PHOTONIC GENERATION OF BINARY DIFFERENTIAL
PHASE-CODED MICROWAVE SIGNALS
Fangzheng Zhang, Xiaozhong Ge and Shilong Pan*
Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing
University of Aeronautics and Astronautics, Nanjing 210016, China
*e-mail address: [email protected]
ABSTRACT
A scheme for photonic generation of binary differential
phase-coded microwave signals is proposed and
demonstrated. In the proposed system, two optical
sidebands are generated and phase modulated by an
electrical signal. Then, the two optical sidebands are
separated by a Mach-Zehnder interferometer (MZI) and
properly delayed between each other before combined at
an optical coupler. By balanced photodetection after the
optical coupler, a binary differential phase-coded
microwave signal is generated. The carrier frequency of
the generated signal is doubled compared with the signal
source in the system, thus the requirement for high
frequency electrical devices is relaxed. The proposed
system is the first microwave photonic signal generator
with differential phase coding, which can find
applications in radar and communication systems.
Keywords: microwave photonics, differential phase
coding, Mach-Zehnder interferometer (MZI).
1. INTRODUCTION
Phase-coded microwave signals are widely used in radar
and communication systems, e.g., to improve the range
resolution by pulse compression in a radar system, phase
coding of the launched radio frequency (RF) signal is
usually applied [1]. Conventionally, the phase-coded
microwave signals are generated in the electrical domain,
which suffers from limited bandwidth and severe
electromagnetic interference. To solve these problems,
photonic generation of phase-coded microwave signals
was proposed and has drawn a lot of attentions because
of the advantages such as large bandwidth, low cost and
low loss, etc. [2]. Many schemes have been proposed to
generate phase-coded microwave signals [3-6]. For
example, a phase-coded microwave signal can be
generated based on optical spectral shaping using a
spatial light modulator (SLM) followed by frequency to
time mapping [3]. This approach is flexible, but the
system is complex and lossy due to bulky spatial optical
devices. Besides, phase-coded microwave signals can
also be generated by introducing a phase difference
between two optical sidebands before heterodyning at a
photodetector (PD) [4, 5].
A problem with the reported photonic microwave
signal generation schemes is that, to realize differential
phase coding, an electrical pre-coding module must be
used, which adds up to the cost and complexity.
Considering that differential phase coding is required in
many cases, it is highly desirable to directly generate
differential phase-coded microwave signals by optical
methods. In this report, for the first time to the best of
our knowledge, we propose a scheme for photonic
generation of a binary differential phase-coded
microwave signal. In addition to the differential phase
coding capability, the requirement for high frequency
electrical devices is relaxed because the carrier
frequency of the generated signal is doubled compared
with the RF source in the system.
2. OPERATION PRINCIPLE
BPD
TLSPC1
MZIPC2
PBS
PolM PM
Delay
OC
RF
Binary
Phase
coded
Microwave
signal
fc-fRF fc+fRFfc
fc-fRF
fc+fRF fc+fRF
fc-fRF
fRF
τ=1/B
B bit/s
Fig 1. Schematic diagram of the proposed differential phase-coded microwave signal generator. TLS: tunable laser source; PC:
polarization controller; PolM: polarization modulator; PM: phase
modulator; MZI: Mach-Zehnder Interferometer; OC: optical coupler; BPD: balanced photo-detector; RF: radio frequency.
Fig. 1 shows the schematic diagram of the proposed
phase-coded microwave signal generator. A continuous
wave (CW) light is generated by a tunable laser source
(TLS). The polarization state of the CW light is adjusted
by tuning a polarization controller (PC1) to have an
angle of 45° to one principal axis of the following
polarization modulator (PolM), which is driven by an RF
signal with a frequency of fRF. After properly setting the
polarization state of the output signal from the PolM by
tuning PC2, optical carrier-suppressed modulation can
be achieved after the polarization beam splitter (PBS) [7].
The obtained signal with two first-order sidebands can
be expressed as
1( ) exp[ 2 ( ) ]
exp[ 2 ( ) ]
c RF
c RF
E t A j f f t
A j f f t
π
π
= +
+ − (1)
2015 14th International Conference on Optical Communications and Networks (ICOCN) @ Nanjing, China
978-1-4673-7373-9/15/$31.00 ©2015 IEEE
where fc is the frequency of the optical carrier and A is
the amplitude of each optical sideband. The frequency of
the two first-order sidebands is fc+fRF and fc-fRF,
respectively. Then, the two sidebands are modulated
by a phase modulator (PM). The PM is driven by an
electrical coding signal with a bit rate of B bit/s, and
the phase modulation depth is π. The phase modulated
optical signal is
2 ( ) exp[ 2 ( ) ( )]
exp[ 2 ( ) ( )]
0 '0 '( )=
'1'
c RF
c RF
E t A j f f t t
A j f f t t
bitt
bit
π ϕ
π ϕ
ϕπ
= + +
+ − +
=
=
(2)
where φ(t) is the phase term caused by phase modulation.
Following the PM, the signal is sent to a Mach-Zehnder
interferometer (MZI) which serves as a comb filter. The
MZI has a time delay of ∆t=1/(4fRF) between its two
arms and thus the free spectral range (FSR) is 4fRF [8].
By tuning the frequency of the optical carrier, the
+1st-order sideband at fc+fRF gets out of the MZI at
output port1 while the -1st-order sideband at fc-fRF goes
through MZI by port2. Thus, the two first-order
sidebands are separated and the obtained two optical
signals are
3,4 exp[ 2 ( ) ( )]c RFE A j f f t tπ ϕ= ± + (3)
Then, a time delay of τ=1/B is introduced between the
two signals. After that, they are combined and interfere
with each other at an optical coupler (OC). The optical
fields at the two output ports of the OC are
5,6 3 4
2[ ( ) ( )]
2E E t E t τ= ± − (4)
When E5 and E5 are sent to a balanced photodetector
(BPD), two electrical currents are obtained as 2 2
1,2 5,6( ) {1 cos[2 2 ( )]}RFi t E A f t tα α π ϕ= = ± × + ∆
(5)
where α is the responsivity of each PD, and
∆φ(t)=φ(t)-φ(t-τ) is the phase difference between two
adjacent bit slots. At the output of the BPD, the DC
components in (7) and (8) are moved, and the obtained
microwave is given by
2
1 2( ) ( ) ( ) 2 cos[2 2 ( )]
RFi t i t i t A f t tα π ϕ= − = × + ∆
(6)
As can be seen from (6), a binary phase coded
microwave signal with a carrier frequency of 2fRF is
generated, of which the phase is determined by the two
adjacent bit slots of the electrical signal driving the PM,
i.e., differential phase coding is realized.
3. SIMULATION AND EXPERIMENT
To verify the feasibility of the proposed differential
phase-coded microwave signal generator, a simulation is
performed based on the setup in Fig. 1 using the
Optiwave Software (Optisystem 12.0). The frequency of
the RF signal driving the PolM is set to 8.6 GHz. An
electrical non-return-to-zero (NRZ) signal with a bit rate
of 0.5 Gbit/s and a pattern of “1100” is applied to the
PM, of which the half-wave voltage is 3 V. To achieve a
phase modulation depth of π, the amplitude of the NRZ
signal is set to 3 V and its waveform is shown in Fig.
2(a). Fig. 2(b) shows the waveform of the generated
phase-coded microwave signal. The carrier frequency of
the obtained signal is 17.2 GHz which is doubled
compared with that of the RF signal driving the PolM.
The phase information is extracted based on the Hilbert
transformation, as shown in Fig. 2(c). As can be seen,
the recovered phase is zero when two adjacent bits of the
signal in Fig. 2(a) are the same, while the phase is π if
the two adjacent bits are different, indicating differential
phase coding is achieved.
Fig 2. Simulation results for (a) the electrical coding signal, (b) the
generated phase-coded microwave signal, and (c) the recovered phase
profile.
Fig 3. Measured optical spectra at (a) output of the PBS, (b) output of the PM, (c) output of the MZI (the solid line and dotted line represent
different output ports), and (d) output of the OC.
To further investigate the performance of the proposed
signal generator, an experiment is carried out based on
the setup in Fig. 1. The TLS (Agilent N7714A) has an
output power of 16 dBm and its frequency can be tuned
by a step of 0.1 GHz. The PolM (Versawave Inc.) has a
bandwidth of 40 GHz and a half-wave voltage of 3.5 V
at 1 GHz, which is driven by an RF source (Agilent
E8267D). The MZI is made by splicing two 3-dB optical
couplers. By controlling the time delay between the two
arms, a specific FSR can be obtained. In the experiment,
the optical spectrum is monitored by an optical spectrum
analyzer (OSA) with a resolution of 0.02 nm. The
temporal waveform is observed through a real time
oscilloscope with a sampling rate of 80 GSa/s.
Encoding
signal
Phase
Amplitude
2015 14th International Conference on Optical Communications and Networks (ICOCN) @ Nanjing, China
978-1-4673-7373-9/15/$31.00 ©2015 IEEE
Fig 4. (a) Waveform of the 0.518 Gbit/s phase-coded 17.2 GHz RF signal in a period of 25.1 ns, (b) the electrical coding signal, (c) the
recovered phase profile, and (d) autocorrelation of the generated signal.
In the experiment, the frequency of the RF signal
driving the PolM is 8.6 GHz. After properly tuning the
polarization state before the PBS, carrier-suppressed
modulation is achieved. The optical spectrum is shown
in Fig. 3(a), where the optical carrier is well suppressed
and a frequency spacing of 17.2 GHz is observed
between the two optical sidebands. The electrical signal
generated by a pulse pattern generator (PPG) has a bit
rate of 0.518 Gbit/s with a pattern of “0000011001010”.
This signal is properly amplified by an electrical
amplifier to achieve a phase modulation depth of π when
applied to the PM. Fig. 3(b) shows the optical spectrum
after the PM. The MZI has a time delay of ~29 ps and an
FSR of 34.4 GHz. By properly tuning the wavelength of
the TLS, the two optical sidebands can be separated by
the MZI, as shown by the solid and dotted curves in Fig.
3(c), respectively. After introducing a time delay of
~1.93 ns between the separated two sidebands by a span
of fiber, the two optical sidebands are recombined at an
OC. The spectrum of the output signal from the OC is
given in Fig. 3(d). After balanced photon detection, a
phase-coded microwave signal is generated.
Fig. 4(a) shows the waveform of the generated 17.2
GHz microwave signal in a period of 25.1 ns. Fig. 4(b)
shows the electrical NRZ signal applied to the PM. The
phase profile of the generated signal is shown in Fig.
4(c), where two phase levels with a difference of 180°
between each other are observed. By comparing the
recovered phase with the electrical NRZ signal in Fig.
4(b), it is found that differential phase coding is
successfully implemented. Since the microwave signal is
generated by optical heterodyning, and due to the
imperfect coupling ratio of the OC, slight amplitude
fluctuation appears as the phase changes in Fig. 4(a). To
check the pulse compression capability, autocorrelation
of the generated microwave signal is calculated with the
result shown in Fig. 4(d). The full width at half
maximum of the compressed pulse is 2.08 ns,
corresponding to a compression ratio of 12.1, and the
peak-to-side lobe ratio (PSR) is 5.93 dB.
4. CONCLUSION
We have proposed a photonic approach to generating
a binary phase-coded microwave signal. Differential
phase coding without pre-coding was achieved which
has not been demonstrated in the previously reported
schemes, and the requirement for high frequency
electrical devices is relaxed. Simulation and experiment
results well verified the feasibility and good performance
of the proposed system, which can be used in future
radar and communication systems.
ACKNOWLEDGMENTS
This work was supported in part by the NSFC Program
(61401201, 61422108), the NSFC Program of Jiangsu
Province (BK20140822, BK2012031), the Open Fund of
IPOC (2013B003), the Postdoctoral Science Foundation of
China (2014M550290), the Jiangsu Planned Projects for
Postdoctoral Research Funds (1302074B), the Fundamental
Research Funds for Central Universities, and a project
funded by the priority academic program development of
Jiangsu higher education institutions.
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2015 14th International Conference on Optical Communications and Networks (ICOCN) @ Nanjing, China
978-1-4673-7373-9/15/$31.00 ©2015 IEEE
