OPTICAL TRUE TIME-DELAY FEEDER
FOR PHASED ARRAY ANTENNAS
USING FIBER BRAGG GRATINGS AND
METAL-FILM REFLECTORS
Jong-Dug Shin, Duk-Hee Bae, and Boo-Gyoun Kim
School of Electronic Engineering
Soongsil University
1 Sangdo-Dong, Dongjak-Gu
Seoul 156-743, Korea
Received 20 September 2004
ABSTRACT: A novel optical true time-delay (TTD) feeder for phased-
array antennas incorporating a fiber Bragg grating (FBG) prism and
optical fibers with metal film deposited on their cleaved ends is pro-
posed. By arranging a metal-film reflector at one end of the fiber delay
line with FBGs written at different locations, one FBG for every fiber
delay line can be saved (compared to FBG-only TTD feeders) and flexi-
bility of wavelength selection is also provided due to the uniform reflec-
tance of the metal film in a broad wavelength range. A TTD feeder with
two fiber delay lines for 10-GHz linear phased-array antennas is built to
steer a beam into three directions at 0° and 30°. The experimental
results of the time delays agree well with those calculated at all the
steering angles. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol
Lett 45: 122–124, 2005; Published online in Wiley InterScience (www.
interscience.wiley.com). DOI 10.1002/mop.20743
Key words: optical true time-delay; fiber Bragg gratings; phased-array
antennas; beam forming networks
1. INTRODUCTION
Optical true time-delay (TTD) feeders for phased-array antennas
(PAAs) offer advantages such as small size, low loss, no electro-
magnetic interference, large instantaneous bandwidth, high reso-
lution, squint-free beam scanning over a broad range of frequen-
cies, multibeam capability, and so forth. Several schemes for
optical TTD feeders have been proposed thus far, including a
fiber-optic prism using high-dispersion-compensation fibers [1],
integrated silica-waveguide switches [2], holographic-grating cou-
plers on top of a glass substrate [3], fiber Bragg grating (FBG)
prisms [4], chirped fiber gratings (CFGs) [5], a combination of
FBGs and CFGs [6], 3D MEMS switches [7], and 2 2 MEMS
switches [8]. Among these schemes, fiber-grating-based TTDs
provide either discrete or continuous beam steering, depending on
the grating types used. Since fiber gratings are made of fiber itself,
they show low insertion loss and polarization-dependent loss.
However, tunable or multiwavelength light sources are required to
utilize wavelength signatures for required time delays. As the
number of gratings per fiber delay line is increased in order to
obtain finer beam steering, the number of source wavelengths
should be increased as well. This means that the performance of
grating-based TTDs is largely dependant upon the control and
switching mechanism of source wavelengths as well as upon the
precision of intergrating distance in fiber delay lines.
In this paper, we propose a novel TTD feeder for PAAs using
a FBG prism and metal-film reflectors deposited directly on the
fiber ends. This structure not only reduces the number of FBGs
required to build the TTD feeders, but also alleviates wavelength
restriction due to the uniform and wide reflectance spectra of the
metal-film reflectors. In section 2, we briefly describe the config-
uration of the proposed TTD feeder system. Experimental results
and discussions on the TTD feeder with two fiber delay lines for
10-GHz linear PAAs capable of steering beams in three directions
at 0° and 30° are presented in section 3. Finally, section 4
concludes this paper.
2. CONFIGURATION OF THE PROPOSED TTD FEEDER
Figure 1 shows the configuration of the proposed TTD feeder for
linear PAAs with N antenna elements.
The relationship between the maximum time delay of the TTD
feeder and the beam-radiation angle is given as T
MAX
=
[( N - 1) d/ c ] sin
max
= ( N - 1) , where c is the velocity of
light in free space,
max
is the maximum radiation angle from
array normal, N is the number of antenna elements, and and d
are the time-delay difference and the distance between adjacent
antenna elements, respectively. d is typically chosen as one-half of
the RF wavelength in order to obtain the maximum gain at the
radiation angle.
At a wavelength of
c
, which can be any wavelength except the
grating wavelengths,
1
,...,
n
, reflection occurs from the metal
films located at the equal distance from the circulators. Therefore,
we obtain broadside radiation at this wavelength. On the other
hand, at one of the grating wavelengths, a time delay (that is, a
radiation angle) is chosen from the prism arrangement of the
corresponding FBGs in the N fiber delay lines.
3. EXPERIMENT AND DISCUSSIONS
We implemented a TTD feeder for 10-GHz PAAs composed of
two antenna elements, as shown in Figure 2.
The TTD was built to have three steering angles, such as 0° and
30°. Therefore, the pair of FBG1s was separated by about 2.5
mm from each other to obtain +30° at 10 GHz. For the pair of
FBG2s, their separation was also 2.5 mm, but the sense was
opposite to obtain -30°, as shown in the figure. Since each pair of
FBGs does not possess the same Bragg wavelength and reflec-
tance, wavelength equalization has been accomplished by using
temperature controllers at 1554.9 nm for FBG1s and 1556.7 nm for
FBG2s, and the reflectance equalization has been obtained by
using variable optical attenuators, as shown in Figure 3(a).
For 0° radiation, Cr/Au film reflectors were placed at the end of
each fiber delay line such that the reflectors were at an equal
distance from circulators. Cr and Au were evaporated on the Figure 1 Schematic diagram of the proposed TTD for linear PAAs
Figure 2 A TTD feeder for 10-GHz PAAs and the experimental setup
for time-delay measurements
122 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 45, No. 2, April 20 2005