Abstract—Terahertz waves can provide in-depth information
on defects for structural health monitoring of composite
materials. This paper describes the technology of a continuous-
wave and a time-domain terahertz system operating on a 2-D and
3-D motion platform to provide 3-D high spatial resolution. The
system as well as the overall detection performance will be
described.
I. INTRODUCTION AND BACKGROUND
NE of the domains in which the terahertz (THz)
technology can be considered as enabling is short-range
in-depth imaging. Using these waves, a contact-free, high-
resolution inspection becomes feasible for typical composite
materials found in aeronautics.
This paper will report on the main results obtained with the
FP7
project
entitled DOTNAC
(Development
and
Optimisation of THz non-destructive testing (NDT) on
Aeronautics Composite Multi-layered Structures [1]) with the
following objectives: (1) the development of a fibre-coupled
Time-Domain System (TDS); (2) the implementation of a
Frequency-Modulated Continuous-Wave system (FMCW)
with electrical cable coupling; and (3) testing them on a series
of calibration and blind samples for evaluation and validation
purposes against conventional NDT techniques such as
radiographic and ultra sound testing, as well as infrared
thermography.
II. FMCW THZ SYSTEM
The implemented FMCW system is an all-electronic THz
system. It consists of three scanning heads with different
frequency ranges (around 100 GHz, 150 GHz, and 300 GHz).
They are used to acquire data by scanning a sample placed in
front of them in reflection mode. Using a homodyne detection,
the amplitude and phase can be measured after demodulating
the received signal. Combining this in-depth information with
a lateral scanning, a 3-D image can be built up. The in-depth
resolution depends on the used bandwidth and the roughness
of the sample surface, and varies between 2 mm and 6 mm
(with an accuracy between 10 µm and 50 µm).
Two configurations have been tested using the FMCW