Terahertz thickness determination with interferometric vibration correction for industrial applications

Abstract

In many industrial fields, like automotive and painting industry, the thickness of thin layers is a crucial parameter for quality control. Hence, the demand for thickness measurement techniques continuously grows. In particular, non-destructive and contact-free terahertz techniques access a wide range of thickness determination applications. However, terahertz time-domain spectroscopy based systems perform the measurement in a sampling manner, requiring fixed distances between measurement head and sample. In harsh industrial environments vibrations of sample and measurement head distort the time-base and decrease measurement accuracy. We present an interferometer-based vibration correction for terahertz time-domain measurements, able to reduce thickness distortion by one order of magnitude for vibrations with frequencies up to 100 Hz and amplitudes up to 100 µm. We further verify the experimental results by numerical calculations and find very good agreement.

Industrial Applications of Terahertz Sensing: State of Play

“…On the one hand, a high signal-to-noise ratio and a large THz bandwidth are required in order to resolve layers of a few micrometer thicknesses at high measurement speeds [78,79]. On the other hand, the system must be cost-effective and robust [80].…”

Terahertz Quality Inspection for Automotive and Aviation Industries

“…The usability of robotic systems to bring the sensor to the device under test has been proven in the past [3,50]. The application of robots and the rough environment lead to the need of insensitivity to vibrations [51,52]. They can lead to severe disturbance of the acquired terahertz signal-especially when applying the most common TDS principle.…”

Frequency Selective Optoelectronic Downconversion of a Terahertz Pulse Using ErAs:In(Al)GaAs Photoconductors

“…P OTONIC terahertz pulsed systems, also known as terahertz time-domain spectrometers (TDS), have become a powerful tool for non-destructive testing. Their wide bandwidth coverage and real-time capabilities have made them valuable tools for thickness determination of opticallyopaque thin layers [1], gas spectroscopy and sensing [2], and quality control of pharmaceuticals [3].…”