Optics Communications 285 (2012) 1819–1824

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Optics Communications

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Improving the performance of electrical duty-cycle division multiplexingwith optimum signal level spacing

Ghafour Amouzad Mahdiraji a,⁎, Ahmad Fauzi Abas b

a Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Photonics and Fiber Optic Systems Laboratory, Centre of Excellence for Wireless and Photonics Networks, Engineering and Technology Complex, University Putra Malaysia, 43400 Serdang,Selangor, Malaysia

⁎ Corresponding author. Tel.: +60 126274711.E-mail address: ghafouram@gmail.com (G. Amouzad

0030-4018/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.optcom.2011.12.022

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 December 2010Received in revised form 2 December 2011Accepted 2 December 2011Available online 17 December 2011

Keywords:Optical fiber communication systemsMultiplexing techniquesDuty-cycle division multiplexing (DCDM)Amplitude distribution controllerDual-drive Mach–Zehnder modulator(DD-MZM)Bias voltage optimization

Performance optimization of 3×10 Gbps conventional electrical-duty-cycle division multiplexing(C-E-DCDM) technique is investigated. It is shown that controlling signal level spacing can optimize itsperformance. Two level spacing optimization techniques, one in electrical domain and another in opticaldomain are examined. In general, performance of the C-E-DCDM is improved significantly using bothapproaches. The results show by optimization, an improvement of around 5.5 dB can be achieved for theC-E-DCDM in terms of receiver sensitivity and optical signal-to-noise ratio using both electrical and opticalmethods. However, chromatic dispersion tolerance in one of the optimization approaches is degraded byaround 34 ps/nm for negative dispersion, while the positive dispersion tolerance improved compared tothe C-E-DCDM.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Wavelength division multiplexing (WDM) is becoming as themain transmission technique for long-haul optical fiber communica-tion systems today [1–3]. Novel modulation formats [4], multiplexingtechniques [5], multilevel signaling [6], dispersion management [7],forward error correction [8], narrow optical filtering [9], pulseshaping [10], and signal level spacing [11] have been used to achievea better transmission performance and a higher capacity.

Recently, duty-cycle division multiplexing (DCDM) is proposed tomultiplex multiple users per WDM channel [12–14]. The techniqueoperates based on return-to-zero (RZ) duty-cycle, where signals ofdifferent users are signed with different percentage of duty-cycle.Then, signals with different duty-cycles are multiplexed or addedtogether in the same time to form a multilevel step down shapesignal. Using this technique, spectral width of RZ-time divisionmultiplexing (RZ-TDM) is reduced. For example, 3×10 Gbps conven-tional RZ-TDM (RZ with 50% duty-cycle) signal has null-to-null mod-ulation spectral width of 120 GHz, whereas, 3×10 Gbps DCDM signalhas 80 GHz [13,15]. This is because n-channel DCDM multiplexedsymbol is divided into n+1 slots, where the shortest pulse has aduration of 1/(n+1) seconds for the case of uniformly duty-cycle

Mahdiraji).

rights reserved.

distribution. However, in n-channel RZ-TDM, the multiplexed symbolis divided into 2n slots (for the case of bit 1) where the shortest pulsewidth is 1/(2n) seconds [15]. In addition, instead of one impulse tran-sition on spectrum of RZ signal, n-channel DCDM provides n numbersof impulse transitions in its spectrum. Considering the impulse withthe lower frequency, which is equal to the symbol rate, the data andclock recovery system, and sampling process can be performed atthe frequency equal to the symbol rate. Furthermore, due to theexistence of only one rising edge transition per DCDM multiplexedsymbol, which is located at the beginning of each symbol; the tech-nique facilitates the symbol level synchronization, where the begin-ning of each symbol can be simply determined by detecting therising edges of the received signals. Thereby, extra overhead bitsrequired in TDM for symbol level synchronization can be neglectedwhen DCDM is implemented. Similar to TDM, DCDM multiplexercan be designed in both optical domain (refer to O-DCDM) [15] andelectrical domain (refer to E-DCDM) [12], however, in both casesthe demultiplexing process performs only in the electrical domain.In addition, E- and O-DCDM signals can tolerate higher chromaticdispersion in comparison to RZ signal due to smaller spectral widthas explained above; however, they suffer from the low receiver sensi-tivity and optical signal-to-noise ratio (OSNR) due to multiple signallevels. The O-DCDM performs slightly better than the E-DCDM, interms of receiver sensitivity and OSNR; however, it required oneintensity modulator per multiplexing user. This makes the transmit-ter of the O-DCDM more costly relative to the E-DCDM, which can

Table 1Q-factors for Eye1 to 6 for C-E-DCDM, EM, and OM at BER 10−9.

Q-Eye1 Q-Eye2 Q-Eye3 Q-Eye4 Q-Eye5 Q-Eye6

C-E-DCDM 5.59 6.70 14.36 6.92 14.26 14.44E-DCDM-EM 5.83 5.91 5.82 6.01 6.10 5.77E-DCDM-OM 5.82 6.41 5.83 6.5 5.96 5.81

1820 G. Amouzad Mahdiraji, A.F. Abas / Optics Communications 285 (2012) 1819–1824

multiplex any number of users with one analog amplitude modulator(AM). Thus, this paper presents an investigation on performance op-timization and enhancement of the E-DCDM; however, performancecomparison of the E-DCDM against the O-DCDM is not within thescope of this paper.

2. Methodology

Fig. 1(a) shows example of the conventional E-DCDM(C-E-DCDM) output signal for three users system. As shown in thefigure, the signal level is spaced equally between different slots. Theequally-spaced signal level is optimum only for the case of thermalnoise, where the noise is not intensity dependent. However, sinceoptical amplifiers are used in the system, the noise is intensity depen-dent. This means that the signals with higher power level experiencehigher noise or signal variations as compared to the one with thelower power. This effect is shown in Fig. 1(b), which represents theeye diagram of the 3-user C-E-DCDM. This eye diagram is obtainedafter passing the DCDM signal over transmission system with opticalamplifiers. Considering the quality of the received signal, the eyeslocated in different levels experienced different Q-factor. Q-factor inthis study is calculated for each eye individually by considering eachof them as a binary signal eye diagram using [16]

Qk;j ¼μ i;j−μ i−1;j

��� ���σ i;j þ σ i−1;j

ð1Þ

where μi,j and σi,j respectively represent the mean and standarddeviation of signal at level i=1, 2, and 3 at sampling point j=1, 2,and 3; and Qk,j is the Q-factor of the eye located at the row k, whichis located between level i and i−1, and sampling point j. As shownin the second row of Table 1, the eyes located in the lower level

0 0.25 0.5 0.75 10

0.25

0.5

0.75

1

Pow

er (

mW

)

0 0.25 0.5 0.75 10

1

2

3

0 0.25 0.5 0.75 10

0.5

1

1.5

0 0.25 0.5 0.75 10

0.25

0.5

0.75

1

Pow

er (

mW

)

0 0.25 0.5 0.75 10

0.5

1

1.5

Time (symbol period)0 0.25 0.5 0.75 1

0

0.25

0.5

0.75

1

Time (symbol period)

Pow

er (

mW

)

(a) (b)

0.667 mW

0.333 mW

1 mW

3

2

1

4

5 6

(e) (f)

0.500 mW0.145 mW

1 mW

(c) (d)

0.506 mW

0.158 mW

1 mW

OM

EM

3 x 10 GbpsC-E-DCDM

Fig. 1. Signal level spacing (left) and eye diagrams (right) for, (a and b) C-E-DCDM,(c and d) EM, and (e and f) OM.

(eyes number 3, 5, and 6 based on Fig. 1(b)), have higher Q-factorof around 14.3 in comparison to the Eye1, which has the worstQ-factor of around 5.59. This difference causes different performancebetween E-DCDM's users. Thus, optimizing the level spacing becomesnecessary. In this paper, two methods for the optimum level spacingdetermination, one electrical and the other optical, are examined.The optical method is based on dual-drive Mach–Zehnder modulator(DD-MZM) and the electrical approach is based on a mathematicalanalysis as discussed in the following section.

3. Simulation setup

Simulation setup of the C-E-DCDM is reported in Ref. [12]. Here, asmall modification at the transmitter side is performed to optimizethe signal level spacing. Fig. 2 shows the simulation setup used inthis study for three-user system. The simulation is performed byusing OptiSystem and Matlab. In this setup, User 1 (U1) to U3, eachwith 10 Gbps, are curved with three RZ modulators. For faircomparison, pseudo-random bit sequence (PRBS) used in this studyis set at 210–1, which is similar to the C-E-DCDM reported in Ref.[12]. The electrical RZ pulse generators are set to produce rectangleshape pulses with different duty-cycles and exponential rising andfalling edge as

ERZ tð Þ ¼1−e− t=crð Þ

; 0≤ t b t11; t1 ≤ t b t2e− t=cfð Þ; t2 ≤ t b tc0; tc ≤ t b T

8>><>>:

ð2Þ

where cr and cf are the rise and fall time coefficient, respectively. t1and t2, together with cr and cf, are determined to generate pulseswith the exact values of rise time and fall time. tc is the duty-cycleduration, and T is the bit period. In this study, the rise time and falltime are set at 5% of the bit period. For simplicity, the duty-cycle isuniformly distributed between different channels, where RZ1, RZ2and RZ3 are fixed at 25%, 50%, and 75% duty-cycles, respectively.Output of the RZ modulators are then synchronously multiplexedusing an electrical adder.

In the C-E-DCDM, the multiplexed signals are modulated over alaser diode (LD) signal by using an analog amplitude modulator(AM). As mentioned earlier, the signal produced by the C-E-DCDMis a step down multilevel signal with equal level spacing as shownin Fig. 1(a and b), which is not optimized especially when opticalamplifiers are used in the system.

In this study, for example, two approaches one in electrical andthe other in the optical domain are used to optimize the level spacingof the C-E-DCDM. The objective of the paper is to show that theperformance of the C-E-DCDM can be improved by using differentapproaches either in electrical or optical domain.

In the electrical method (EM), the equation y(t)=xm(t) is used tocontrol the signal level spacing, where x(t) is the signal with equallevel spacing output from the multiplexer shown in Fig. 2. m is con-sidered as an coefficient that controls the signal level spacing. y(t) isthe desired signal whose level spacing is modified according to them value. Signal y(t) is then externally modulated over a distributedfeedback (DFB) laser with 10 MHz linewidth that operates at1550 nm, using an ideal AM. The analog modulator is used because

(a)

(b)

Fig. 2. E-DCDM system with the electrical ADC (block (a)), and optical ADC( block b).

-5 -4 -3 -2 -1 00

1

2

3

4

5

6

Voltage (V)

Abs

orpt

ion

(dB

) an

d P

hase

(ra

dian

s)

PhaseAbsorption

Fig. 3. Dependency of the absorption and phase of the optical signal as a function ofapplied voltage for each arm of DD-MZM at splitting ratio of 1.3 [17].

1821G. Amouzad Mahdiraji, A.F. Abas / Optics Communications 285 (2012) 1819–1824

it provides an optical output signal Opout(t), with the same level spac-ing format as the electrical input signal y(t) as follows

Opout tð Þ ¼ Opin tð Þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1−MIð Þ þMI⋅y tð Þð Þ

pð3Þ

where Opin(t) is the optical input signal (here the LD signal).MI is themodulation index between 0 and 1, which in this study it is fixed at 1.In this equation, the electrical input signal is normalized between0 and 1.

On the other hand, for optical method (OM), a multiple-quantum-well based DD-MZM [17–19] is used to control the signal levelspacing [11]. In this case, block (b) represented by a discrete line inFig. 2 replaces the block (a). In this method, the DD-MZM consistsof an input Y-branch splitter, two arms with independent driveelectrodes, and an output Y-branch combiner. The optical carriersignal is split into two arms of the interferometer from the inputY-branch. The on- and off-state is achieved when there is constructive(no phase shift) and destructive (π radian phase shift) interferencebetween the two signals at the output Y-branch. In a good approxi-mation, the output signal from the modulator is the sum of the fieldsat the output of the two arms. The optical output signal (Opout) from amodulator with the same input and output Y-branch splitting ratio isgiven by [17–19]

Opout V1; V2ð Þ ¼ E01þ Sr

Sr e−L 0:5Δαa V1ð ÞþjΔβ V1ð Þð Þ þ e−L 0:5Δαa V2ð ÞþjΔβ V2ð Þð Þþjϕ0

h i

≡ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiI V1; V2ð Þp

ejΦ V1 ; V2ð Þ

ð4Þ

where E0 is the input optical signal to the modulator; Sr=P1/P2 is theY-branch power splitting ratio; 0.5Δαa is the attenuation constant;Δβ is the phase constant; L is the interaction length of the modulatorarm; ϕ0 is zero radians for a conventional modulator and π radian fora phase-shift modulator; V1 and V2 are the voltages applied to arms 1and 2, respectively; I is the intensity of the optical signal; Φ is thephase. The Vi (for i=1, 2) is defined as

Vi tð Þ ¼ Vbi þ V mod1; 2 υ tð Þ ð5Þ

where Vbi is the bias voltage; Vmod 1, 2 is the peak-to-peak modulationvoltage of the input signal applied on arms 1 and 2; and υ(t) is theinput modulation waveform with a peak-to-peak amplitude of oneand average value of zero. Since voltage of the input modulatingwaveform (υ(t)) is considered to be normalized, thus, the peak-to-peak magnitude of the input modulation waveform applied on arms1 and 2 of the modulator can be changed by Vmod 1, 2. Fig. 3 illustratesthe dependency of the absorption (Δαa) and phase (Δβ) of the opticalsignal on applied voltage for π phase-shift modulator. For DD-MZM inthis study, ϕ0=π radian and L=600 μm are considered. The modelalso utilized a dual-drive (push–pull) modulation, where ΔV1=−ΔV2 [17]. Thus, in this study, the level spacing is controlled bychanging the parameters of the DD-MZM, namely bias voltages (Vb1

and Vb2), the voltage of modulating signal applied on arm 1 and 2(Vmod 1, 2), and the splitting ratio (Sr).

The modulated signal (DD-MZM or IM) is then transmitted overthe transmission link that include a booster and pre-amplifier thatare based on Erbium doped fiber amplifier (EDFA) with 4 dB noisefigure. An optical attenuator between the booster and pre-amplifieris used for back-to-back receiver sensitivity and OSNRmeasurements.Standard single mode fiber (SSMF) is used before the optical attenu-ator for chromatic dispersion tolerance measurement. The receivedsignal is then detected with a p-i-n photo detector (PD) followed bya low-pass filter (LPF). In this simulation, the PD responsivity is setat 0.8 A/W, the dark current is 10 nA, and the thermal noise powerspectral density level is 10−22 W/Hz. In general, all the noises thatare considered in the simulation include thermal noise, shot noise,and the signal-amplified spontaneous emission (ASE) noise. Thegain of the pre-amplifier before PD is kept at a value so that theoutput is lower than the PD maximum power. The noise from thePD is mitigated using the electrical Gaussian LPF with transferfunction H(f) of [20]

H fð Þ ¼ αexp −0:5 ln2fB

� �2N� �ð6Þ

where α is the insertion loss, B is 3-dB bandwidth of the filter, N is thefilter order, and f is the signal frequency. In this simulation, α isconsidered to be minimum (0 dB), B is set at 30 GHz, and N is 1. Tocompare performance of these systems against the C-E-DCDMreported in Ref. [12], the optical band-pass filter (OBPF) is left outfrom the simulation setup. However, performance of the system isexpected to be improved by using OBPF. The demultiplexing processis performed similar to the C-E-DCDM [12], where using a clockrecovery circuit, 10 GHz clock is recovered by detecting the 10 GHzimpulse transition from the received signal spectrum. Then, the

Fig. 5. Splitting ratio optimization of DD-MZM.

1822 G. Amouzad Mahdiraji, A.F. Abas / Optics Communications 285 (2012) 1819–1824

received signals are sampled using three samplers each oscillating at10 GHz, where by putting appropriate delay line, the 1st, 2nd and 3rdsamplers take sample at the best point (which provides the higherQ-factor) around Ts/8, 3Ts/8 and 5Ts/8 s per symbol (where Ts issymbol duration), respectively. Decision and regeneration are per-formed by comparing every sampled value with three thresholds.Since signal of U1 is RZ with 25% duty-cycle (or pulse width of Ts/4), thus its data can be easily determined using information extractedfrom the 1st and 2nd sampling points. If the amplitude of the 1stsample is equal to the 2nd sample, bit 0 is recovered. If the amplitudeat the 1st sampling point is greater than the 2nd sampling point, bit 1is discovered. A similar approach is used for regenerating U2 (with50% duty-cycle) by considering the information extracted from the2nd and 3rd sampling points. U3 (with 75% duty-cycle) is recoveredby considering the amplitude of only the 3rd sampling point. Detailof the regeneration rules can be found in Refs. [12,14,15]. Bit errorrate (BER) in this paper is calculated by using the probability oferror method based on eye diagrams as reported in Refs. [12,14,15].

4. Results and discussion

4.1. Level spacing optimization

Fig. 4 shows the BER of 3×10 Gbps E-DCDM as a function of thecoefficient m used in electrical approach. The result is obtained byvarying the coefficient value while the optical signal-to-noise ratio(OSNR) of the system was fixed at a value close to the systemrequired OSNR. With coefficient value m=1.68, the system operatesin the optimized state, where all the three users have similar BER.At this coefficient value, the optimum level spacing for the zero,first, second, and third levels relative to the maximum level are 0,15.8, 50.6, and 100%, respectively, as shown in Fig. 1(c).

For optimizing the performance of O-DCDM using DD-MZM, firstthe effect of modulator splitting ratio is tested at fixed Vb1=−0.5 Vand Vb2=−1.5 V as shown in Fig. 5. As illustrated, in all varioussplitting ratios, the minimum BER is obtained when the Vmod 1, 2 isaround 1 V, which is equivalent to Vmod 1, 2=Vb1−Vb2. At the sametime, the best system performance is achieved at the splitting ratioof 1. Secondly, the effect of bias voltages on performance ofO-DCDM is investigated. For this purpose, two different conditionsfor bias voltages are considered, where in the first case, Vb1=−Vb2,while in the second case, the two bias voltages are independent.Considering the first condition with Vb1=−Vb2, Fig. 6(a) shows thenormalized modulator output intensity as a function of Vb1, whilethe splitting ratio is 1 and Vmod 1, 2=1.2 V. The peak value of thenormalized intensity due to the absorption that occurs in each armof the modulator is less than one. As illustrated in the figure, the Vπ

or the voltage required to obtain π radians phase shift in each armof the modulator is around 3.35 V, which can also be observed fromthe phase dependent of the modulator as shown in Fig. 3. In addition,as illustrated in Fig. 6(a), the quality of O-DCDM eye diagram is chan-ged by the change of bias voltages, while the optimum eye is aroundVb1=0.6 V (thus, Vb2=−0.6 V). By changing the input modulationvoltage (Vmod 1, 2), the position of the optimum eye is slightly shifted

1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.212

10

8

6

Coefficient m

-log(

BE

R)

Fig. 4. Level spacing optimization for EM.

accordingly as shown in Fig. 6(b). According to the result in Fig. 6(a)and the relationship observed from Fig. 5 (Vmod 1, 2=Vb1−Vb2), theoptimum performance in the case of for example Vmod 1, 2=2 and0.4 V is expected to be around Vb1=1 and 0.2 V, respectively, whileVb2=−Vb1. For verification, the performance of O-DCDM system asa function of bias voltages and various Vmod 1, 2 is shown inFig. 6(c). As illustrated in the figure, for all Vmod 1, 2 values, the mini-mum BER is obtained when Vmod 1, 2=Vb1−Vb2. This is similar to theearlier case. In addition, all the curves at different Vmod 1, 2 show thesame system performance. This means the optimum bias andmodulation voltage can be easily achieved by satisfying the relation-ship, Vmod 1, 2=Vb1−Vb2. Considering this relationship, Fig. 6(d)shows performance of the 3-user O-DCDM system as a function ofVb1. The range of Vb1 is from −1.5 to 1.5 V (or around −0.45Vπ to0.45Vπ), which provides very similar and minimum BER, except atVb1=0 V (modulator is in off-state). The value of Vmod 1, 2 in thisrange is −3 to 3 V (or 0.9Vπ to −0.9Vπ), while Vb2=−Vb1. This isthe optimum operation range of DD-MZM for 3-user O-DCDM systemat the condition Vb1=−Vb2. In specific, as illustrated in the figure, ataround Vb1=−0.5 V, Vb2=0.5 V, and Vmod 1, 2=−1 V (or Vb1=−0.15Vπ, Vb2=0.15Vπ, and Vmod 1, 2=−0.3Vπ), BER of the system isslightly lower compared to the other points. Thus, these bias andmodulation voltages are considered as the optimum points for thecondition Vb1=−Vb2. For the second condition, where the twobias voltages are varied independently, Fig. 7 shows the effect of Vb1

on performance of O-DCDM at different value of Vb2, while theVmod 1, 2=Vb1−Vb2. There are several optimum points, which pro-vides almost the same performance as compared to the optimumpoint in the first condition (Vb1=−Vb2). However, BER at Vb1=−1.5 V and Vb2=1 V (or Vb1=−0.45Vπ and Vb2=0.3Vπ) is slightlybetter (around 0.3 dB difference in terms of OSNR) as compared tothe other points and also when compared to any points obtained inthe first condition. Thus, the bias values (Vb1=−1.5 V andVb2=1 V) with splitting ratio of 1 and Vmod 1, 2=Vb1−Vb2 are usedin this study as the optimum condition for calculating the perfor-mance of O-DCDM. At this optimum condition, signal level spacingin the O-DCDM compared to the maximum signal level is spacedaround 0, 14.54, 50.01, and 100% for the zero, first, second, andthird levels, respectively, as shown in Fig. 1(e).

4.2. Receiver sensitivity and OSNR

Fig. 8(a) shows the performance of 3×10 Gbps E-DCDM systemoptimized with the EM and OM, as a function of the received power.For comparison purposes, the performance of the worst user in theC-E-DCDM is presented. The receiver sensitivity is obtained byswiping the optical attenuator value (Fig. 2) and measuring the

(a)

(b)

(c)

(d)

Fig. 6. Bias voltage optimization of DD-MZM for the first condition (Vb1=−Vb2),(a) examples of eye diagrams at different Vb1, (b) the effect of input single voltage(Vmod 1, 2) on modulator output intensity as a function of Vb1, (c) BER of the systemas a function of bias voltages at various Vmod 1, 2, and (d) the optimum range of biasvoltages for the condition Vb1=−Vb2.

Fig. 7. Bias voltages optimization for the second condition, where the bias voltagesvaried independently.

1823G. Amouzad Mahdiraji, A.F. Abas / Optics Communications 285 (2012) 1819–1824

total average power received after the attenuator by using an opticalpower meter. From the results, at optimum condition, in both EM andOM, all the three users show almost similar performance. Referring toBER of 10-9, 30 Gbps E-DCDM required a received power of around−28.85 dBm, while around −23.4 dBm for the worst channel usingthe C-E-DCDM. This result confirms that using an appropriateapproach, performance of the C-E-DCDM can be improved up toaround 5.5 dB. Fig. 1(b), (d), and (f) shows the eye diagram of theE-DCDM based on the conventional structure, EM, and OM, respec-tively. All the three eye diagrams are obtained at BER of 10−9, whiletheir received powers were set at their receiver sensitivity value.The Q-factor of Eyes 1 to 6 for all the three systems are calculatedand presented in Table 1. As shown in the table, in C-E-DCDM, theQ-factor between the best eye (Eye6) and the worst eye (Eye1) variesby around 8.85. Whereas, by optimizing the signal level spacing usingthe EM and OM, the Q-factor variation between the best and theworst eyes is reduced to around 0.33 and 0.69, respectively. Thisresult confirms a good level of optimization for both methods.

Fig. 8(b) shows the BER of the E-DCDM system optimized with theEM and OM, as a function of OSNR. Similar to the receiver sensitivity,the OSNR of the system is obtained by swiping the optical attenuatorvalue. Then, the OSNR value is measured after the pre-amplifier and

(a)

(b)

Fig. 8. Receiver sensitivity (a) and OSNR (b) of E-DCDM before and after optimizedwith EM and OM.

-150 -120 -90 -60 -30 0 30 60 90 120 15012

9

6

3

0

Fig. 9. Chromatic dispersion tolerance of E-DCDM before and after optimized with EMand OM.

1824 G. Amouzad Mahdiraji, A.F. Abas / Optics Communications 285 (2012) 1819–1824

before the PD by using a WDM analyzer at a resolution bandwidth of0.1 nm, where the OSNR is calculated by subtracting the signalspectrum peak power from the noise spectrum peak power. Accord-ing to the results, in both approaches, all three users show almostsimilar performance. With both EM and OM showing very similarperformance, OSNR in OM is slightly better than EM with onlyabout 0.1 dB, which is negligible. At BER of 10−9, the E-DCDM systemoptimized with OM required around 23.3 dB OSNR, which is around5.5 dB better than the C-E-DCDM that required an OSNR of around28.85 dB for the worst user. At this OSNR and the above mentionedreceiver sensitivity, the average and peak power received by the PINPD are around −0.11 and −7.8 dBm using the EM, and around−0.08 and−7.7 dBm using the OM, respectively. The result confirmsthat performance of the C-E-DCDM can be optimized and improvedsignificantly by controlling the signal level spacing.

4.3. Chromatic dispersion tolerance

Fig. 9 shows the chromatic dispersion tolerance of 30 GbpsE-DCDM optimized with OM and EM. For comparison purpose,chromatic dispersion tolerance of the worst user in the C-E-DCDM isshown in this figure. After optimization, all three channels showalmost similar performance for both EM and OM with the maximumof around ±2 ps/nm difference between the best and the worst userat BER 10−9, except U3 with 75% duty-cycle, which experiences somefluctuation. Considering performance of the worst user, 30 GbpsE-DCDM optimized with EM can tolerate around ±93 ps/nm of thechromatic dispersion, which is almost similar to the C-E-DCDM.However, E-DCDM optimized with OM shows different dispersiontolerance in positive and negative dispersion, which are around+100 and −59 ps/nm, respectively.

5. Conclusion

Optimization of 3×10 Gbps E-DCDM system has been successfullyreported. It is shown that controlling the signal level spacing is a keyelement for achieving the objective. By optimizing signal levelspacing, performance of the C-E-DCDM is improved significantly inboth the optical and electrical approach. However, dispersiontolerance is slightly reduced relative to the C-E-DCDM. The resultconfirms the effect of signal level spacing optimization technique inoptimizing the performance of the C-E-DCDM. This finding willopen up new efforts in pushing towards a better E-DCDM transmitterand receiver design.

References

[1] A.H. Gnauck, G. Charlet, P. Tran, P.J. Winzer, C.R. Doerr, J.C. Centanni, E.C. Burrows,T. Kawanishi, T. Sakamoto, K. Higuma, Journal of Lightwave Technology 26 (2008)79.

[2] S. Ahsan, M.S. Lee, S. Newaz, A. Syed, Journal of Networks 6 (2011) 18.[3] A.H. Gnauck, P.J. Winzer, S. Chandrasekhar, X. Liu, B. Zhu, D.W. Peckham, Journal

of Lightwave Technology 29 (2011) 373.[4] T. Tokle, M. Serbay, J.B. Jensen, W. Rosenkranz, P. Jeppesen, OFC/NFOEC

2008–2008 Conference on Optical Fiber Communication/National Fiber OpticEngineers Conference, 2008.

[5] G. Bosco, A. Carena, V. Curri, P. Poggiolini, F. Forghieri, IEEE Photonics TechnologyLetters 22 (2010) 1129.

[6] M. Nakazawa, S. Okamoto, T. Omiya, K. Kasai, M. Yoshida, 2010 Conference onOptical Fiber Communication, Collocated National Fiber Optic EngineersConference, OFC/NFOEC 2010, 2010.

[7] O.V. Shtyrina, M.P. Fedoruk, R.S. Bhamber, S. Boscolo, S.K. Turitsyn, J. Schwartz, S.Barnes, Optics Communications 283 (2010) 2263.

[8] Y. Miyata, K. Sugihara, W. Matsumoto, K. Onohara, T. Sugihara, K. Kubo, H.Yoshida, T. Mizuochi, 2010 Conference on Optical Fiber Communication, Collocat-ed National Fiber Optic Engineers Conference, OFC/NFOEC 2010, 2010.

[9] G. Gavioli, E. Torrengo, G. Bosco, A. Carena, S.J. Savory, F. Forghieri, P. Poggiolini,IEEE Photonics Technology Letters 22 (2010) 1419.

[10] C. Del Rio Campos, P.R. Horche, A. Martin-Minguez, Optics Communications 284(2011) 1346.

[11] A. Malekmohammadi, M.H. Al-Mansoori, G.A. Mahdiraji, A.F. Abas, M.K. Abdullah,Optics Communications 283 (2010) 3145.

[12] G.A. Mahdiraji, M.K. Abdullah, A.M. Mohammadi, A.F. Abas, M. Mokhtar, E. Zahedi,Optics and Laser Technology 42 (2010) 289.

[13] G.A. Mahdiraji, M.K. Abdullah, M. Mokhtar, A.M. Mohammadi, A.F. Abas, S.M.Basir, R.S.A.R. Abdullah, Photonic Network Communications 19 (2010) 233.

[14] G.A. Mahdiraji, M.K. Abdullah, M. Mokhtar, A.M. Mohammadi, R.S.A.R. Abdullah,A.F. Abas, Optical Review 16 (2009) 422.

[15] G.A. Mahdiraji, A.F. Abas, M.K. Abdullah, A. Malekmohammadi, M. Mokhtar,Japanese Journal of Applied Physics (JJAP) 48 (2009) 6.

[16] S. Ohteru, N. Takachio, IEEE Photonics Technology Letters 11 (1999) 1307.[17] J.C. Cartledge, Journal of Lightwave Technology 18 (2000) 647.[18] J.C. Cartledge, Journal of Lightwave Technology 17 (1999) 1142.[19] J.C. Cartledge, IEEE Journal of Selected Topics in Quantum Electronics 9 (2003)

736.[20] L.W. CouchII, Digital and Analog Communication Systems, 6th ed. Prentice-Hall,

USA, 2001.