http://www.fraunhofer.de/en/press/research-news/2016/april/terahertz-technology.html
Terahertz is a new technology in which nondestructive
testing of components and surfaces is possible. Until now, these devices and,
in particular, the sensor heads have been expensive and unwieldy. Researchers
at Fraunhofer have now succeeded in making sensor heads more compact and, thus,
cheaper, which facilitates their handling considerably. First prototypes are
already being used for the production of plastic pipes. They are also very well
suited for analyzing coatings of fiber composites. From April 25 to 29, 2016,
these new sensor heads are being presented at the Hannover Messe (Hall 2, Stand
C16/C22).
The researchers at Fraunhofer have packaged the transmitting and receiving unit for terahertz radiation into a handy little sensor head with a diameter of only 25 and a length of 35 millimeters
The researchers at Fraunhofer have packaged the transmitting
and receiving unit for terahertz radiation into a handy little sensor head with
a diameter of only 25 and a length of 35 millimeters.
More than ten years ago, terahertz technology was the next
big thing. Back then there was a lot of talk about ‚nude‘ body scanners. People
thought that the devices that were being installed at airports were going to
make revealing images of passengers. Moreover, scientists were hoping to
develop measurement systems for material testing and inspection of components
using terahertz radiation. Despite all these great expectations, the
long-awaited breakthrough for terahertz technology did not come about. Compared
to the conventional methods currently used for non-destructive testing, such as
X-ray or ultrasound, terahertz technology is simply too expensive, unwieldy and
overall impractical.
Measuring system with the innovative sensor head
The latest developments of the Fraunhofer Institute for
Telecommunication, Heinrich Hertz Institut, HHI in Berlin , could now give terahertz technology
a decisive boost. The research team of Thorsten Göbel, who heads the Terahertz
Research Group at HHI, has succeeded in developing terahertz devices that for
the first time are made from standard and therefore low-cost components and are
also relatively easy to handle. At the Hannover Messe, the experts will be
presenting a terahertz measuring system with an innovative sensor head, which
allows easy testing of various components, such as plastic tubes. The principle
employed by Fraunhofer HHI for generating terahertz radiation is based on an
optoelectronic method. Using a special semiconductor, laser light pulses are
converted into electrical terahertz pulses that are only one billionth of a
second long.
The reason for the lack of success of the terahertz
technology until now is mainly due to the required properties of the
semiconductors being used. These could only be achieved with materials that
required an illumination with a wavelength of 800 nanometers. Both the
terahertz system‘s laser and optical components are too expensive and not
robust enough for industrial use when using this rather exotic wavelength.
Commonly used wavelength standard
“Therefore, we have developed a semiconductor that can be
stimulated with a laser light centered around a wavelength of 1.5 micrometers,”
says Göbel. “In optical communications, this wavelength is standard, which is
why there is large number of inexpensive and high-quality optical components
and lasers on the market.”
However, one obstacle needed to be overcome on the way to
creating an affordable and handy terahertz system for testing materials. Until
now, the sensor head for scanning components was too big and heavy for easy
handling. The reason: the terahertz transmitter and the receiver were two
separate components that had to be mounted in a casing with great effort and
precision. The main disadvantage of this arrangement was that samples can only
be measured at an angle. Therefore, the object had to be exactly in focus of
the transmitter and receiver so that the terahertz signal sent from the
transmitter via the sample could be displayed on the receiver. If the distance
between the sensor head and the sample changes, for example due to vibrations,
it becomes more difficult to measure.
The Fraunhofer HHI experts have solved this problem by
manufacturing an integrated chip that can transmit and receive simultaneously.
Now it is possible to use a single optical lens that focuses on the object,
which allows for a flexible operating distance. The researchers packed this
transmitting and receiving unit, the transceiver, into a handy little sensor
head with a diameter of only 25 and a length of 35 millimeter. The device will
be presented at the Hannover trade fair.
Moreover, these terahertz sensor system prototypes have been
in use by manufacturers of plastic pipes for some time now. These sensors are
directly used in the production line to monitor the thickness of the pipe
walls. If the walls are too thin, the pipe becomes unstable. If they are too
thick, valuable plastic is wasted. Up to now, the production of plastic pipes
has been monitored using an ultrasound system. Since ultrasound cannot measure
correctly in the air, water is needed. Similarly to the ultrasound gel doctors
use, the water works as a coupling medium between the ultrasound sensor head
and the pipe. Therefore, the nearly 250-degree Celsius pipes must be dragged
through a water tank. In addition, the ultrasound technology also fails with
so-called intelligent pipes that are constructed from a wide-range of layers of
different materials.
Another future application is the verification of paints and
coatings on fiber reinforced composite materials. Today, it is possible to use
handy eddy current equipment on metallic substrates, such as sheet metal for
the automotive industry. However, this method fails on poorly conducting fiber
composites. “The need for a reliable measurement process is enormous,” says
Göbel, “because the market for composite materials is growing in the
automotive, aircraft and wind energy industries.”
Although the new terahertz sensor system is
constructed from inexpensive standard optical components, it is currently more
expensive than, for example, ultrasonic devices, which are manufactured in
quantities of many hundreds of thousands. “However, the price will drop in the
future when volume production starts picking up,” Göbel predicts. Given the
advantages of the measuring method and the current advances, Göbel believes
that the terahertz method will be successfully established in the coming years
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