Sheng-Hui Ding

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.

Real-time terahertz scanning imaging by use of a pyroelectric array camera and image denoising.

A high-resolution large-area terahertz (THz) scanning imaging system is demonstrated based on a 124×124 pyroelectric array camera and a CO(2) pumped continuous-wave THz laser. By applying a scanning mechanism to the real-time imaging setup, images of large-area targets were accomplished. Self-made resolution charts were employed to test the resolution. In order to improve the image quality, the noise in the images was studied and modeled, and then the performance of several denoising methods was compared with real-time THz original images. The experimental results show that, with the help of anisotropic diffusion, noise can be effectively suppressed, and the results are visually pleasant even when there is great attenuation. Those results greatly confirm application potentials of THz imaging using pyroelectric cameras in the field of concealed object detection.

High-resolution terahertz reflective imaging and image restoration

We present a high-resolution terahertz (THz) reflective imaging system, operating at 2.52 THz, that employs a continuous-wave THz gas laser and a pyroelectric detector. The spatial resolution was evaluated from the systems modulation transfer function and tested by scanning a series of resolution targets. To further improve the image quality, Lucy-Richardson method was adopted to restore the scanning result. With the scanning spot profile measured using knife edge method, a satisfying restoration result can be obtained. Finally, the systems performance was observed by imaging some different test objects.

Continuous-wave terahertz scanning image resolution analysis and restoration

Resolution of continuous-wave CW terahertz scanning im- age is limited by many factors among which the aperture effect of finite focus diameter is very important. We have investigated the factors that affect terahertz THz image resolution in details through theory analysis and simulation. On the other hand, in order to enhance THz image res- olution, Richardson-Lucy algorithm has been introduced as a promising approach to improve image details. By analyzing the imaging theory, it is proposed that intensity distribution function of actual THz laser focal spot can be approximatively used as point spread function PSF in the res- toration algorithm. The focal spot image could be obtained by applying the pyroelectric camera, and mean filtering result of the focal spot image is used as the PSF. Simulation and experiment show that the algorithm implemented is comparatively effective.

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.

A high pixel and fast THz imaging system

The scanning imaging system has low frame rate due to the long period of point-by-point scanning especially for the large-sized object, while the array imaging system with relatively high frame rate is employed to image for small-sized objects due to the limit of the detector size. In this paper, a novel imaging system applying the scanning imaging setup and a 124×124 array detector is constructed and the software based on Visual C++ is designed to control the scanning process, collect image data, display the imaging result and the image mosaic process, and store the complete image. Utilizing the 2.52-THz laser generated by the SIFIR-50FPL as the imaging light source, preliminary imaging experiments are done. The imaging results are basically clear and image stitching effect is comparatively satisfying. Comparing with the point-by-point scanning imaging system, the frame rate is improved. Comparing with the array imaging, the setup is able to image for large objects utilizing the scanning system.

CW THz scanning transmission imaging for concealed object detection

In the paper, the two-dimensional THz imaging methods are described. The SIFIR-50 FPL Far-Infrared Laser is used as the THz source. The output frequency is 2.5THz in the experiment, because the THz laser operates steadily at this frequency. The P4-42 detector works at room temperature and offers relatively high sensitivity. The software of THz imaging system is self-designed, and it plays a crucial role in this imaging system because it controls nearly all the operations of this system, including the two-dimensional scanning, image data collection, image data storage, image display and image processing. Utilizing this setup, THz transmission images of concealed objects are obtained. In the experiment, a bottle cap and a plastic board covered by reflective materials are chosen as the imaging objects; paper and Teflon are placed before the object to test the transmission imaging effect. The experimental results show that this imaging system can generate clear images.

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.

Study on continuous-wave THz confocal scanning image restoration

Terahertz (THz) confocal scanning imaging is a combination of confocal scanning microscope and THz imaging, which could effectively improve the imaging resolution of the imaging system. The size of the confocal pinholes has a great influence on the systems resolution. When the confocal pinholes are removed from the optical path, blurred imaging results are obtained. Lucy-Richardson method is applied to restore the blurred image and improve the image quality. The restoration result is compared with the imaging result when small pinholes were employed.

Preliminary measurement on transmission properties of SiO 2 matrix ceramic composite

CO<inf>2</inf>-pumped THz laser was used to measure a new SiO<inf>2</inf> matrix ceramic composite samples transmittance. Experimental transmission data reported here were obtained respectively at wavelengths of 118.83μm, 122.4μm, 158.51μm, 184.31μm and 214.58μm.