

利用THz-ABCD（Terahertz Air Breakdown Coherent Detection）系统对同一厂家生产的牛黄上清片（有效期至2019年3月/有效期至2019年12月）分别进行压片测量,对时域信号进行处理分析,得到它们在0.5~7 THz之间的有效特征吸收谱线,分析结果表明：在2.37 THz之后,过期药品和正常药品的吸收幅值差距比较明显。与此同时利用太赫兹时域光谱技术对3种不同产地（川贝母、平贝母、浙贝母）的贝母中药进行测量和分析,得到了3种贝母在0.5~6 THz之间的有效特征吸收谱,不同类型的贝母呈现不同的吸收光谱。结果表明,宽谱太赫兹时域光谱技术在药品控制和检测中有一定的应用前景。     

基于LabVIEW的太赫兹时域光谱分析软件

针对目前太赫兹光谱仪的数据采集和数据处理软件不能兼容的情况,设计了一个基于LabVIEW软件平台的THz时域光谱分析软件。主要研究了THz时域光谱系统的反射式以及透射式样品光学信息提取算法、支持向量机分类算法、以及偏最小二乘等分析算法以及它们的LabVIEW实现。该软件采用全LabVIEW代码实现,能够实现样品光学信息提取,物质分类识别以及多组分样品浓度分析,并且具有很高的移植性,可扩展性,与数据采集软件具有很好的兼容性。利用该光谱分析软件,分析了三种塑化剂的THz时域光谱信号,得到了它们在THz波段的折射率和吸收系数, 结果表明三种塑化剂可以用THz时域光谱完全区分,软件运行稳定可靠。     

基于流形学习和支持向量机的太赫兹谱分类

太赫兹时域光谱技术是一门新兴光谱检测技术,广泛应用于安检及反恐、生物医学和食品质量检测等方面。太赫兹谱的分类识别技术是太赫兹光谱检测技术的一个重要环节。由于受到噪声的影响,太赫兹谱可能在高维空间中成复杂的非线性分布,传统的分类方法难以取得理想的分类效果。流形学习和支持向量机都是当前机器学习领域的研究热点,都采取了核方法来解决非线性问题,正因为两者之间有很多共通之处,将这两种方法充分结合提出了一种称之为ISOMAP-SVM的新算法。这种新算法拥有比传统的支持向量机算法更快的训练速度和更好的分类效果。实验结果表明利用新算法可以实现对不同种类药品的识别,为太赫兹光谱技术用于药品的检测和识别提供了一种新的有效方法。     

THz-TDS signal analysis and substance identification via the conformal split

A terahertz time-domain spectroscopy(THz-TDS)imaging system can obtain high-dimensional signals with substance fingerprint information.By introducing geometric algebra,a novel signal analysis approach to THz-TDS signals is developed based on an optical physical mechanism.Using this approach,signals are represented with vectors in the high-dimensional real vector space.Geometric distribution properties and algebraic relationships of THz-TDS signals are deduced.It is proved that every complex refractive index of substances relates to a unique 2-blade,the vectors corresponding to the samples of the same substance are collinear and belong to the intrinsic 2-blade of the substance.When decomposed through the conformal split with respect to a 2-blade,THz-TDS signals of high dimensionality can be related to vectors in a 2-dimensional subspace.Based on the conformal split properties we deduced,two criteria for substance identification on the basis of THz-TDS signals are proposed.Accordingly,a novel substance identification method via the conformal split is presented.In the method,the 2-blade related to each "known" substance is calculated with two vectors corresponding to THz-TDS signals measured from samples of the substance but with dierent thicknesses.Using the conformal split with respect to those 2-blades,an identified vector corresponding to a THz-TDS signal is linearly related to the vector in a 2-dimensional subspace.The substance of a sample can be identified using criteria on the projected vectors in the subspaces.This method can contribute to accurate classification and identification.Finally,two experiments are presented that show the feasibility and accuracy of this method.     

采用拍频检测的内嵌型微环传感器研究

提出了一种内嵌型微环传感结构。利用内嵌型结构产生的高Q值滤波峰,采用拍频检测技术,可以实现高灵敏度传感器。采用传输矩阵法分析了内嵌微环半径对滤波峰位置及相邻滤波峰之间关系的影响。内嵌微环的滤波峰产生了分裂,其波长间距很小,且随折射率变化波长间距也会变化。利用该器件构建了拍频检测系统,并对该器件进行传感分析。仿真结果表明,该结构的传感灵敏度为1×10-4折射率单位(RIU),其频率偏移为4 MHz/1×10-4RIU。     

Gain enhancement of the ultra‐wideband tapered slot antenna using broadband gradient refractive index metamaterial

In this article, a broadband gradient refractive index (GRIN) metamaterial is designed and used to enhance the gain of the tapered slot antenna (TSA). The proposed GRIN is implemented using nonresonant parallel‐line unit cell with different refractive index values. GRIN lens are placed in front of the tapered slot direction in the direction of x‐axis. The designed GRIN metamaterials have broad bandwidth (2‐12 GHz) characteristics due to the nonresonant parallel‐line elements which is suitable for the ultra‐wideband frequency band. The measurement results indicates that the radiated beam becomes more directive with narrow beam width. The measured reflection coefficient is below ?10 dB over the frequency bandwidth of 3‐11 GHz. The peak gain of TSA is obtained up to 14 dB at 10 GHz using GRIN lens.     

3D printed dielectric lens for the gain enhancement of a broadband antenna

A novel 3D printed dielectric lens to enhance antenna gain parameters is presented. The lens is fabricated using a fused deposition method (FDM) which is a cost‐effective and an efficient 3D printing technique. Poly‐methyl methacrylate (PMMA) is used as a dielectric material due to its good RF properties. The thickness of the dielectric lens is 14 mm and provides a gain enhancement of up to 6.9 dBi over a wide frequency range. The dielectric lens is designed and computationally analyzed to demonstrate refractive index value close to zero. It has been shown that impedance‐matched near‐zero refractive index lens geometry eliminates strong reflections, and consequently enhances the antenna gain. A correlation is established between the individually, stacked unit cell layers and near‐zero refractive index cut‐off frequencies. The claim is substantiated through measured results using a broadband Vivaldi antenna. A gain enhancement of up to 6.9 dBi is recorded for the bandwidth from 13.5 to 24 GHz. An excellent correlation is reported between the measured and simulated results.     

薄包层长周期光纤光栅的折射率传感特性

长周期光纤光栅(LPFG)可用于折射率传感,同时利用折射变化可以实现谐振波长的调谐.不同的光纤包层半径和光栅周期对折射率灵敏度有较大影响.利用光波导的耦合模理论分析了LPFG的折射率传感特性,给出LPFG的折射率灵敏度的解析表达式,给出了利用不同包层模时的折射率灵敏度.利用三层介质光纤的纤芯模本征方程计算了薄包层LPFG的折射率灵敏度.结果表明,基于不同包层模的LPFG的谐振波长随折射率的变化有红移,也有兰移;基于低阶模序包层模的LPFG,折射率灵敏度较小,中间模序最大,而高阶模序则较大,薄包层LPFG对折射率具有更大的灵敏度.     

Absorption-based fiber optic surface plasmon resonance sensor: a theoretical evaluation

In this paper, an optical absorption based fiber optic surface plasmon resonance (SPR) sensor has been studied theoretically. The theoretical treatment is based on Kretschmann’s SPR theory and the Lorentz model that expresses a damped harmonic oscillator is included in the treatment for optical absorption in the sensing layer. The optical source considered is an un-polarized collimated beam. The light is coupled to the fiber using a microscope objective that focuses the beam at the center of the input face of the fiber. The effects of the parameters related to the sensing region, the light source and the optical fiber on the sensitivity and the operating range of the SPR sensor have been studied with the help of numerical calculations and computer simulations. It has been found that the excitation frequency in absorption-based fiber optic SPR sensor is an important parameter. The sensitivity is better for the lower off-resonance excitation frequency. The sensitivity and the operating range of the sensor are better for large value of the core diameter. The optimization of numerical aperture of the fiber, film thickness and the length of the sensing region is required to achieve the maximum sensitivity. Further, the increase in the extinction coefficient of the sample increases the sensitivity of the sensor while the decrease in the width of its absorption spectrum increases the sensitivity. The sensitivity and the operating range of the sensor are better for small values of the refractive index of the absorbing sample.     

Terahertz free‐space dielectric property measurements using time‐ and frequency‐domain setups

Based on a simple and effective calculation method, two free‐space measurement setups are employed to investigate the dielectric properties of various materials at terahertz (THz) frequencies. One setup involves THZ time‐domain spectroscopy (THz‐TDS) at a frequency range of 0.4 to 1 THz. The other setup comprises a vector network analyzer (VNA) with pairs of VAN extenders (VNAXs) and diagonal standard gain horns (SGHs) at a frequency range of 0.22 to 1.1 THz. The calculation method is verified for the THz‐TDS system and employed in the VNA system for the first time. Dielectric properties, including refractive indices, power absorption coefficients, relative permittivities, and loss tangents, are calculated from measured transmission data. Several materials, including printed circuit boards and 3D printing materials, are characterized to verify the calculation method and compare the measurement setups.     

Fast surface plasmon resonance imaging sensor using Radon transform

Surface plasmon resonance (SPR) imaging technique is proposed in which a diverging laser beam at given frequency was used to illuminate the entire sensor surface in Kretschmann-Raether configuration. A pattern of dark intensity line on bright background is observed corresponding to the SPR dip at an angular range depending on the refractive index of the adjacent analyte and monitored by a two-dimensional CCD detector. A novel Radon transform based detection algorithm for the SPR line pattern is proposed, which is non sensitive to the laser speckle noise and improves the accuracy.     

Surface plasmon resonance based fiber optic sensor for the detection of low water content in ethanol

A fiber optic sensor utilizing surface plasmon resonance (SPR) has been fabricated for the detection of low content of water in ethanol. The sensor utilizes spectral interrogation technique for operation. The resonance wavelength has been found to vary linearly with water content in the range 0-10% with sensitivity of 1.149 nm per percentage of water. The results are in agreement with the refractive index variation of ethanol-water mixture. The sensor has a water resolution of 0.145% which is better than the evanescent wave absorption sensor reported for the similar study. The sensor will find application in determining the low water content in ethanol which is used as a bio-fuel and in the field of medicine and organic chemistry.