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JOURNAL OF LIGHTWAVE TECHNOLOGY 1

Alternative Modulation Formats in Gb/sWDM Standard Fiber RZ-Transmission Systems

Anes Hodzic, Beate Konrad, and Klaus Petermann, Senior Member, IEEE

AbstractA comparison of carrier-suppressed return-to-zero(CSRZ) and single sideband return-to-zero (SSB-RZ) formats ismade in an attempt to find the optimum modulation format forhigh bit rate optical transmission systems. Our results show thatCSRZ is superior to return-to-zero (RZ) and SSB-RZ with respectto signal degradation due to Kerr nonlinearitiesand chromatic dis-persion in wavelength division multiplexing (WDM) as well as insingle-channel 40-Gb/s systems over standard single-mode fibers(SSMF). It is shown that CSRZ enables a maximum spectral ef-ficiency of approximately 0.7 (b/s)/Hz in a

Gb/s WDMsystem with equally polarized channels. Furthermore, the CSRZformat in

Gb/s WDM systems shows no further signaldegradation compared to single-channel transmission.

Index TermsCommunication system nonlinearities, opticalfiber communication, optical modulation, optical pulse generation,wavelength division multiplexing (WDM).

I. INTRODUCTION

I N order to achieve wavelength division multiplexing(WDM) systems with high spectral efficiency, it is attractiveto operate at bit rates of 40 Gb/s per channel [1][3]. In conven-

tional standard-fiber transmission lines, the return-to-zero (RZ)

and nonreturn-to-zero (NRZ) formats are the two modulation

formats most often used. Recent analysis and investigations

[4], [5] have shown that RZ turns out to be superior compared

to conventional NRZ systems [5], at least as long standardsingle-mode fibers are used as transmission media. On the

other hand, because of the narrower optical spectrum of the

NRZ format, NRZ enables higher spectral efficiency in WDM

systems compared to RZ in the linear regime. As alternatives

to RZ and NRZ several other modulation formats like car-

rier-suppressed return-to-zero (CSRZ) [7][9], single-sideband

RZ (SSB-RZ) [9][11], and duobinary modulations [12][14]

have been proposed. There are different factors that should

be considered for the right choice of modulation format, such

as spectral efficiency, power margin, and tolerance against

group-velocity dispersion (GVD) and against fiber nonlinear

effects like self-phase modulation (SPM), cross-phase modula-

tion (XPM), four-wave mixing (FWM), and stimulated Raman

scattering (SRS).

In this paper, we analyze 40-Gb/s WDM RZ-, CSRZ-, and

SSB-RZ-based transmission systems over standard-fiber lines.

So far, it has been shown [7][9] that CSRZ has a larger tol-

erance toward the degradation of signal quality with respect

Manuscript received April 10, 2001; revised January 10, 2002.The authors are with the Fachgebiet Hochfrequenztechnik, Technische Uni-

versitt Berlin, Berlin 10587, Germany (e-mail: [email protected]).Publisher Item Identifier S 0733-8724(02)02557-4.

to SPM and GVD compared to RZ. CSRZ format reduces the

nonlinear impairments [7] in SSMF-based dispersion-managed

lines. RZ- and CSRZ-based WDM systems have been analyzed

for spectral efficiencies up to 0.4 (b/s)/Hz [7] and [9] or 0.8

(b/s)/Hz for orthogonal polarization between adjacent channels

[2]. Results of [7], [9] indicated that the CSRZ format keeps

SPM tolerance high, even in the WDM configuration, and that

there is no excess penalty caused by the XPM- and FWM-in-

duced nonlinear crosstalk.

In this paper, we will demonstrate the maximum achievable

spectral efficiency in Gb/s WDM systems with equally

polarized channels over SSMF for RZ, CSRZ, and SSB-RZmodulation formats. Thus, it is theaim of this paper to determine

an optimal RZ-like modulation format in order to improve trans-

mission characteristics and to achieve higher spectral efficiency

in RZ-based 40 Gb/s/ch WDM transmission systems. The inves-

tigated modulation formats were studied regarding their proper-

ties with respect to nonlinear effects and chromatic dispersion

anddue to these impairments themaximum limit of spectral effi-

ciency for RZ, SSB-RZ, and CSRZ Gb/s systems willbe

determined. The tolerance of allmodulation formats to SPM and

GVD in 40-Gb/s transmission systems will also be investigated.

For each format, the optimal fiber type and dispersion-compen-

sating scheme will be recommended. This paper is organizedin three parts. First, the theoretical description and demonstra-

tion of signal generation is presented. Thereby, the general fea-

tures (spectrum, dispersion tolerance) of all modulation formats

will be described. In the second part, the single-channel 40-Gb/s

system will be considered and the impacts of single-channel ef-

fects are analyzed. Finally, the limiting effects and performance

of Gb/s WDM systems will be presented.

In order to investigate the performance differences between

these three modulation formats, two different evaluation criteria

have been used: the system penalty and the eye-opening penalty

(EOP). These two evaluation criteria provide different insights

into transmission characteristics. The system penalty evaluationenables the comparison of results of numerical simulations with

experimental results and yields the information about achiev-

able bit error rate (BER) of the transmission. On the other hand,

with system penalty, it cannot be clearly distinguished between

different effects, such as GDV, SPM, and XPM, that occur in the

fiber. For this purpose, the EOP evaluation can be very helpful

for the investigation of different transmission regimes (linear

and nonlinear) and for different modulation formats.

The system penalty is defined as the difference of receiver

sensitivity at 10 BER between back-to-back (BTB) and the

0733-8724/02$17.00 2002 IEEE


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2 JOURNAL OF LIGHTWAVE TECHNOLOGY

transmission case, as shown in (1) at the bottom of the page.

Generally, in numerical simulations, the BER is simulated with

a relatively low number of bits in order to reduce the computing

time. Thus, the assumption is made that the noise is Gaussian

distributed. This assumption is not valid any more in presence

of intersymbol interferences (ISI). In order to make a BER esti-

mation taking into account the ISI effects for numerical investi-

gation, the technique described in [15] is used in this paper.

The EOP is defined as

EOP [dB]

Eye opening back to back

Eye openingafter transmission (2)

The EOP of 1 dB is chosen as a maximum limit for the trans-

mission quality. EOP of 1 dB represents 80% eye opening.

II. GENERATION OFMODULATIONFORMATS

The generation of different modulation formats can be

achieved in different ways [7], [9]. In this paper, different

modulation formats have been realized as depicted in Fig. 1.

Fig. 1(a) presents the generation of RZ signals. The light of

the continuous wave (CW) pump is externally modulated in

MachZehnder interferometer (MZI) with a 40-Gb/s NRZ

electrical signal. The model used for numerical simulation of

MZI is based on [16]. A random bit word of length 2 is used

to generate the electrical RZ signal by filtering rectangular

RZ-coded pulse with a filter of bandwidth equivalent to 80%

of the bit rate. The first MZI is biased at the quadrature point.

The final signal forming of the RZ pulses takes place in the

second modulator. The second modulator is also biased at

the quadrature point. Thus, the second modulator is driven

with a 40-GHz sine-clock signal. The 40-Gb/s RZ signal

spectrum is illustrated in Fig. 2(a). It shows the typical RZ

signal spectrum with a spectral width of 80 GHz between the

first two sidebands. The modulator parameters are set such that

all three modulation formats have the same duty cycle in spite

of different methods of generation and different signal forms,

indicating the same FWHM pulsewidths for all investigated

modulation formats. The duty cycle for all three modulation

formats amounts to .

The generation of CSRZ and SSB-RZ signals is presented in

Fig. 1(b) and 1(c) and can be mathematically described as

(3)

Thus, and describe the optical input and output

fields of MZI 2. represents the optical field of a 40-Gb/s

Fig. 1. Generation of 40-Gb/s signals (a) RZ , (b) CSRZ, and (c) SSB-RZ.

NRZ signal, which is generated after MZI 1. and represent

the phases of two modulator arms. is biased with bias direct

current (dc) voltage of . and are defined as

(4)

(5)

and are the amplitudes of a sine-clock signal (

) and represents the phase difference between the two

sine-clock signals. is voltage, which is required in the mod-

ulator for a -phase shift.MZI 1 generates a 40-Gb/s NRZ optical signal through the

external modulation of the CW pump. The final signal forming

takes place in MZI 2, which is driven with sine-clock signals [7],

[8]. Depending on bias points of the second modulator, different

modulation formats can be realized.

System penalty [dB] Receiver sensitivity (back to back)

Receiver sensitivity (transmission) (1)


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HODZICet al.: ALTERNATIVE MODULATION IN Gb/s WDM STANDARD FIBER RZ TRANSMISSION 3

(a)

(b)

(c)

Fig. 2. Optical spectra of 40-Gb/s modulation formats (a) RZ, (b) CSRZ, and(c) SSB-RZ.

For CSRZ signal generation, the second modulator is biased

at the zero point ( ) and . equals .

The frequency of the sine clock is GHz (the half

bit rate). The mathematical representation of generated CSRZ

signal is depicted in (6)(8) as

(CSRZ) (6)

(CSRZ) (7)

(CSRZ)

(8)

The CSRZ spectrum is presented in Fig. 2(b). The carrier com-

ponent of the CSRZ signal spectrum is suppressed and the spec-

tral width between two first sidebands amounts to 40 GHz. This

represents a spectral reduction with factor of two, compared to

spectral width between two first sidebands in the RZ case.

For generation of SSB-RZ signals, both amplitudes of the

sine-clock signals are equal and amount to

. The phase difference is set to . The second

modulator is biased at the point. The frequency of the sine

clock is GHz for the generation of 40-Gb/s

SSB-RZ signals. The generated spectrum of an SSB-RZ signal

is represented in Fig. 2(c), and the final mathematical descrip-

tion is (9)(11), shown at the bottom of the next page. Through

the modulation in MZI 2, the left sideband of the signal is sup-

pressed, as can be seen from Fig. 2(c). Due to suppression of one

sideband, an improved transmission characteristic compared to

RZ signals can be achieved. At the same time, it is expected that

WDM systems with higher spectral efficiency can be realized.

III. SINGLE-CHANNEL 40-GB/SSYSTEMS

In this section, the behavior of different modulation formats

in a 40-Gb/s singlechannel transmission at 1550 nm is analyzed.

Our main focus is set on the system impairments due to SPM

and interaction between SPM and GVD. For the single-channel

analysis, the system from Fig. 3 is used. Fig. 3 represents an

Gb/s CSRZ-based WDM system. The only difference

between modulation formats is the second modulation stage

(MZI 2). For the single-channel investigations, just one channel

at 1550 nm is considered. In this case, an optical Bessel filter

of the sixth order with a 3-dB filter bandwidth of 60 GHz re-

places the multiplexer (MUX) and the demultiplexer (DMUX),

as shown in Fig. 3, respectively. For all investigations in this

paper, an amplifier spacing of 80 km is used. and in each

SSMF span are fully compensated by the dispersion compen-

sating fiber (DCF) in a postcompensating scheme. The post-


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compensating scheme is used in the single-channel case be-

cause of better system penalties compared to the precompen-

sating scheme in 40-Gb/s RZ transmission [6]. Pre- and post-

compensation will be discussed further in Fig. 8. The values for

the nonlinearity and dispersion values of the used SSMF and

DCF fibers are given in Table I. The attenuation of the trans-

mission line is compensated within the span with an inline er-

bium-doped fiber amplifier (EDFA). The inline EDFA consists

of two EDFAs, one before and one after the DCF fiber. The input

power in the DCF fiber has been kept smaller than 5 dBm

so that the nonlinear effects in the DCF fiber can be neglected.

In order to study the influence of the fiber nonlinearities, the

amplified spontaneous emission (ASE) noise in the EDFAs is

not considered for single-channel transmission. The received

channel is directly detected and electrically filtered. After the

photodiode in the receiver, a Bessel low-pass filter of the third

order with 28-GHz electrical bandwidth is placed. This yields a

conversion of the received signals to NRZ in the electrical do-

main. For the numerical investigation, random bit sequences of

length 2 are used. As described in the Appendix, this number

of bits is sufficient for a proper investigation of the transmission

characteristics. The pulse propagation is solved numerically by

the split-step Fourier method [17]. Numerical simulations are

realized with the simulation tool VPItransmission Maker 4.0

[18]. The aforementioned single-channel system configuration

is used for all investigations to be described.

A. Maximum Penalty for Single-Channel Transmission

For lower channel powers, the maximum transmission length

is determined by the accumulated noise in the system. For

higher channel powers, the system behavior is limited through

the nonlinear effects in the fiber. In the case of a cascaded

single-channel transmission, the most dominant nonlinear

effects are Kerr nonlinearities as expressed by SPM. The results

of numerical investigations for the SPM limit for different mod-

ulation formats are shown in Fig. 4, displaying the maximum

span count with maximum eye-opening penalty of 1 dB

as a function of power launched into the fiber. From Fig. 4,

it can clearly be seen that the CSRZ and SBB-RZ have better

(SSB - RZ) (9)

(SSB - RZ) (10)

(SSB - RZ)

(11)


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Fig. 3. System setup for 2 Gb/s CSRZ-based WDM transmission system over SSMF fiber.

TABLE I

FIBERPARAMETERS

Fig. 4. Maximum transmission length at 1 dB EOP for different input powersin 40-Gb/s single-channel system over SSMF.

properties due to SPM than RZ. It turns out that the product

of input channel power ( ) and maximum span count is

a nearly constant for each modulation format, which will be

denoted . can be used as an indicator of system

tolerance for SPM effects. can be defined in logarithmic

units as [19]

[dBm] [dBm] (12)

Thus, CSRZ tolerates 2 and 6 dB more power per channel than

RZ and NRZ transmissions, respectively. These advantages are

further justified in Fig. 5. Fig. 5 shows the system penalty of

single-channel transmission for different input powers with RZ,

Fig. 5. System penalty for different input powers in a 4 2 80 km SSMF40-Gb/s channel transmission.

CSRZ, and SSB-RZ format, respectively, in a 40-Gb/s system

fora constant length(320 km yieldsfour 80-km spans)of SSMF.

In the linear regime (for smaller input powers), all three modula-

tion formats show almost the same transmission characteristics.

The differences between different modulation formats become

evident for higher input power. In this case, CSRZ and SSB-RZ

modulation provide an improvement of the system penalty of

5 dB compared to conventional RZ transmission. This im-

provementis especially important in long-haul systems enabling

higher power per channel and the better optical signal-to-noise

ratio (OSNR).


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(a)

(b)

(c)

Fig. 6. Dispersion tolerance for different modulation formats (a) RZ,(b) CSRZ, and (c) SSB-RZ. The parameter is the eye-opening penalty, indecibels.

B. Dispersion Tolerance

The maximum dispersion tolerance of each modulation

format is investigated. A higher dispersion tolerance is impor-

tant, for example, in WDM systems in which the dispersion

compensation is realized at the central wavelength and where

dispersion slope compensation is another important issue.

It could be expected that, due to chromatic dispersion of

transmission fibers, the modulation format with the narrowest

signal spectrum will exhibit the highest dispersion tolerance.

The transmission is again considered over four spans. The

amount of residual dispersion has been changed through the

(a)

(b)

(c)

Fig. 7. Optimum fiber type for 40-Gb/s single-channel transmission over4 2 80 km in (a) RZ, (b) CSRZ, and (c) SSB-RZ systems. The parameter is theeye-opening penalty, in decibels.

length variation of DCF over all four spans. Dispersion toler-

ances of all modulation formats at different input powers (0,

3, 6, and 9 dBm) are investigated. As a base for the disper-

sion tolerance a comparison at the 1-dB EOP is considered.

The results are presented in Fig. 6. It can be seen that CSRZ

[see Fig. 6(b)] possess the highest dispersion tolerance of about

70 ps/nm, which is almost 30-ps/nm higher then the dispersion


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tolerance of SSB-RZ and RZ modulations. Dispersion curves

in CSRZ and RZ cases show a symmetrical behavior and have

a minimum EOP at a residual dispersion of 0 ps/nm. Thus, it

is verified that the full compensation of dispersion is the best

choice for RZ and CSRZ single-channel transmission systems.

In the SSB-RZ case, the dispersion curves are asymmetrical and

the minimum EOP value is reached at 10 ps/nm residual dis-

persion. This can be explained with the asymmetrical spectrum

of SSB-RZ signals due to the suppression of one sideband [see

Fig. 2(c)]. Thus, the SSB-RZ modulation format shows no sig-

nificant improvement of dispersion tolerance compared to the

RZ case. From Fig. 6, it can be seen again that the RZ modula-

tion is more sensitive to nonlinearities.

C. Optimum Dispersion of Transmission Fiber

The next important question by the implementation of new

modulation format in 40-Gb/s transmission systems is the

optimum fiber for each modulation format. The investigation

is made with the same system configuration as in section B

over 4 80 km spans. Thus, different chromatic dispersions at

1550 nm in the fiber and different fiber effective areas ( )

are considered in order to find out the optimum fiber type for

40-Gb/s single-channel transmission. At the same time, we

considered the ASE noise in EDFAs, which are used for optical

amplification. The noise figure of each EDFA amounts to 4

dB. The results of this investigation are presented in Fig. 7.

From all three diagrams in Fig. 7, it can be seen that for all

modulation formats investigated, a fiber type with large

is advantageous, which indicates smaller impact of nonlinear

effects. Thus, the optimum chromatic dispersion in all three

cases is approximately 6 to 8 ps/nm km. In order to enable a

comparison of different commercially available fiber types,their main parameters are included in diagrams. For future

40-Gb/s single-channel transmission systems, nonzero-dis-

persion-shifted fibers (NZDSF) with large effective area and

larger chromatic dispersion are, thus, advantageous in systems

with either RZ, CSRZ, or SSB-RZ modulation formats. SSMF

shows similar characteristics as large-area NZDSF. Due to the

fact that higher chromatic dispersion enables better suppression

of FWM effects (especially in systems with smaller channel

spacing), it can be expected that in WDM transmissions, SSMF

shows even better performances than NZDSF with large .

Therefore, in the further investigation, we concentrate on

SSMF transmission.

IV. WDM Gb/s SYSTEMS

For considering a Gb/s WDM system, it is sufficient

to consider just four adjacent channels for system analysis, be-

cause main distortions are due to the interaction between adja-

cent channels. The analyzed transmission systems consist, gen-

erally, of four spans of SSMF fiber (total length 320 km), but

the results of this study can easily be extended to a higher span

number. For investigating Gb/s systems over SSMF fiber

with CSRZ, SSB-RZ, and RZ modulation formats, the system

setup in Fig. 3 is used. In each channel, there are different sta-

tistically independent random bit sequences of length 2 and

(a)

(b)

(c)

Fig. 8. Optimum dispersion-compensating scheme in 4 2 40 Gb/s WDMsystem (0.8-nm channel spacing) over 320-km SSMF in (a) RZ, (b) CSRZ, and(c) SSB-RZ systems. The parameter is the eye-opening penalty, in decibels.

identical bit sequences are used for the analysis of all modula-

tion formats. In order to investigate the impact of multichannel

effects (XPM, FWM, SRS) in each system, different channel

spacings (200, 100, 80, and 60 GHz) are considered. An equal


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channel spacing is used between all channels. For all consid-

ered channel spacings, Bessel filters of the sixth order with op-

tical 3-dB bandwidth of 60 GHz per channel are used in MUX

and DMUX, respectively. For each system, the worst channel

is evaluated, which turns out to be one of the two middle chan-

nels (channel 2), because this channel shows the strongest im-

pair-ments due to nonlinear channel interaction (XPM, FWM,

SRS) caused by neighboring channels.The optimum dispersion-compensating scheme is deter-

mined for each modulation format considering an channel

spacing of 100 GHz. As evaluation criteria the EOP is used.

The results are shown in Fig. 8, which represents the amount

of precompensation versus input power per channel. It can be

clearly seen that, in all three cases, the full postcompensation

(0% of precompensation) represents the optimum dispersion

compensating scheme for Gb/s WDM transmission.

According to these results, further investigation has been made

with fully postcompensated WDM systems.

Fig. 9 shows results of numerical simulations for each

modulation format. In this investigation, the ASE noise is

not considered, in order to concentrate on the nonlinear

effects (XPM, FWM). Thus, indicates the input power

per channel. In Fig. 9(a), the EOP for different input power

is shown in a 4 40 Gb/s RZ system. It can be seen that

the minimum channel spacing in the RZ case amounts to

60 GHz. For even smaller channel spacings, the eye of the

signal becomes fully closed. This implies a maximum spectral

efficiency of 0.66 (b/s)/Hz. This spectral efficiency can be

reached with CSRZ format [see Fig. 9(b)], even with fewer

penalties. SSB-RZ [see Fig. 9(c)] posseses a lower spectral

efficiency [ 0.5 (b/s)/Hz] compared to other modulations,

which is caused by the smaller tolerance of SSB-RZ toward

narrow-filtering effects due to the suppression of one signal

sideband. It is expected that the spectral efficiency of the

SSB-RZ modulation format can be further improved through

use of unequal spacing between the channels [20].

The limiting effects for each modulation format are dif-

ferent. In Fig. 9(a), two regions are distinguished for RZ. The

first region occurs for channel spacing greater than 80 GHz.

The system limitations in this region are caused only by

single-channel limitations (SPM beside GVD) and there are

no further impairments due to multichannel effects (XPM,

FWM). At smaller channel spacing ( 80 GHz) multichannel

effects (FWM, XPM) occur. Thus, the XPM yields a stronger

impact than FWM due to the high chromatic dispersion in

the fibers. For the SSB-RZ case [see Fig. 9(c)], it can be seen

that the power tolerance at 1-dB EOP gets extremely worse

for channel spacings smaller than 80 GHz. This indicates thatmultichannel effects in SSB-RZ are more severe than those in

RZ or CSRZ, yielding a minimum channel spacing of 70 GHz

for the SSB-RZ system. For CSRZ format, no impairments

due to FWM or XPM at different channel spacings could be

detected. This means that CSRZ 4 40 Gb/s WDM system

shows no additional impairments due to nonlinear effects

compared to the single-channel CSRZ system, as long as

there is no significant spectral overlap between the channels.

If some improved narrowband filtering is used, it can be

expected that even smaller channel spacing (e.g., 50 GHz) can

be realized in CSRZ transmission. These results are supported

by experimental results for WDM systems with 100-GHz

(a)

(b)

(c)

Fig. 9. Eye-opening penalty for different input powers at different channelspacing in 4 2 40 Gb/s WDM system over 320 km SSMF in (a) RZ, (b) CSRZ,and (c) SSB-RZ formats.

channel spacing, as shown in [7]. Thus, CSRZ combines the

best features of all three modulation formats in 4 40 Gb/s

WDM systems over SSMF fibers.

V. CONCLUSION

In conclusion, CSRZ showed the best tolerance to the SPM

effect, as well as the largest dispersion tolerance among all mod-

ulation formats investigated. Gb/s CSRZ systems over

standard single-mode fiber may be realized with the same per-

formance as single-channel 40-Gb/s systems, up to a spectral ef-

ficiency of approximately 0.7 (b/s)/Hz. The results of this paper


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(a)

(b)

Fig. 10. Optimum length of random bit words for numerical investigation of

40 Gb/s transmissions over 42

80 km SSMF (a)

versus number of bits(input power dBm) (b) EOP versus number of bits.

can be used for upgrading existing SSMF-based optical net-

works, as well as for the design of new high bit rate networks

with higher capacity and improved spectral efficiency. Further

work is still required with respect to the usage of NZDSF in the

transmission line, improved dispersion compensation (hybrid

compensation), prechirping (through chirp in external modula-

tors), distributed optical amplification (Raman amplifier), and

improved optical filtering (AWG filters).

APPENDIX

The optimum number of bits for the numerical simulation isdetermined for both evaluation criteria (EOP and BER) used in

this paper. The determination of the optimum (sufficient) bit

number is important in order to find the best compromise be-

tween the computing time and the error of the numerical simu-

lations. The investigation for the optimum bit number is made

by using a system setup for 40-Gb/s RZ single-channel trans-

mission over 4 80 km SSMF. The factor is determined

according to [15], as shown in Fig. 10(a). The random bit words

with lengths from 2 to 2 bits are considered. Each bit word

consists of equal number of marks and spaces. Forcalculation of

each point in Fig. 10(a), 20 different noise distributions are con-

sidered, so that each point represents the mean value of different

noise distributions. From this figure, it is evident that a random

word length of 2 bits is sufficient for a correct numerical simu-

lation of 40-Gb/s transmission. There are almost no differences

of values between 2 and 2 bit words.

For the optimum number of bits in the EOP case [see

Fig. 10(b)], the same setup without consideration of noise in

the system is used. The same random bit words are used as

in the case. From Fig. 10(b), it can be clearly seen thatfor EOP values below 1 dB, the differences between different

numbers of bits vanish, becoming larger for higher input power

values. In this case, there is very little difference between 2

and 2 , so that 2 can be considered as the optimum for the

numerical analysis of EOP in 40-Gb/s transmission systems.

REFERENCES

[1] A. Frbert, G. Mohs, S. Splter, J. -P. Elbers, C. Frst, A. Schpflin,E. Gottwald, C. Scheerer, and C. Glingener,7 Tbit/s (176 2 40 Gbit/s)bi-directional interleaved transmission with 50 GHz channel spacing, inProc. ECOC, Mnchen, Germany, 2000, PD 1.3, pp. 12.

[2] T. Ito, K. Fukuchi, K. Sekiya, D. Ogasahara, R. Ohhira, and T. Ono,6.4 Tbit/s (160 2 40 Gbit/s) WDM transmission experiment with 0.8

Bit/s/Hz spectral efficiency, in Proc. ECOC, Mnchen, Germany, 2000,PD 1.1, pp. 12.

[3] S. Bigo, A. Bertaina, Y. Frignac, S. Borne, L. Lorcy, D. Hamoir, D.Bayart, J. -P. Hamaida, W. Idler, E. Lach, B. Franz, G. Veith, P. Sil-lard, L. Fleury, P. Guenot, and P. Nouchi, 5.12 Tbit/s (128 2 40 Gbit/s)transmission over 3 2 100 km of TeralightTM fiber, in Proc. ECOC,Mnchen, Germany, 2000, PD 1.3, pp. 12.

[4] S.-G. Park, A. H. Gnauck, J. M. Wiesenfeld, and L. D. Garrett,40-Gb/stransmission over multiple 120-km spans of conventional single-modefiber using highly dispersed pulses,IEEE Photon. Technol. Lett., vol.12, pp. 10851087, Aug. 2000.

[5] D. Breuer and K. Petermann,Comparison of NRZ- and RZ- modula-tion format for 40 Gbit/s TDM standard-fiber systems, IEEE Photon.Technol. Lett., vol. 9, pp. 398400, Mar. 1997.

[6] C. M. Weinert, R. Ludwig, W. Pieper, H. G. Weber, D. Breuer, K. Peter-mann, and F. Kppers,40 Gb/s and 4 2 40 Gb/s TDM/WDM standardfiber transmission,J. Lightwave Technol., vol. 17, pp. 22762284, Nov.

1999.[7] Y. Miyamoto, A. Hirano, K. Yonenega, A. Sano, H. Toba, K. Murata,and O. Mitomi, 320Gbit/s (8 2 40 Gbit/s) WDMtransmissionover 367km with120 km repeater spacing using carrier-suppressed return-to-zeroformat,Electron. Lett., vol. 35, pp. 20412042, Nov. 1999.

[8] Y. Miyamoto,40 Gbit/s transport system: Its WDM upgrade,inProc.OFC, Baltimore, MD, 2000, ThW4-1, pp. 323325.

[9] Y. Kobayashi, K. Kinjo, K. Ishida, T. Sugihara, S. Kajiya, N. Suzuki,and K. Shimizu, A comparison among pure-RZ, CS-RZ and SSB-RZformat, in 1 Tbit/s (50 2 20 Gbit/s, 0.4 nm spacing) WDM transmissionover 4,000 km,inProc. ECOC, Mnchen, Germany, 2000, PD 1.7, pp.12.

[10] M. Sieben, J. Conradi, and D. E. Dodds,Optical single sideband trans-mission at 10 Gbit/s using only electrical dispersion compensation, J.

Lightwave Technol., vol. 17, pp. 17421749, Oct. 1999.[11] R. Olshansky, Single sideband optical modulator for lightwave sys-

tems,U.S. patent 5 301058, 1994.[12] A. J. Price and N. le Mercier,Reduced bandwidth optical digital inten-

sity modulation with improved chromatic dispersion tolerance, Elec-tron. Lett., vol. 31, pp. 5859, Jan. 1995.

[13] K. Yonenaga, S. Kuwano, S. Norimatsu, andN. Shibata, Optical duobi-narytransmissionsystemwith no receiversensitivitydegradation,Elec-tron. Lett., vol. 31, pp. 302304, Feb. 1995.

[14] K. Yonenaga, M. Yoneyama, Y. Miyamoto, K. Hegimoto, and K.Noguchi, 160-Gbit/s WDM transmission experiment using four40-Gbit/s optical doubinary channels, in Proc. OFC, San Jose, CA,1998, TuI2, pp. 4951.

[15] C. J. Anderson and J. A. Lyle,Technique for evaluating system perfor-mance using in numerical simulations exhibiting intersymbol inter-ference,Electron. Lett., vol. 30, pp. 7172, Jan. 1994.

[16] F. Koyama and K. Iga,Freqeuncy chirping in external modulators,J.Lightwave Technol., vol. 6, Jan. 1988.

[17] G. P. Agrawal,Nonlinear Fiber Optics, 2nd ed. San Diego, CA: Aca-demic, 1995.

[18] VPItransmission Maker 4.0, VPI Systems, Inc., Holmdel, NJ.


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[19] J.-P. Elbers and A. Frbert,Reduced model to describe SPM-limitedfiber transmission in dispersion-managed lightwave systems,J. Select.Topics Quantum Electron., vol. 6, pp. 276281, Mar./Apr. 2000.

[20] W. Idler, S. Bigo, Y. Frignac, B. Franz, and G. Veith,Vestigal side banddemultiplexing for ultra high capacity (0.64 bit/s/Hz) transmission of128 2 40 Gb/s channels, in Proc. OFC, Anaheim, CA, 2001, MM3,pp. 13.

Anes Hodzicwas born in Sjenica, Bosnia and Herce-gowina, in 1975. He received the Dipl.-Ing. degreein electricalengineering fromthe Technische Univer-sitt Berlin, Berlin, Germany, in 1999. He is currentlypursuing the Dr.-Ing. degree at the Technische Uni-versitt Berlin.

His research interests include design criteria forhigh-bit rate transmission systems.

Beate Konrad was born in Hameln, Germany,in 1974. She received the Dipl.-Ing. degree inelectrical engineering from the University Hannover,Hannover, Germany, in 1998.

Since 1998, she has been a Research Associateat the Institut fr Hochfrequenztechnik-und Hal-bleiter-Systemtechnologien, Technical UniversityBerlin, Berlin, Germany, where she is engaged inresearch work on high-speed fiber-optic communi-cation systems.

Klaus Petermann (M76SM85) was born inMannheim, Germany, on October 2, 1951. Hereceived the Dipl.-Ing. and Dr.-Ing. degrees in elec-trical engineering from the Technische UniversittBraunschweig, Braunschweig, Germany, in 1974and 1976, respectively.

From 1974 to 1976, he was a Research Associateat the Institut fr Hochfrequenztechnik, TechnischeUniversitt Braunschweig, where he worked on op-

tical waveguide theory. From 1977 to 1983, he waswith AEG-Telefunken, Forschungsinstitut Ulm, Ger-many, wherehe was engaged in research on semiconductor lasers, optical fibers,and optical fiber sensors. In 1983. he became a Full Professor at the Tech-nische Universitt Berlin, where his research interests are optical fiber commu-nications and integrated optics. He is an Associate Editor of IEEE P HOTONICSTECHNOLOGYLETTERS and a member of the board of the Verean DeutscherElectrotechniker (VDE).

Dr. Petermann is a member of the Berlin-Brandenburg Academy of Science.In 1993, he received the Leibniz Award from the Deutsche Forschungsgemein-schaft.

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alter_mod_WDM.pdf