Active MMW/Terahertz Security System Based on Bessel Beams

The novel concept of the security system based on THz Bessel beams is offered. The system is based on a novel THz diffractive optics for scanning the person (without the application of THz laser) and on a sensitive scheme for the detection of the reflected and scattered THz radiation. The development of enabling technology, namely, sensitive detector arrays and Millimeter wave/THz diffractive optics, will allow building compact, easy-to-use millimeter wave/THz imaging systems without expensive cost THz laser. The scanning properties of diffractive optics for Bessel beam are investigated. 1. Introduction The novel technology of a new class of imaging systems for detecting threats, explosives, pathogens, and chemicals hidden by a person or inside an object such as letters or luggage is based on active imaging concepts implementing terahertz (THz) or/and millimeter wave (MMW) radiation. In combination with advanced sensors based on diffractive optics with Bessel beams this will lead to an increased level of security at public places, for example, airports or private places as a room. 2. The Principles of Security Systems As well known [1] there are three essential components to a laser security system: a laser, a detector, and sensing electronics. The laser is a concentrated light source that puts out a straight line “pencil beam” of light (or IR) of a single color. The detector is sensitive to light and puts out a voltage when the laser light hits it. The detector is connected to the sensing electronics. When the laser beam is interrupted and cannot reach the detector, its voltage output changes, and the electronics sense the change and put out a warning signal. Usually you want to protect the perimeter of a room, or at least the entrances. So laser security systems start with a laser pointing to a small mirror. The first mirror is angled to direct the beam to a second small mirror, and so on until the final mirror directs the beam to the detector. If the beam is interrupted anywhere between the laser and the detector, the electronics will put out the warning signal. Disadvantages of laser security systems are as follows.(1) The laser beam is visible by eye or special (e.g., night-vision) systems.(2) The laser beam is visible in smoke.(3) The work of laser security system can be destroyed by high-absorption smoke and so forth. But a THz/MMW waves are less sensitive to smoke and invisible by eyes. For example, under identical dust concentrations, the IR signal at 1.5？μm wavelength is significantly more attenuated than the THz signal (625？GHz), which shows