【作者】 于洋；

【导师】 张朝晖；

【作者基本信息】 北京科技大学 ， 控制科学与工程， 2022， 博士

【摘要】 基于相干性、高穿透性以及低光子能量等特性,太赫兹光谱技术在公共安检及无损检测领域具有极高的应用潜力与价值。但是,目前在太赫兹光谱测试中,被测试的固态材料被要求制作成表面平行且光滑的标准形态。然而,在安检等实际应用场合中,非标准形态样品不可避免,且很难进行标准制样。因此,样品的非标准形态是阻碍太赫兹光谱技术实际应用进程的一项主要因素。本文将常见的非标准形态样品分为规则形态样品和不规则形态样品两类,在表面粗糙的不规则形态样品和表面具有凹陷特征的规则形态样品的光谱传递函数和光学参数提取方面进行了研究,本文主要内容如下:(1)在高斯随机粗糙表面对太赫兹透射光谱的影响研究中,通过划分粗糙表面微元建立了基于菲涅尔透射过程的模型,在表面形态参数已知的二维粗糙面上对模型值进行求解。分析了模型中透射系数和相位的概率分布情况,归纳了入射太赫兹波频率、表面粗糙度与相关长度这三项因素造成太赫兹波幅值衰减的原因,并得到粗糙程度和光谱传递函数失真程度之间的定量关系。选用在太赫兹波段无吸收特性的聚四氟乙烯为样品进行实验,实验结果证实了模型及相关结论的准确性。(2)在高斯随机粗糙表面样品太赫兹透射谱光学参数提取的研究中,引入了表面粗糙信息,建立了粗糙表面样品的阶梯型太赫兹波透射模型,模型将太赫兹波在样品内部的传播光程与样品表面粗糙度相互关联。在实验部分中,分别选择在太赫兹波段没有吸收特性的聚四氟乙烯和有吸收特性的α-乳糖和L-组氨酸为样品。实验结果表明在已知粗糙度和平均厚度的条件下,该模型的提取算法与传统光学参数提取算法相比能够有效降低粗糙度的影响,较为准确的提取出样品的折射率和吸收系数。该研究为粗糙表面样品光学参数的提取提供了一种新的方法。(3)在规则凹陷表面对太赫兹透射光谱的影响研究中,对凹陷表面样品进行了基于高斯光学的太赫兹波透射过程建模,使用小孔拟合方法对实验系统的太赫兹高斯光束参数进行测量。在实验部分中,选用聚四氟乙烯为实验材料,将太赫兹波传递函数的理论模型值和实验值进行对比来验证模型的适用情况。通过实验证实了在表面有凹陷等缺损情况时对太赫兹波进行高斯光束建模的必要性,并由此推论出在与太赫兹波传播方向平行和垂直两个维度方向上,凹陷深度和凹陷半径对太赫兹透射光谱的定量影响作用。本研究一方面可用于太赫兹波在塑料等非极性材料表面缺陷的无损检测中;另一方面,可用于设计样品表面形态,使其具有期望的特定光谱传递函数。更多还原

【Abstract】 Terahertz spectroscopy technology shows great potential and value in public security inspection and non-destructive testing based on coherence,penetrability,and low-energy photon characteristics.However,in the current terahertz spectroscopy test,the solid-state material is required to be made into a standard shape with a parallel and smooth surface.On the contrary,samples with unconventional morphology are inevitable,and sample preparation methods cannot be performed,especially in practical situations such as security inspection.Therefore.the sample’s morphology will hinder the application process of terahertz spectroscopy.The samples with common unconventional morphology are divided into regular shape and irregular shape samples in this paper.The optical parameters extraction and terahertz spectral transfer function of irregular shape samples with a rough surface and regular shape samples with concave features on the surface is studied.The main contents and innovations of this paper are as follows:(1)In the study of the Gaussian random rough surface influence on terahertz transmission spectra,a model based on the Fresnel transmission process is established.In this model,the rough surface is divided into microelements,and the model value is obtained by rough surface with known morphological parameters.The model’s transmission coefficient and phase probability distribution are analyzed,and the mechanism for terahertz wave amplitude attenuation caused by frequency,roughness,and correlation length are summarized.Above all,the quantitative relationship between the ratio of roughness and correlation length and the distortion degree of the spectral transfer function is obtained.Polytetrafluoroethylene without absorption characteristics within the terahertz band is selected as the experimental material,and the experimental results confirm the accuracy of the model and related conclusions.(2)In the study of extracting the optical parameters of Gaussian random rough surface samples in terahertz transmission spectra,surface roughness is introduced,and we establish a stepped terahertz transmission model which makes terahertz wave path associated with surface roughness.The model includes the average thickness,roughness,refractive index,and extinction coefficient.In the experiment,polytetrafluoroethylene without absorption characteristics and α-Lactose and Lhistidine with absorption characteristics within the terahertz band are selected as samples.The experiment results show that the model extraction algorithm can effectively reduce the influence of roughness.It can extract the sample’s refractive index and absorption coefficient accurately compared with the traditional algorithm under the condition of known roughness and average thickness.This research provides a new method to extract optical parameters of samples with a rough surface.(3)In the study of the influence of regular concave surface on terahertz transmission spectra,we establish a model according to the transmission process of terahertz wave on the concave surface based on Gaussian beam optics.The terahertz Gaussian beam parameters are measured by the small-aperture fitting method.In the experiment,polytetrafluoroethylene is selected as the experimental material.The theoretical model value of terahertz transfer function is compared with the experiment value to verify the model’s applicability.The necessity of treating terahertz wave as Gaussian beam in the case of defects on the surface is confirmed by experiments,and it is inferred that the quantitative influence of concave depth and concave radius on terahertz transmission spectra in the parallel and perpendicular propagation direction of the terahertz wave.On the one hand,this research is meaningful in non-destructive testing of surface defects of plastic products by terahertz spectra;On the other hand,it is valuable in designing the sample’s surface morphology to have a specific spectral transfer function.更多还原