Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Real-time layer thickness measurement with terahertz

17.04.2018

Terahertz is a key technology for non-destructive material testing. Major advantages are examinations without physical contact and the analysis of complex systems with a low contrast range, in which established methods like ultrasonic usually fail. The Fraunhofer Heinrich Hertz Institute HHI has successfully developed a cost-efficient and compact measurement system that conducts real-time measurements of the thickness of multi-layer systems for the first time, an essential technology e.g. for lacquering lines. The new terahertz measurement system T-Sweeper will be presented live at the Hannover Messe trade fair from April 23 to 27, 2018 (Fraunhofer booth C22, hall 2).

Terahertz radiation was already a major topic a good ten years ago, in connection with body scanners for inspecting passengers during airport security screening. Terahertz radi-ation was also to be used in measurement systems for material testing and inspection of components. However, in spite of high expectations the breakthrough for terahertz tech-nology took a long time.


The photograph shows the T-Sweeper and the fast tunable laser source Finisar® WaveSource™ as part of the setup used to demonstrate the layer thickness measurement.

© Fraunhofer HHI

Compared to classical methods for non-destructive material testing today, such as X-ray radiation and ultrasonic, terahertz was long considered simp-ly too expensive. In recent years however, great progress has been made: An increasing number of systems has been introduced with designs based on standard components, making them significantly more economical to build. These developments are already raising the interest of potential users. System costs certainly will have to be cut further and the complexity of the systems will have to be reduced before terahertz technologies are established as an instrument in industrial process monitoring.

Cost-efficient and in real-time

Current developments at the Fraunhofer HHI could mean a considerable boost for the terahertz technology. A research team led by Björn Globisch, head of the terahertz re-search group at Fraunhofer HHI, has successfully developed a measuring device that re-places the previously used femtosecond pulse laser with compact continuous-wave laser technology.

The fact that this continuous-wave measurement system is capable of eight measurements per second is key to allow real-time measurements without the use of a pulsed laser for the first time. The scientists will present this terahertz measurement sys-tem at the Hannover Messe trade fair and will give a live demonstration of non-contact layer thickness measurement for various multi-layer systems.

The principle employed by the Fraunhofer HHI to generate terahertz radiation is based on an optoelectronic process. Using a special semiconductor component, the beat of two continuous-wave lasers is converted into terahertz radiation, which corresponds exactly to the difference frequency of the two lasers.

The lack of significant success of terahertz technology in the past is largely due to the properties of the semiconductors used. These properties could initially only be achieved with materials that required illumination at a wavelength of 800 nanometers. This rather exotic wavelength means both the lasers and the optical components of the terahertz system are much too expensive and not robust enough for industrial applications.

Widespread wavelength standard

"This is why we developed a semiconductor that can be excited by laser light at a wave-length of 1.5 micrometers," explains Globisch. "This matches the wavelength standard in optical telecommunications technologies, which means there are plenty of cost-effective and high-quality optical components and lasers available." On the way to a competitively viable terahertz system for material testing, however, another obstacle had to be over-come: The pulse laser, on which all conventional real-time capable terahertz systems are based, is a decisive cost factor in existing systems. Femtosecond lasers are not only tech-nologically complicated and expensive on their own, terahertz spectrometers using pulsed lasers require additional optomechanical components that have to be precisely produced and adjusted in highly complex procedures.

One alternative is continuous-wave spectroscopy, in which continuous-wave radiation is generated instead of a terahertz pulse. Two continuous-wave laser sources are mixed and their beat signal is converted into terahertz radiation in a special semiconductor de-vice. The wavelength of the terahertz radiation generated can be easily modified by changing the relationship of the laser wavelengths to one another. Continuous-wave systems have two decisive advantages compared to pulsed terahertz systems: First, the laser sources themselves are more compact and less expensive; second, no optomechan-ical components are needed in order to operate the system.

Continuous-wave terahertz systems are already available, but they need periods ranging from several seconds to a matter of minutes to capture a measured signal. In contrast, industrial applications often operate like this: In the production, a robot arm moves over measurement points on painted/coated components and measures the coating thickness. Since the production cycle has to be maintained, there is only little time per measuring point. Until now, the measurement speed of continuous-wave terahertz systems was too slow to be viable in non-destructive testing applications.

Fraunhofer HHI has solved this problem using lasers that can be adjusted extremely fast (Finisar® WaveSource™) and by modifying electronics, data capture and algorithms to accommodate the high speeds required. This combination has increased measurement speeds by a factor of 160 compared to previous systems. This has made it possible for the first time to conduct material testing in real-time with continuous-wave terahertz sys-tems. This system will be presented at the Hannover Messe trade fair in a live demon-stration.

Application example: layer thickness measurement

Inspection of paints and coatings is an important application of non-contact terahertz measurement technologies. Here, layer thickness measurement makes up a significant part of quality assurance and production monitoring. Thus, for example, minimum thick-ness is ensured, consumption of resources is optimized by reducing the amount of excess material used and subsequent rectifications are avoided. Today, manual eddy current devices can be used with metallic substrates such as automotive sheet metal. This meth-od however, fails on poorly conductive fiber-reinforced composites. Ultrasound finds ap-plication in this context, but at the costs of mechanical contact. "There is a substantial need for a reliable measurement technique," says Globisch, "because the market for fiber-reinforced composites is growing in the automotive, aeronautics and wind power sectors." This is where the major advantage of the terahertz technology lies: Measure-ments are non-destructive and contactless, so that even coatings that are not completely dried can be measured as well. In addition, the quality of the results is not dependent on temperature and multi-layer systems can be directly resolved. Finally, terahertz radiation is non-ionizing and not harmful to humans due to its low power.

Although the new terahertz measurement system can be built using cost-effective stand-ard optical components, it is currently even more expensive than for example ultrasonic devices, which are manufactured in quantities of several hundreds of thousands. "How-ever, the prices will become similar with future series production," Globisch predicts. The advantages of the measurement technique and the current progress have convinced the researcher that the terahertz method will establish itself as a measurement technique for non-destructive material testing in the years to come.

At Hannover Messe, Fraunhofer HHI acts as a representative for the FMD.

Weitere Informationen:

https://www.hhi.fraunhofer.de/en/press-media/press-releases.html

Anne Rommel | Fraunhofer-Institut für Nachrichtentechnik, Heinrich-Hertz-Institut, HHI

More articles from Power and Electrical Engineering:

nachricht Failures in power grids: Dynamically induced cascades
25.05.2018 | Technische Universität Dresden

nachricht Beyond the limits of conventional electronics: stable organic molecular nanowires
24.05.2018 | Tokyo Institute of Technology

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Powerful IT security for the car of the future – research alliance develops new approaches

The more electronics steer, accelerate and brake cars, the more important it is to protect them against cyber-attacks. That is why 15 partners from industry and academia will work together over the next three years on new approaches to IT security in self-driving cars. The joint project goes by the name Security For Connected, Autonomous Cars (SecForCARs) and has funding of €7.2 million from the German Federal Ministry of Education and Research. Infineon is leading the project.

Vehicles already offer diverse communication interfaces and more and more automated functions, such as distance and lane-keeping assist systems. At the same...

Im Focus: Molecular switch will facilitate the development of pioneering electro-optical devices

A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.

The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...

Im Focus: LZH showcases laser material processing of tomorrow at the LASYS 2018

At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.

At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...

Im Focus: Self-illuminating pixels for a new display generation

There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?

At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...

Im Focus: Explanation for puzzling quantum oscillations has been found

So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics

Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

In focus: Climate adapted plants

25.05.2018 | Event News

Save the date: Forum European Neuroscience – 07-11 July 2018 in Berlin, Germany

02.05.2018 | Event News

Invitation to the upcoming "Current Topics in Bioinformatics: Big Data in Genomics and Medicine"

13.04.2018 | Event News

 
Latest News

In focus: Climate adapted plants

25.05.2018 | Event News

Flow probes from the 3D printer

25.05.2018 | Machine Engineering

Less is more? Gene switch for healthy aging found

25.05.2018 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>