Abstract-Broadband stepped-frequency modulated continuous terahertz wave tomography for non-destructive inspection of polymer materials

Xiaoxuan Zhang, Qijia Guo, Tianying Chang, Hong-Liang Cui,


https://www.sciencedirect.com/science/article/pii/S0142941818320683

An all-solid-state electronic three-dimensional terahertz tomography system designed specifically for non-destructive inspection of polymer materials is demonstrated, which is capable of determining the positions and shapes of hidden defects accurately. The imaging radar system, based on stepped-frequency modulated continuous wave (SFMCW), with center frequency at 180 GHz, bandwidth of 60 GHz, and average power 0.5 mW, is tested against thick Teflon (polytetrafluoroethylene, PTFE) plates with internal voids as samples. The locations and shapes of the hidden holes are obtained in both electromagnetic simulation and experimental measurements with a three-dimensional image reconstruction algorithm, which features advantages of accurate reconstructed image details and fast computation speed, demonstrating that the terahertz imaging radar system combined with the algorithm developed is capable of detecting internal defects of thick polymers. Key resolution parameters are established experimentally in detail, demonstrating 2.5 mm and 1.7 mm range resolutions in free space and in Teflon separately, and lateral spot diameters ranging from 3.4 mm to 8 mm at imaging distances from 5 mm to 60 mm.

Abstract-Nondestructive imaging of hidden defects in aircraft sandwich composites using terahertz time-domain spectroscopy

Qiang Wang, Xinyi Li, Tianying Chang, Jin Zhang, Lingyu Liu, Hongbin Zhou, Jinpeng Bai,



https://www.sciencedirect.com/science/article/pii/S1350449518306091

Aircraft sandwich composites are superior to ordinary composites and have been largely used in the manufacture of military aircraft. This study attempts to investigate the efficacy of terahertz (THz) time-domain spectroscopy (TDS) imaging technology in detecting hidden defects in aircraft glass fiber (GF) sandwich composites. The defects of various sizes, such as debonding, delamination, and multi-delamination, at different depths in GF A-sandwich structure composites with foam core, C-sandwich structure composites with honeycomb core, and sheet-to-sheet cementing structure composites were evaluated. The THz frequency-domain amplitude and time-domain amplitude imaging algorithms were used to visualize the defects simulated by ultrathin double-layer Teflon inserts. And the suitable image processing methods which include wavelet-based fusion and multiscale edge representation were employed. With a combination of high-resolution THz C-scan and B-scan imaging, both the horizontal size and location, and the vertical depth and thickness of the defects were obtained in three dimensions. This study experimentally demonstrated that THz imaging technology can clearly detect various hidden defects in aircraft GF sandwich composites through reflection or transmission imaging mode.

Abstract-Reliable Origin Identification of Scutellaria Baicalensis Based on Terahertz Time-Domain Spectroscopy and Pattern Recognition

Jie Liang, Qijia Guo, Tianying Chang, Ke Li, Hong-Liang Cui



https://www.sciencedirect.com/science/article/pii/S0030402618311574

An effective approach for identification of the origin of Scutellaria baicalensis, an essential member of the family of Chinese herbal medicine and known to be an effective anti-inflammatory, is proposed based on terahertz time-domain spectroscopy (THz-TDS) and pattern recognition. Terahertz absorption spectra of Scutellaria baicalensis collected from its main growth areas in China, including Inner Mongolia, Shanxi and Shaanxi are investigated using the proposed method, in the frequency range from 0.2 to 1.7 THz. To reduce the dimensionality of the original spectral data and extract useful features of the data, principal component analysis is employed. The matrix of the selected principal component scores is fed into a classification model established by support vector machines. We use the particle swarm optimization to optimize the parameters of the classification model to achieve an identification rate of 95.56% for the samples, demonstrating that terahertz time-domain spectroscopy combined with particle swarm-support vector machines approach can be efficiently utilized for automatic identification of the origin of Scutellaria baicalensis.

Abstract-Terahertz spectroscopic study of aeronautical composite matrix resins with different dielectric properties

Qiang Wang, Xinyi Li,  Tianying Chang,, Qiuping Hu, Xiaohui Yang,

https://www.sciencedirect.com/science/article/pii/S0030402618305047

This study was to investigate the spectral characteristics of aeronautical composite matrix resins (ACMRs) that differ in dielectric properties in terahertz frequency range. Terahertz time-domain spectroscopy was employed to examine the spectral absorption and dielectric dispersion characteristics of five ACMRs of three major types, namely, epoxy resins 3238A and BA9916, cyanate ester (CE) resin 9915, and bismaleimide (BMI) resins QY8911 and QY9611, between 0.2 THz to 1.6 THz. The refractive index n$$, the absorption coefficient α$$ and the real part ε^′${}^{}$ and imaginary part ε^″${}^{}$ of the dielectric constant of each resin were calculated. On this basis, the relaxation process of dipoles was theoretically analyzed using the Debye model. The results showed that, ε^″${}^{}$ of each epoxy resin was higher than that of each BMI resin, while ε^″${}^{}$ of the CE resin was the lowest. The CE resins had the lowest ε^′${}^{}$ and exhibited the highest and most stable dielectric performance, followed by the BMI resins, whereas the epoxy resins exhibited fluctuating and slightly less stable dielectric performance. This study gives, for the first time, the basic parameters of five ACMRs (epoxy, CE and BMI resins) in terahertz frequency range, and provides an important reference for THz nondestructive testing of aeronautical composites.

Abstract-Hilbert-Transform-Based Accurate Determination of Ultrashort-Time Delays in Terahertz Time-Domain Spectroscopy

Tianying Chang ;  Qijia Guo ;  Lingyu Liu ;  Hong-Liang Cui

http://ieeexplore.ieee.org/document/7951024/

Accurate determination of time delays is crucial for distinguishing a sample's different interfaces in terahertz time-domain spectroscopy (THz-TDS), especially for ultrathin samples or interfacial gaps. In this paper, Hilbert transform (HT) is invoked, along with posttransform signal spectral estimation of several varieties, including the conventional, that based on multiple-signal classification spectrum estimation (HT-MUSIC), and that based on autoregressive spectrum estimation using Yule–Walker law (HT-AR), to determine the sample's different interfaces. The results are compared with the traditional method of obtaining time delays in THz-TDS, along with analysis of the resolution and accuracy, as well as advantages and deficiencies of the various approaches. It is demonstrated that HT is an effective method for determining different interfaces of an ultrathin sample by simulation and experiments. The simulation results show that MUSIC-HT has the best resolution, at about 0.35 ps and AR-HT has the best accuracy, whose error is less than 0.075 ps. Experimental results for ultrathin air gaps and polymer films confirmed that MUSIC-HT can distinguish time delays as short as 0.44 ps from the THz time-domain spectra.

Abstract-Dielectric properties of coal in the terahertz frequency region of 100–500 GHz

• Wei Fan^a, 
• Tianying Chang^a, b, 
• Lingyu Liu^b, 
• Hong-Liang Cui^a

• ^a College of Instrumentation and Electrical Engineering, Jilin University, Changchun, Jilin 130061, China
• ^b Institute of Automation, Shandong Academy of Sciences, Jinan, Shandong 250103, China
• 
  

http://www.sciencedirect.com/science/article/pii/S0016236116309942

The dielectric properties of anthracite and bituminous coals were investigated in the terahertz (THz) frequency region from 100 GHz to 500 GHz. We developed two types of THz material measurement systems that can be operated in this frequency region, including one based on a vector network analyzer (VNA) with frequency extension modules, and another based on a THz time-domain spectroscopy (TDS) system. Employing the free-space configuration, we obtained the variation of dielectric property of coals in the frequency region of 100–500 GHz. By comparing the VNA and TDS systems, we evaluated the continuity and consistency of the two systems and verified the dielectric property measurement results of coal in the entire frequency region. We first outlined the fundamental theory of dielectric property with both methods, followed by estimating the measurement error in consideration with the system stability, the time span of the time-domain gating, and measurement uncertainty. We evaluated the dielectric property of coal samples through the measured results of the VNA and TDS systems. Comparison is also made of the variation of measured dielectric property with the lower THz frequency region (W-band) in our previous work. The results show that different coal type exhibited different variation with increasing frequency in the THz band considered.

Abstract-A High Precision Terahertz Wave Image Reconstruction Algorithm

Qijia Guo 1, Tianying Chang 1,2,*, Guoshuai Geng 1, Chengyan Jia 1 and Hong-Liang Cui 1

1

School of Instrumentation Science and Electrical Engineering, Jilin University, Changchun 130012, China

2

Institute of Automation, Shandong Academy of Sciences, Jinan 250014, China

*

Correspondence: Tel.: +86-186-1251-9976

Academic Editors: Vincenzo Spagnolo and Dragan Indjin

http://www.mdpi.com/1424-8220/16/7/1139/htm

With the development of terahertz (THz) technology, the applications of this spectrum have become increasingly wide-ranging, in areas such as non-destructive testing, security applications and medical scanning, in which one of the most important methods is imaging. Unlike remote sensing applications, THz imaging features sources of array elements that are almost always supposed to be spherical wave radiators, including single antennae. As such, well-developed methodologies such as Range-Doppler Algorithm (RDA) are not directly applicable in such near-range situations. The Back Projection Algorithm (BPA) can provide products of high precision at the the cost of a high computational burden, while the Range Migration Algorithm (RMA) sacrifices the quality of images for efficiency. The Phase-shift Migration Algorithm (PMA) is a good alternative, the features of which combine both of the classical algorithms mentioned above. In this research, it is used for mechanical scanning, and is extended to array imaging for the first time. In addition, the performances of PMA are studied in detail in contrast to BPA and RMA. It is demonstrated in our simulations and experiments described herein that the algorithm can reconstruct images with high precision.