Optics Communications 281 (2008) 5811–5818

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Optics Communications

journal homepage: www.elsevier .com/locate /optcom

Optimizing amplifier spacing to improve performance in RZ-rectangular pulsebased 10 Gb/s single channel dispersion managed optical communication system

Manjit Singh a,*, Ajay K. Sharma b, R.S. Kaler c

a Department of Electronics and Communication Engineering, University College of Engineering, Punjabi University, Patiala, Punjab, Indiab Department of Electronics and Communication Engineering, National Institute of Technology, Jalandhar, Punjab, Indiac Department of Electronics and Communication Engineering, Thapar University, Patiala, Punjab, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 May 2008Received in revised form 3 September 2008Accepted 3 September 2008

Keywords:Amplifier spacingDispersion compensationTiming jitterBit error rate

0030-4018/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.optcom.2008.09.008

* Corresponding author. Fax: +91 0175 3046324.E-mail address: manjitsing@gmail.com (M. Singh).

In this paper, we optimize the inter-amplifier spacing in combination with duty cycle of RZ data formatand EDFAs power so that link length of system can be maximized. The results for EDFA amplifier place-ment in 10 Gbps single channel dispersion managed optical communication system have been presented.By increasing the length of standard single mode fiber of dispersion 16 ps/nm/km in proportion to theincrease in length of compensating fiber of dispersion �80 ps/nm/km, the pre-, post- and symmetrical-dispersion compensation schemes of the system have been compared. Further, schemes are observedat 8, 10 and 12 dBm values of EDFA power in the link with different duty cycle values of RZ optical pulsein the range of 0.2–0.8 with step size of 0.2 in relation to amplifier spacing to get lower value of bit errorrate and timing jitter. The graphical results obtained show strong relationship among duty cycle of RZoptical pulse, EDFA power and, dispersion compensation scheme.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Conventionally, NRZ (non-return-to-zero) modulation formathas been used in long-haul transmission systems [1,2]. These sys-tems are based on the fact that fiber dispersion and nonlinearitiesare detrimental effects. NRZ is used advantageously as it providesminimum optical bandwidth and minimum optical peak powerper bit interval for given average power. However, with increasedbit rates it has been shown that RZ (return-to-zero) modulationformats offer certain advantages over NRZ, as they tend to be morerobust against distortions [3]. For instance, RZ modulation is moretolerant to non-optimized dispersion maps than NRZ schemes [4].This can be explained by the reason that optimum balancing be-tween fiber nonlinearities and dispersion is dependent on pulseshape. A RZ modulated signal stream consists of a sequence of sim-ilar pulse shapes, whereas a NRZ modulated stream does not. Thedispersion tolerance of a signal stream can be derived from thesuperposition of dispersion tolerance of individual pulse shapes.In fact, for majority of cases, the best results of WDM transmissionexperiments regarding distance-bit rate product have beenachieved using RZ modulation formats in both terrestrial andtransoceanic systems [4].

For designing of a system point of view, impairments from opti-cal transmission needs to be understood and what are the ways to

ll rights reserved.

reduce them, how the receiver affects signal and whether it canimprove the performance. Comparison of modulation formatsCRZ, RZ and NRZ formats in generic undersea system using noisefree simulations has already been done [5]. The influence of electri-cal filters under these formats has been analyzed. First an optimi-zation procedure was performed over a wide range of parametersto achieve the best performance for each format in a given systemand then studied physical properties and limitations of the for-mats. It was found that during transmission, rapid stretching andcontractions, while in the receiver, concentration of pulse energyin the center of bit slot decreases intersymbol interference. How-ever, to achieve higher spectral efficiency, it is necessary at somepoint to sacrifice these two properties of RZ formats in favor of for-mats like NRZ with smaller spectral bandwidth [5,6]. So, RZ dataformat with its selected duty cycle value could be watchful inimproving the system performance.

Another problem in cascaded optical communication systems athigher data rates is timing jitter. Timing jitter expressions havebeen presented analytically using moment method with theassumption of a chirped Gaussian pulse in dispersion-managed(DM) light-wave systems [7]. The extent of pre- and post-disper-sion compensation of a low-power system employing RZ formatcan minimize timing jitter along fiber link [7–9]. It is proposed thatamplifier spacing could be a solution to avoid Gordon–Haus timingjitter. A 10-Gbps return-to-zero pulse transmission in cascadedcommunication systems using dispersion compensation of stan-dard monomode fiber with large amplifier spacing has been exam-

5812 M. Singh et al. / Optics Communications 281 (2008) 5811–5818

ined. It has been shown that pulse distortions due to Kerr nonlin-earity were significantly diminished by symmetrical ordering ofcompensation sections when total number of pre- and post-disper-sion compensation sections is equal and repositioning of these sec-tions is not critical [10]. It has been also demonstrated by usingvariational analysis that noise-induced Gordon–Haus timing jitterin a DM soliton transmission system can be substantially reducedby appropriate placement of amplifiers. For short maps which re-quire only two amplifiers, the amplifier position can greatly reduceinduced Gordon–Haus timing jitter (by a factor of nearly 4) and in-crease system performance. In contrast, use of many amplifiers perperiod has a relatively small (<20%) impact on systems perfor-mance due to jitter, since noise contribution effectively averagesover amplitude fluctuations per period. In either case, optimal per-formance is achieved by keeping amplifiers away from the mid-point of anomalous dispersion fiber where pulse experiences peakpower. Finally, shorter map (two amplifiers) can boost perfor-mance by 40% per amplifier in comparison with longer maps(m > 4) considered [11]. Thus RZ data format though widely stud-ied but simultaneous effect of duty cycle of RZ optical pulse, in lineEDFAs (erbium doped fiber amplifiers) power and respective inter-amplifier spacing in DM optical communication system are re-quired to be seen [1–11]. Such investigations would be useful topredict the longest length of optical link under above parameters.

The authors have already presented an investigation on NRZdata format giving comparison of different dispersion compensa-

Optical link with pre compensat

Optical link with post compensa

EDFA Booster

Four inline EDF

DCF SSMF SSMF

Optical link with symmetrical-com

EDFA Booster

Four inline EDF

DCF DCF SSMF

EDFA Booster

Four inline EDF

SSMF SSMF DCF

10 Gb/s single channel optical communication system

Optical l

schemLaser

Source

RZ Driver

Sin2 MZModulator

BessDisplays to measure BER, Q value and Timing Jitter

Data source

(a)

(b)

(c)

Fig. 1. 10 Gbps single channel optical communication system model of optical link usingD = 16 ps/nm/km considered with three dispersion compensation schemes highlighted i

tion schemes [12]. Here, the study is further extended through thispaper to find optimum inter-amplifier spacing in combinationwith duty cycle of RZ data format and EDFAs power so that linklength of system could be maximized. It would result into leastnumber of amplifiers and thus economical for the use of repeatedspans over long distances. In addition, pre-, post- and symmetri-cal-dispersion compensation with the use of DCF (dispersion com-pensating fibers) have been closely investigated to have BER (biterror rate) and timing Jitter under control. The results present per-formance at 0.2–0.8 varied duty cycle of RZ optical pulse launchedinto fiber link consist of SSMFs (standard single mode fiber) andproportionate DCF whereby fiber loss is compensated by typicallypowered EDFAs (8, 10 and 12 dBm) after each fiber type. Theinvestigations compute maximum possible link length and com-ments on optimum inter-EDFAs spacing for other parameters indi-rectly in terms of DCF length in each trial. The details of simulatedoptical communication system and its other important parametersassumed are defined in Section 2. In Section 3, comparative resultsare reported for the trials and finally in Section 4, conclusions havebeen drawn.

2. Simulations

The block diagram of communication system used is shown inFig. 1, whose optical link details are separately shown in figure partFig. 2a–c for pre-, post- and symmetrical-compensation methods,

ion scheme

tion scheme

As

DCF

pensation scheme

As

SSMF

As

DCF

model considered in each compensation scheme

ink as per the dispersion compensation

e shown in figure part a, b and c

PIN Detector el filter

dispersion compensating fiber of D = �80 ps/nm/km and standard single mode fibern figure parts as (a) pre (b) post and (c) symmetrical.

Pre Compensation

Post Compensation

Symmetric Compensation

EDFA Booster

Four inline EDFAs

DCF DCF SSMF SSMF

Distance

+16 -80

Variable DCF Length

SSMF Length

= 5 x DCF length

Variable DCF Length

SSMF Length

= 5 x DCF length

EDFA Booster

Four inline EDFAs

DCF SSMF SSMF DCF

EDFA Booster

Four inline EDFAs

SSMF SSMF DCF DCF

Distance

+16 -80

Variable DCF Length

Variable DCF Length

Variable DCF Length

Variable DCF Length

SSMF Length SSMF Length

= 5 x DCF length = 5 x DCF length

Distance

Dis

pers

ion

(p

s/nm

/km

) D

ispe

rsio

n

(ps/

nm/k

m)

Dis

pers

ion

(p

s/nm

/km

)

+16 -80

SSMF Length

= 5 x DCF length

SSMF Length

= 5 x DCF length

(a)

(b)

(c)

Fig. 2. Schematic placement of EDFAs (shown by dark triangles) for the control oftiming jitter in dispersion compensated 10 Gbps single channel optical communi-cation system using dispersion compensating fiber (D = � 80 ps/nm/km) whereproportionate length (i.e. 5 � DCF length) of standard single mode fiber (D = 16 ps/nm/km) is taken in three dispersion compensation schemes: (a) pre (b) post and (c)symmetrical.

M. Singh et al. / Optics Communications 281 (2008) 5811–5818 5813

respectively, using standard and dispersion compensating fibers.The main Fig. 1 shows transmitter section, consists of data source,electrical driver, laser source and amplitude modulator. Datasource produces pseudo random bit pattern at 10 Gb/s bit rate usedin each pre-, post- and symmetrical-dispersion compensationscheme to obtain statistical independence of results. Electrical dri-ver is important component that generates desired data transmis-sion format. It converts a logical input signal of a binary sequence(consisting zeros and ones) into an electrical signal. Duty cycle(0.2–0.8) of RZ data format is adjusted in the investigation to studyits comparative importance. Laser source generates laser beam at1550 nm. Its output and the output of electrical driver are givento a modulator. The output of modulator is fed to optical linkthrough an EDFA acting as a booster amplifier. Optical link is de-fined as pre-, post- and symmetrical-compensations according tothe order of spans placed. Two spans are considered so that thereare two DCFs each �80 ps/nm/km dispersion and two SSMFs eachwith 16 ps/nm/km dispersion. Their lengths are proportionatelyvaried to have a dispersion managed system for each of the trial.Optical signal is amplified after both types of fibers with EDFAsin each pre- and post-dispersion compensation scheme over onespan so there are total of five EDFAs in link. In the first case, optical

communication system is pre-compensated by DCFs of negativedispersion against SSMFs over the span. In the second case, opticalcommunication system is post-compensated by DCFs of negativedispersion against SSMFs over the span.

In order to compare the two compensation configurations, wedefine equivalent symmetrical-compensation dispersion configu-ration in third case whereby the system is symmetrically compen-sated by two DCFs of negative dispersion against two SSMFs inbetween with EDFAs after each type of fiber. So there are five ED-FAs for this configuration also. Length of optical link can be esti-mated through a simple relation [=2 � (length of SSMF) = 2 �5 � (length of DCF)] in each case. Thus, the link length can be com-puted indirectly by keeping different lengths of DCF fiber indirectlyas shown in from Figs. 3–11. Optical signal is detected at receiverby PIN detector and is passed through electrical filter and final out-put is observed on BER meter, Q meter to read corresponding val-ues which are subsequently plotted. Laser is of type CW Lorentzianwith laser center emission frequency 1550 nm (193.4145 THz).Amplitude modulator used is of type sine square with excess lossof 3 dB. Simulated bit rate is of 10 GHz. EDFAs are of fixed-outputpower type with noise figure of 4.5 dB each. Electrical filter is ofBessel type with 3 dB bandwidth equal to 8 GHz. PIN diode usedin detector has response 0.875. For comparison, 1 mW signalpower is fed into modulator then power of each optical amplifierin optical communication link is simultaneously kept at 8, 10and 12 dBm to find a power level to achieve desired performanceand subsequently Optimum amplifier spacing. All fiber nonlinear,birefringence and polarization mode dispersion effects are consid-ered in simulations. PMD coefficient of both SSMF and DCF is0.1 ps/km0.5. Attenuation and nonlinear coefficient for DCF is0.6 dB/km and 1.8 W�1 km�1 and that of SSMF is 0.2 dB/km and1.2 W�1 km�1, respectively.

3. Results and discussion

Second order chromatic dispersion of SSMFs is compensatedwith DCFs in each of three models considered. Standard relationD1L1 + D2L2 = 0 may be used to verify compensation, where Di

and Li are the first dispersion parameter and length of the respec-tive SSMFs and DCFs. In our system DDCF = �80 ps/nm/km,DSMF = 16 ps/nm/km and if LDCF = x, then LSMF = 5x. Thus consider-ing DCF length variable, one can find proportionate SSMF length.This idea will result in dispersion managed system at each valueof DCF length considered which is used here in this paper also.Third order dispersion can cause dispersion to small extend andneglected in present discussion. For single channel light-wave sys-tems, dominant nonlinear phenomenon that limits system perfor-mance is self-phase modulation (SPM). If launched power overamplified link is satisfying the relation (1) of peak power thenSPM due to phase accumulation over multiple amplifiers is of littleconcern [7]

Pin < 0:1a=ðcNAÞ ð1Þ

where Pin (W) is input peak power, a (dB/km) is attenuation, c(W�1 km�1) is nonlinear coefficient, NA is number of amplifiers inlink. A generalized equation of pulse propagation called nonlinearSchrodinger equation (NLSE) with neglecting third order dispersion,has the form (2)

oAozþ i

b2

2o2Aot2 ¼ �

a2

Aþ icjAj2A ð2Þ

where A(z,t) is slowly varying amplitude of pulse envelope, b2 isgroup velocity dispersion, a and c play same role as defined forEq. (1). Because of nonlinear nature of Eq. (2), it is usually solvednumerically. Calculation of propagation in optical fibers is

Fig. 3. Performance of the pre-dispersion compensated system at RZ pulse dutycycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 8 dBm in terms of (a) BERversus DCF length (km) and (b) timing jitter versus DCF length.

Fig. 4. Performance of the post-dispersion compensated system at RZ pulse dutycycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 8 dBm in terms of (a) BERversus DCF length (km) and (b) timing jitter versus DCF length (km).

5814 M. Singh et al. / Optics Communications 281 (2008) 5811–5818

performed by standard split-step algorithm with adaptive step-size[7]. Sequence lengths of 1024 bits are used to decrease error lessthan ±1 dB in the calculation Q factor and corresponding accuracyBER and timing jitter. In order to observe dependence of duty cycleof RZ optical pulse on the output of communication link, duty cyclevalues are varied 0.2–0.8 in 0.2 step size. Five fixed-output gain ED-FAs are used in link whose gains are changed simultaneously in thelink to provide output power 8–12 dBm in steps of 2 dBm to ob-serve the role of EDFAs. All graphs drawn in the manuscript, havehorizontal plots indicating permissible limit of BER = 10�9 and tim-ing jitter = 16 ps to decide system performance.

Graph between BER and duty cycle value at 8 dBm fixed-outputpower of EDFA is shown in Fig. 3a. It gives indication that pre-dis-persion compensation scheme is sharply affected by duty cycle andlink length. It is assumed that system is only useful i.e. ifBER < 10�9. For pre-compensation if duty cycle is changed from0.2 or 20% to 0.8 or 80%, the corresponding total viable link lengthcomputed is from 400 km to 450 km. Thus, pre-dispersion com-pensation scheme results into possible optical link length atrespective duty cycle in round bracket are 400 km (0.8), 420 km(0.6, 0.4), 450 km (0.2). These results are also listed in Table 1 whilecorresponding timing jitter variations are plotted in Fig. 3b. It indi-cates timing jitter can be improved by selecting low value of dutycycle. Other two schemes post- and symmetrical-dispersion com-pensation at EDFA power 8 dBm perform well for the range of duty

cycle as shown in Figs. 4 and 5 indicated in Table 1 as well. Post-dispersion compensation scheme gives small sensitivity to duty cy-cle as shown in Fig. 4a. Timing jitter has the effect of duty cycle andlink length as indicated in Fig. 4b. Here, smaller duty cycle valuemeans smaller value of timing jitter. Result of symmetrical-disper-sion compensation scheme at 8 dBm shows that system gives lessdependency on duty cycle also. It gives desired performance of thesystem and results into link length of 470 km and 400 km for post-and symmetrical-dispersion compensation scheme respectively.After these maximum lengths mentioned, system gives poor per-formance because of accumulation of nonlinearities SPM and ASEnoise.

Further, effect of same EDFAs power is observed by keeping it10 dBm, in three schemes and its results are plotted in graphs asshown in Figs. 6–8. Pronounced effect of duty cycle is observedin pre-compensation scheme. The minimum link length at arespective duty cycle value in bracket is 20 km (0.6) while themaximum value of link length is 42 km (0.2). These results arementioned in Table 1 and its variation in Fig. 6a. Higher link lengthof 42 km is obtained at the low value of duty cycle. So, lower valueof duty cycle will be advisable. In the Fig. 6b, corresponding timingjitter variations are shown. At 10 dBm EDFA power, results of Figs.7 and 8 show that post- and symmetrical-dispersion compensationschemes show less sensitivity and provide guaranteed link lengthof 470 km (any), 420 km (0.8) in respective order.

Fig. 5. Performance of the symmetrical-dispersion compensated system at RZ pulseduty cycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 8 dBm in terms of (a)BER versus DCF length (km) and (b) timing jitter versus DCF length (km).

Fig. 6. Performance of the pre-dispersion compensated system at RZ pulse dutycycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 10 dBm in terms of (a) BERversus DCF length (km) and (b) timing jitter versus DCF length (km).

M. Singh et al. / Optics Communications 281 (2008) 5811–5818 5815

Theoretically, if we substitute parameters of our model consid-ered (NA = 5, aSMF = 0.6 dB/km, cSMF = 1.8 W�1 km�1, aDCF = 0.2 dB/km, cDCF = 1.2 W�1 km�1) in relation (1). Threshold power com-puted to cause SPM is Pth = 11.7 dBm for SMF while 8.8 dBm isfor DCF. It shows that shifting EDFA power from 10 to 12 dBm, dis-tortion in the link is due to SPM nonlinearity of single mode fiber. Itis particularly because of launched power which exceeds thresholdlimit of 11.7 dBm. Moreover, the power is effectively depends onthe average value of signal power thus duty cycle and compensa-tion scheme shows certain safe regions of operation dependingon dispersion scheme and other parameters. Fig. 9a shows thatat 12 dBm EDFAs power, the pre-compensation scheme case is nolonger give viable region of operation. Fig. 10a show that post-dis-persion compensation scheme gives the largest link length at anyduty cycle value except at 0.8 (80%), even in comparison to sym-metrical-dispersion compensation scheme as shown in Fig. 11a.Accumulation of jitter may be observed in Figs. 10b and 11b forrespective case.

This behaviour is because SPM nonlinearity has been in-creased greatly, while going from 0.1 to 0.9 duty cycle, moreoveras we move from RZ pulse shape to very near NRZ pulse shape.In RZ format, each optical pulse represents bit 1, shorter than bitslot and its amplitude returns to zero before the bit duration is

over. In NRZ format, optical pulse remains ON throughout the bitslot and its amplitude does not drop to zero between two ormore successive 1 bits. As a result, pulse width varies dependingon bit pattern, whereas it remains same in the case of RZ format.An advantage of NRZ format is that bandwidth associated withbit stream is smaller than that of RZ format by a factor of 2 sim-ply because on–off transitions occurs fewer times. However, itsuse requires tighter control of pulse width and may lead to bitpattern dependent effects if optical pulse spreads during trans-mission. It is the cause of deterioration of BER at high duty cyclevalue visible in each BER versus duty cycle plot of this paper.But NRZ format is often preferred in practice because of a smal-ler signal bandwidth associated with it. Comparatively the use ofRZ format in the optical domain helps in the design of highcapacity light-wave systems [3–4]. Here, it is found that thereis always a limit for higher value of duty cycle of optical pulseon the basis of BER and timing jitter to be kept under controlbut there is also a limit on the smaller value of duty cycle onthe basis of its spectral bandwidth. Thus considering spectral as-pect of duty cycle of optical pulse only higher side limit of dutycycle should be used which is shown in Table 1. In every situa-tion, optimum choice of inter-amplifier spacing could be a solu-tion or alternatively appropriate duty cycle of RZ optical pulselaunched at the transmitter.

Fig. 7. Performance of the post-dispersion compensated system at RZ pulse duty cycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 10 dBm in terms of (a) BER versus DCFlength (km) and (b) timing jitter versus DCF length (km).

Fig. 8. Performance of the symmetrical-dispersion compensated system at RZ pulse duty cycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 10 dBm in terms of (a) BERversus DCF length (km) and (b) timing jitter versus DCF length (km).

Fig. 9. Performance of the pre-dispersion compensated system at RZ pulse duty cycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 12 dBm in terms of (a) BER versus DCFlength (km) and (b) timing jitter versus DCF length (km).

5816 M. Singh et al. / Optics Communications 281 (2008) 5811–5818

Fig. 10. Performance of the post-dispersion compensated system at RZ pulse duty cycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 12 dBm in terms of (a) BER versusDCF length (km) and (b) timing jitter versus DCF length (km).

Fig. 11. Performance of the symmetrical-dispersion compensated system at RZ pulse duty cycles values 0.2, 0.4, 0.6 and 0.8 when EDFA power = 12 dBm in terms of (a) BERversus DCF length (km) and (b) timing jitter versus DCF length (km).

Table 1Depicting the highest useful amplifier spacing (i.e. at BER = 10�9) for each dispersion compensation scheme for single channel optical communication system considered

Dispersioncompensationscheme

Optimum amplifier spacing in terms of DCF length at various EDFA powers with RZ rectangular optical pulse duty cycle in brackets i.e. DCF_Len (dutycycle)

8 dBm 10 dBm 12 dBm

Pre 40 km (0.8); 42 km; (0.6, 0.4) 45 km (0.2); i.e.DCF_Len P 40 km) link length = 400 km

25 km (0.8); 20 km (0.6); N.F. (0.4); 42 km (0.2); i.e.DCF_Len P 20 km) link length = 200 km

Not feasible (any value)

Post 47 km; (any value); i.e. DCF_Len P 47 km )link length = 470 km

47 km; (any value, except 0.8); i.e. DCF_Len P47 km ) link length = 470 km

47 km (any value, except 0.8) i.e.DCF_Len P 47 km ) link length = 470 km

Symmetrical 40 km (0.8); 42 km (0.6, 0.4); 44 km (0.2); i.e.DCF_Len P 40 km ) link length = 400 km

42 km (0.8); 43 km (0.6); 44 km (0.4); 45 km (0.2);i.e. DCF_Len P 42 km ) link length = 420 km

37 km (0.8); 40 km (0.6); 42 km (0.4); 45 km(0.2); i.e. DCF_Len P 37 km ) linklength = 370 km

Link length may be computed indirectly in the present context as = 10 � DCF_Len. Maximum link length is also computed in each case and written in respective cell of table.NF stands for ‘not feasible’.

M. Singh et al. / Optics Communications 281 (2008) 5811–5818 5817

4. Conclusions

The performance of pre-, post- and symmetrical-dispersioncompensation schemes is compared by taking 8, 10 and12 dBm fixed-output power EDFAs in single channel optical com-munication link at 0.2–0.8 (step size 0.2) duty cycle of RZ optical

pulse. There is significant relationship among duty cycle value ofRZ optical pulse, power level of in line fixed amplifiers, linklength and dispersion compensation scheme used for opticalcommunication system. From results, we have shown that pre-compensation scheme performs better at EDFA power 8 dBm. Itis highly sensitive to selection of duty cycle as well as EDFA

5818 M. Singh et al. / Optics Communications 281 (2008) 5811–5818

power and link length varies widely in it. In the case of post-compensation, it is observed that inter-amplifier spacing is lessaffected by the variations in EDFA power and duty cycle. Themaximum link length of 470 km is obtained in post-dispersioncompensation scheme and is independent of five in line 8–12 dBm powered EDFAs. Alternatively, it gives maximum lengthof DCF 47 km or SSMF 235 km at any duty cycle except 0.8among considered values 0.2–0.8. Symmetrical-compensationscheme is second the best performing case, it gives maximumlink length 450 km with 0.2 duty at EDFA power 10 dBm but varywidely with duty cycle. We recommend that to achieve maxi-mum link length, post-dispersion compensation scheme, andsmall duty cycle value must be used being more resilient to non-linearity and timing jitter. It also makes our system more insen-sitive to EDFA power fluctuations.

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