International Journal of Terahertz Science and Technology
  TST >> Vol.8, No.3, September 2015: PP. 113-118

A 330 GHz active terahertz imaging system for hidden objects detection

C. C. Qi *, G. S. Wu, Q. Ding, and Y. D. Zhang
China Communication Technology Co., Ltd., Baotian Road No. 1, Building 37, Chentian Industrial Zone,
Xixiang, Baoan District, Shenzhen, PR.China
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Abstract: A 330-GHz terahertz active imaging system has been designed for personal concealed objects detection. The study concerns both the optimization of a terahertz transceiver and the development of an optomechanical system. Unlike passive imaging systems, active imaging systems will emit a strong enough illumination source (generally approximating to 1 mW) to penetrate through thick clothes, and to generate a high signal-to-noise ratio (SNR) to overcome large signal clutter and speckle caused by a scene with a diversity of angles of incidence, surface roughness, and clothes layers. On the other hand, active imaging systems are insensitive to temperature change, and thus it is not easily impacted by ambient environment. Therefore the detector performance requirement is extremely lowered. In our active imaging systems, a heterodyne coherent detection technology as applied in others groups and a frequency-modulated continuous wave (FMCW) technology adopted from the radar field have been utilized to achieve a mm-scale resolution. In the emission end, the emission chain generates a fast chirp source of 20-21.25 GHz which is then doubled, amplified, 3 times passively doubled, and transmitted at 320-340 GHz by the horn antenna. The emission bandwidth of 20 GHz ensures sub-cm range resolution. In the receiving end, the reflected signal of 320-340 GHz plus a shift frequency proportional to the target range (for example 40 MHz) is mixed via a sub-harmonic mixer with a 160.48-170.48 GHz multiplier chain. In the sub-harmonic mixer end, an intermediate frequency (IF) signal of 960 MHz\40 MHz is generated and transmitted to in-phase and quadrature (I/Q) module. After frequency downconversion, a 40 MHz signal including phase and amplitude information of the detected objects is digitized. To obtain a scan area of 2.0 m (L) x 1.0 m (W) with about 7.5 mm resolution within 5 seconds, the tilt angle is designed as 2.875 degree with a rotation speed of 3768 rpm and a vibration speed of 5.6750/s. Both the rotation and tilt axes of the scanner are controlled by commercial motor drivers. About 26667 Image data are collected continuously during the scanning with the transceiver triggered by an optical encoder on the rotational axis. Using the Delaunay algorithm, a THz-image with gray or color maps is finally obtained. In short, the active terahertz imaging system can be applied in personal concealed weapon and contraband surveillance in the airport and customs.

Keywords: Terahertz imaging, Transceiver, Optomechanical scanning system, Multiplier chain.

Received: 2015-8-18

Published: 2015-9-29

Cite this article:
C. C. Qi *, G. S. Wu, Q. Ding, and Y. D. Zhang. A 330 GHz active terahertz imaging system for hidden objects detection[J]. International Journal of Terahertz Science and Technology, 2015, Vol.8, No.3: 113-118.  DOI:10.11906/TST.113-118.2015.09.11




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