Influences of real-world conditions on terahertz stand-off detection: simulation and experiment
Terahertz spectroscopy has become a well-established tool under laboratory conditions but terahertz stand-off detection in the real world is hampered by a number of influences related to the environment and the sample properties such as air humidity, clothes, surface roughness and angle of detection. We have performed experiments and simulations in order to probe the limits of terahertz stand-off detection under real-world conditions.
In a THz reflection measurement, dielectrics such as clothes as well as the surface properties of a sample typically show up as broadband structures caused by absorption, interference or scattering. In order to recover the chemical signature of potential hazards, we study the nature of these effects and derive characteristics that help us to distinguish between the signatures and the fakes.
Water vapor absorption affects the THz spectra in a different way. Its absorption lines are very strong, however, since the properties of the atmosphere are known to the operator of the THz setup, these effects can be arithmetically removed unless the signal falls below the signal-to-noise ratio. We have checked various models well-known in meteorology covering the infrared and the microwave region of the electromagnetic spectrum
Different path lengths and humidities are measured and simulated to check the feasibility limits of stand-off detection. The following figure shows simulated transmission spectra with varied path length (1m, 2m, 5m, 10m) for a constant humidity (30%), temperature (20°C) and pressure (1013 hPa).
For the variation of the humidity (1%, 30%, 60%, 90%) the path length (1m), the temperature (24°C) and pressure (1013 hPa) were kept constant.
The figure shows a typical transmission spectra measured for a THz path length of 1 m at 28.8 % humidity. This proves that the simulation agrees with the experiment very well.
terahertz (THz), measurement, simulation, transmission spectrum, water vapour, absorption lines, stand-off detection, security application
 "Terahertz time-domain spectroscopy of atmosphere with different humidity", T. Yuan, H. Liu, J. Xu, F. Al-Douseri, Y. Hu, and X.-C. Zhang, SPIE, Orlando, FL, USA, Vol. 5070, No.1, pp. 28-37 (2003)
 "Terahertz time-domain spectroscopy of water vapor", M. van Exter, Ch. Fattinger, and D. Grischkowsky, Opt. Lett., Vol. 14, No.20, pp. 1128-1130 (1989)
 "Simulation and Experiment of Terahertz Stand-Off Detection", S. Wohnsiedler, M. Theuer, M. Herrmann, S. Islam, J. Jonuscheit, R. Beigang, and F. Hase, Proc. SPIE, Vol. 7215, 72150H (2009)