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This paper reviews recent progress of active millimeter wave (mmW) imaging techniques for personnel security screening. With the ability to penetrate clothing and the promised millimeter scale spatial resolution, mmW imaging has been widely pursued for personnel surveillance without the usual concern for radiation safety. A brief development history of mmW imaging is introduced. An overview of imaging research and developmental achievements, along with antenna considerations based on quasi-optical geometry, phased reflector array, monostatic array, and multistatic array, is provided. Design considerations related to practical applications, including system calibrations, walk-through system (WTS), portable system, and compressive sensing (CS), are also discussed. INDEX TERMSMillimeter wave imaging, personnel security screening, review.

The ability to break Kirchhoff’s law is of fundamental importance in thermal radiation. Various nonreciprocal emitters have been proposed to break the balance between absorption and emission. However, the thicknesses of the nonreciprocal materials are usually larger than 1/10 times of the wavelength. Besides, the previous proposed nonreciprocal emitters are complex, thus they can hardly be fabricated in experiment to verify the Kirchhoff’s law for nonreciprocal materials. In this paper, we investigate the nonreciprocal thermal radiation of the magnetic Weyl semimetal (MWSM) film atop of the metal substrate. It is found that the strong nonreciprocal radiation at the wavelength of 9.15 µm can be achieved when the thickness of the MWSM film is 100 nm. The enhanced nonreciprocity is attributed to the Fabry-Perot resonances. The results indicate that the MWSM film is the promising candidate to engineer the ultrathin and simple nonreciprocal thermal emitters. What is perhaps most intriguing here is that the proposed structure can be more easily fabricated in experiment to verify the Kirchhoff’s law for nonreciprocal materials.

In recent years, our research group has been devoting substantial efforts to the research and development of active all-solid-state electronic terahertz (THz) continuous wave imaging systems for nondestructive testing, which is currently benefitting from the increasing amount of transmitting power, high performance/cost ratio and adaptability to engineering. In this paper, an in-house developed broadband linear frequency modulated continuous wave (LFMCW) three-dimensional (3D) THz imaging system is described, and two sets of experimental platforms are set up to assist the planning and completion of the research, including a narrow-band LFMCW 3D imaging radar and a wide-band stepped-frequency modulated continuous wave 3D imaging radar. For 3D imaging systems, to cope with demanding scenarios and to achieve excellent imaging performance, various reconstruction algorithms are explored. The first is a spectral refinement and correction approach based on fast Fourier transform and modern spectral estimation for accurate thickness measurement. The second is the synthetic aperture radar imaging algorithm for surface detection or internal detection of objects with lower refractive index. The third is a 3D reconstruction algorithm based on half space Green's function and the exploding source model for the interior detection of materials with higher refractive index. The fourth is the frequency interference algorithm combining phase unwrapping to measure uneven and nonplanar surfaces. Exploiting these systems, along with the associated experimental platforms and reconstruction algorithms, we successfully implemented non-destructive testing for objects with various defects and of different materials, such as polymer boards with voids, and foam with inclusions.INDEX TERMS Continuous wave imaging sensors, nondestructive testing, three-dimensional reconstruction, all-solid-state electronics.

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To date, the existing terahertz super-resolution reconstruction methods based on deep-learning networks have achieved noteworthy success. However, the terahertz image degradation process needs to fully consider the blur and noise of the high-frequency part of the image during the network training process, and cannot be replaced simply by interpolation, which has high complexity. The terahertz degradation model is systematically investigated, and effectively solves the above problems by introducing the remaining channel mechanism into the deep-learning network. On the one hand, an image degradation model suitable for the terahertz imaging process is adopted for the images in the training dataset, which improves the accuracy of network training. On the other hand, the residual channel attention mechanism is introduced to realize the adaptive adjustment of the dependence between network channels, which results in the network being more focused on the restoration of high-frequency information, thereby supporting the extraction of high-frequency edge details in the image. In addition, experimental results demonstrate that this method successfully improves the peak signal-to-noise ratios, and offers clearer edge details and a better overall reconstruction effect. We believe that this work may provide a new possibility to improve the resolution of terahertz images.

We demonstrate a three-component fiber optic seismometer for seismic observation, with no electronics in the sensor probe, but relying exclusively on optical fiber in the form of an unbalanced Michelson interferometer, with state-of-the-art performance characteristics. The fiber optic seismometer system is installed in the seismic observation cave at the Earthquake Administration Bureau of Jilin Province, in Changchun, Jilin, China, and has been operating without interruption for more than a year. Two minor earthquakes of different magnitudes recorded by the three-component fiber optic seismometer are analyzed and discussed in detail from the perspectives of time domain and frequency domain signal quality and fidelity. In addition, the recording results of the fiber optic seismometer are compared with those of the traditional electrical-mechanical seismometer co-located at the observation site. Actual observational results of the seismic events show that the three-component fiber optic seismometer can record the seismic waves clearly, with comparable performance as the conventional seismometer. INDEX TERMS Optical fiber applications, optical interferometry, seismic measurements, vibration measurement. YUE YANG received the B.S. degree from the College of Instrumentation and Electrical Engineering, Jilin University, in 2017, where she is currently pursuing the Ph.D. degree. Her research interests are fiber optical communications and sensing.