Terahertz Camera for Remote Detection of Material Defects and Biological and Chemical Substances
- Technical University of Denmark (DNSC Space and Photonics departments)
- SynView GmbH
- European Commission, Seventh Framework Programme (7th)
Imaging at terahertz frequencies has received much attention and has progressed rapidly in the last years due to large-scale funding programs, mainly in US and recently EU, focusing on terrestrial applications such as security enforcement. THz imaging systems offer unique properties of being able to identify objects through a range of materials, excluding metal, and have the ability to identify biological and chemical substances. For terrestrial security applications, threat objects can be viewed through clothing, cardboard and other obscuring materials. Other areas of THz imaging have addressed the food sector, communications and avionics. In the later case THz imaging at lower frequencies down to mm-wave frequency range can be used to provide visibility through fog and poor weather conditions during landing approach. A very thorough recent article on THz imaging has been recently presented by the consortium.
For stand-off measurements it is required to achieve a sufficient sensitivity of the imaging system. Therefore, the possible solutions are either active photonic systems or active and passive electronic systems. It has been demonstrated by numerous groups that a high spatial resolution is achievable at relatively low frequencies of e.g. 94 GHz and 200 GHz, which is beneficial in terms of atmospheric conditions, system complexity, and availability. Key issue is the provision of high signal power at the operating frequencies to overcome the inevitable path loss in stand-off measurements. Electronic systems currently outperform photonic systems by at least two orders of magnitude in output power, which converts to an improvement of 20 dB in sensitivity, irrespective the receiver performance.
However these electronic imaging systems are deprived of biological and chemical substance detection capabilities, which are available in photonic CW and pulsed systems. This is owing to the possibility of wideband signal generation either with pulses or via tuning. Photonic systems with wide tuning range are still limited in power, in spite of the developments in the area of LiNbO3emitters. Therefore, currently the most powerful systems are based on pulsed techniques.