Introduction and comparison of millimeter wave and terahertz technology

In the current market of new human body security devices, in addition to the widespread concern of millimeter-wave human body instruments, there is also an emerging technology that has received widespread attention, namely terahertz imagers. In fact, there are coincidences between the terahertz imager and the millimeter wave imager in the band. The main method to distinguish the two methods is whether to use active imaging or passive imaging. Active imaging uses a holographic imaging system to illuminate the human body through millimeter-wave sources. Sub-wavelength imaging resolution can be achieved based on synthetic aperture technology. Passive imaging uses traditional optical focusing imaging methods, consisting of mirrors, lenses, and highly sensitive detectors. The light path enables rapid detection of the terahertz wave emitted by the human body itself.

In fact, the two definitions of millimeter wave and terahertz wave are to describe electromagnetic waves from the perspective of wavelength and frequency, respectively, and are not two completely different technologies. As shown in Fig. 1, the wavelength of the millimeter wave is between 1 mm and 10 mm, and the frequency of the terahertz wave is between 0.1 THz (100 GHz) and 10 THz, that is, the wavelength is between 0.003 mm and 3 mm (the coincidence region is the wavelength of 1 mm to 3 mm). In the electromagnetic range from 100 GHz to 300 GHz, in general, the terahertz wave has a shorter wavelength and a higher frequency than the millimeter wave. If it is the same imaging mechanism, terahertz can obtain clearer imaging. However, in the higher frequency terahertz band, due to the limitation of the cutoff frequency of the electronic device and the lack of a mature solid-state circuit transceiver module, the active imaging based on the holographic system will be more difficult to implement than the passive imaging. Active human body security devices currently on the market generally use millimeter wave bands of less than 100 GHz, while passive terahertz imaging can achieve detection of terahertz waves greater than 100 GHz with high sensitivity detectors in order to improve resolution.

In summary, due to the problem of ionizing radiation, X-ray imaging technology can be applied to some special occasions, but may not be suitable for the inspection channel. For millimeter waves and terahertz, passive imaging can provide rapid imaging. However, the image resolution is usually on the order of centimeters, so it is not suitable for fine inspection in the travel inspection channel, and the active millimeter wave products are generally used for inspection.

Future proactive terahertz technology will become a more advanced imaging method, but it is still in the laboratory stage, and there is no product based on this technology on the market. (Author: Jin Yingkang Pu)