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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 8, AUGUST 1999 991

NRZ Versus RZ in 1040-Gb/s Dispersion-ManagedWDM Transmission Systems

M. I. Hayee, Member, IEEE, and A. E. Willner, Senior Member, IEEE

Abstract We compare nonreturn-to-zero (NRZ) withreturn-to-zero (RZ) modulation format for wavelength-division-multiplexed systems operating at data rates up to 40 Gb/s. Wefind that in 1040-Gb/s dispersion-managed systems (single-modefiber alternating with dispersion compensating fiber), NRZ ismore adversely affected by nonlinearities, whereas RZ is moreaffected by dispersion. In this dispersion map, 10- and 20-Gb/ssystems operate better using RZ modulation format becausenonlinearity dominates. However, 40-Gb/s systems favor theusage of NRZ because dispersion becomes the key limitingfactor at 40 Gb/s.

Index TermsNRZ/RZ modulation, optical dispersion manage-ment, optical fiber communication, optical fiber nonlinearities,

wavelength-division multiplexing.

FOR wavelength-division-multiplexed (WDM) systems in

which the data rate is 10 Gb/s/channel, the deleteri-

ous effects of dispersion and nonlinearity must be managed

to achieve transmission over any appreciable distance [1].

Dispersion management, utilizing alternating fiber segments

of opposite dispersion values, is a key technique that keeps

the total accumulated dispersion low while suppressing most

nonlinear effects. In dispersion-managed systems utilizing

single-mode fiber (SMF) and dispersion compensating fiber

(DCF), the positive dispersion of SMF can be compensated

by the large negative dispersion of DCF. In this scenario:1) four-wave mixing (FWM) is significantly reduced [1], [2]

and 2) overall dispersion accumulation is minimized over a

fairly wide wavelength range. However, self-phase modulation

(SPM) and cross-phase modulation (XPM) still degrade the

system performance. A recent study shows that in a 40-Gb/s

single-channel system, return-to-zero (RZ) modulation is more

optimal than NRZ in combating SPM [3] despite the fact that

SPM is enhanced in RZ due to the higher pulse peak power [4].

Other reports show that RZ can take advantage of soliton-like

pulse compression to perform better than NRZ for propagation

in SMF [5], [6].

An issue still remains as to determining whether NRZ

or RZ is the more optimal modulation scheme for use inWDM dispersion-managed systems. Intuitively, the following

suppositions can be made: 1) due to higher peak power, RZ

may suffer more nonlinearities, and due to shorter pulsewidth,

RZ may suffer more dispersion and 2) due to a longer pulse

Manuscript received June 2, 1998; revised April 16, 1999.M. I. Hayee is presently with Tyco Submarine Systems, Eatontown, NJ

07724 USA.A. E. Willner is with the Department of Electrical Engineering-Systems,

University of Southern California, Los Angeles, CA 90089-2565 USA.Publisher Item Identifier S 1041-1135(99)05929-7.

time and longer interaction time between wavelengths, NRZ

may suffer more nonlinearities. In this letter, we compare NRZ

with RZ modulation for dispersion-managed systems (SMF

alternating with dispersion compensating fiber) operating at

data rates up to 40 Gb/s. In 1040 Gb/s systems, we find

that NRZ is more adversely affected by nonlinearities whereas

RZ is more affected by dispersion. Typically, 10- and 20-

Gb/s systems are limited mostly by nonlinearities, whereas

40 Gb/s systems are limited mostly by dispersion. Taken

together, these two statements suggest that: 1) 10- and 20-

Gb/s systems, in general, operate better using RZ modulation

because nonlinearity dominates and 2) 40-Gb/s systems, beinglimited mostly by dispersion, favor the usage of RZ for few

channels but require NRZ as the number of channels increases.

Our modeled system consists of alternate fiber spans of 50

km of SMF ps/nm km, and dispersion slope

ps/nm k m) and 10 km of DCF ps/nm km,

and dispersion slope ps/nm km). Each fiber span is

followed by an optical amplifier with a noise figure of 6 dB to

compensate for fiber loss (0.2 dB/km for SMF and 0.5 dB/km

for DCF). The launched power in SMF and DCF is same.

The overall dispersion slope is 0.0125 ps/nm km. We have

considered 16 WDM channels with uniform channel spacing

(0.8 nm for 10- and 20-Gb/s systems, and 1.6 nm for 40-

Gb/s systems) with the center of the wavelength range beingthe point of complete dispersion compensation. A pseudoran-

dom 64-bit data stream for each WDM channel is encoded

using either nonreturn-to-zero (NRZ) or RZ (50% duty cycle)

modulation. The transmitter chirp is assumed to be zero and

bandwidth for NRZ and RZ is assumed to be and ,

respectively, where is the bit rate. The combined electric

field of all 16 channels is then propagated through the fiber,

and propagation of the resultant optical field is simulated by

solving the nonlinear Schrodinger equation using a split-step

Fourier analysis [1].

The total field approach is adopted for our simulation model,

and the simulated spectral range is more than three times thebandwidth occupied by the WDM channels. Note that a vari-

able step size is chosen for the simulations such that the nonlin-

ear phase shift does not exceed 1 mrad/step, and the maximum

step size is 100 m [1]. The affective areas of SMF and DCF

are assumed to be 70 and 22 m , and the included nonlinear

processes are SPM, XPM, FWM, and stimulated Raman

scattering. At the receiver, each WDM channel is optically de-

multiplexed with a bandpass filter of and , respectively,

for NRZ and RZ modulation formats. Each demultiplexed

channel is then electrically low-pass filtered with a filter

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992 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 8, AUGUST 1999

Fig. 1. The Q factor of the worst of 16 channels versusaverage-power/channel for NRZ and RZ systems at (a) 10 Gb/s, (b)20 Gb/s, and (c) 40 Gb/s.

bandwidth . The system performance evaluation is based

upon the eye closure penalty of each channel [1][3], [5].

To compare NRZ and RZ systems, we first vary the average

power per channel for a 16-channel system with and without

including nonlinear effects; note that nonlinear effects are

excluded from the simulation by making the nonlinearitycoefficient zero. The resulting eye closure penalty of the

poorest-performing channel is shown in Fig. 1(a)(c), re-

spectively, for 10-, 20-, and 40-Gb/s systems. The poorest

performing channel is channel #7 or Channel #8 in the cases of

10 and 20 Gb/s and is channel #1 or channel #16 in the case of

40 Gb/s. In 10- and 20-Gb/s systems, the eye closure penalty

is large for lower channel powers because the EDFA noise

is dominant [Fig. 1(a) and (b)]. For higher channel powers,

the eye closure penalty decreases since the signal power now

overcomes the EDFA noise. However, at the highest channel

powers being considered, the eye closure penalty increases as

a function of power due to nonlinearity. The penalty due to

nonlinearity is more severe in NRZ as compared to RZ. SinceRZ performs better than NRZ in both 10 and 20 Gb/s systems,

we can conclude that RZ is less affected by nonlinearity than is

NRZ. This is because: 1) isolated RZ pulses take advantage of

soliton-like pulse compression in SMF, whereas the variable

number of adjacent 1s in NRZ are nonuniformly degraded

by nonlinearity, thereby causing the rail of 1s to spread [5],

[7], and 2) long strings of 1s in NRZ as compared to RZ

have a much longer cross-wavelength interaction time, thereby

producing more severe penalty from nonlinearity [7], [8].

In a 40-Gb/s system [see Fig. 1(c)], the narrower RZ pulses

are more susceptible to dispersion; note that dispersion-based

Fig. 2. The Q factor for each of 16 channels modulated at (a) 10 Gb/s, (b)20 Gb/s, and (c) 40 Gb/s. The average power/channel is 0.3 mW.

penalties grow inversely as the square of the pulsewidth [8].

Moreover, the required larger channel spacing of 1.6 nm for

40-Gb/s transmission produces more dispersion in the end

channels since the total wavelength range is increased. An

indication that RZ is more affected by dispersion can be

derived from the fact that 40-Gb/s RZ systems have a largereye closure penalty as compared to NRZ systems for low

and moderate channel powers [Fig. 1(c)]. For higher channel

powers, the penalty for NRZ increases because of nonlinearity,

whereas no appreciable change in penalty is noticed for the

RZ system.

To isolate the degrading effects on individual channels,

the penalties of all 16 channels are shown in Fig. 2(a)(c),

respectively, for 10-, 20-, and 40-Gb/s systems. The penalties

are shown with and without including nonlinear effects, and

the average power per channel is 0.3 mW. This power value

is chosen since ASE noise is overcome but nonlinear effects

have not begun to dominate [Fig. 1(a) and (b)]. The penalty

differential for all 16 channels at 10- and 20-Gb/s is negligiblefor NRZ and RZ when nonlinearities are not included. When

nonlinearities are included, the penalty in NRZ increases more

than RZ for all the channels. In a 16-channel 40-Gb/s system,

the channel distribution in penalty is different for NRZ and

RZ because dispersion dominates in RZ and affects the end

channels more.

In general, the nonlinearity induced penalty for all the

channels in 10- and 20-Gb/s NRZ systems increases with the

transmission distance. The eye closue penalty as a function of

distance for the poorest-performing channel in a 16-channel

system is shown in Fig. 3(a)(c), respectively for 10, 20,

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HAYEE AND WILLNER: NRZ VERSUS RZ IN 1040-Gb/s DISPERSION-MANAGED WDM TRANSMISSION SYSTEMS 993

Fig. 3. The Q factor of the worst channel versus transmission distance for16 channel NRZ and RZ systems at (a) 10 Gb/s, (b) 20 Gb/s, and (c) 40 Gb/s.The average power/channel is 0.3 mW.

and 40 Gb/s; note that the DCF length is not included in

transmission distance. The penalty is larger for NRZ than

for RZ at 10 and 20 Gb/s when nonlinearity is included.

Furthermore, the penalty increases with distance in NRZ more

rapidly as compared to RZ, thereby showing effectiveness of

RZ modulation format in combatting nonlinearities. However,the penalty increases for 40 Gb/s more quickly with distance

in RZ as compared to NRZ. Moreover, there is a negligible

change in the penalty for RZ when nonlinearity is included,

indicating that the penalty is mainly due to dispersion. For

the 40-Gb/s NRZ system, the penalty increases slightly due

to nonlinearities.

The dispersion slope of DCF is an important parameter that

influences the wideband performance of dispersion-managed

WDM systems. Therefore, we analyze in Fig. 4, the NRZ and

RZ systems by varying the slope of dispersion for each 10-km

span of DCF while keeping the slope of each 50-km span of

SMF fixed at 0.075 ps/nm km. The eye closure penalty of

the poorest performing channel (Channel #1 or Channel #16)versus dispersion slope of DCF is shown in Fig. 4. When the

combined accumulated dispersion slope has a small nonzero

value (the regime where dispersion accumulation in all the

channels is minimal and nonlinearity is the main degrading

effect), RZ has a lower penalty than NRZ [Fig. 4(a)(c)]

showing that RZ is less affected by nonlinearity than NRZ.

Fig. 4. The Q factor versus dispersion slope of DCF for (a) 10 Gb/s, (b) 20Gb/s, and (c) 40 Gb/s NRZ and RZ systems. The average power/channel is0.3 mW and dispersion slope of SMF is + 0.075 ps/nm2 1 km.

However, RZ systems have a smaller operational window

in terms of dispersion slope of DCF as compared to NRZ

[Fig. 4(a)(c)] because RZ modulation is more susceptible to

dispersion due to high modulation bandwidth.

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