Multilayer Thickness Measurements below the Rayleigh Limit Using FMCW Millimeter and Terahertz Waves

Abstract

We present thickness measurements with millimeter and terahertz waves using frequency-modulated continuous-wave (FMCW) sensors. In contrast to terahertz time-domain spectroscopy (TDS), our FMCW systems provide a higher penetration depth and measurement rates of several kilohertz at frequency modulation bandwidths of up to 175 GHz. In order to resolve thicknesses below the Rayleigh resolution limit given by the modulation bandwidth, we employed a model-based signal processing technique. Within this contribution, we analyzed the influence of multiple reflections adapting a modified transfer matrix method. Based on a brute force optimization, we processed the models and compared them with the measured signal in parallel on a graphics processing unit, which allows fast calculations in less than 1 s. TDS measurements were used for the validation of our results on industrial samples. Finally, we present results obtained with reduced frequency modulation bandwidths, opening the window to future miniaturization based on monolithic microwave integrated circuit (MMIC) radar units.

Terahertz Quality Inspection for Automotive and Aviation Industries

“…In the millimeter wave and lower terahertz regime, FMCW systems provide a high dynamic range of up to 70 dB at measurement rates of several kilohertz in connection with higher signal penetration depth than pulsed terahertz systems. Recently, we demonstrated the suitability of such systems for thickness measurements and proved that layer thicknesses even below the Rayleigh resolution limit can be resolved by taking advantage of a model-based signal processing technique [23,24]. In a basic approach, modeled versions of the measurement signal are computed a priori based on estimated properties of the multilayer material system under test.…”

“…In this way, the minimum layer resolution of the measurement system can be improved significantly by proper consideration of the a priori information. Multiple reflections can be taken into account by using the transfer-matrix method for the calculations of the signal models [24].…”

“…In order to prove the feasibility of the approach, we compared the measurement results from a multilayer structure obtained with a broadband terahertz TDS system as a reference, and an FMCW terahertz system operating between 70 and 110 GHz [24]. Figure 7 shows the results of TDS and FMCW measurements along a three-layer tube wall, which consists of a dense recyclate foam structure, sandwiched between two thin layers of polyvinyl chloride (PVC).…”

Millimeterwave Radar Systems for In-Line Thickness Monitoring in Pipe Extrusion Production Lines

“…Radar sensors can be used in various fields of nondestructive testing, including e.g., the thickness estimation of multilayer plates [11] or the identification of electromagnetic material properties [12]. The basic principle is the estimation of time-of-flight differences that occur from This work is licensed under a Creative Commons Attribution 4.0 License.…”

Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4 THz bandwidth

“…Owing to the S-DSH technique, an active phase or amplitude modulation—the standard technique for coherent cw THz measurements so far—is no longer required. This technique can be regarded as an optoelectronic analog of frequency-modulated continuous-wave (FMCW) radar 27 , 28 . Our THz spectrometer achieves a bandwidth of 4 THz, a peak DR of 117 dB, and a measurement speed of 200 Hz, i.e., a performance comparable to that of state-of-the-art THz TDS systems, yet without their complexity.…”