Abstract

We compare a state-of-the-art terahertz (THz) time domain spectroscopy (TDS) system and a novel optoelectronic frequency domain spectroscopy (FDS) system with respect to their performance in layer thickness measurements. We use equal sample sets, THz optics, and data evaluation methods for both spectrometers. On single-layer and multi-layer dielectric samples, we found a standard deviation of thickness measurements below 0.2 µm for TDS and below 0.5 µm for FDS. This factor of approx. two between the accuracy of both systems reproduces well for all samples. Although the TDS system achieves higher accuracy, FDS systems can be a competitive alternative for two reasons. First, the architecture of an FDS system is essentially simpler, and thus the price can be much lower compared to TDS. Second, an accuracy below 1 µm is sufficient for many real-world applications. Thus, this work may be a starting point for a comprehensive cross comparison of different terahertz systems developed for specific industrial applications.

Highlights

  • Terahertz (THz) spectroscopy is an interesting sensing technology for many applications in material and structural analysis, compound identification, and testing [1, 2]

  • We found that the standard deviation of both time domain spectroscopy (TDS) and frequency domain spectroscopy (FDS) is always lower than 2%

  • For five single layer PET and Kapton foils with thicknesses between 23 μm and 350 μm, we found standard deviations smaller than 0.4 μm with FDS and below 0.2 μm with TDS

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Summary

Introduction

Terahertz (THz) spectroscopy is an interesting sensing technology for many applications in material and structural analysis, compound identification, and testing [1, 2]. One of the key applications for THz spectroscopy is the thickness measurements of paint and coating layers. Time domain spectroscopy (TDS) is almost exclusively used for this kind of. Journal of Infrared, Millimeter, and Terahertz Waves (2021) 42:1153–1167 applications, as these systems are mature and commercially available. THz TDS offers high acquisition speed and high THz bandwidth. The latter enables thickness measurements of sub-mm dielectric layers [3,4,5] and the detection of defects in polymers, foams, and other non-conductive materials [6]

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