Active real-time imaging system employed with a CW 460GHz gyrotron and a pyroelectric array camera
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Citation
Seong-Tae Han et al. “Active real-time imaging system employed
with a CW 460-GHz gyrotron and a pyroelectric array camera.”
Infrared, Millimeter, and Terahertz Waves, 2009. IRMMW-THz
2009. 34th International Conference on. 2009. 1-2. ©2009
Institute of Electrical and Electronics Engineers.
As Published
http://dx.doi.org/10.1109/ICIMW.2009.5324787
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Institute of Electrical and Electronics Engineers
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Thu May 26 08:46:23 EDT 2016
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http://hdl.handle.net/1721.1/59372
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Detailed Terms
Active Real-Time Imaging System employed with a CW 460-GHz
Gyrotron and a Pyroelectric Array Camera
Seong-Tae Hana, Antonio C. Torrezanb, Jagadishwar R. Sirigirib, Michael A.
Shapirob, and Richard J. Temkinb
a
Korea Electrotechnology Research Institute, Ansan, 426-170 Korea
b
Massachusetts Institute of Technology, Cambridge, MA 02139 USA
Abstract—We report experimental testing of an active real-time
imaging system useful for many practical applications, such as
fast security check, food safety inspection, etc. The system
consists of a 460-GHz gyrotron capable of producing 16 W in
continuous wave operation and a pyroelectric array camera with
124-by-124 pixels. The detailed results obtained from the
proof-of-concept experiment with the system will be presented.
T
I. INTRODUCTION
ERAHERTZ imaging has received constantly growing
interest as a promising method to detect concealed
weapons on people at security-check points and for
non-destructive inspection for food because of its safety against
radiation hazards1. Nevertheless, the lack of a powerful source
and a more sensitive array detector has been an obstacle to
realizing such a potential imaging modality in the real world.
Conventional THz active imaging systems employ a focused
beam scanned over the object to overcome the obstacle, which
results in long scanning time to take a frame of the image.
Electro-optic (EO) sampling technique converting the pulsed
THz field from a photoconductive antenna to optical intensity
using an EO crystal for recording by CCD camera was applied
to reduce the acquisition time, and high speed THz imaging
was successfully demonstrated in real-time2. On the other hand,
terahertz active real-time imaging is demonstrated by use of a
micro-bolometer
focal
plane
array
camera
with
single-frequency continuous- wave (CW) sources, such as
far-infrared gas laser (2.52 THz) 3 and quantum cascade laser
(4.3 THz) 4.
In this study, we propose an alternative method to realize
terahertz active real-time imaging, especially suited for
applications such as fast security-check and quality control of
dry or frozen food in a production line at a video rate. Those
applications require CW radiation sources with superior output
power to illuminate the entire inspection area with wide
field-of-view while maintaining the power density above the
detection level of a focal plane array camera operating with low
sensitivity at room temperature.
II. PROOF-OF-CONCEPT EXPERIMENTS
The quasi-optical system designs for the proposed active
terahertz real-time imaging system are depicted in Figure 1.
The system employs a 460 GHz CW gyrotron capable of
producing a maximum of 16 watts in CW operation with a 13
kV 100 mA electron beam6. Second harmonic operation of the
gyrotron eases the magnetic field requirement and results in a
978-1-4244-2120-6/08/$25.00 ©IEEE.
relatively compact system.
The TEM00–like output beam from the gyrotron is expanded
by an off-axis parabolic mirror after a corrugated waveguide,
and the collimated beam illuminates a test object. Transmitted
or reflected beam is captured by a Teflon lens into the active
area of the pyroelectric camera7.
The camera consists of an array of 124-by-124 LiTaO3
pyroelectric sensors (originally designed for laser applications)
with a spacing of 100 Pm between each pixel and a motorized
chopper over the sensor array. The chopper enables the
pyroelectric crystals to detect the CW beams by the changes in
signal. This target would become more sensitive to the terahertz
signal by introducing a filter of thin polyethylene or Styrofoam
to get rid of background infrared radiation possibly
overwhelming the signals.
Fig.1. Layouts for the active terahertz real-time imaging system. The
terahertz beam (solid profile) is expanded and collimated by the
off-axis parabolic mirror, and irradiates the objects under test. The
scattered energy (dotted line) from the each part of the object is
imaged by the pyroelectric detector array.
Transmission through clothing decreases approximately as
the frequency of operation increases near 1 THz though the
resolution increases. Hundreds of GHz signal would be more
suited for better penetration along with sufficient resolution.
Generally, the approach to achieving source power at around
this frequency range has been either to use multipliers to
generate radiation from RF sources or to translate down in
frequency from the optical region using a laser and nonlinear
medium. There are exceptions to this trend in that the gyrotron
has been available for many years and can provide adequate
high power for the applications of interest between 0.1 and 1
THz.
The sensitivity of the array detectors operating at room
temperature is relatively low, for example, the sensitivity of the
pyroelectric camera is about 300-mW/cm2 at around 1 THz7.
To overcome the shortage of sensors, various methods of
generating CW THz waves could be taken into consideration,
including frequency multiplied microwave source8, backward
wave oscillators9, quantum cascade lasers4, optically pumped
gas lasers3, photomixing10, and parametric oscillators11.
However, it is most effective when a gyrotron is used as the
irradiation source in conjunction with the sensor to take
real-time images at the video rate, due to its inherent high
power capability in the frequency ranges with good output
beam pattern for covering a wide inspection space5. The
gyrotron is capable of frequency selection at the tube designers’
will accounting for the effect of atmospheric window and
minimal attenuation of materials under inspection. A CW
system tuned to a spectral window between atmospheric
absorption lines is easier to operate at longer standoff distance.
The inherent stability of the gyrotron with a narrow line-width
is another figure of merit5 to be mentioned.
To demonstrate the possibility of real-time stand-off
detection of concealed weapons on people at the security-check
point, and identification of a foreign substance in visually
opaque dry or frozen food coming out of a production line, we
present real-time videos of a moving envelop containing
metallic letters inside with the setup of transmission and
reflection, respectively. Figure 2 shows the detected images of
the letters made of metallic foil and hidden inside a paper
envelope. These results show that it is possible to detect
metallic objects hidden in clothing or similar materials.
frequency9 faster than the chopper frequency. Most gyrotrons
are not capable of tuning their operation frequency, but the one
we have developed has the capability of frequency tuning by
about 1 GHz5 at around 460 GHz in the second harmonic
operation. This is a unique feature of the gyrotron used in this
experiment.
III.
CONCLUSION
As a demonstration of the capability of a real-time imaging
system to see through visually opaque material, we presented
videos of a moving envelope containing metallic letters inside
with the setup of transmission and reflection, respectively.
This research is different from others’ in that a powerful CW
gyrotron is used as the radiation source to overcome the
shortcomings of an insensitive pyroelectric array camera
operated at room temperature. To the authors’ knowledge, it is
the first time that a gyrotron has been used in an imaging
configuration.
Even in this early-stage laboratory experiments, we were able
to see that it might be possible to meet the compelling needs
such as security screening and quality control, i.e. fast security
check and identification of defects such as inclusions of foreign
substance, cracks, holes, and deformations in dry or frozen
food in real time. Ultimately, it is hoped that this proposed
technology gains more widespread acceptance in future
systems for security sector and quality control.
ACKNOWLEDGEMENTS
This work was supported in part by the National Institute of Health (NIH) and
NIBIB under contract EB004866.
REFERENCES
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Fig.2. Captured images from the real-time videos taken in the setup of
transmission and reflection, respectively. Letters identified by the
terahertz imaging system are contained in the visually opaque envelop.
The thickness of each stroke of the letters is about 2-3 mm.
Detector arrays also have the disadvantage of being severely
affected by distortions and aberrations in the focusing system.
In Fig. 2, an etalon effect (bright and dark interference fringes)
inherent in the images taken with a coherent, monochromatic
radiation source is clearly visible. Reflective optical elements
instead of the lens might reduce spurious reflections and
consequently reduce the etalon effect. Also, it will increase the
available power for the detection of the pyroelectric crystals
otherwise dissipated through absorption by the lens. Covering
with partially absorbing medium around the system would be a
bit helpful.
Another promising measure is demonochromatizing the
source to reduce coherence length by modulating the
978-1-4244-2120-6/08/$25.00 ©IEEE.
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