Design of an Optical Terahertz Generator
N. Khan, A. Vickers, N. Abas and A.R. Kalair
Detection of an improvised explosive device (IED) is a significant challenge for security agencies. Last ditch strategy in dealing with terrorists is to maintain an upper edge over crime science in the market. Security officials use dogs, metal detectors, and scanners to sniff out chemical signatures. IED detection devices are available yet 17,000 to 20,000 civilians succumb to death every year. Detection devices and imaging scanners locate buried landmines and concealed weapons at checkpoints but are unable to recognize the suicide bombers in large crowds. Man is an intelligent machine who changes defence strategies with time depending on the feedback of sensing organs. Detection of an adaptive suicide bomber in crowded public places, or car bombs on busy roads in large cities, is still a big challenge requiring innovative research. Hazardous materials (HAZMAT) and metal detectors often fail to recognize high energy materials (HEM) based home-made explosives (HME) wrapped in plastics. Standoff lasers identify HAZMAT by decoding the vapour spectroscopic signatures at 1 m to 1 km distances but, fail to scan the concealed IEDs. The terahertz band (100 GHz to 10 THz, corresponding to the 1 mm to 30.0 mm wavelengths) falls between the high microwave (1 mm to 30 mm) and the low infrared (IR) (0.7 to 30.0 mm) bands. Terahertz photons are reflected off mirrors like light, penetrate tarps like X-rays and are detected by dipole antennas like microwaves. Terahertz waves can readout concealed IED due to their penetrating yet non-ionizing properties. Dogs and robot borne detectors recognize explosives by a physical approach to the site, but spectroscopic and imaging devices from the remote. Lasers facilitate standoff detection of landmines and X-ray scanners, image vehicles, but terahertz are used to scan drivers. Quantum cascade laser (QCL), dipole antennas, time multiplexed laser pulses, impact avalanche ionisation transit time (IMPATT) and Gunn diodes based terahertz sources and detectors come in 1 mW to 1 W ranges which require more extensive research for high power designs. Photoconductive switches have low pulse energies, vacuum electronic devices are limited to lower frequencies, and free electron lasers are too expensive. Difference frequency generation (DFG) based terahertz sources due to high conversion efficiency and repetition rate seem to be low-cost options under research worldwide. This work reviews state of the art terahertz technology and introduces the design of an innovative wavelength and time division multiplexed (WTDM) terahertz wave source.
Keywords: Laser, improvised explosive device (IED), dipole antenna, detectors, terahertz, microwaves, infrared (IR), optical, spectroscopy