Terahertz Applications – Paper Humidity and Thickness Measurements
- Terahertz radiation can determine paper humidity and thickness
- Extreme sensitivity with phase-sensitive measurements
- Effects of a single sheet can be quantified
- Recommended system: TOPTICA’s cw THz packages
An emerging application of terahertz radiation is industrial process control. In paper production lines, for instance, a variety of parameters has to be monitored, including temperature, humidity and paper area density.
Terahertz-based measurements might provide the long sought-after alternative to commonly used radioactive emitters: Terahertz radiation is hardly attenuated by paper, but strongly absorbed by water . More precisely, both the transmittance and the terahertz phase vary with the humidity level of the sample. The figure shows the drying process of a sheet of tissue paper, monitored at a frequency of 200 GHz. After approx. 20 minutes, the sample was sprayed with a detergent solution, which is seen as a sudden step in both transmission and phase data. During the following hour, the two parameters converge towards their initial values as the sample dries .
In another study  we were able to show that terahertz radiation could be used to determine the thickness of paper samples. Employing a coherent detection technique, we demonstrated that the terahertz phase varied linearly with the total thickness of the paper samples. Different area / mass densities (70 g/m² and 80 g/m²) resulted in significantly different slopes. Even the effect of a single sheet of paper could be accurately quantified.
 D. Banerjee et al., Opt. Express 16:12, 9060 (2008).
 T. Göbel et al., Techn. Messen 75:12, 623 (2008).
 A. Deninger et al., Proc. TeraTech, Osaka (2009).
- Standard Package: DFB lasers for cw terahertz generation
- High Power Extension for best terahertz SNR
- Phase Modulation with twin fiber-stretcher
- cw Terahertz Spectroscopy Extension: Leading-edge photomixer technology
- Ultrafast fiber lasers for time-domain terahertz applications
- TeraFlash: Time-domain spectroscopy platform