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630 NATURE PHOTONICS | VOL 3 | NOVEMBER 2009 | www.nature.com/naturephotonics
industry perspective TECHNOLOGY FOCUS
TERAHERTZ IMAGING
Revealing hidden defects
Irl Duling and David Zimdars
With new laser sources and detectors coming onto the market, terahertz imaging is starting to become a valuable
tool for non-destructive testing, process control and quality inspection.
Traditionally dicult to access, the terahertz
(THz) region of the electromagnetic
spectrum (0.1–10 THz), is higher in
frequency than most electronic sources
and lower in frequency than most optical
sources. However, recent advances
in photonics and electronics are now
enabling the development of compact
yet sophisticated THz imaging systems.
For instance, new semiconductor growth
structures have yielded continuous wave
(CW) lasers — specically quantum
cascade lasers — that emit at frequencies
in the THz region, although these require
cryogenic cooling.
Despite these advances, detectors in this
spectral range face challenges. Electronic
detectors, such as Schottky-diode-based
detectors, provide phase information
but suer from ‘skin eect’ losses at THz
frequencies. Photon detectors, such as
bolometers, measure thermal background
noise and provide no phase information.
As a result, both of these detectors require
THz powers of nearly 1 mW, making them
several orders of magnitude less sensitive
than detectors used in the visible part of
the spectrum.
Terahertz imaging is typically applied
in one of two ways: passive or active
imaging. In the passive approach, a small
one-dimensional (1D) array of detectors is
used to measure the THz radiation either
emitted by an object or reected from
another thermal source, such as the sun or
deep space. Although this approach gives
little information about the identity of an
object other than its exterior prole, it
requires only detection technology (there
is no need for an external source) and can
image an object many metres away.
In contrast, active imaging uses both
a THz source and a detector. When the
two are phase-locked, this technique
yields the highest sensitivity while also
providing phase (or timing) information.
is approach allows for either reective
or transmissive imaging. e image is
typically collected one pixel at a time
by scanning the THz beam across the
target. For reective congurations, the
transmitter and receiver are oen combined
as a ‘transceiver’; that is, the transmitter
and receiver are moved together across
the target. is approach can be applied to
either time-domain (pulsed) or CW THz
sources, although time-domain sources
are preferred because they avoid problems
associated with multipath interference and
range ambiguity. ese problems related
to CW sources can be mitigated — at the
cost of increased complexity — by using
a frequency-modulated CW (FM-CW)
source. FM-CW systems that can image
objects at distances of many metres have
been constructed, but the scanners are
large and therefore generate images at very
low speed.
e active imaging method that
provides the most information and the
highest signal-to-noise ratio — without
any range ambiguity — is pulsed, or
time-domain (TD), terahertz imaging.
Here, THz pulses are produced by using
ultrashort laser pulses to excite a specially
fabricated photoconductive switch.
e switch is connected to an antenna
that radiates the THz pulse to the optical
imaging system. e short THz pulses
generated can be used like high-frequency
radar to image the prole, the internal
structure and the spectroscopic signature
of the object.
Pulsed THz imaging is being adopted
for non-destructive testing (NDT)
applications in military, aerospace and
industrial settings, including for the
analysis of pipes (Fig. 1), radar domes
(Box 1) and art (Box 2). A key capability of
this type of imaging is that it can be used
to generate 2- or 3D subsurface images of
many materials that are otherwise opaque.
For example, NASA is currently using TD-
THz reection-based NDT to examine the
interface between the space shuttle external
tank and the spray-on foam insulation,
to detect voids and disbonds. NASA are
also using pulsed THz imaging to identify
features beneath the thermal protection
system of the space shuttle.
In NDT, THz pulses are directed
towards the sample as the transmitter
and receiver are ‘raster scanned’ in a
2D grid. At each point in the grid, the
electric eld of the pulse is recorded as a
function of time. Imaging is performed
either in transmission or reection
modes, each providing dierent but
complementary information.
In transmission mode, the value of each
pixel is computed by numerical analysis of
the pulse; for example, either by integrating
the power over a specic frequency range
or by analysing the time-of-ight of the
pulse. e 2D TD-THz transmission
images are similar to a transmission
radiograph, but TD-THz imaging typically
has more methods of analysis available
than radiography. For example, images
based on power measurements are related
to the chemical and physical constitution
of the sample, whereas time-of-ight
measurements are proportional to the
total amount of material and its index of
refraction. Both types of image can be
derived from the same 2D data map.
Figure 1 | A terahertz scanner checking the
integrity of a pipeline. The scanner magnetically
attaches to the pipe and crawls around its
circumference collecting data, allowing breaks or
areas of delamination to be detected.
PICOMETRIX
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© 2009 Macmillan Publishers Limited. All rights reserved
NATURE PHOTONICS | VOL 3 | NOVEMBER 2009 | www.nature.com/naturephotonics 631
industry perspective TECHNOLOGY FOCUS
Pulsed THz reection imaging provides
additional capability to investigate the
structure of a sample. In reection mode,
portions of the THz pulse will return
from any structural interface that has a
change in its refractive index. Because
the speed of light within the sample is
nite, a series of interfaces will result
in a series of pulses that have a spacing
proportional to the distance between the
interfaces, in the return waveform. In many
ways, TD-THz reection tomography is
a non-contact electromagnetic analogue
to pulsed ultrasound imaging. However,
in addition to structural information,
TD-THz reection images can also
provide information on the spectral or
material composition of a sample, owing
to changes in the spectral content of the
reected THz pulses.
e basic technology and method
of pulsed THz imaging was developed
by Lucent Bell Laboratories and other
researchers in the early 1990s. However,
early TD-THz instrumentation was large
and conned to an optical table, and
the early commercial systems required
the object to be brought to a small
imaging chamber within the instrument.
Today, the most advanced commercial
TD-THz instruments are portable, with
interchangeable bre-optic coupled remote
scanning sensors. is new generation of
instruments allows the short-pulse laser,
optical delays and signal processing to
be deployed in a 19-inch rack-mounted
control unit, while the THz transmitters
and receivers can be remotely located and
are freely positioned. ese sensors can
be used to scan large stationary objects
using either robotically controlled gantries,
crawlers or hand-held devices. e bre-
optic coupled TD-THz instrumentation
platform provides a powerful, compact,
rugged and highly exible system for
Ground-based radar installations are
oen protected from the elements by large
domes called radomes (Fig. B1a). ese
domes must be transparent to the radar
frequency, while being strong enough to
withstand weather conditions. A typical
dome is constructed of a ‘sandwich’ of
breglass and structural foam. If an object
strikes the radome, it is possible for the
breglass to separate from the foam,
causing delamination. is can lead to
structural weakness and eventual collapse.
In addition, the void of the delamination
can become lled with water (water
intrusion), disrupting the radar beam.
Typical defects of interest are 10–15 cm
in diameter.
Similarly, airborne radar is protected
in the nose of an aircra. Although smaller
than ground-based radomes, these have a
similar construction and must therefore
be inspected periodically for damage
and water intrusion.
e current method of radome
inspection is a manual ‘tap test’
inspection of the dome surface. As
expected, the measurement is prone
to operator-dependent variability and
does not provide any way to record
the measurement, for comparison at
a later date.
Fortunately, pulsed THz systems oer
an attractive solution to manual inspection.
Individual measurements can be made
at high speed (up to a kilohertz), and
information can be obtained on the size,
depth and water intrusion of the damage.
It is possible to scan the THz beam over a
wide area of the dome and inspect much
more rapidly than using a tap test. By
encoding the scanner, image maps can be
produced for an archival health record of
the radome.
e scanner used in a terahertz radome
inspection system is connected to the main
control unit by a bre-optic cable. is link
carries optical and electrical signals back
and forth to the scanner (Fig. B1b). e
scanner has a 15-cm scan-width and can
be rolled across the surface of the dome at
speeds of up to 12.7 cm–1.
A dry delamination and a water
intrusion are pictured, respectively, in
Figs B1c,d. e water intrusion appears
black, owing to the absorption of the
terahertz energy. Advanced analysis of the
waveform can provide automated detection
of delamination.
With the speed and objectivity of
terahertz radome inspection, the health
history of a radome can be collected
and stored. As defect growth is tracked,
maintenance can be scheduled on a
controlled basis, reducing costs and
ensuring reliability.
a
c
b
d
Figure B1 | Terahertz imaging is proving valuable for non-destructive testing of radomes, the protective
domes enclosing radar installations. a, A typical radome. b, Picometrix’s mobile terahertz radome
scanner. c,d, Terahertz scans showing delamination (c) and water intrusion (d) of a radome.
Picometrix craig Steiner
Picometrix Picometrix
Box 1 | Radome imaging
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© 2009 Macmillan Publishers Limited. All rights reserved
632 NATURE PHOTONICS | VOL 3 | NOVEMBER 2009 | www.nature.com/naturephotonics
industry perspective TECHNOLOGY FOCUS
implementing a broad range of pulsed
THz measurements.
Modern TD-THz imaging systems
consist of one or more user-interchangeable
remote TD-THz measurement sensors; an
intelligent core control unit; control and
analysis soware; and optional accessories
such as a motion controller and gantry.
An important component of the
TD-THz control unit is the rapid-scan
waveform time base. It is desirable that the
system scans a waveform for as long a time
record — and as rapidly — as possible.
e time delay in air is 6.6 ps mm–1 for
reection. erefore, a longer recording
time translates to a larger depth record.
For example, a 320-ps scan window can
sample the internal structure of an object
up to 48 mm thick. Within this range,
the image slice can be located as deep in
the object as attenuation and the optical
system will allow. Faster repetition rates
allow for the collection of images in a
shorter time. Industry-leading waveform
scanners are 320-ps long at 100 Hz, and
80-ps long at 1 kHz. e 1 kHz systems,
when coupled with a high-speed line
scanner, can collect cross-section images at
video rates.
Imaging sensors that use bre-optic
pigtailed transmitter and receiver modules
are manufactured using the same methods
and standards as telecommunications laser
modules. ese miniature components have
integrated modulator and pre-amplier
electronics that enhance the signal-to-noise
ratio and reject external electromagnetic
interference. ey permit imaging sensors
that are lightweight, rugged and entirely
solid-state, with no internal lasers or
moving parts.
e recent arrival of robust, eld-
deployable pulsed THz systems has
opened the doors to exciting new imaging
applications outside the laboratory setting.
By measuring both amplitude and phase,
we can now see below the surface of objects
and generate high-contrast, quantitative
tomographic images of their structure.
e eld of pulsed THz NDT has just
begun and promises to bear much fruit
in the coming years. Already, the elds
of aerospace engineering, composite
inspection, multilayer manufacturing,
radome repair and art conservation have
recognized the unique imaging capabilities
of pulsed THz imaging. Applications now
extend across the full range of process
control, quality inspection and NDT into
medical testing, security applications and
spectroscopy. e fact that pulsed THz
imaging systems are exible, easy to use
and portable is crucial in enabling these
high-value applications. ❐
Irl Duling and David Zimdars are at Picometrix
LLC, 2925 Boardwalk, Ann Arbor, Michigan
48104, USA.
e‑mail: iduling@picometrix.com
In the eld of art restoration, it is
important to catalogue the structure,
materials and history of a work of art. Each
time restoration work is done there is a
trade-o between restoring the art to its
original state and risking further damage.
Modications that have been made
to a painting, either by conservators or
the artist themselves, can be observed
by using X-ray, infrared, UV or, more
recently, THz imaging. By using pulsed
THz imaging, not only are the variations
in reectivity of the pigments observable,
but also the internal layer-structure of the
art can be mapped. Using time-domain
THz imaging to separate the various layers
of a particular artefact can reveal hidden
structure that is impossible to see without
destroying the artwork.
In the town of Vif, outside of
Grenoble, France, is a small church where
researchers have discovered 14th century
frescoes on walls covered with plaster.
Local conservators hope THz imaging can
help them ‘expose’ the beautiful artwork
without removing the plaster. Indications
are that the variation in pigment
reectivity can be seen even under the
plaster covering.
In the prestigious Uzi Gallery in
Florence, Italy, a pulsed THz scanner
was used during the restoration of the
Polittico di Badia by Giotto di Bondone
to study the location of the gold leaf
under the pigments, the presence of metal
in some of the symbolic elements and,
more importantly, the internal structure
of the painting itself. ese panels were
constructed by carving both the painting
surface and frame from a single piece of
wood. e surface was then smoothed
with a layer of gypsum, covered with
canvas and more gypsum, and then
painted. All of these layers are suspected
to be there but cannot be seen with
current non-destructive technology. e
pulsed THz system (Fig. B2a) was able
to reveal these layers (Fig. B2b). Because
the prole of each layer was able to be
measured, this allowed the weave of the
canvas and the tool marks in the wood
to be imaged. is level of internal detail
had never before been observed, and this
was performed with a non-contact, non-
destructive imaging system.
Much more is le to investigate.
Researchers at the Louvre in Paris, France,
are examining the wood grain of panels
to perform precise dating, and attempting
to examine fragile written documents
without disturbing them. e availability
of pulsed THz systems that are portable
and have bre-optic coupled heads to
allow imaging of stationary objects, has
given art conservators a powerful new
tool in their pursuit of art investigation
and restoration.
ab
Time (ps)
Distance (mm)
40
50
60
70
80
90
100
–15020 40 60 80 100
Figure B2 | Pulsed THz imaging for art restoration. a, Set-up of an imaging system. b, The internal,
hidden structure of a painting, revealed by THz imaging.
Picometrix
Box 2 | Imaging antiquities
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© 2009 Macmillan Publishers Limited. All rights reserved
