# Mail Inspection Based on Terahertz Time-Domain Spectroscopy
## 2 System Design
### Closed-Loop Dry Air Purging  
One of the main reasons why stand-off detection of explosives over several tens of meters is still a challenge, is the signal absorption due to water vapor in the environmental air [26]. Within a relatively tiny mail inspection system, the overall beam-path length is much lower and hence the influence of water vapor is less. Still, a significant drop of the dynamic range at the main water absorption lines can be observed [27]. Figure 7 shows a simulation of the effect of water vapor on the dynamic range for an 80 cm long beam path in 40% relative humidity (rh) of the environment (black curves). The complete spectral detection window is strongly rugged by the sharp absorption lines--at some single frequencies for e.g., around 1.4 THz or 1.86 THz the dynamic range of the terahertz signal gets completely lost. Due to the fact that, in general, solid-state materials as drugs and explosives have relatively broad absorption bands compared to the narrow water lines (see Figure 3), this is not critical for the function of the system itself.  
Figure 4: Mail inspection system based on terahertz TDS [25]. An automated XY-scanner accepts the envelopes to be inspected.  
However, the shorter the beam path in humid air, the lower the influence of water absorption to the detected spectra and the higher the overall system performance. Hence, nearly the complete terahertz beam path of 80 cm--except the inner part of the sandwich structure where the sample is moved within the terahertz focus (about 8 cm)--is covered with a transparent housing. This is sketched in Figure 8. The housing is made out of poly-carbonate since its water storage capability is lower than polymethyl methacrylate (PMMA). The terahertz windows to the humid region (sample area) are realized by two thin polytetrafluoroethylene (PTFE) foils, each. Although the housings of the upper beam path and the lower beam path are more or less hermetically sealed, a slight leakage can be detected. Hence, a closed loop setup is realized by the serial arrangement of the beam path's housings, an air filter filled with zeolite and a small air circulation pump.  
Figure 5: Schematic of the side swap mechanism. **(a,b)** Top view and perspective view of the beam path configuration reaching the left side of the sample, respectively. **(c,d)** Top view and perspective view of the beam path configuration reaching the right side of the sample, respectively.  
Figure 6: Side swap mechanism. Two 45\({}^{\circ}\) plane mirrors (forming a prism-like reflector) are located on a servo-driven linear stage to enable the scanning of a DIN C4 envelope with a reduced system footprint. The switching of the terahertz beam path to achieve the two possible measurement locations (**a** and **b**, respectively) is indicated.  
Since the covered volume is kept as small as possible, the pump system is able to reduce the relative humidity in the closed loop setup within approx. 10 min from ambient condition to below 3% rh. This procedure is only needed when powering up the system after a longer shut down. To ensure dry and stable measurement conditions, the pump is permanently running in a low power mode even when the humidity level inside the system is lower than 3% rh. Therefore, only 10% of the terahertz beam path is left within humid air. Figure 7 shows the difference between purged (red curves) and not purged (black curves) conditions to the dynamic range of the system. As a result, the influence of the water absorption is much lower which enhances the quality of the detected spectra and the overall system performance, too.