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Investigations are ongoing to develop cheaper THz time-domain spectroscopy (THz-TDS) systems. In this context, many studies have been published in which multimode laser diodes (MLDs) are used as light sources in THz- TDS systems. There is another method for cost-reduction. Photomixing employs two single-mode LDs and does not require a pulse laser, and then realizes a reduced-cost system. A comprehensive report has been presented by examining the work done up to now. In these studies, it has been shown that the characteristics of MLDs, the effects of antenna structures on THz signal production, the difficulty of collimation of the MLD radiation beam, and the contribution of fiber-based systems are of utmost importance. It has also been determined that mode spacing has a significant role in THz radiation generation. In addition, successful SN ratio enhancement and imaging studies have shown that a more compact and cheaper THz-TDS system can be established using MLDs as a light source. Macor and RC51HF samples have been analyzed using a MLD-TDS created in the same way as in previous studies and results are given. [ABSTRACT FROM AUTHOR]

Terahertz technology is still an evolving research field that attracts scientists with very different backgrounds working on a wide range of subjects. In the past two decades, it has been demonstrated that terahertz technology can provide a non-invasive tool for measuring and monitoring the water content of leaves and plants. In this paper we intend to review the different possibilities to perform in-vivo water status measurements on plants with the help of THz and sub- THz waves. The common basis of the different methods is the strong absorption of THz and sub-THz waves by liquid water. In contrast to simpler, yet destructive, methods THz and sub-THz waves allow for the continuous monitoring of plant water status over several days on the same sample. The technologies, which we take into focus, are THz time domain spectroscopy, THz continuous wave setups, THz quasi time domain spectroscopy and sub-THz continuous wave setups. These methods differ with respect to the generation and detection schemes, the covered frequency range, the processing and evaluation of the experimental data, and the mechanical handling of the measurements. Consequently, we explain which method fits best in which situation. Finally, we discuss recent and future technological developments towards more compact and budget-priced measurement systems for use in the field. [ABSTRACT FROM AUTHOR]

In industrial paper production, online monitoring of a range of quality parameters is essential for ensuring that the performance and appearance of the final product is suitable for a given application. In this article, two optical sensing techniques are investigated for non-destructive, non-contact characterization of paper thickness, surface roughness, and production defects. The first technique is optical coherence tomography based on a mid-infrared supercontinuum laser, which can cover thicknesses from ~20–90 μm and provide information about the surface finish. Detection of subsurface voids, cuts, and oil contamination was also demonstrated. The second technique is terahertz time domain spectroscopy, which is used to measure paper thicknesses of up to 443 μm. A proof-of-concept thickness measurement in freely suspended paper was also demonstrated. These demonstrations highlight the added functionality and potential of tomographic optical sensing methods towards industrial non-contact quality monitoring. [ABSTRACT FROM AUTHOR]