High-resolution FMCW millimeter-wave and terahertz thickness measurements

We have adapted the FMCW radar technique to perform high-resolution thickness measurements within the millimeter-wave and terahertz frequency domain. High signal modulation bandwidths of several 10 GHz conform to millimeter resolution limits as well as micrometer accuracies. However, for our target application - the thickness measurement of single- and multi-layer plastics such as tube walls - the adapted approach for FMCW radar distance measurements is insufficient. Thick layers restrict the penetration depth of high frequency signals. Therefore, operation frequencies in the millimeter-wave or lower terahertz regime are required, which provide reduced modulation bandwidths and hence limit the resolution in the order of approximately one to several millimeters. Simultaneously, fine layers have to be separated. In this contribution, we present a correlation approach to overcome the Rayleigh resolution limit including first promising results for single and multi-layer structures.