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The use of a Terahertz laser in a noninvasive body scanner system to detect weapons, both metallic and chemical is proposed.  Such lasers produce light that is in the range of .3 to 4 Terahertz, and wavelengths ranging from 75mm to 1000mm and would generate power levels well below 1 Watt.

The Terahertz region of light reflects from metals and heavy organic molecules, some of which are used in explosives, but Terahertz light is highly absorbed by water and certain other explosives containing polar molecules.  It is also transparent to clothing, paper, and most packaging materials, including plastic and cardboard [1], [2].

The Terahertz laser produces a beam that is non ionizing and has power energy levels that are orders of magnitude lower than x-rays therefore, making the Terahertz laser safe for exposure to human tissues.

There are several THz sources, but the quantum cascaded laser is the proposed laser of choice in the weapon scanning gun due to its small size, low power requirements, and tunability.   Quantum cascaded lasers are similar in size to semiconductor diode lasers, which can be much smaller than a quarter.  The quantum cascaded lasers function off of voltages below 20 Volts, which makes them ideal for use in a handheld portable system [1]. The tunability of the quantum cascaded laser makes the use of multiple lasers to perform sensing at multiple points in a system possible. Furthermore, these lasers have a high signal to noise ration up to the order of 100,000. [2]

The proposed scanning system would consist of a rectangular array of quantum cascaded lasers arranged in the scanner gun.  Each laser would be tuned to a different frequency.  The scanner gun would be directed at the materials/person being checked for weaponry.  The reflected beam intensity would be dependent on the substance that the incident beam encountered.

The reflected beam would be detected using heterodyne detection.  Multiple detectors would be arranged in such a way as to collect the different beam frequencies reflected and mix them with local oscillators.   The signals detected by the photodetectors would be processed to determine if there are any suspect substances in the materials or on the person being tested.

Reference

Linden, Kurt J,and William R. Neal, “Terahertz Laser Based Standoff Imaging System,” IEEE Computer Society, Proceedings for the 34th Applied Imagery Recognition Workshop,April 2005, Washington DC.

Mukherjee, Ramit, and Krishneendu Chakraborty, AnikBatabyal, Jayanta Das, and Subhan Kr. Bhagat,  “Concealed Weapon Detection Using Terahertz Technology, Kalpana Chawla Space Technology Cell  Indian Institute of Technology, Kharagpur, Dec. 2014.