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Spatial resolution and spectral contrast are two major bottlenecks for non-destructive testing of complex samples with current imaging technologies. We use a three-dimensional terahertz (THz) imaging system to obtain the internal structure of the sample, and exploit the wavelet transform algorithm to improve the spatial resolution and the spectral contrast. With this method, the longitudinal resolution of terahertz imaging system can be improved to the wavelength comparable thickness, while the x-y plane resolution can be as high as 0.2 mm0.2 mm, which benefits from the point-to-point scanning on the x-y plane. In this three-dimensional terahertz imaging system, the Syn View Head 300 with light source/detector frequency of 0.3 THz is used for two-dimensional scanning (x-y direction) of the sample, and the linear frequency modulated continuous wave technique is used to obtain the reflected terahertz light intensity at different depths (z axis) of the sample. When the sample is thin, the upper and lower interface reflection peaks are difficult to distinguish due to broad peak width of the THz source. To solve this problem efficiently, continuous wavelet transform (CWT) is used. In recent years, CWT is applied widely because of its particular mathematical properties in the feature signal recognition. Since the Gaus2 wavelet basis is better to highlight the peak signal, we choose it for CWT. After CWT, one scale of the wavelet coefficients is chosen for three-dimensional data reconstruction, for which the widths of the reflection peaks are narrower and the noise signals are weaker. That means if we reconstruct the three-dimensional wavelet coefficient data on the chosen scale, the three-dimensional image of the tested sample will be enhanced. In order to demonstrate that, the three-dimensional images reconstructed by wavelet coefficients are compared with those by original data. The tested sample has holes inside with different depths. Based on the original three-dimensional THz image, it is hard to locate the top of 4 mm deep hole (1 mm deep photosensitive material plate), while the top of the inner 4 mm deep holes (the bottom of the 1 mm deep photosensitive material plate) can be distinctly located and the noises are greatly reduced based on the three-dimensional images reconstructed by wavelet coefficients. With this method, the longitudinal resolution of terahertz detection systems can be improved to 1 mm that is comparable to the wavelength, which demonstrates advantages of this method.
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Keywords:
- terahertz /
- non-destructive testing /
- wavelet transform
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[17] Mallat S, Hwang W L 1992 IEEE Trans. Inform. Theory 38 617
[18] Yin X X, Ng B W H, Ferguson B, Abbott D 2009 Digit. Signal Process. 19 750
[19] Weg C A, Spiegela W V, Hennebergerb R, Zimmermannb R, Roskos H G 2009 Proceedings of Terahertz Technology and Applications II San Jose, September 15-19, 2008 p72150F-1
[20] Anastasi1 R F, Madaras E I 2006 Proceedings of Nondestructive Evaluation and Health Monitoring of Aerospace Materials, Composites, and Civil Infrastructure IV San Diego, March 6, 2005 p356
[21] Hong J C, Kim Y Y, Lee H C, Lee Y W 2002 Int. J. Solids. Struct. 39 1803
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[1] Deng Y Q, Xing Q R, Lang L Y, Chai L, Wang Q Y, Zhang Z G 2005 Acta Phys. Sin. 54 5224 (in Chinese) [邓玉强, 邢岐荣, 郎利影, 柴路, 王清月, 张志刚 2005 54 5224]
[2] Yang Z G, Liu J S, Wang K J 2013 Journal of OptoelectronicsLaser 24 1158 (in Chinese) [杨振刚, 刘劲松, 王可嘉 2013 光电子激光 24 1158]
[3] Zhou S F, Reekie L, Chan H P, Luk K M, Chow Y T 2013 Opt. Lett. 38 260
[4] Sanchez A R, Karpowicz N, Xu J Z, Zhang X C 2006 Proceedings of the 4th International Workshop on Ultrasonic and Advanced Methods for Nondestructive Testing and Material Characterization Dartmouth, June 19, 2006 p67
[5] Stoik C D, Bohn M J, Blackshire J L 2008 Opt. Express 16 17039
[6] Dong J L, Kim B, Locque A, Keon P M, Declercq N, Citrin D S 2015 Compos. Part B 79 667
[7] Wietzke S, Jrdens C, Krumbholz N, Baudrit B, Bastian M, Koch M 2007 J. Eur. Opt. Soc. -Rapid 2 07013
[8] Jrdens C, Scheller M, Wietzke S, Romeike D, Jansen C, Zentgraf T, Wiesauer K, Reisecker V, Koch M 2010 Compos. Sci. Technol. 70 472
[9] Ren J J, Li L J, Zhang D D, Qiao X L, Lu Q Y, Cao G H 2016 Appl. Opt. 55 7024
[10] Yasui T, Yasuda T, Sawanaka K, Araki T 2005 Appl. Opt. 44 6849
[11] Sanchez A R, Heshmat B, Aghasi A, Naqvi S, Zhang M J, Romberg J, Raskar R 2016 Nat. Commun. 7 12665
[12] Yan G F, Markov A, Chinifooroshan Y, Tripathi S M, Bock W J, Skorobogatiy M 2013 Opt. Lett. 38 2200
[13] Cheng B B, Li H P, An J F , Jiang K, Deng X J, Zhang J 2015 Journal of Terahertz Science and Electronic Information Technology 13 843 (in Chinese) [成彬彬, 李慧萍, 安健飞, 江舸, 邓贤进, 张健 2015 太赫兹科学与电子信息学报 13 843]
[14] Di Z G, Yao J Q, Jia C R, Bing P B, Yang P F, Xu X Y 2011 Laser Infrared 41 1163 (in Chinese) [邸志刚, 姚建铨, 贾春荣, 邴丕彬, 杨鹏飞, 徐小燕 2011 激光与红外 41 1163]
[15] Ge X H, L M, Zhong H, Zhang C L 2010 J. Infrared Milli. Wave. 29 15 (in Chinese) [葛新浩, 吕默, 钟华, 张存林 2010 红外与毫米波学报 29 15]
[16] Chen P F, Tian D, Qiao S J, Yang G 2014 Spectrosc. Spect. Anal. 34 1969 (in Chinese) [陈鹏飞, 田地, 乔淑君, 杨光 2014 光谱学与光谱分析 34 1969]
[17] Mallat S, Hwang W L 1992 IEEE Trans. Inform. Theory 38 617
[18] Yin X X, Ng B W H, Ferguson B, Abbott D 2009 Digit. Signal Process. 19 750
[19] Weg C A, Spiegela W V, Hennebergerb R, Zimmermannb R, Roskos H G 2009 Proceedings of Terahertz Technology and Applications II San Jose, September 15-19, 2008 p72150F-1
[20] Anastasi1 R F, Madaras E I 2006 Proceedings of Nondestructive Evaluation and Health Monitoring of Aerospace Materials, Composites, and Civil Infrastructure IV San Diego, March 6, 2005 p356
[21] Hong J C, Kim Y Y, Lee H C, Lee Y W 2002 Int. J. Solids. Struct. 39 1803
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