Report Zelalem Hailu

8/2/2019 Report Zelalem Hailu

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OPTICAL

TRANSMISSION -2

VIRTUAL LAB

(optsim)SIMULATION

REPORT

P o l i t e c n i c o d i T o r i n o

http://www.polito.it/

3 / 1 4 / 2 0 1 2

BY ZELALEM HAILU GEBEYEHU

This virtual lab simulation is done to analyze the effect of

chromatic dispersion and fiber nonlinearity on the

performance of optical communication systems. more over

the characteristics and performance of different modulation

formats has been evaluated with and without the presence of

chromatic dispersion and nonlinear effect . the effect on the

performance due to the type of optical filter used at the

receiver and signal pulse types at transmitter is also included.

The simulation is specifically done by having a target bit error

probability 10e-3.


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INTRODUCTION

This virtual lab simulation is done to analyze the effect of chromatic dispersion and fiber nonlinearity on the

performance of optical communication systems. more over the characteristics and performance of

different modulation formats has been evaluated with and without the presence of chromatic dispersionand nonlinear effect . the effect on the performance due to the type of optical filter used at the receiver

and signal pulse types at transmitter is also included. The simulation is specifically done by having a target

bit error probability 10e-3.

SHORT OVERVIEW OF HELPFUL THORIES FOR THIS VERTUAL LAB

SIMULATION.

Optical communications can be described as the transmission of information by modulation of a carrier

frequency that is generated by an optical source, and the recovery of that information by means of anoptically sensitive receiver. Now a days, optical communication is the dominating technology for high

bandwidth and data rate demanding communication delivery service. It is also the most successful choice

for high bitrate long haul transmission system. On the other hand ,there so many impairments which

degrade the performance of optical fiber communication, among which chromatic dispersion and fiber non

linearity are the main ones. In order to maintain the good performance of fiber communication link, one

has to deal with nonlinearity and chromatic dispersion by using advanced modulation formats and different

compensation schemes.

CHROMATIC DISPERSION

Chromatic dispersion is a broadening of the input signal as it travels down the length of the fiber. The

concept to consider when talking about chromatic dispersion should be optical phase. It is important to

mention optical phase before any explanations of chromatic dispersion or group delay because of their

mathematical relationship. Group delay is defined as the first derivative of optical phase with respect to

optical frequency. Chromatic dispersion is the second derivative of optical phase with respect to optical

frequency. These quantities are represented as follows:

dispersion degrade the optical modulated signal, reducing the data carrying capacity through pulse-

broadening. Chromatic dispersion results from a variation in propagation delay with wavelength.

FIBER NONLINEAR EFFECT

Nonlinear effects in optical fibers occur due to change in the refractive index of the medium with optical

intensity. The power dependence of the refractive index is responsible for the Kerr-effect. Depending upon

the type of input signal, the Kerr-nonlinearity manifests itself in three different effects such as Self-PhaseModulation (SPM), Cross-Phase Modulation Cross-Phase Modulation (XPM) and Four-Wave Mixing (FWM).


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SELF-PHASE MODULATION (SPM)

The higher intensity portions of an optical pulse encounter a higher refractive index of the fiber compared

with the lower intensity portions while it travels through the fiber. In fact time varying signal intensity

produces a time varying refractive index in a medium that has an intensity dependent refractive index. The

effective refractive index of a nonlinear medium can be expressed in terms of the input power (P) and

effective core area (Aeff ) as,

The nonlinear effects depend on ratio of light power to the cross-sectional area of the fiber. This temporally

varying index change results in a temporally varying phase change. The optical phase changes with time in

exactly the same way as the optical signal . Since, this nonlinear phase modulation is self-induced the

nonlinear phenomenon responsible for it is called as self-phase modulation.

CROSS PHASE MODULATION (XPM)

SPM is the major nonlinear limitation in a single channel system. The intensity dependence of refractive

index leads to another nonlinear phenomenon known as cross-phase modulation (XPM). When two or

more optical pulses propagate simultaneously, the cross-phase modulation is always accompanied by SPM

and occurs because the nonlinear refractive index seen by an optical beam depends not only on the

intensity of that beam but also on the intensity of the other copropagating beams. In fact XPM converts

power fluctuations in a particular wavelength channel to phase fluctuations in other copropagating

channels. The result of XPM may be asymmetric spectral broadening and distortion of the pulse shape.

FOUR-WAVE MIXING (FWM

FWM is the phenomenon experienced in multichannel WDM system. SPM and XPM are significant mainly

for high bit rate systems, but the FWM effect is independent of the bit rate and is critically dependent on

the channel spacing and fiber dispersion. Decreasing the channel spacing increases the four-wave mixing

effect and so does decreasing the dispersion.

VERTIUAL LAB _ONE

SET UP AND PARAMETRS USED

Modulation formats

IMDD-NRZ: electrical rectangular pulses, shaped using a 5 poles Bessel filter having -3 dB

bandwidth equal to RB

IMDD-RZ: electrical RZ rectangular pulses with 50% duty-cycle, shaped using a 5 poles Bessel filter

having -3 dB bandwidth equal to 3RB

PSBT: electrical NRZ rectangular pulses, shaped using a 5 poles Bessel filter having -3 dB bandwidth

equal to 0.25RB

DPSK: electrical NRZ rectangular pulses, shaped using a 5 poles Bessel filter having -3 dB bandwidthequal to RB

External modulation, use a realistic sin2 model


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Receiver parameters

Optical filter

Optical matched filter

Super Gaussian 2nd order with Bo=0.4 nm

Photodiode: PIN with 0.55 A/W TIA: ideal

Electrical filter

No filter in case of optical matched filter

Bessel 5 poles with bandwidth Be=0.8Rb

OBJECTIVE:

To verify the eye-diagram and spectra for every modulation format.

To verify BER vs. OSNR behavior for each modulation format using the ideal optical matched filter

and the realistic SG2 optical filter.

Evaluate the target OSNR for the target BER in the two filtering scenarios

For each modulation format, evaluate the BER (and Q) vs. Dacc curve. For the Dacc value giving 2

dB Q-penalty, evaluate the corresponding OSNR penalty. For the PSBT, redo this analysis

considering the bandwidth of the Tx filter equal to 0.2Rb and 0.3Rb.

BLOCK DIAGRAM OF LAB SET UP

SECTION ONEABSTRACT: this section includes the verification of eye diagram and optical spectrum of signal for

different modulation formats, namely IMDD-NRZ/RZ,DPSK-NRZ ,and PSBT. Furthermore I tied to figure out

the change in the eye diagram while using different optical filters( matched and supper Gaussian) and

different bitrates.

RESULT AND DESCUSION

The following eye diagrams and optical spectrum are taken from OPTSIM simulation for IMDD-NRZ for

bitrates 10.7 Gb/s and 42.65 Gb/s by using matched and super Gaussian optical filter.


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From the above picture it is clearly seen that the eye diagram of IM-DD RZ pulse with the rail at the bottom

representing the sequence of data 0.The optical spectrum of RZ pulse has also stronger carriers at the

center with broader continuous part which arises due to the narrow RZ pulses. Note that Compared to NRZ

pulse spectrum the continuous portion of RZ pulse spectrum is wider. this is because , 50% duty cycle RZ

pulse has a bit duration exactly equal to half of the bit duration of its NRZ counterpart. Hence RZ has

broader spectrum compared to NRZ.

As it is expected, for 42.67 Gb/s ,the eye diagram is comparably worse than the 10.7 Gb/s one. Specially

while using super Gaussian optical filter ,part of the signal fall out of the bandwidth of the filter and it is

going to be filtered out. This makes the eye diagram worse.

The following figure shows eye diagram of PSBT which is taken right after the Bessel electrical filter .it

shows the three level of the signal.

The following eye diagram is taken at the receiver for PSBT and DPSK 10Gb/s system. Here the signal of

PSBT are no more three level. It is two level because of the filter action and the receiver should detect it as

0 or 1. The optical spectrum are also shown as follows. The spectrum of PSBT is narrow because of small

bandwidth band width of electrical filters used at the transmitter

DPSK-NRZ-MATCHED (10.7Gb/s) DPSK-NRZ-SUPERGAUSSIAN (10.7Gb/s) DPSK-NRZ (10.7Gb/s)


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DPSK-NRZ-MATCHED (42.65Gb/s) DPSK-NRZ-SUPERGAUSSIAN (42.65Gb/s) DPSK-NRZ (42.65Gb/s)

PSBT-MATCHED (10.7Gb/s) PSBT-SUPERGAUSSIAN (10.7Gb/s) PSBT (10.7Gb/s)

PSBT-MATCHED (42.65Gb/s) PSBT-SUPERGAUSSIAN (42.65Gb/s) PSBT (42.65Gb/s)

Optical spectrum of PSBT is narrower compared to the NRZ and RZ- IMDD type. But the spectrum of DPSK

has nearly the same bandwidth as the NRZ one. For PSBT and DPSK, there is no carrier at the center of the

optical spectrum. This is because ,they use a carrier suppressed modulation format at which the mean for 0

and 1 is zero.

Conclusion:

Compared to NRZ, RZ has wider optical spectrum

Compared to all ,PSBT has the narrower optical spectrum

Higher bitrate signals have higher bandwidth and wider optical spectrum

For 40.65 Gb/s, the eye diagram is somewhat closed compared to 10.7Gb/s, this is due to the fact

that the optical filter bandwidth is not wide enough to let pass the whole part of signal spectrum. For 40.65 Gb/s ,the eye diagram obtained by using matched filter and Gaussian filter is more or less

similar. Because the matched filter bandwidth approaching that of Gaussian one.

SECTION TWO

ABSTRACT: This part more focus on comparing the performance of different modulation formats interms of OSNR need to achieve the target BER, that is 10 e-3 in our case. Moreover the difference in


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performance while using matched and super Gaussian filter in different bitrate system is included here in

this section. Hence, the OSNR penalty induced to the system because of using supper Gaussian optical

filter rather than matched filtering is going to be explained.


The following BER Vs. OSNR graph is the one which is taken from OPTSIM simulation. It is for IMDD-NRZ

and IMDD-RZ modulation formats for different bitrates ,10.7 Gb/s and 42.65 Gb/s.

the following result tables are extracted from the above graphs for IMDD NRZ and RZ ( 42.67 Gb/s and 10.7 Gb/s) for

both supper Gaussian and matched optical filter based receivers.

TABLE 1. IMDD-NRZ

TABLE 2. IMDD-RZ

As we can see from table.1,for IMDD-NRZ modulation scheme, the system which comprises super Gaussian

optical filter for smaller bit rate experience certain OSNR penalty(2.37 dB) compared to optical matched

filter based system to achieve the target BER. This difference arises because of the wide bandwidth of the

super Gaussian filter let pass certain amount of noise which has remarkable impact for the usable signal

degradation. But in the case of high bit rate system (42.65 Gb/s) both filter type based system show almost

the same performance. This is due to the fact that the bandwidth of the matched filter is exactly equal tothe bit rate , hence for 42.67 Gb/s the bandwidth of the matched filter approaches to that of the


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bandwidth of supper Gaussian filter. This results both system experience more or less the same

performance.

But as table 2 shows, for IMDD RZ in the case of small bitrate , the systems comprises both filter type show

nearly equal performance. Remembering that 50 % duty cycle RZ modulation scheme has a bandwidth

which can be twice of the bandwidth of its NRZ counterpart, the matched filter which we are going to useshould have wide bandwidth to exactly fit or matched with the signal pulse shape. That means the band

width of the matched filter becomes more close to that of the bandwidth of the supper Gaussian filter. This

finally results both matched and super Gaussian filter based 10.7 Gb/s system perform more or less

equally. But for higher bit rate ,because of the smaller bit duration or wide bandwidth of the RZ pulse

,certain portion of the signal falls out of the supper Gaussian filter bandwidth, this results worsening of

performance compared to matched filter. note that the bandwidth of matched filter is equal to as that of

the RZ pulse.

the following BER Vs. OSNR graph is taken from the virtual lab simulation for NRZ-DPSK and PSBT

modulation for bitrates 10.7Gb/s and 42.65 Gb/s.

The following result tables are extracted from the previous OSNR Vs. BER graph of DPSK and PSBT

Just like I encountered for IMDD ,in the case of DPSK and PSBT, for higher bit rate, supper Gaussian and

matched filter based system have nearly the same performance . but for lower bitrate , having matched

filter in our system show certain advantages to achieve the needed target BER with relatively small OSNR.

This happens for the same reason explained for the case of IMDD.


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In the case of PSBT, using filter having different bandwidth at the transmitter shows me differentperformance as listed in the above table . Among them the system that use 0.25 Rb is the one which

enables to get better performance. So, 0.25*Rb is the optimized Bessel filter bandwidth which should be

used to shape the signal at transmitter in the case of PSBT.

Generally, as I have seen so far the performance of different modulation formats , namely: IMDD, DPSK,

PSBT from the OPTSIM simulation result on the previous result tables and graphs, one can realize the

superior performance of DPSK over the others. The next good system is the one which based on IMDD

modulation formats if the comparison is conducted in non-dispersive linear regime. The following table

contains target OSNR taken from the previous BER Vs. OSNR graph of those modulation formats.it could be

the compact summarization of the performance of those modulations in terms of needed OSNR to achievetarget BER.

The narrow bandwidth(0.25 Rb) of the filter used at the transmitter puts PSBT in the worst place in terms

of sensitivity among the three. The reason for this is ,part of the signal cut by the Bessel filter at transmitter,hence the receiver needs big OSNR to achieve the needed BER. the other reason is ,the narrow spectrum

of PSBT is more subjected to inter symbol interference.

CONCLUSION

Among those modulation formats DPSK is the best to get the target BER with minimum OSNR. PSBT is the worst one, it needs high OSNR compared to others to achieve the target BER.

In comparison to supper Gaussian based system, A system comprises matched filter based receiver

shows better performance for 10.7 Gb/s system.

But for 42.67 Gb/s system, both systems which contain matched and super Gaussian filter hasnearly equal performance in terms of needed OSNR to achieve the given BER.

SECTION THREE

ABSTRACT: In this part I am going to analyze performance evaluation of different modulation formats by

introducing accumulated dispersion which is large enough to create a 2 dB Q penalty from which I originally

obtained by using the target OSNR obtained in section two. Furthermore, The comparison of their


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performance in linear dispersive regime is explained to differentiate which modulation format is more

resistant for dispersion.


The following graph shows Q vs. accumulated dispersion graph for IMDD NRZ and RZ modulation withsuper Gaussian and matched filter for 10.7 and 42.65 Gb/s .

The following Q Vs. accumulated dispersion graphs taken from simulation for PSBT and DPSK modulation.

The following result table extracted from the previous Q Vs. Dacc graphs of those modulation formats.it

containsthe amount of accumulated chromatic dispersion which can induce 2 dB Q penalty with respect to

non-dispersive regime for those modulation format.


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As shown on the table, for IMDD modulation, NRZ shows better resistance to chromatic depression

compared to RZ pulse. This is due to the fact that RZ signal format has larger bandwidth than NRZ. Hence,

different spectral slice in the wide bandwidth of RZ pulse experience different group velocity that introduce

widening of pulse shape. So that RZ signals are more victim of chromatic dispersion compared to NRZ

signals.

On the other hand another comparison can be drawn in terms of optical filters used at the receiver.

Systems that contain super Gaussian filter is more subjected to chromatic dispersion than matched filter .

the reason for this is, if we use matched filtering in dispersive system, since its width is equal to the signal,

the boarding of the pulse because of the chromatic dispersions causes the part of the signal to fall outside

of the matched filter.

We can see also another scenario to deal the characteristics of chromatic dispersion for different bitrate

systems. high bitrate systems are more exposed for chromatic dispersion. This is because bandwidth of the

system is equal to twice of bite rate ,that is, high bitrate systems have higher bandwidth. On the other

hand having higher bandwidth experience high group velocity delay (chromatic) dispersions which causes

widening of pulse shape.

From the previous result table , it is also possible to visualize that which modulation type is more tolerant

to dispersion. As we can see from the result , to reach 2dB Q penalty ,PSBT needs very big accumulated

dispersion ,that is, PSBT modulation scheme is more tolerant of chromatic dispersion. This is because of the

smaller bandwidth of signal in the case of PSBT. Since the shaping Bessel filter bandwidth at the

transmitter is narrow (0.25 RB) the pulse of the PSBT modulated signal possess small bandwidth. In general,

it is recommended to use PSBT modulation format for long haul high bitrate optical fiber communication.

DPSK is the second ranked dispersion tolerant modulation type while RZ- IMDD is the least tolerant one.

Finally , here I go to analyze the OSNR penalty induced by accumulated chromatic dispersion for each

modulation formats. The following figure shows Q Vs. OSNR graph of those modulation types to verify the

OSNR penalty come up due to the accumulated chromatic dispersion which introduce 2 dB Q penalty .


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Here this result table is taken from the above graphs to compare the performance of those modulation

formats to withstand in the combat of dispersion. The comparison is based on OSNR penalty caused by the

accumulated dispersion.

PSBT is again the brave one in front of chromatic depression by achieving small OSNR penalty for higher

accumulated dispersion. this is also due to the narrow bandwidth of its modulated signal. The DPSK format

also shows better resistance than the IMDD for chromatic dispersion .

PSBT could perform in different way depend on the bandwidth of the Bessel filter used at the transmitter.

To get more dispersion tolerant PSBT system and to minimize OSNR penalty caused by accumulated

dispersion, the bandwidth of the shaping filter at the transmitter should be optimized properly. The


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following table taken from the result of virtual lab simulation for three different bandwidth of shaping filter

at transmitter to optimize the performance of PSBT modulation.

As we can see from the result, the one which is based on filter having bandwidth equal to 0.25*Rb is the

better one while which use 0.2*Rb is the worst.

in conclusion, including PSBT modulation in dispersive system has positive trade of to stand with the

combat of destructive chromatic dispersion effect in optical fiber communication system. And because of

its good dispersion tolerance it is also recommended to use PSBT in long haul transmission systems.

CONCLUSION:

PSBT is more dispersion tolerant modulation format.

0.25*Rb is the optimum bandwidth of Bessel filter to be used at the transmitter in the case of PSBT

DPSK is also the second ranked for dispersion tolerance.

Compared to NRZ, RZ modulation scheme is more subjected to dispersion because of its wide

bandwidth.


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VERTIUAL LAB _TWO

LAB SET UP AND PARAMETRS USED

Transmitter

Lorentzian laser with power = 0 dBm IMDD-NRZ: electrical rectangular pulses, shaped using a 5 poles Bessel filter having -3 dB

bandwidth equal to RB

External modulation, use a realistic sin2 model

Transmitted power PTx set through the use of an ideal fixed output power optical booster

amplifier with noise figure = 6 dB

PTx = 5 -> 15 dBm step 2 dB

Line

Fiber

L = 200 km

Loss = 0.2 [dB/km]Dispersion: D = 6 [ps/nm/km]

Nonlinearity: = 2 [1/W/km]

Dispersion compensation Unit inserted after the fiber

Ideal fiber grating (no loss)

Dacc=-LDperccomp/100

Attenuator placed after the DCU introducing Loss = PTx dB in order to have the same receiver

power level with PTx variation.

Receiver optical amplifier inserted after the attenuator before the photo detection: fixed gain

completely recovering the fiber loss (Gain=LLoss) with noise figure = 6 dB

Receiver parameters Optical filter: Super Gaussian 2nd order with Bo=30 GHz


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Photodiode: PIN with 0.7 A/W

TIA: ideal

Electrical filter: Bessel 5 poles with bandwidth Be=0.8Rb

OBJECTIVE:

To verify the joint effect of SPM and chromatic dispersion

To see the effect of anomalous and normal dispersion reign with and without SPM.

To obtain the optimum percentage of dispersion compensation in linear and nonlinear regime.

ABSTRACT: This OPTSIM simulation lab report explains the joint influence of chromatic dispersion andnonlinear effect(SPM) on optical communication system performance . in general, it shows to what level of

percentage dispersion should be compensated in nonlinear and linear regime to achieve better

performance of communication.


The following graph shows the Q Vs. percentage of dispersion compensation for anomalous dispersion

regime having a quantity of +6 ps/nm/km and -6ps/nm/km respectively.

D=+6 ps/nm/km D=-6 ps/nm/km

The following result table is taken from the above graphs. Because of simulation uncertainity,Some of the

values are a little bit deviated from my expectation .


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From the above result, we can see that to get better performance in anomalous dispersive nonlinear

regime, the percentage of dispersion compensation should be lower than 100%. The reason for this is, in

the case of anomalous dispersion regime, the red shifted parts travels more slowly, and moves toward the

pulse center. Similarly, blue shifted one travels more quickly, and also moves toward the center of the

pulse. Therefore, effect of chromatic dispersion and SPM act in different directions, resulting in a

compression of the pulse. In the range of anomalous dispersion, nonlinearity and dispersion induced

chirpings are going to partially compensate each other. So that ,to have good system performance oneshould not over compensate chromatic dispersion in order to save the system from SPM effect.

From the previous graphs we can also observe that the percentage of dispersion compensations is lower

for high power transmitted pulses compared to low power transmitted pulses. This is due to the fact that

,nonlinearity (SPM) is directly proportional to the power of transmitted signal . hence, for higher power

signals, we need less percentage of dispersion compensation to compensate SPM.

But for small transmitted power signal, non-linearity is not the dominant effect because the power is not

that much enough to create Kerr effect (SPM). So the dominant effect that we should take care of is

dispersion. So we need to take counter measure to compensate dispersion with high percentage of

compensation.

The second Q Vs. percentage of chromatic dispersion compensation graph is taken from virtual lab

simulation. It indicates the Q value for different amount of dispersion compensation for normal dispersion

regime having dispersion quantity -6 ps/nm/km. The optimum dispersion compensation is the value at

which we can get higher Q value. But for normal dispersion regime as indicated in the above graph , , the

case is vice versa, that is ,instead of compression of the pulse ,widening of signal pulse width come up.

in the normal dispersion regime ,the chirping due to dispersion has a similar effect to that of the chirping

due to SPM. Thus, in this regime the chirping due to dispersion and SPM act in the same direction and lead

to larger broadening of the pulse than the dispersion alone, or we can say that the effect of dispersion and

SPM add up. So to reduce this cumulative effect ,the percentage of dispersion compensation should be

greater than 100%. Meaning that we would have positive residual dispersion that can act against SPM.

The other thing we should notice from the previous graph is, the optimum percentage of dispersion

compensation is larger for high power transmitted pulses than that of low power ones. This is because as

we know high power of the signal favor the uprising of SPM. Thus we need higher percentage(greater than

100%) of dispersion compensation to have large enough residual positive dispersion which is going to

counter act on SPM.

So in optical fiber communication design, the joint effect of chromatic dispersion and fiber nonlinear effectshould be considered.


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the following figure which is taken from virtual lab simulation shows the optimum percentage of dispersion

compensation for linear regime. In dispersive linear regime, we have only the effect of chromatic dispersion

on our communication performance.

D=+6 ps/nm/km D=-6 ps/nm/km

So to get good performance we have to cancel out all effect of chromatic dispersion. Hence the optimum

percentage of dispersion compensation in this case is 100% for both anomalous and normal dispersion.

CONCLUSION

During anomalous dispersion the phase shift induced by chromatic dispersion and SPM compensate

each other. So that the optimum compensation is less than 100%.

But in normal dispersion regime, the phase shift caused by the chromatic dispersion and SPM add

up. To minimize the impact the optimum percentage of compensation is greater than 100%.

In linear dispersive system chromatic dispersion is the one we should deal with.so to avoid its

negative effect completely, it should be compensated 100%.

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