Yun-Da Li

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.

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 systems imaging capability. Compared with THz focal plane images, the image quality of the reconstructed images are improved a lot. The results show that the systems 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.

Experimental research on terahertz Gabor inline digital holography of concealed objects

Terahertz (THz) radiation has the characteristics of penetrating nonmetallic and nonpolar materials that are opaque to visible light, which makes THz digital holography have an application potential of imaging concealed objects with certain barriers. A CO(2) pumped continuous THz Gabor inline digital holographic imaging system was utilized to conduct experimental researches on imaging concealed objects. Paper, Teflon, a plastic express envelope, and silicon wafers were used as barriers. High-quality reconstructed images were obtained. Compared with the reconstruction results without any barriers, the results verify the feasibility of THz Gabor inline digital holography in imaging concealed objects.

Resolution measurement of a 2.52 THz continuous-wave terahertz scanning imaging system

The resolution of a terahertz scanning imaging system based on a CO2 pumped continuous-wave terahertz laser at 2.52 THz was measured. A self-made resolution test chart was employed and scanned. The imaging results show that the systems resolution is 0.6 mm. The experiment results are also compared with the results of another point-by-point scanning imaging system. The study can make a contribute to the development and application of THz imaging technique.

Autofocusing research on terahertz off-axis digital holography

Terahertz (THz) digital holography is a combination of THz technology and digital holography which has drawn wide attention. Exact measurements of real recording distances in terahertz (THz) digital holography are important. Autofocusing experimental researches have been conducted based on the holograms in THz off-axis digital holography. Five criterion functions are utilized in this paper. Autofocusing performance of these criterion functions has been analyzed and compared by changing the numbers of the superposition frames of holograms. Experiment results show that five criterion functions all have preferable autofocusing performance with the 5-frame averaged holograms; standard deviation–based correlation criterion function has the worst autofocusing performance.

2D reconstruction of terahertz Gabor inline digital holography

Terahertz imaging can make up the defect of imaging opaque samples in visible light domain. Digital holography is a new technology for extracting full information of the original object. In the paper, the improved angular spectrum (AS) algorithm is coulping the original AS algorithm with direct current (DC) suppression method, apodization and piecewise-nonlinear transformation. The reconstruction characteristics of the algorithm have been studied by numerical analysis and experimental researches. The experimental results validate the application value of the algorithms in improving 2D reconstructed image quality in terahertz Gabor inline digital holography.

Calculation and analysis of Hu moment of Continuous-Wave Terahertz reflecting image

Terahertz (THz) radiation can penetrate nonmetallic materials and nonpolar materials, and has low photon energy, therefore THz imaging has drawn more attention. The moment function of an image has wide applications on pattern recognition and target classification. The paper has applied image segmentation to Continuous-Wave THz point-to-point scanning reflecting image depending on Otsus method. The Hu moments of the original image and the segmentation image were calculated, and the mean value and relative error were also obtained. It can provide technical support for THz image pattern recognition by use of moment invariants.