Photonic Generation and Detection of W-Band Chirped Millimeter-Wave Pulses for Radar

Abstract

Based on the frequency-to-time mapping approach, we generate frequency-modulated millimeter-wave (MMW) pulses with central frequencies up to the W-band by a shaped optical pulse excitation of an MMW photonic transmitter with an ultrawide band photodiode as its key component. A coherent detection is achieved via a terahertz time-domain spectroscopic setup. Two different kinds of chirped MMW waveforms are generated; one is a linearly chirped sinusoidal pulse and the other is produced by a frequency-stepped modulation. Through appropriate optical spectral design, the frequency-chirped MMW pulses with instantaneous frequencies sweeping from 120 to 60 GHz, and a time-bandwidth product of ~ 25 is experimentally demonstrated.

Full text

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 16, AUGUST 15, 2012 1437
Photonic Generation and Detection of W-Band
Chirped Millimeter-Wave Pulses for Radar
Jim-Wein Lin, Chun-Liang Lu, H.-P. Chuang, F.-M. Kuo, Jin-Wei Shi,
Chen-Bin Huang, and Ci-Ling Pan, Fellow, IEEE
Abstract— Based on the frequency-to-time mapping approach,
we generate frequency-modulated millimeter-wave (MMW)
pulses with central frequencies up to the W-band by a shaped
optical pulse excitation of an MMW photonic transmitter with
an ultrawide band photodiode as its key component. A coherent
detection is achieved via a terahertz time-domain spectroscopic
setup. Two different kinds of chirped MMW waveforms are
generated; one is a linearly chirped sinusoidal pulse and the
other is produced by a frequency-stepped modulation. Through
appropriate optical spectral design, the frequency-chirped MMW
pulses with instantaneous frequencies sweeping from 120 to
60 GHz, and a time-bandwidth product of ∼25 is experimentally
demonstrated.
Index Terms—Millimeter wave (MMW), photonic transmitter
(PT), pulse shaping, terahertz (THz).
I. INTRODUCTION
IN RECENT years, pulsed radio-frequency (RF)
technology has attracted considerable attention due
to potential applications in modern radars, spread-spectrum
communications and microwave computed tomography
[1]–[3]. Specifically, in a modern radar system, pulsed
transmitted signals are desirable to obtain range information.
The radar pulses are usually frequency-chirped or
phase-modulated to increase the time-bandwidth product
(TBWP) and compressed at the detection end to improve the
range resolution while maintaining a high average power.
State-of-the-art electronic technology can produce chirped
microwave pulses; however, the central frequency and the
scanning bandwidth of the generated pulses are usually
limited to several GHz [4]. At the present stage, efforts have
been made toward utilizing the millimeter-wave (MMW)
band and beyond [5], which can offer better spatial and
Manuscript received May 3, 2012; revised June 5, 2012; accepted June
15, 2012. Date of publication June 25, 2012; date of current version July 31,
2012. This work was supported in part by the National Science Council under
Grant NSC 98-2221-E-007-025-MY3, Grant NSC 98-2221-E-007-026-MY3,
and Grant NSC-100-M-2112-007-007-MY3.
J.-W. Lin and C.-L. Pan are with the Department of Physics, National Tsing
Hua University, Hsinchu 30013, Taiwan (e-mail: d907913@oz.nthu.edu.tw;
clpan@phys.nthu.edu.tw).
C.-L. Lu, H.-P. Chuang, and C.-B. Huang are with the Institute of Photonics
Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan (e-mail:
vic770707@hotmail.com; s80164@hotmail.com; robin@ee.nthu.edu.tw).
F.-M. Kuo and J.-W. Shi are with the Department of Electrical
Engineering, National Central University, Zhong-li 32001, Taiwan (e-mail:
975201125@cc.ncu.edu.tw; jwshi@ee.ncu.edu.tw).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2012.2205914
temporal resolution than low-frequency microwave radars.
As a solution, photonically assisted techniques based on
optical pulse shaping have been proposed for microwave
radar [6], [7]. Photonic synthesis of chirped MMW waveform
in a spectral filter, followed by linear frequency-to-time
mapping (FTM) conversion in a dispersive medium, allows
one to obtain self-imaging optical waveforms, which are
subsequently transferred into electrical signals through a
high-speed photodiode (PD). Recently, a high TBWP of ∼90
based on incoherent source processing has been proposed
by using nonlinear dispersive elements [8]. However, due to
the limited operating bandwidth imposed by the photonic
processing and available photodiode, the central frequencies
are typically less than few tens of GHz. In order to push
the carrier frequency over MMW, we recently reported a
photonic scheme with a W-band (75–110 GHz) photonic
transmitter that allows both chirped MMW generation and
ultrahigh TBWP by a heterodyne-beating using two lasers [9].
Nevertheless, this system is restricted to ∼kHz repetition
rate because of the laser sweeping time. Moreover, direct
detection of chirped MMW signals is still out of reach due to
the bandwidth limitation (∼60 GHz) of commercial real-time
scope.
In this letter, we propose and present a novel scheme where
the photonic synthesis is utilized to generate chirped MMW
pulses at W band. Coherent detection of such arbitrarily
structured MMW waveforms was performed using the
terahertz time-domain spectroscopy (THz-TDS) [10]. The
detection bandwidth is thus extended to the THz regime. This
allows us to directly measure the temporal waveforms and
extract the spectra of chirped MMW signals. Two examples
are carried out to demonstrate the full reconfigurability of our
approach. It shows promising results that can permit high-rate
applications such as high-speed radar scanning and imaging.
II. EXPERIMENTAL SETUP
Fig. 1 shows the experimental setup of the THz-TDS
system [11] for chirped MMW pulses generation and
detection. A mode-locked Er:doped fiber laser serves as a
high-rate (100 MHz) and broadband coherent source (∼150 fs,
∼30 nm bandwidth). Short pulses from one branch of laser
are sent into a reflective Fourier-transform (FT) pulse shaper
for spectral filtering [12]. A 640-pixel liquid crystal modulator
(LCM, CRI SLM-640-D-NM) is utilized that allows us to
impress arbitrary filter functions onto the optical spectrum.
1041–1135/$31.00 © 2012 IEEE

1438 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 16, AUGUST 15, 2012
Reflective FT
Pulse Shaper
EDFA PT
SMF
2
3
1Lensed fiber
Delay
Receiver
MLFL
Horn antenna
Coupler Circulato
r
60 80 100 120 140
10
-2
10
-1
10
0
MMW field (a.u.)
Fre
q
uenc
y(
GHz
)
~80 GHz
10 dB
Fig. 1. Experimental setup of THz-TDS system for chirped MMW pulse
generation and detection. Inset: Measured frequency response of our photonic
transmitter. PT: photonic transmitter.
The shaped spectrum then undergoes wavelength-to-time
mapping in a length of SMF. Note that the design of tailored
spectra in this approach is significantly different from our
previous work [11] that can not perform arbitrary waveform
synthesis. An erbium-doped fiber amplifier (EDFA) is used
after the SMF to provide adjustable optical power. The
dispersive pulses are then injected into a high-speed photonic
transmitter (PT) [13], which is composed of a high-power
PD and a planar antenna through a lensed-fiber.
The PD in our PT module is a high-power near-ballistic
uni-traveling-carrier photodiode (NBUTC-PD) module with a
high saturation current-bandwidth product. The whole module
is flip-chip bonding packaged on an aluminum-nitride (AlN)
substrate with good thermal conductivity for high-power
operation. To detect the MMW signals, we use a broadband
InGaAs photoconductive dipole antenna (TERA15-DP25,
Menlo Systems). The measured frequency response of our
PT is shown in the inset of Fig. 1. It can be clearly seen that
the designed resonant frequency of the PT is around 80 GHz
and the 10 dB bandwidth is ∼80 GHz (60–140 GHz).
III. RESULTS AND DISCUSSION
We now present several representative waveforms generated
in our system. In the first example, a linearly chirped MMW
pulse is demonstrated, which is commonly used in modern
radar systems. The spectra of the tailored optical pulses are
centered at 1550 nm with a spectral bandwidth of ∼40 nm.
A 200 m-long single-mode-fiber (SMF) is employed. The
bandwidth and the total chromatic dispersion of the fiber
stretcher (∼3.4 ps/nm) enable a time aperture of approximately
136 ps for our MMW waveforms. As a reference, Fig. 2(a) and
(b) shows the filtered optical spectrum and the corresponding
MMW signal for generation of a burst sinusoidal waveform,
respectively. The spectrum is shaped with a constant period
of ∼3.6 nm. After dispersive stretching the measured MMW
signal exhibit a quasi-sinusoidal waveform with a slow-varying
envelope. The instantaneous frequency of the generated MMW
pulse is also plotted in Fig. 2(b) in circles, which is obtained
by calculating the reciprocal of the period for each cycle.
A linear curve fitting shows that the oscillating frequency
is ∼82 GHz, which agrees quite well with the predicted
value. To demonstrate the ability to modulate our waveforms,
0 50 100 150 200
0.0
Time (ps)
60
80
100
120
Amplitude (a.u.)
Fre
q
uenc
y(
GHz
)
1530 1540 1550 1560 1570
0.0
0.5
1.0
Intensity (a.u.)
Wavelen
g
th
(
nm
)
(a)
(b)
(c)
(d)
0 50 100 150 200
0.0
Time (ps)
Amplitude (a.u.)
60
80
100
120
Fre
q
uenc
y(
GHz
)
1530 1540 1550 1560 1570
0.0
0.5
1.0
Intensity (a.u.)
Wavelen
g
th
(
nm
)
Fig. 2. 82 GHz sinusoidal burst. (a) Filtered optical spectrum with a period
of ∼3.6 nm. (b) ∼82 GHz MMW signal and linearly frequency-modulated
waveform. (c) Filtered optical spectrum with linearly increasing periods from
2.35 to 4.83 nm. (d) Linearly chirped MMW pulse.
Fig. 2(c) and (d) shows an example of a broadband linearly
frequency-modulated waveform. The filtered spectrum in
Fig. 2(c) has been patterned to exhibit a sinusoidal intensity
modulation with linearly increasing periods. The periods are
selected to be from 2.35 nm to 4.83 nm within 11 cycles. As
can be seen in Fig. 2(d), the measured MMW signal shows a
linearly chirped waveform sweeping from 117 GHz to 68 GHz
in the main pulse lobe, with an equivalent frequency chirp rate
of 41.7 GHz/100 ps. Note that the frequency range is slightly
lower than the theoretically predicted values (125–61 GHz)
due to the Gaussian-like spectrum and the finite bandwidth
of our PT. A TBWP of ∼6.6 is obtained for the generated
chirped MMW pulse.
To further illustrate the capability to generate higher TBWP
MMW waveforms in our system, we decrease the periods of
the spectral modulation to accommodate more MMW cycles
in one pulse. Limited by the spectral resolution of our pulse
shaper, we only demonstrate a 3-step chirped MMW pulse
in this letter. Step-chirped microwave pulses have also been
widely used in modern radar for its high range resolution
without imposing large instantaneous bandwidth on the radar
system [14]. Again, an 80 GHz MMW burst sinusoidal wave-
form (not shown) is first generated by reducing the spectral
period to ∼0.74 nm in combination with a 1-km-long SMF.
The time aperture is increased to ∼425 ps. Figure 3 shows
an example of a broadband frequency-modulated waveform
composed of ∼120/80/60 GHz cycles. The filtered spectrum
in Fig. 3(a) is patterned to exhibit 3-step sinusoidal intensity
modulation with 12 cycles each in period from ∼0.5 nm to
∼0.74 nm, to ∼1 nm. Note that the spectral envelope has been
modified as a square-like pulse for MMW power equalization.
The amplitude of intensity modulation at 80 GHz is reduced
by a factor of 2 to 3 to compensate for the frequency response
of our PT, which peaks around 80 GHz. As can be seen
in Fig. 3(b), the measured MMW waveform has an abrupt
frequency step from 120 GHz to 80 GHz within the main
pulse lobe. Again, the amplitude and frequency fluctuation
during the transition is due to the finite bandwidth of our
PT and can be alleviated through smoothly varying frequency
modulation. For 60 GHz regime, the amplitude and frequency
display strong oscillatory behavior. It might be attributed to

LIN et al.: PHOTONIC GENERATION AND DETECTION OF W-BAND CHIRPED MMW PULSES FOR RADAR 1439
1530 1535 1540 1545 1550 1555 1560
0.0
0.5
1.0
Intensity (a.u.)
Wavelength (nm)
-300 -200 -100 0 100 200 300
-1
0
1
Frequency (GHz)
Amplitude (a.u)
Time (ps)
0
80
160
(a)
(b)
Fig. 3. Step-chirped MMW pulse generation. (a) Filtered optical spectrum
composed of three periods of ∼0.5/0.74/1 nm. (b) Corresponding MMW
signal.
60 80 100 120 140
0
1
Normalized Amplitude
Frequency (GHz)
60
80
120
Fig. 4. Broadband MMW field spectrum showing three main peaks at ∼120,
∼80, and ∼60 GHz (solid line). Dashed line: 80 GHz sinusoidal burst. Dotted
line: Frequency response of PT.
the interference between the residual 80 GHz and 120 GHz
signals, which is caused by the echoes in our THz receiver.
It should be noted that the 60 GHz signal still exists in the
MMW waveform. Figure 4 shows the FT spectrum (the solid
curve) of the step-chirped MMW pulse. Three main peaks
centered at ∼120/80/60 GHz are clearly seen. For comparison,
the frequency response (the dotted curve) of our PT and that
for an 80 GHz quasi-sinusoidal MMW (the dashed curve) is
also shown in Fig. 4. The weakest 60 GHz MMW signal
is due to the fact that such frequency is around the cut-
off frequency (59.01 GHz) of the WR-10 waveguide, which
could be compensated through additional power equaliza-
tion. The field strength of MMW pulses is estimated to be
∼30 V/cm. Our results thus indicate the feasibility
of frequency-sweeping MMW radar with bandwidth of
∼60 GHz, leading to a TBWP of ∼25 for the generation of
step-chirped MMW pulses. The maximal TBWP is limited by
the spectral resolution of the pulse shaper, the total number
of pixels in the LCM, as well as the available bandwidth
of the PT. Compared with incoherent photonic processing,
the present highly coherent system is attractive because of
its superior signal-to-noise ratio (>1000 in one scan) without
distortions induced by the electric modulator. In addition, this
approach allows full reconfigurability of a pulse in terms of
central frequency, step level and chirp rate by the proper design
of the spectral filtering pattern. With a large amount of
modulation cycles in the optical spectrum, a large TBWP can
be expected.
IV. CONCLUSION
Based on the frequency-to-time mapping (FTM) approach,
we generate frequency-modulated MMW pulses with cen-
tral frequencies up to the W-band by shaped optical pulse
excitation of a MMW photonic transmitter (PT) with an
ultrawide band photodiode as its key component. Coherent
detection was achieved via a terahertz time-domain spectro-
scopic setup. Through appropriate optical spectral design, the
frequency-chirped MMW pulses with instantaneous frequen-
cies sweeping from 120 GHz to 60 GHz and a time-bandwidth
product (TBWP) of ∼25 is experimentally demonstrated. Our
approach provides a potential solution for the MMW radars
with reconfigurable chirp control, which can be used in a wide
range of commercial, military and scientific applications.
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