A transfer matrix modification for accurate terahertz FMCW thickness measurements

Millimeter and terahertz waves offer novel solutions for nondestructive testing such as imaging and layer thickness measurements within dielectrics. For the thickness measurement of plastics, we use the frequency modulation continuous wave technique within fully electronic terahertz systems. The available modulation bandwidth inherently restricts the resolution to several millimeters. Our correlation approach, which compares the acquired measurement data with a signal model, overcomes this limit for predefined measurement conditions. However, to obtain high measurement accuracies - especially in the case of compact multilayer structures and dielectric coatings on conductive substrates - beam propagation aspects such as multiple reflections between the boundary surfaces of the layers have to be considered. Therefore, we adapt the Transfer Matrix method to our measurement scheme with optimizations with regard to computation complexity.