Practical millimeter-wave holographic imaging system
with good robustness
Yukun Zhu (朱玉琨)
1,2,
*, Minghui Yang (杨明辉)
1
, Liang Wu (吴 亮)
1
,
Yun Sun (孙 芸)
1
, and Xiaowei Sun (孙晓玮)
1
1
Key Laboratory of Terahertz Technology, Shanghai Institute of Microsystem and Information Technology,
Shanghai 200050, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*Corresponding author: ykzhu@mail.sim.ac.cn
Received June 7, 2016; accepted July 22, 2016; posted online August 30, 2016
A practical millimeter-wave (MMW) holographic imaging system with good robustness is developed for the
detection of concealed weapons at security checkpoints, especially at the airport. The system is used to scan
the passenger and detect any weapons hidden in their clothes. To reconstruct the three dimensional image,
a holographic MMW imaging algorithm based on aperture synthesis and backscattering is presented. The system
is active and works at 28–33 GHz. As a practical imaging system, the robustness is analyzed in detail in terms of
the peak signal-to-noise ratio.
OCIS codes: 110.3010, 110.6880.
doi: 10.3788/COL201614.101101.
With the growing threat of terrorism around the world
nowadays, security detection of airplane passengers is
becoming more and more important
[1]
. Conventional sys-
tems for security detection include metal detectors for
personnel and x ray scanners for luggage. Compared with
these methods, millimeter-wave (MMW) imaging is more
effective and safe. MMW imaging combines the advan-
tages of both optical and microwave imaging systems,
particularly the high resolution of optical imaging due
to the short wavelength and penetration through most
clothing of microwave imaging. Moreover, holographic
techniques can be easier to achieve in the MMW band
than optic waves, as we can get the amplitude and phase
of the MMW signal easily. Additionally, the extreme
development of MMW chips, modules and communication
and system techniques make MMW security detection
possible
[2–9]
.
MMW imaging can be classified as a passive imaging
system and an active imaging system. There is no illumi-
nating source in a passive system
[10–17]
. It only records the
intensity of objects and works through these signals. Scat-
tering information of the target can be obtained because
different electromagnetic strengths are emitted due to the
different temperatures. An active system transmits MMW
signals, receives them reflected by the objects
[18–24]
, and
interferes them with the transmitting ones. Then, both
the amplitude and phase can be recorded.
Because of the advantages of MMW imaging systems,
we present an active MMW holographic imaging system.
A practical system has been realized in our lab, and good
robustness of the system is achieved. When it comes to the
word “practical,” we mean the system can work even if
some useful electromagnetic information is missing, which
could happen in practical, complicated circumstances. In
other words, the system does not just work in a perfect lab
environment. The less electromagnetic data is necessary
when reconstructing the image, the more practical the sys-
tem is. First, the imaging system is described in detail, and
then the imaging algorithm is presented. Next, an analysis
of the robustness of the imaging system through the peak
signal-to-noise ratio (PSNR) is demonstrated. Finally,
conclusions are drawn at the end of the Letter.
A practical implementation of an MMW holographic
imaging system is performed. It takes quite a long time
to scan a single transceiver over the whole aperture,
and it is costly and complex to make two-dimensional
arrays without mechanical scanning. Taking the working
time and the cost of the system into account, we choose to
scan a line of transmitting and receiving antennas array
row by row. A line of transceivers is scanned over the aper-
ture that has the target to collect the data. Although the
transmitting and receiving antennas are separate, they are
at approximately the same location, and may be assumed
to be at the middle point. The diagram of our MMW
holographic imaging system is shown in Fig.
1, and the
prototype of the system is shown in Fig.
2.
This system can reconstruct a three-dimensional (3D)
image of the target as wideband frequency (28–33 GHz)
signals are recorded. A high-stability MMW frequency
source is applied here to achieve the transmitting signal.
We choose 64 frequencies at an interval of 80 MHz. Two
levels of SP8T switches are adopted to achieve a row of
64 T/R units. The switching and moving of the antenna
array is controlled by a logic circuit. We record both the
in-phase and quadrature signals (called I and Q, respec-
tively) after mixing the receiving signals reflected by
the target with the transmitting ones from MMW oscilla-
tors, that is to say, both the amplitude and phase can be
obtained. Then, the intermediate frequency (IF) signals
are digitized by an eight-unit analog to digital (AD)
COL 14(10), 101101(2016) CHINESE OPTICS LETTERS October 10, 2016
1671-7694/2016/101101(5) 101101-1 © 2016 Chinese Optics Letters