Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array

Real-time, continuous-wave terahertz imaging is demonstrated with a 10 mW, 2.52 THz (118.8 microm) far-infrared gas laser and a 160 x 120 element microbolometer camera. The microbolometer camera is designed for wavelengths of 7.5-14 microm but retains sensitivity at terahertz (THz) frequencies. The setup has no moving parts, and transmission-mode THz images can be obtained at the video rate of 60 frames/s. The peak signal-to-noise ratio is estimated to be 13 dB for a single frame of video, acquired in 16 ms. With this setup, THz imaging through a FedEx envelope is demonstrated, showing the feasibility of real-time mail screening.     

基于数学形态学的弱点状运动目标的检测

提出了一种新的基于数学形态学的红外图像序列中弱点状运动目标的非参数检测算法。采用数学形态学抑制背景杂波干扰和增强目标,用沿时间轴投影和二维空域搜索代替复杂的时空三维搜索形成组合帧,然后在每条可能的轨迹上将进行目标能量累加,实现了一种快速检测前跟踪(TBD)检测算法。仿真实验表明:在恒虚警概率条件下,该检测算法能高效地检测信噪比约为2的弱点状运动目标,检测性能对噪声分布不敏感,能精确地得到目标的即时位置和速度信息,适合于实时图像处理和目标探测,具有很高的实用价值。     

基于太赫兹量子级联激光器的实时成像研究进展

太赫兹(THz)实时成像是THz技术中颇具潜力的一个领域,具有成像速度快、成像分辨率高等特点,基于THz量子级联激光器(QCL)的实时成像系统是其中最重要的一种,系统体积小、重量轻、成像信噪比高等特点使其在实际应用中具有独特的优势。本文主要介绍了THz QCL器件及其实时成像系统的研究进展,采用超半球高阻硅透镜改善了THz QCL的输出激光,实现了准高斯光束输出,搭建了基于二维摆镜消干涉技术的THz实时成像系统,单帧成像光斑面积45mm×30 mm,实现了对刀片、药片的实时成像演示,成像分辨率优于0.5 mm;最后对成像系统激光源、成像光路和探测端的改进以及成像效果的改善方面进行了综述,并探讨了THz实时成像系统未来的发展趋势及其在材料分析和生物医学成像方面的应用前景。     

Terahertz real-time imaging with balanced electro-optic detection

We propose a method for two-dimensional terahertz (THz) real-time imaging which systemically integrates the THz balanced electro-optic sampling technique and the dynamic subtraction technique. The proposed method can effectively improve the signal to noise ratio and the spectrum measurement accuracy of the imaging system. Experiment results demonstrate the advantage of the method.     

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。     

Terahertz Real-Time Off-Axis Digital Holography with Zoom Function

We present a terahertz(THz) real-time digital holographic system with zoom function worked at 0.17 THz. The magnification factor ranges from 1 to 2. In the imaging experiment, the resolution is 2 mm with the magnification factor of 1.2. A metal sheet with F-shaped hollow is used as a sample, and its THz holograms are reconstructed by our developed algorithm based on the angular spectrum theorem, and the qualities of the THz images under different conditions are compared.     

A novel frame shift and integral technique for processing low-light-level moving image sequence

A new method for processing low-light-level moving image sequence is proposed, in which (1) a novel algorithm based on difference processing has been firstly developed to determine the motion parameters of moving image sequence, (2) the spatial relativity of the frames within moving image sequence can be then well established after frame shifting according to motion parameters obtained above, (3) finally, frame integration method can be applied to each processed frame, thereby increasing the signal-to-noise ratio of low-light-level moving image sequence. Experiments have been carried out to verify the validity of the proposed method, which show that the parameters obtained by the developed algorithm coincide well with the actual values, and the SNRs of the moving image sequences were effectively increased with the proposed method, indicating that the method offers significant advantages in the enhancement of low-light-level moving image sequence.     

基于THz光谱技术的1,3-二硝基苯挥发气体检测方法研究

针对工业生产过程中1,3-二硝基苯挥发气体突发性泄露等情况的发生,设计了一种基于太赫兹(THz)特征光谱吸收技术的检测方法,该方法可通过1,3-二硝基苯在THz波段的特征波长对痕量剧毒有害气体进行检测。系统设计了双气室结构,为THz波提供差分数据,一组为标准空气,另一组为待测样气。由1,3-二硝基苯在THz波段的四个主吸收带0.635,0.912,1.095和1.435THz可根据对应吸收带吸收系数的比例关系得到对应波长上的吸收幅值,最终反演气体浓度。经过对两种传统检测方法的实验分析和对THz吸收检测方法的仿真研究可知,色谱法检测速度无法实现实时检测,红外吸收法对环境湿度要求很高,需经干燥处理,而THz吸收法对环境湿度要求较低,且具备实时性,更适合于实际应用。     

基于平均吸收的太赫兹波振幅成像研究

提出了一种基于一定频率内平均吸收的太赫兹(THz)波振幅成像新方法。太赫兹波频率在0.1～10 THz之间,波段位于红外和微波之间。太赫兹波成像技术的一个显著特点是信息量大,如何对每个样品点的大量信息进行处理提取有用信息重构出样品的图像是一项关键技术。选用中间挖空有“THz”字样的白纸为样品作太赫兹波成像研究,首先探讨了时域和频域上几种常用太赫兹波振幅成像方法所反映的样品信息及其特点,进一步使用提出的基于一定频率内平均吸收的太赫兹波振幅成像新方法对样品进行图像重构。实验结果表明这种新方法可以很好的反映样品的真实信息,反映了样品在一定频率范围内由于吸收而引起的效果的综合,与吸收系数和厚度相关,离散效应得到了很好的消除,相对几种常用的太赫兹波振幅成像方法能够得到更清晰的图像。此新方法尤其适用于结构简单的样品,能够成为几种常用振幅成像方法的有力补充。     

Continuous-wave terahertz digital holography by use of a pyroelectric array camera

Terahertz (THz) digital holography is realized based on a 2.52?THz far-IR gas laser and a commercial 124 × 124 pyroelectric array camera. Off-axis THz holograms are obtained by recording interference patterns between light passing through the sample and the reference wave. A numerical reconstruction process is performed to obtain the field distribution at the object surface. Different targets were imaged to test the system's imaging capability. Compared with THz focal plane images, the image quality of the reconstructed images are improved a lot. The results show that the system's imaging resolution can reach at least 0.4 mm. The system also has the potential for real-time imaging application. This study confirms that digital holography is a promising technique for real-time, high-resolution THz imaging, which has extensive application prospects.     

Real-time flow imaging by removing texture pattern artifacts in spectral-domain optical Doppler tomography

We present a new, simple method to suppress texture pattern artifacts induced by the optical heterogeneity of tissues to improve the performance of flow imaging for real-time phase-resolved optical Doppler tomography. The method performs transverse scanning of the probe beam in the forward and then reverse directions, and it takes average of the spatial phase changes between them to obtain the final velocity image. It relies on the fact that the phase changes between successive axial scans due to the optical heterogeneity of the sample are time independent, while those due to the moving particles are time dependent. We experimentally demonstrate this method by real-time imaging of a flow phantom.     

T-ray computed tomography

We demonstrate a tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional (3D) structures in the far-infrared. This THz-wave computed tomography (T-ray CT) system provides sectional images of objects in a manner analogous to conventional CT techniques such as x-ray CT. The transmitted amplitude and phase of broadband pulses of THz radiation are measured at multiple projection angles. The filtered backprojection algorithm is then used to reconstruct the target object, including both its 3D structure and its frequency-dependent far-infrared optical properties.     

Measurement of Refractive Index for High Reflectance Materials with Terahertz Time Domain Reflection Spectroscopy

A method to measure the refractive index for high reflectance materials in the terahertz range with terahertz time domain reflection spectroscopy is proposed. In this method, the THz waveforms reflected by a silicon wafer and high reflectance sample are measured respectively. The refractive index of the silicon wafer, measured with the THz time domain transmission spectroscopy, is used as a reference in the THz time domain reflective spectroscopy. Therefore, the complex refractive index of the sample can be obtained by resorting to the known reflective index of the silicon and the Fresnel law. To improve the accuracy of the phase shift, the Kramers-Kronig transform is adopted. This method is also verified by the index of the silicon in THz reflection spectroscopy. The bulk metal plates have been taken as the sample, and the experimentally obtained metallic refractive indexes are compared with the simple Drude model.     

0.14 THz高分辨力成像雷达信号处理

为适应0.14 THz超高分辨雷达实时成像的需求,开发了基于CPU+GPU+FPGA的硬件架构和成像处理算法,算法以距离-多普勒为原型,引入L类维格纳分布变换提高横向分辨力,用Keystone变换方法对越距离单元徙动进行校正,并开发了系统非线性补偿算法。在载频0.14 THz、带宽5 GHz雷达样机上进行了逆合成孔径雷达成像试验,获得了3 cm3 cm的成像分辨力和实时成像能力,验证了信号处理方法的有效性。     

Continuous-wave terahertz in-line digital holography

A terahertz (THz) in-line digital holography project has been proposed based on a CO₂ pumped 2.52?THz continuous-wave laser and a pyroelectric-array camera used as the detector. The THz Gabor in-line digital holograms have been obtained and then the high resolution reconstruction of THz in-line digital holography was realized. The resolution of an in-line digital holography system has been tested by the use of a series of objects. High-quality and high-resolution reconstructed images have been obtained, and the real lateral resolution is higher than 0.2?mm. It can be inferred from the results that the THz Gabor in-line digital holography system has the abilities of real-time and high-resolution imaging.     

Study of drying process of paint by dynamic speckle with B/D pixel counting technique

The aim of the work is the assessment of drying process of latex paint by dynamic Laser speckle method. The basic concept of dynamic speckle technique is described. Laser light scattering by water borne sample is a time dependent surface activity. The variation of laser speckle intensity is due to the change of refractive index and the particle movements of the latex paint. A novel method, B/D counting technique to measure the dynamic activity of drying paint using co-occurrence matrix of Time History of Speckle Pattern (THSP) is presented. The result of drying process of latex paint by dynamic laser speckle method is compared with the gravimetric method and agreed well.     

A Novel Woodpile Three-Dimensional Terahertz Photonic Crystal

A novel woodpile lattice structure is proposed. Based on plane wave expansion （PWE） method, the complete photonic band gaps （PBGs） of the novel woodpile three~dimensional （3D） terahertz （THz） photonic crystal （PC） with a decreasing symmetry relative to a face-centred-tetragonal （fct） symmetry are optimized by varying some structural parameters and the highest band gap ratio can reach 27.61%. Compared to the traditional woodpile lattice, the novel woodpile lattice has a wider range of the filling ratios to gain high quality PBGs, which provides greater convenience for the manufacturing process. The novel woodpile 3D PC will be very promising for materials of THz Junctional components.     

Object based dual watermarking for video authentication

Video cameras have been widely installed in public facilities for the surveillance applications. So, video authentication has becoming increasingly attractive. This paper presents a dual watermarking for video authentication based on moving objects. For each frame, the frame index, as a watermark is first embedded into the moving objects of the corresponding frame using a reversible watermarking method, aiming to detect the temporal tampering. Then the principle content and the details of the moving objects combined with the authentication code, as the other watermark, are embedded into the frame for spatial tampering location and recovery. Specially, a synthesized frame method is proposed for lossless recovery of moving objects and effective extraction of frame index. Statistical analysis and experiment results show that the proposed method can locate spatial, temporal and spatio-temporal tampering accurately. The spatial tampered regions can be recovered approximately and the moving objects can be restored completely when the tampered area is limited.     

Fourier domain optical coherence tomography using optical demultiplexers imaging at 60,000,000 lines/s

We describe high-speed Fourier domain optical coherence tomography (OCT) using optical demultiplexers (ODs) for spectral dispersion. The OD enables separation of a narrow spectral band of 14 GHz (0.11 nm) from a broadband incident light at 256 different frequencies in 25.0 GHz intervals centered at 192.2 THz (1559.8 nm). OCT imaging of 60,000,000 axial scans per second was achieved through parallel signal acquisition using 256 balanced photoreceivers to simultaneously detect all the output signals from the ODs in a Fourier domain OCT system. OCT imaging at a 16 kHz frame rate, 1100 A-lines per frame, 3 mm depth range, and 23 microm resolution was demonstrated using a resonant scanner for lateral scanning.     

移动坐标系FDTD方法在脉冲波传播中的应用

通过在时域有限差分(FDTD)方法中引入移动坐标系,解决了一般FDTD方法处理脉冲长距离传播时面临的困难。推导了移动坐标系下的场方程和FDTD差分格式,并将该方法应用于模拟波导中电磁脉冲的传播。计算结果表明,在传播过程中,脉冲波包沿传播方向逐渐展宽,脉冲的峰值场强变小,同时出现了色散现象,脉冲中传播较慢的成分逐渐移向尾部,产生带尾态,这与一般FDTD方法得到的计算结果接近。多个频率的计算结果表明,该方法与一般FDTD方法得到的数据的相对误差小于0.1%,验证了方法的可行性和正确性。