Offset frequency locking is widely applied in laser detection, frequency stabilization of lasers, laser accurate
measurement and laser spectroscopy. In the paper, an offset frequency locking system based on Field Programmable
Gate Array (FPGA) is designed and applied for cavity-dumped CO<sub>2</sub> waveguide laser with two channels and common
electrodes. A scheme for all-digital frequency discriminator in the system is described in the paper. The frequency
discriminating precision of single pulse is improved by designing a high speed counter which could count both the rising
edges and falling edges of signals. The multi-pulse discrimination and the high probability mean filtering algorithm are
used to further improve the discriminating precision. At the same time, the algorithm of variable step length and
segmental approximation is used to improve the speed and precision of frequency modulation. The experimental results
show that with the method of multi-pulse discrimination the beat frequency is stabilized within ±10MHz.
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.
THz transmission imaging is considered as a promising detection measure and imaging method with wide application
prospect in security inspection and counter-terrorism. And high-speed array imaging is a very important direction. In this
paper, 2.45-THz transmission imaging experiments are made by applying a 124×124 array detector. The 2.45-THz CW
CO2-pumped laser is used as illumination source and its output power is about 17mW. Some undersize articles (gasket,
screw and nut) are chosen as objects. THz imaging experiments are respectively demonstrated through 1-2 pieces of
paper and two kinds of envelopes to show the imaging effect. The original data is acquired and some image processing
methods (5-frame average, median filtering, etc) are used to improve the image effect. Finally the THz images obtained
are basically clear. The experimental results show that utilizing the setup designed for 2.45-THz imaging, the images are
fast generated and objects' contours are clear through paper and envelop.
With the deepening of research on the terahertz imaging, real-time imaging is becoming more and more important. Right after Massachusetts Institute of Technology demonstrated 2.52-THz CW imaging by use of a 160×120 element microbolometer camera with spectral coverage from 7.5-14 μm, the single frequency CW THz imaging using the focal-plane array has become a new research direction. In this paper, 124×124 array transmission imaging is
demonstrated. The 1.63-THz CW CO<sub>2</sub>-pumped laser with the output power of 45mW is used as the illumination source,
and a pyroelectric camera is adopted as the array detector with spectral response range of 1.06μm - >1mm. In the
imaging experiments, RMB watermark, gasket and key are chosen as objects and several pieces of paper are placed
before the objects to test the transmission imaging effect. The experimental results show that utilizing the experimental
devices designed in this paper the 1.63-THz images generated are basically clear and objects' contours are clear. The images of gasket and key can be generated penetrating at most 4 pieces of B5 paper in the transmission distance of 0.5m when the power is 45mW and the focal length of polyethylene lens is 120mm.