Fast FMCW Terahertz Imaging for In-Process Defect Detection in Press Sleeves for the Paper Industry and Image Evaluation with a Machine Learning Approach.

We present a rotational terahertz imaging system for inline nondestructive testing (NDT) of press sleeves for the paper industry during fabrication. Press sleeves often consist of polyurethane (PU) which is deposited by rotational molding on metal barrels and its outer surface mechanically processed in several milling steps afterwards. Due to a stabilizing polyester fiber mesh inlay, small defects can form on the sleeve's backside already during the initial molding, however, they cannot be visually inspected until the whole production processes is completed. We have developed a fast-scanning frequenc-modulated continuous wave (FMCW) terahertz imaging system, which can be integrated into the manufacturing process to yield high resolution images of the press sleeves and therefore can help to visualize hidden structural defects at an early stage of fabrication. This can save valuable time and resources during the production process. Our terahertz system can record images at 0.3 and 0.5 THz and we achieve data acquisition rates of at least 20 kHz, exploiting the fast rotational speed of the barrels during production to yield sub-millimeter image resolution. The potential of automated defect recognition by a simple machine learning approach for anomaly detection is also demonstrated and discussed.

Terahertz thickness determination with interferometric vibration correction for industrial applications.

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.

Interferometry-aided terahertz time-domain spectroscopy.

Terahertz time-domain spectroscopy as well as all optical pump-probe techniques with ultrashort pulses relies on the exact knowledge of an optical delay between related laser pulses. Classical realizations of the measurement principle vary the optical path length for one of the pulses by mechanical translation of optical components. Most commonly, only an indirect measurement of the translation is carried out, which introduces inaccuracies due to imprecise mechanics or harsh environment. We present a comprehensive study on the effect of delay inaccuracies on the quality of terahertz spectra acquired with time-domain spectroscopy systems and present an interferometric technique to directly acquire the optical delay simultaneously to the terahertz measurement data. This measurement principle enables high-precision terahertz spectroscopy even in harsh environment with non-systematic disruptions.

Compact fiber-coupled terahertz spectroscopy system pumped at 800 nm wavelength.

Photonic terahertz (THz) technology using femtosecond (fs) lasers has a great potential in a wide range of applications, such as non-destructive testing of objects or spectroscopic identification of chemical substances. For industrial purposes, a THz system has to be compact and easily implementable into the particular application. Therefore, fiber-coupled THz systems are the key to a widespread use of THz technology. In order to have flexible THz emitters and detectors near infrared fs light pulses have to be sent through optical fibers of considerable length. As a consequence, the fiber's dispersion has to be compensated for and nonlinear effects in the fiber have to be minimized. A fiber-based THz time-domain spectroscopy system of high stability, flexibility, and portability is presented here.

A pulsed THz imaging system with a line focus and a balanced 1-D detection scheme with two industrial CCD line-scan cameras.

We present a pulsed THz Imaging System with a line focus intended to speed up measurements. A balanced 1-D detection scheme working with two industrial line-scan cameras is used. The instrument is implemented without the need for an amplified laser system, increasing the industrial applicability. The instrumental characteristics are determined.