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TuF3
Nonlinear tolerance of differential phase sh ift keying
modulated signals reduced by XPM
B. Spinnler
Siemens AG, C o p r o l e Technology, Otlo-Hohn-Rins 6.D-81739 Munich Germany
€-mil:
emho rdSp in l e~s i emen r . com
N.
Hecker-Denschlag,
S.
Calabrb, M. H e n , C.J . Weiske, E.-D. Schm idt
Siemenr AG, ICNOpricol Solut ionr,Hofmmm ~rasse 1, D-81359 Munich Gennony
D. van de n Borne, G.-D. Kho e, H. de Waard t
R Griffim,
S.
Wadsworth,
Bookhilm Technology PLC .,Cowell. Tmvcesier.Norihonls, “ 12 8EQ England
Abstract: We show
that
in order to maintain the high nonlinear tolerance of the DQPSK
modulation format, XPM from neighboring 00K-modulated channels must be avoided. The
negative imp act on IOGb/s DQPSK ch annels is higher than at 2OGb/s.
0 2003 Optical Societyof America
OClS codes:
(060.4510)optical communications,(060.5060) hase dd s t i o n
COBRA Imtilufe.
Eindhoven
Universiv of Technology, 7‘he Nerherlondr
1. Introduction
Advanced phase modulation formats have been very successful in extending the reach and capacity of ultra long
haul, high capac ity, optical transmission systems [1,2]. T his success is not only due
to
the use of balanced receivers
for a gain of 3dE at the receiver, but also that D(Q)PSK modu lation is less sensitive to non-optimum dispersion
com pensation [3] and also allows for a larger input power before nonlinear effects degrade the signal quality [4,5].
While the high nonlinear tolerance of systems running solely with D(Q)PSK can be exploited for extending the
reach of high capacity systems
or
increasing amplifier spacing, the effect of mixed usage, i.e. combinin g OOK and
D(Q)PSK modulation formats at different wavelengths, on the individual channel tolerances still needs to be
determined. For instance mixed usage can occur when installed systems are upgraded with new m odulation formats
or when meshed networks come into operation. In such scenarios, wavelength channels with both D(Q)PSK and
OOK modulation formats can become nearest neighbors and the nonlinear interactions must be understood. The
rohusmess of binary DPSK in presence
of
NRZ modulated channels with
100
GH z spacing for transmission over
SSMF has been discussed in [6]. n this paper we discuss the reduction of the no nlinear tolerance of NRZ-DQ PSK
because of
XPM
kom NRZ-O OK neighboring channels at a
50 GHz spacing.
50 GHz
spacing is used instead of 100
GHz
spacing as it represents the standard today for OOK systems and is more likely to be limited by XF’M effects.
2. Experimental se tup
The two exp erimental setups
shown
in
fig.
l a and l b were used
to
compare the transmission performance of five
wavelength channels on a 50
GHz
grid over
100km SSh4F.
The m iddle chan nel was always modulated with a 20
Gb/s DQPSK (IO GsymboM). In the first setup (fig. la) the four remaining channels were also 20 Gbis DQPSK
modulated. In the second setup (fig. Ib) these channels were 10 Gb/s OOK modulated. We chose half the bit rate for
the OOK chann els in order to have similar spectral width of the two types of formats. Therefore, this bit rate seem s a
“natural” choice con sidering IO Gb/s OOK systems in operation today. The DQPSK m odulation was achieved with a
GaAs-based modulator [4] which is com posed of two M ach-Zehnder interferometers (MZI) offset biased around the
zero transmission point so that with a
10
Gb/s m odulation, a phase difference between bits of
II
is obtained. Each of
the
MZI
are mod ulated with the same pseudo-random bit seq uence with a length 2”-1 , but a delay of 22 bits between
the two allows for a decorrelation of the bits, since a precoder
was
unavailable. The two
MZIs are
combined with an
added d 2 phase shifter in a ‘super’
MZI
structnre. We used a tunable dispersion compensator in order to optimize
performance. Full com pensation was foun d to be optimal for DQPSK. The total input power of the five wavelength
channels was varied before the transmission fiber to study the effect of the XPM. At the end of line, we used a 0.2
nm passband filter to extract the channel under consideration. Differential demodulation was performed by an optical
one-symbol delayed
MZI.
The two
outputs
of the demodulator were differentiallydetected with a balanced receiver
and a limiting differential amplifier.In order
to
measure a
BER,
the receiver is given the corresponding bit sequence
TuF3
for detection of either the I or Q channel. At the output of the differential amplifier, a noise loading experiment was
performed to determine the BERs of the middle channel
as
a function of the per channel power and
OSNR.
In the mixed modulation setup we additionally adjusted the polarization of the neighbo ring channels
so
that they
were either parallel or orthogonal to the middle DQPSK chann el under consideration in order to better observe the
XPM effect. In the DQPSK-only setup all channels had an identical polarization. All channels have equal power
independent of the modulation format.
Fe. . Experimental top with a) 5 DQPSK channels,
b)
one DQPSK
channel sa d 4
OOK haoarls
3.
Results
In
order to assess the effect of XPM on the middle DQPSK channel, we made measurements and simulations for
both systems shown in fig 1.We evaluated the BER of the DQPSK channel
for
various values of the transmitted
power. Fig.
2
shows the measured BER ersus OSNR for total input powers rang ing from
7
o 18.5 dBm. The figures
in the fist row show results obtained by measurements, the figures in the second row show the corresponding
simulation results. The figures in the left column (a and d) present the BER for the system with five DQPSK
channels, the figures in the m iddle column (b and e) show the BER or the system with one DQ PSK channel and four
OOK neighbors (all polarizations aligned parallel), and the figures in the right column (c and 9 how the re sults for
the system with one DQ PSK chann el and four OOK neighbors (polarizations of the
OOK
channels orthogonal to that
of the middle DQPSK channel).
Le t us first discuss the measurement results. It can be see n that in all cases the higher the launch power the more
the performance is degraded by XPM from the neighboring channels. However, the results differ widely if we
compare them q uantitatively. When we in crease the launch power
from 7
o 16 dBm the
OSNR
penalty for the
DQPSK -only system is about
2.5
dB at BER= If the po larization of the OOK neighboring channe ls is parallel to
the DQPSK ch annel the penalty is 2 dB for 7 Bm launch power and the penalty for 16 dBm launch power could not
be determined at because the signal is significantly degraded. When the polarization of the OOK neighboring
channels is orthogonal to the DQPSK channel, the performance is equal to that of the DQPSK-only system for
7
dBm launch power, and the OSNR penalty for 16 dBm launch power is about 6dB.
The correspond ing simulation results are shown in the second row of fig.
2.
BE& above 10.’ were measured
directly using Monte-Carlo simulation. For lower BERs we employed the tail extrapolation technique. While the
results differ quantitatively h m he measurements because we did not include all relevant impairments into the
simulation
(e.g. in the simulation we did not include laser phase noise and used ideal
filters,
delmodulators and clock
recovey), the general trend
in
the measurements and the simulations is the same. We observe only a slight
degradation for an increase in the launch power when we use DQPSK fo r the neighbo ring channels. If we use OOK
with parallel polarization with respect to the middle DQPSK chann el for the neighbo ring channels, the degrad ation is
dramatic. Ifwe use OOK with orthogo nal polarization with respect to the midd le DQPSK chan nel, the degradation is
still larger than in the DQPSK-only system, but far less than in the case with parallel polarized OOK neighbors.
These results
are
in agreement with our measurements and support our claim that the performance of DQPSK
depend s very much on the type of the m odulation format and polarization of the D QP SK s neighboring channels.
A prevalent measure to further enhance the dispersion tolerance of DQPSK is the reduction of the data rate. In
order to investigate the influence of the data rate we repeated the simulations corresponding to the setup shown in
fig. I h with the DQPSK data rate reduced
to 10 Ghls. The neighboring channels again use 10 Gbls OOK with the
same polarization (worst case). Fig. 3 com pares the simulation results for the cases
20
Gbls (fig. 3a) and 10 Gbls
(fig. 3b). While for low inp ut power the re is the usual 3
dB gain in favour of the 10 Gh/s system, we ohserve that for
higher input power the
‘IO
Gbls system performs even worse than the 20 Gh/s system. Hence, the XPM from OOK
neighbors is more detrimental for the 10 Gb/s channel than for the
20
Gbls chann el. This effect has to he taken into
account when an
OOK
channel is to
be replaced by a “more tolerant” DQPSK ch annel with the same data rate.
TuF3
Fig. 2. BER of middle
DQPSK channel ver301 OSNR 4 nd
d) 5 DQPSK rbannrls b)
md
e) DQPSK and 4 OO K
cb~nnc l s p o h r i u t i o a
of d l channels paraUel), c) and r)
DQPSK and
4
OO K rbaomb (polal iut ion
of
OO K
channels
ortboaoml
to middle DQPSKchnneI) . a ) -
c) Measurements, d)- l)irnulatioar
Fe. . Performnrc ef DQPSK wltb 4 10
GWSOOK eighboring
r h ~ n n r l s
polarization of . rbrmels panUeI). Th e
DQPSK channe l rums s t 20GWa (a)
and 10 Gb/s (b), mpeclively.
4. Conclusions
Since practically all systems today use OOK modulation, the tolerance to interference generated by neighboring
OOK channels will become a major criterion for the deploym ent of optical alternative modulation schemes.
In
this
paper we addressed the, for practical reasons, very important case of 10 and 20 Gb/s DQPSK channels being turned
on in a 50 GHz
WDM
grid nearby 10 Gb/s OOK channels. We showed by measurements and simulations that a
middle
20
Gbis DQPSK channel surrounded by four OOK chan nels suffers
from
a very large OSNR p enally even for
moderate launch powers w hen the polarizationof the neighboring channels is aligned parallel
to
the middle channel.
Part of this degradation is recovered when the polarization of the neighboring channels is orthogonal to the m iddle
DQPSK channel. But this cannot
be
guar antee d unless polarization interleaving is emptoyed at the transm itter site.
Furthermore, we showed by means of simulations that a
10
Gb/s DQPSK channel is even
more
vulnerable to the
effect of XPM
ko m OO K
neighboring channels for moderate to high launch power. However, elaborate dispersion
maps can be chosen such that XPM interfercnce is minimized. A verification by measurements and thorough
investigation of this issue represent an open field for future work.
5.References
[I]C. Rasmussener al . “DWDM
4OG
tmmmirsion over transPacific distances (10,wOhn)using CSRZ-DPSK,
enhanced
FECad all-llaman
amplified 100bn UltraWave fiber spans”, OFC 2003,PDII.
[Z] B. hu r 01. “6.4-?WE 160 42.7 Ws) transmission
with 0.8
b i t l a spectral efficiency over 32 IW km of
fiber using CSRZ-DPSK
format
“
FC 2W3, D19.
[PI H. l . rcssurero1. “ 1 . 6 ~ ~ (~ x 4 0 G b / s ) D P S K h a n s m i u i o n w i t h d i r s td c t e c t i on ” ,
COC
2003.paper8.1.2.
[4]R.A. tiffin
er o/. IO GW s optical di&rential
qusdrahln
phase shifl ey (DQPSK)mmmission using W A I G a A s ntegration” OFC 2002
FD6.
[SI C. Wnc l 01. “RZ-DQPSK formatwithhigh spechal efficiency
and high obustness towards fiber nonlkaritics‘’, ECOC
2W2, aper 9.6.6.
[ 6 ] M. Rohdc er al., “ R o b m s ofDPSK direct detection transmissionformat in standard fib= WDM systems”, l&on. Lett Vol. 36,
No.
I7,Aug. oW,
p.
1483-4