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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

 

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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.

 

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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,

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p.

1483-4