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162 CHINESE OPTICS LETTERS / Vol. 8, No. 2 / February 10, 2010
Study of terahertz interferometric imaging using optical
techniques
Yuntao He (ÛÛÛ777)
∗
, Yuesong Jiang (ôôôttt), Yuedong Zhang (ÜÜÜÀÀÀ), and Guoli Fan (IIIwww)
School of Electronic and Information Engineering, Beihang University, Beijing 100191, China
∗
E-mail: taoyunhe@ee.buaa.edu.cn
Received October 30, 2009
Principles of terahertz (THz) interferometric synthetic aperture imaging with heterodyne and optical tech-
niques are presented. A THz interferometric experiment based on optical up-conversion is set up. The
received THz signal is modulated into an optical carrier and transmitted in a fiber. To simulate phase
differences between two THz receivers, the output of receiver is divided and a phase shifter is placed before
electro-optical modulation (EOM). Interferometric spectra of these mo dulated optical signals are examined
at different phase shifts. Otherwise, carrier suppression and phase error calibration are discussed for THz
interferometric synthetic aperture imaging.
OCIS co des: 110.3175, 040.2235, 280.6730.
doi: 10.3788/COL20100802.0162.
Defined by frequencies from 0.1 to 10 THz, terahertz
(THz) radiation is opening the door to a wide variety
of applications. Waves in this gap are able to pene-
trate materials such as plastics, cloth, packaging, paper,
and many organic compounds, including human tissues,
but strongly absorbed by metals and other inorganic
substances
[1]
. This means that imaging in THz fre-
quencies can potentially reveal the presence of concealed
objects such as knives and guns and also smuggled items
such as drugs, alcohol, and money. Without the hazards
associated with ionizing radiation, THz radiation com-
puted tomography (T-rays CT) has been developed to
substitute for X-rays
[2]
. So THz imaging can be widely
used in astronomy, military, non-destructive testing
(NDT), healthcare, aerospace, and communication
[1−6]
.
Compared with the long history of microwave and opti-
cal imaging, THz imaging system has just emerged very
recently. The THz wave imaging system was demon-
strated by using THz time-domain spectroscopy (TDS)
by Hu et al.
[7]
. These initial images have inspired a great
deal of excitement and much of the subsequent develop-
ment of THz imaging systems and techniques
[8−12]
. The
simplest and most prevalent THz imaging configuration
is the single-point measurement using a single transmit-
ter and detector pair. To exceed diffraction limitation
and increase the resolution, synthetic aperture technol-
ogy was widely adopted in microwave, and optical region
was proposed in THz imaging system
[9−11]
. The THz
synthetic aperture imaging system was set up for astron-
omy in Ref. [13]. A synthetic phased-array THz imaging
system was demonstrated by O’Hara et al.
[9,10]
. Federici
has been dedicated in real-time THz interferometric and
synthetic aperture imaging for many years
[11,14−16]
, and
an imaging array acquired video-rate images with four
THz receivers was reported recently
[16]
. The principle of
this approach is based on the fact that the coherent prod-
uct (correlation) of the signal from pairs of THz receivers
measured at different antenna-pair spacings (baselines)
yields a sample point in the Fourier transform of the field
of view (FOV), and the FOV itself is reconstructed by
inverting the sampled points. To implement this, THz
frequencies are down-converted and N*(N -1)/2 correla-
tors are need for an N-element array
[17−19]
.
An alternative approach of imaging is to convert
the received signals up to the optical carrier, trans-
mit them in fiber and image them by coherent optical
beam forming
[20−23]
. There are many advantages such as
easiness of realizing real-time imaging, light weight and
small scale, and low cost
[24]
. For THz synthetic aperture
interferometric imaging based on optical technique, a key
problem is to preserve the phase differences of each two
receiver pairs. So the phase errors must be calibrated
and the useless part of carrier should be filtered before
beam forming.
In this letter, principles of THz synthetic aperture
imaging with or without optical techniques are discussed.
An experiment is set up to demonstrate the primary in-
terferometric process and phase differences maintaining.
The relations between interfered results and the phase
difference are analyzed. And two important issues, phase
error calibration and carrier suppression, are discussed
and demonstrated.
The process of THz interferometric synthetic aperture
imaging is described as follows. Firstly, the scattered
and radiated THz waves received from FOV are con-
verted to an intermediate frequency (IF) and transport
along coaxial cables. Secondly, containing the ampli-
tudes and phases of incident THz wave, these IF signal
pairs are cross-correlated and integrated to yield spa-
tial frequency samples
[14,17−19]
. Finally, the image of
FOV is reconstructed by a proper inverse algorithm
and post treatment. Figure 1 shows a scheme of THz
synthetic aperture imaging system with frequency down-
conversion.
Imaging system of this scheme has been widely used
in radio astronomy at all kinds of frequencies includ-
ing THz wave band. For a non-redundant N-element
interferometric imaging array, there are N*(N -1)/2 inde-
pendent baselines (spatial frequency samples). It means
that to figure out all the cross-correlations, N*(N -1)/2
correlators are needed, which is square multiple of the
receiver number. Besides, it would result in increas-
ing bulk and electromagnetic interference especially in
a complex environment for signals are transmitted in
1671-7694/2010/020162-05
c
° 2010 Chinese Optics Letters