Proceedings of IC-NIDC2010
IMPROVEMENT OF PERFORMANCE IN 40 GB/S RZDQPSK TRANSMISSION SYSTEM WITH
ALTERNATE-POLARIZATION
Lujiao Li, Yaojun Qiao, Yuefeng Ji
Key Laboratory of Information Photonics and Optical Communications(IPOC), Ministry of Education(MOE),
Beijing University of Posts and Telecommunications(BUPT), Beijing 100876, P.R. China
jiaoer10@163.com, qiao@bupt.edu.cn, jyf@bupt.edu.cn
Abstract
In this paper, we show that, for the first time to the
best of our knowledge, the alternate polarization of
adjacent symbols in 40Gbit/s RZ-DQPSK
transmission system can improve significantly
system performance through suppression of intrachannel nonlinear effects. Simulation results show
that about 4dB reduction of the non-linear threshold
(NLT) and 2dB increase of the maximum Q factor
for all transmission distances greater than 1200km,
respectively, by using alternate polarization (APol)
RZ-DQPSK compared to the standard RZ-DQPSK.
Keywords: alternate-polarization; DQPSK; 40
Gb/s; inter-channel nonlinear
1 Introduction
The ever increasing demand for bandwidth
constantly drives researchers to upgrade the
capacity of WDM transport systems. This requires
increased bit rate per wavelength up to 40 Gb/s or
more. In high bit rates, such as 40 Gb/s and beyond,
transmission is mostly in the pseudo linear regime.
In this regime, intra-channel nonlinear effects are
generally the dominating degradation limiting
transmission reach [1]. Due to the high chromatic
dispersion of SSMF, 40 Gb/s signals transmit in a
pulse-overlapped regime. The major nonlinear
distortion comes from intra-channel four-wave
mixing (IFWM), which generates echo pulses and
pulse amplitude distortion, and intra-channel crossphase modulation (IXPM) which produces timing
jitter by the fiber dispersion [2].
Modulation formats employing bit-to-bit alternate
polarization (APol), in which adjacent bits have
orthogonal
polarizations,
have
attracted
considerable attention, because of its robustness
against nonlinear impairment. In principle,
compared to that produced by co-polarization,
FWM between two orthogonally polarized signals
is significantly suppressed, and the effect of XPM
between the two orthogonally polarizations is
reduced to one half. The APol between adjacent
bits has been shown to effectively overcome
limitations arised from intra-channel nonlinearities
in OOK systems [2-4]. In recently reported
researches, computer simulations and experiments
have shown that this advantage also adapt to DPSK
systems [5-7]. The return-to-zero (RZ) differential
quadrature phase shift keying (DQPSK) modulation
format has been attracting considerable interest in
high-speed channel transmissions, owing to its
transmission line rates at twice the baud rates, and
because of its narrow signal spectrum, high spectral
efficiency (SE) can be realized which, in turn,
increases the total capacity of WDM transmission
systems. Moreover, it has high tolerance to intrachannel nonlinear effects such as XPM and FWM
[8-9].
In this paper, we combine symbol-to-symbol
alternate polarization with DQPSK modulation
format. It is shown numerically, for the first time,
that applying APol to RZ-DQPSK is highly
effective in suppressing the intra-channel nonlinear
distortions at 40Gbit/s system.
2 Implementation of APol-DQPSK
2.1 APol-DQPSK data encoding and decoding
(a)
(b)
Fig.1. (a) Schematic of differential precoder for
APol-DQPSK. (b) APol-DQPSK signal.
In the APol-DQPSK, DQPSK data is firstly
precoded using a precoder, as is shown in Fig. 1(a).
Each tributary signal (in-phase I or quadrature Q)
has a delayed feedback loop. Delay time is two bit
periods, while one bit in the conventional DQPSK,
to ensure the same polarization of the two
1
interfering bits in the differential encoder. The
similar delay set was described in the APol-DPSK
[6]. At the receiver, differential detection is
performed, and one symbol that corresponds to two
bits delay interferometer (DI) is used to demodulate
either I tributary signal or Q tributary signal in the
conventional DQPSK. In contrast, a two symbols
(or four bits) delay should be adopted so that copolarized, instead of being orthogonally polarized,
symbols could be compared.
2.2 The APol-RZ-DQPSK signals generation
The signal generation of APol-RZ-DQPSK is
shown in Fig. 2(a). DQPSK signal is obtained first
through the use of an integrated LiNbO3 MachZehnder modulator (MZM) that is driven by 20
Gb/s in-phase and quadrature drive signals. The
nested MZM adopts push-pull operation on both
sub-MZMs. An additional pulse carver with a 20
GHz clock signal converts the NRZ-DQPSK signal
to 50% duty cycle RZ-DQPSK. In order to realize
the symbol-to-symbol APol-RZ-DQPSK signal, a
polarization modulator (Fig. 1(b) of Ref. [3]) is put
after the data modulator to perform polarization
modulation. Some part of phase and polarization
information of APol-RZ-DQPSK signal is
illustrated in Fig. 1(b).
transmitted. An Erbium-doped fiber amplifier
(EDFA) is used to adjust the loop launch power.
The transmission loop contains N spans of SSMF,
and in each span DCF is used to fully compensate
for chromatic dispersion. The parameters of SSMF
and DCF are shown in Table 1. The effect of PMD
is not considered in these simulations due to the
intrinsic lower PMD tolerance of all APol formats
[10]. Two EDFAs with 6 dB noise figure are used
to compensate the fiber loss as well as to control
the SSMF and DCF launch power. To produce
sufficient nonlinear distortions an equal input
power is used for both the SSMF and DCF fiber
[11]. At the receiver, the I and Q components of the
APol-RZ-DQPSK signal are demodulated using a 2
symbols (4 bits) delay interferometer. The DI is
followed by an integrated pair of balanced photodetector. After detection, the error rate analysis is
carried out.
Table 1. Fiber parameters.
SSMF
DCF
Fiber attenuation (dB/km)
0.2
0.5
Dispersion (ps/nm/km)
16
-80
Nonlinear refractive index
1.3
5.2
80
16
80
35
-20 2
(10 m /W)
Length (km)
2
Effective core area (um )
4 Simulation Result
Fig.2. 40 Gb/s APol-RZ-DQPSK signal generator.
-1
3 Simulation Setup
-2
The performance of the 40 Gb/s APol-RZ-DQPSK
system is investigated using numerical simulations
by VPIsystems’ VPItransmissionMaker™ WDM
V7.6.
-4
log10 (BER)
-3
-5
-6
-7
2800km w/ APol
2800km w/o APol
2400km w/ APol
2400km w/o APol
2000km w/ APol
2000km w/o APol
-8
-9
-10
-11
-9
-7
-5
-3
-1
Pin (dBm)
1
3
5
Fig. 4. BER versus launch power for 40 Gb/s RZDQPSK transmission with and without APol.
Fig. 3. Simulation setup. (EDFA: Erbium-doped
fiber amplifier, DCF: dispersion compensation fiber,
DI: delay interferometer, PD: photo-detector,
BERT: bit error ratio test)
The
schematic
of the
APol-RZ-DQPSK
transmission system is shown in Fig. 3. In the
transmitter, the APol-RZ-DQPSK signal with a
pseudo-random data sequence of length 216 bits is
Fig. 4 shows the simulated BER of different
transmission distances (2000 km, 2400 km, 2800
km) versus launch power for 40 Gb/s RZ-DQPSK
transmission with and without alternate polarization.
For lower input powers, the system is nearly linear,
and the system performance with and without
alternate polarization is similar. However, as launch
power increases, a significantly reduction of BER is
observed with alternate polarization. Since at high
input powers the system is limited by intra-channel
2
distortions, however, RZ-DQPSK with alternate
polarization, in which adjacent symbols have
orthogonal polarization, can apparently decrease
the intra-channel nonlinear impairments.
Fig. 5 shows the minimum BER and nonlinear
threshold (NLT) versus the transmission distance
for 40 Gb/s transmission system. The NLT, defined
as the launch power that gives a BER<10-3. In Fig.
5, the minimum BER is obtained by adopting the
best input power at each distance. For every
transmission length, with APol offers about 2 dB
performance improvement, and 4 dB increases of
NLT. The increased NLT can lead to longer
transmission reach.
residual dispersion on the BER of 40 Gb/s RZDQPSK after 1600 km transmission is depicted in
Fig. 6. As seen in the figure, for lower Dres, both
with and without alternate polarization have worse
performance since at low Dres, the pulse spreading
will be smaller and the interaction of the stronger
self–phase modulation (SPM) and optical
amplifiers noise generates larger GordonMollenauer effect [12]; for high Dres, the system
performance is also worse, since in this case, the
pulse spreading will be increased and the effects of
IFWM and IXPM become larger. The best BER
performance is obtained at Dres=30 ps/nm (of the
three different Dres). Fig.6 also shows that the APolRZ-DQPSK performance is better than the standard
RZ-DQPSK for different residual dispersion. We
also note that the performance of Dres=30 ps/nm and
60 ps/nm is almost the same for the APol-RZDQPSK signal. The probable explanation is that
both of the two Dres can effectively suppress SPM,
andmeanwhile, the penalties of IFWM and IXPM
by the different broadened pulse of different Dres
are efficiently reduced using the symbol-to-symbol
alternate polarization.
5 Conclusions
Fig. 5. Minimum BER and NLT versus
transmission distance for 40 Gb/s transmission with
and without APol.
-1
Dres=0ps/nm w/ APol
Dres=0ps/nm w/o APol
Dres=30ps/nm w/ APol
Dres=30ps/nm w/o APol
Dres=60ps/nm w/ APol
Dres=60ps/nm w/o APol
-2
-3
log10 (BER)
-4
We demonstrated numerically, for the first time, the
use of alternate polarization to suppress the
dominant intra-channel nonlinear impairments in a
40 Gb/s RZ-DQPSK transmission system. The
results show that the system performance can be
significantly improved by applying alternate
polarization: about 2 dB and 4 dB increase in the Q
factor and NLT, respectively.
6 Acknowledgements
-5
-6
This work was supported in part by NSFC
(60932004),
National
863
Program
(2009AA01Z256, 2009AA01A345), National 973
Program (2007CB310705), and the SRFDP of
MOE (200800130001), P. R. China.
-7
-8
-9
-10
-11
-9
-7
-5
-3
-1
1
3
5
Pin (dBm)
Fig. 6. Measured BER of different residual
dispersion versus launch power for 40 Gb/s
transmission with and without APol at 1600 km.
As a comparison, two different dispersion maps
were used in the recirculation loop: (i) inline
dispersion fully compensation, namely, residual
dispersion (Dres) =0 ps/nm, no dispersion precompensation and post-compensation, and (ii)
inline dispersion partially compensation. In this
case, a dispersion compensation fiber (DCF)
provides dispersion pre-compensation of -500
ps/nm,
and
DCF-based
dispersion
postcompensation was applied to make the net
dispersion of the entire system zero (Dres=30 ps/nm
and 60 ps/nm in Fig. 6). The effect of different
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