Experimental Research On The Improvement Of Cw-thz 2d Imaging Quality And Speed

Terahertz (THz) radiation has the advantages of penetrating through many nonmetallic and nonpolar materials, high spectral resolving power, and low photon energy. Therefore THz technology has promising application foreground in safety inspection, food and health detection, remote sensing and environmental monitoring, especially in detection and imaging for human bodies and biological tissue. CO2-pumped continuous wave THz imaging system has great application potential for its compact fabric, high output power and low cost, comparing with pulsed THz time-domain imaging system.Imaging quality and speed are two important factors for an imaging system. Imaging quality could be judged by analyzing its resolution, detection ability for hidden object, image contrast, and signal-to-noise ratio. When the laser system and detector are fixed, imaging quality and speed could be improved by changing optical system and imaging model. It is helpful for noise suppression and visual effect improvement by adopting image processing methods after imaging experiments.In this paper, utilizing CO2-pumped continuous wave THz laser system, based on the preliminary researches on THz point-by-point scanning imaging and array imaging, original imaging systems are improved and new high-resolution setups in both transmission and reflection mode are constructed utilizing teflon lens and a series of off-axis parabolic reflectors. Self-made resolution test sample is designed based on the printed circuit board method, so that system resolution could be correctly evaluated by imaging the test sample according to Rayleigh criterion. With the help of energy distribution data of imaging spot measured by Knife Blade Method, imaging process of spot through test sample could be simulated and the resolution could be further calculated. Imaging experiments on different kinds of objects are conducted through common wrapping and covering material. By comparing with original imaging results, better imaging quality and stronger penetration power of the systems designed in this paper will be demonstrated. At the same time, original imaging control software interface is improved by modifying defects and introducing new functions.Real-time array imaging system with high resolution is constructed by use of thermal imager as array detector, and real-time imaging experiments in both transmission and reflection mode are conducted. By imaging for the resolution test sample, system resolution capability is evaluated. As noise has great impact on image quality, multi-frames of images are collected in order to suppress noises and image processing methods are employed for image enhancement. The performance of the array detector employed is also tested and the result is not satisfying. Unfortunately, at present, few of the array detectors are designed right for THz waveband. Due to array detector's wide response range, low sensitivity at THz band and high noises, imaging quality is badly influenced and limited to low levels.Point-by-point scanning imaging has better quality, but the imaging speed is limited by the size of object, response speed of detector and translational speed of scanning device. Array imaging could produce real-time images; however, the size of object is limited by the active area of detector. In order to improve the imaging speed, point scanning system and array detector are integrated to be THz array scanning system called in this paper. Point-by-point scanning is updated to region scanning and image stitching. The control software interface of this system is designed in VC++in order to set parameters, control imaging process, and realize image acquisition, stitching, display and storage. Based on array imaging setup,2D translational platform is introduced and imaging experiments on different objects through different materials are conducted utilizing this fast and high-resolution system. The characteristics of THz images with sub-images of different sizes are analyzed. And image quality is improved by multi-frame mean and edge overlap.In addition, some defects in point scanning images, such as stripes and saw-tooth, are analyzed and solutions are figured out. Different image processing methods are adopted according to different features of point-by-point scanning images and array images. Finally, imaging systems of point-by-point scanning and array scanning are compared.