COL 12(5), 051101(2014) CHINESE OPTICS LETTERS May 10, 2014
Optical phase control for MMW sparse aperture
upconversion imaging
Yuntao He (ÛÛÛ777)
1∗
, Haiping Huang (°°°²²²)
1
, Yuesong Jiang (ôôôttt)
1
,
and Yuedong Zhang (ÜÜÜÀÀÀ)
2
1
School of Electronic and Information Engineering, Beihang University, Beijing 100191, China
2
Beijing Institute of Space Mechanics and Electricity, Beijing 100190, China
∗
Corresponding author: yuntaohe@buaa.edu.cn
Received November 26, 2013; accepted March 5, 2014; posted online April 30, 2014
The random phase errors of the optical carriers are discussed and controlled for passive millimeter-wave
sparse aperture (PMMW SA) upconversion imaging. A two-channel model is set up for analyzing charac-
teristics of the phase errors, and an active optical control technique based on stochastic parallel gradient
decent algorithm (SPGD controller) is proposed to calibrate the phase errors. To demonstrate the feasi-
bility of the SPGD controller, simulations are performed and an experimental system with a two-channel
fiber array is set up. Simulation and experiment results show that the SPGD controller can effectively
and rapidly compensate the phase errors of the opt ical carrier, and the accuracy of the phase control is
sufficient for imaging systems.
OCIS codes: 110.0110, 250.0250, 140.0140, 030.0030.
doi: 10.3788/COL201412.051101.
Passive millimeter wave (PMMW) imaging has shown
significant potential over other technology capable of
penetr ating through low visibility conditions and ob-
scurations caused by cloud, fog, smoke, sandstorms,
and clothing. So PMMW imager could be widely used
in homeland security, defense, and aviation safety
[1,2]
.
However, the angular resolution of such an imager is
limited by classical diffraction theor y, ∆θ = λ/D, where
D is the circular aperture diameter and λ is the free-
space wavelength of the imager. Sparse aperture (SA)
imaging techniques can obtain higher res olution by us -
ing many discrete apertures compared with the single
large-aperture imaging technique. With regard to the
traditional down-conversion imaging techniques utilizing
a mixer to reduce the MMW s ignal frequency with elec-
tronic techniques, it is still suppressed by many techni-
cal problems, including high-sensitivity phase detectors,
routing and a large array of complex cross-correlators ,
and interconnects for a real-time signal processing
[3,4]
.
To circumvent these obstacles, several novel techniques
of pass ive SA MMW imaging using optical upconversion
techniques have been proposed
[2−7]
. Unlike conventional
techniques, the optical upconversion technique realizes
imaging by modulating the target MMW signals into
optical sidebands and forming the target images on the
optical domain. Although this technique shows signif-
icant potential for PMMW applications , a number of
technical challenges must be overcome for such a sys-
tem to be implemented
[8,9]
. Such challenges include:
1) New wide-band MMW signal processing techniques
should be developed
[10−12]
; 2) The target signal must
be routed from each node to a central processor without
dispersion traveling through the fiber; 3) The optically
upconverted signal experiences phase error induced by
random variations in antenna positioning, atmospheric
artifacts, s tress-change in fiber, the fluctuations of the
laser and so on. This letter focuses on a technique to
ensure phase stability.
Minor variations in effective path length will cause
dramatic changes in optical phase, making the coherent
imaging techniques unfeasible
[8]
. In practical applica-
tions, it is necessa ry for us to restr ict the change fro m
fiber length to be les s than λ/10 to get a high quality
reconstructed image
[3]
. This would require the optical
lengths to be controlled within 0.1 µm. Fortunately, the
phase effects can be acco unted for by actively compen-
sating for induced changes in the optical path.
Several active phase control methods have been pro-
posed to circumvent the phase effects in other kinds of
optical system
[12−19]
, such as heterodyne technique, mul-
tidither technique, redundant spacing calibration (RSC)
and s tochastic parallel gradient decent SPGD algorithm.
This letter presents a technique called the optica l car-
rier interference calibration by using SPGD algorithm to
calibrate the phase errors in the fiber of an optical up-
conversion imaging system. To implement the algorithm,
a program needs to execute a fter converting the phase
errors to its re lated voltages.
Firstly, the sources of phase errors are analyze d with
a two-channel phase errors model, and the calibration
algorithm of SPGD is theoretically analyzed. Secondly,
simulations are performed to verify the feasibility that
the algorithm can calibrate the phase errors. In the
end, the experimental validation of the algorithm is per-
formed.
The schematic diagram of the optical upconversion
imaging system is illustrated in Fig. 1. The target ra-
diation signals received by the a ntenna array are modu-
lated onto coherent optical carriers. After up-converting
the signal onto sidebands, the optical signals are se-
quentially transmitted through p olarization-maintaining
(PM) fibers and a topolo gy-maintained fiber array that
matches the geometry of the antenna arr ay at the fib e r
tails. Then the light is co llimated and transmitted into
1671-7694/2014/051101(6) 051101-1
c
2014 Chinese Optics Letters