raman amplification design in wdm systems

8/13/2019 Raman Amplification Design in Wdm Systems

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Raman Amplification Design in WDM

Systems

Definition

Raman amplification is based on stimulated Raman scattering (SRS), a nonlineareffect in fiber-optical transmission that results in signal amplification if opticalpump waves with the correct wavelength and power are launched into the fiber.

Overview

This tutorial gives an introduction into the complex design issues of wavelengthdivision multiplexing (WDM) systems applying Raman amplification. It firstpresents an overview of traditional WDM systems, predicts problems that mightarise for future configurations, and shows how Raman amplification could be ofhelp. Then, a behavioral description of SRS is provided, and repeater designs arediscussed. Finally, several system examples are shown to demonstrate typicalfields of applications of Raman amplification.

Topics1. Traditional Configuration of WDM Systems

2. How Can Raman Amplification Be of Help?

3. Erbium-Doped Fiber versus Raman Amplification

4. Raman Amplification in Wideband WDM Transmission

5. Raman Amplification to Build Bidirectional WDM Systems

6. Raman Amplification and Fiber Nonlinearities

7. Conclusion

Self-Test

Correct Answers

Glossary


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1. Traditional Configuration of WDMSystems

A typical configuration of a point-to-point WDM system is comprised mostly of

the following:

A number of optical transmitters

An optical multiplexer

Spans of optical transmission fiber, such as standard single-mode fiber(SSMF)

Optical amplifiers, usually erbium-doped fiber amplifiers (EDFAs)

Dispersion compensating devices, like spans of dispersion

compensating fiber (DCF) or chirped fiber Bragg gratings (FBGs)

An optical demultiplexer

A number of optical receivers

Figure 1shows the layout of such a WDM system.

Figure 1. Typical WDM Transmission Link

The dramatically increasing service demand driven by the rapid growth of theInternet generates new challenges for WDM system designers. Common designapproaches reach their limits, and the usage of comprehensive modelingtechniques becomes more and more important. Additionally, to achieve thedemanding targets created by the application-oriented business developments,future systems must comply with upgraded performance criteria, such as thefollowing:

Transmission of higher total data capacities through increased channelbit rates and number of wavelength-multiplexed channels

Cost-reduction by allowing longer amplifier spacing and, thus,reduction of the number of EDFAs per optical link


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Reduction of signal distortion to allow transmission over longer all-optical transmission links

There are several new design approaches to meet these criteria, including thefollowing:

New transmission windows in wavelength domain

New types of optical amplifiers covering a very high signal bandwidthto increase data capacity

Bidirectional WDM transmission allowing suppression of nonlinearfiber interactions

All of these techniques require a thorough understanding of the underlyingphysical effects and the interplay between diverse optical devices to judge theirimpact on system performance measures, using numerical simulation tools.

2. How Can Raman Amplification Be ofHelp?

One of the most recent and interesting developments includes the constructiveusage of the so-called Raman effect in optical fibers. A Raman amplifier usesintrinsic properties of silica fibers to obtain signal amplification. This means thattransmission fibers can be used as a medium for amplification, and hence thatthe intrinsic attenuation of data signals transmitted over the fiber can becombated within the fiber. An amplifier working on the basis of this principle is

commonly known as a distributed Raman amplifier (DRA).

The physical property behind DRAs is called SRS. This occurs when a sufficientlylarge pump wave is co-launched at a lower wavelength than the signal to beamplified. The Raman gain depends strongly on the pump power and thefrequency offset between pump and signal. Amplification occurs when the pumpphoton gives up its energy to create a new photon at the signal wavelength, plussome residual energy, which is absorbed as phonons (vibrational energy) asshown inFigure 2.

Figure 2. Energy States during SRS


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As there is a wide range of vibrational states above the ground state, a broadrange of possible transitions are providing gain. This is shown inFigure 2bymeans of the shaded region. Generally, Raman gain increases almost linearlywith wavelength offset between signal and pump peaking at about 100 nm andthen dropping rapidly with increased offset.Figure 3shows a typically measured

Raman gain curve. The usable gain bandwidth is about 48 nm.

Figure 3. Typical Raman Gain Curve versus Wavelength Offset

The position of the gain bandwidth within the wavelength domain can beadjusted simply by tuning the pump wavelength. Thus, Raman amplificationpotentially can be achieved in every region of the transmission window of theoptical transmission fiber. It only depends on the availability of powerful pump

sources at the required wavelengths. The disadvantage of Raman amplification isthe need for high pump powers to provide a reasonable gain.

This opens a new range of possible applications. It is possible, for instance, topartially compensate fiber attenuation using the Raman effect and, thus, toincrease the EDFA spacing. The Raman pump wave can be conveniently placed atthe EDFA locations. This saves costs as less EDFAs are needed on the link, andthe number of sites to be maintained is reduced.

Another application of the Raman effect is given with hybrid EDFA/Ramanamplifiers characterized by a flat gain over especially large bandwidths.Repeaters can be built that compensate the nonflatness of the EDFA gain with amore flexible Raman gain. Multiwavelength pumping could be used to shape theRaman gain such that it equalizes for the EDFA gain shaping.

Also, the Raman effect on its own might be used for signal amplification intransmission windows that cannot be covered properly by EDFAs. Somefrequency regions of a wideband WDM signal could be amplified by commonEDFA structures, while others are amplified using the Raman effect and proper


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pumping. The upgrade of already existing systems by opening anothertransmission window where Raman amplification is applied could be anattractive application.

3. Erbium-Doped Fiber versus RamanAmplification

Raman amplifiers offer several advantages compared to EDFAs, including thefollowing:

Low noise buildup

Simple design, as direct signal amplification is achieved in the opticalfiber, and no special transmission medium is needed.

Flexible assignment of signal frequencies, as Raman gain depends onthe pump wavelength and not on a wavelength-sensitive materialparameter of the medium, such as the emission cross-section of dopantin the erbium-doped fiber (EDF).

Broad gain bandwidth is achievable by combining the Ramanamplification effect of several pump waves that are placed carefully inthe wavelength domain.

However, despite the many advantages of Raman amplification, there can besome degradation effects. For example, not only the specially launched pumpwaves but also some of the WDM channels may provide power to amplify the

other channels. This would result in power exchange between WDM channelsand thus cross-talk leading to signal degradation.

These negative effects occur in unidirectional and bidirectional WDMtransmission. So for accurate analysis of advanced WDM systems, it is crucial tomodel all Raman interactions. Additionally, degrading effects like spontaneousRaman scattering and backward Rayleigh scattering have to be considered.

Table 1gives an overview of important characteristics of Raman and EDFamplifiers. Note that hybrid amplification schemes, using Raman and EDFamplification in concatenation, can be designed to take advantage of both types.


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Table 1. Comparison of Raman and Doped-Fiber AmplifierCharacteristics

Characteristic Doped-Fiber Amplifier Raman Amplifier

Amplification Band depends on dopant depends on availability of

pump wavelengthsAmplification Bandwidth 20 nm, more for multiple

dopants/fibers48 nm, morefor multiple pump waves

48 nm, more for multiplepump waves

Gain 20 dB or more, depending onion concentration, fiberlength, and pumpconfiguration

411 dB, proportional topump intensity andeffective fiber length

Saturation Power depends on gain and materialconstants

equals about power ofpump waves

Pump Wavelength 980 nm or 1480 nm forEDFAs

100 nm lower then signalwavelength at peak gain

Raman amplifiers are topologically simpler to design than doped-fiber amplifiers,as the existing transmission fiber can be used as a medium if properly pumped.However, the selection of pump powers and wavelengths, as well as the numberand separation of pumps, strongly determines the wavelength behavior of Ramangain and noise.

When building distributed Raman amplifiers, designers face the question ofusing forward or backward pumping (or even both) with respect to signalpropagation. The backward pumping scheme is most commonly used as it offersseveral advantages. Pump noise strongly affects the WDM signals to be amplified

if forward pumping is applied, as the Raman process is nearly instantaneous.When the Raman pump wave has slight random power fluctuations in time,which is almost always the case, individual bits might be amplified differentially,which leads to amplitude fluctuations or jitter. If backward pumping is applied,power fluctuations of the Raman pump will be averaged out, as each individualbit will see several milliseconds of the Raman pump wave.Figure 4shows thegeneral setup of a backward pumped DRA and the counter-propagation of signaland pump.

Figure 4. Backward-Pumped Raman Amplifier ShowingCounter-Propagation of Pump Wave and Signal


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Hybrid (EDF and Raman) amplification has been used successfully in recentdesigns to obtain the necessary optical signal-to-noise ratio (OSNR) for high-capacity dense wavelength division multiplexing systems (DWDM) or to achievevery large amplifier spacing in, for example, festoon applications.Figure 5showsa possible design of a hybrid EDF/Raman amplifier. The doped fiber is pumped

remotely via the transmission fiber where Raman amplification occurs.

Figure 5. Hybrid EDF/Raman Amplifier

The transversal power distribution of the signal over an amplified fiber span isstrongly dependent on the applied amplification scheme and can be controlled bythe Raman pump power and pump direction.Figure 6shows the transversalspan power profile employing different hybrid EDF/Raman amplificationschemes.

Figure 6. Span Power Profile for EDFABased Systems (1),

System Using Hybrid Schemes with Backward RamanAmplification Only (2), and Bidirectional Raman Amplification

(3)

By properly selecting pump laser wavelengths, transmission fiber lengths, andtypes, many optimization targets can be reachedflattening of the EDFA gainthrough an optimized design of the frequency-dependent Raman gain, for

example. Optimization can be achieved using numerical simulation.


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4. Raman Amplification in Wideband WDMTransmission

This example demonstrates the design of a distributed Raman amplifier for ultra-

wideband WDM transmission, using multiple pumps to achieve a gain flatnessover an 80-nm signal bandwidth as designed after the work of Kidorf et al. Asmentioned earlier, a very wideband flat amplification can be achieved byselecting launched powers and emission wavelengths of the Raman pumpsproperly.Figure 7shows the general design setup.

Figure 7. Design Setup for Wideband Raman Amplifier

Evaluation

One hundred test carriers are used to sample the Raman gain response over abandwidth of approximately 82 nm. Each launched with an average power of 3dBm into 60 km SSMF. The accumulated fiber attenuation is completelycompensated using the SRS effect of eight counter-propagating Raman pumps.

At the receiver, 100 power detectors are used to evaluate the Raman gainresponse at the output of the fiber.Figure 8shows the optical spectrum at thereceiver. The gain ripple is less than 2 dB over 81 nm. Note that there is still

enough power margin to introduce a gain-flattening filter at the output of thefiber span to achieve a total gain ripple of less than 0.5 dB.


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Figure 9. Spectral Distribution of the Eight Raman Pumps at the

Backward Input of the Fiber (Red) and the Forward Output of

the Fiber (Blue)

Second, there are strong pump-to-pump interactions, as the Raman pumps arespaced over 86 nm for which the Raman efficiency is already very large. Pumpsemitted at the very low wavelengths amplify the WDM signal band as well as thepumps at the higher wavelengths.

Figure 10. Propagation of the Eight Raman Pumps over the

Fiber

Figure 10shows the pumps' power profile along the fiber. Starting with almostequal pump powers at the far end of the fiber, the pumps at the higherwavelengths are first amplified by the pumps at the lower wavelengths. Further


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down the fiber, when the power of the low-wavelength pumps is reduced due toenergy transfer to high wavelengths and fiber attenuation, the effect of pump-to-pump amplification is reduced. As can be concluded fromFigure 10, interactionof the different pump waves is nonnegligible.

5. Raman Amplification to BuildBidirectional WDM Systems

This example demonstrates the bidirectional WDM signal transmission andRaman pumping to compensate attenuation in the transmission fiber.Figure 11shows the design.

Figure 11. Bidirectional DWDM System Exploiting C and L BandsUsing Hybrid EDF/Raman Amplification

In this system configuration, the C band is employed for signal transmission inone direction and the L band to propagate signals in the opposite direction. Thefiber attenuation is partly compensated by the distributed Raman amplifier.Accordingly, applying the backward pumping scheme for each band requires theL-band pump to be placed at the same fiber end as the C-band transmitter andvice versa. Obviously, this configuration implies the bidirectional pumping.

When modeling such systems, it is crucial that the power exchange between theco- and counter-propagating signals and pumps (pump-to-pump, pump-to-

signal, and signal-to-signal) is accurately considered. Therefore, simplifiedapproaches neglecting, for example, pump depletion are not suitable. Accuratemodeling is only possible if all bidirectional interactions are modeled.

Typical signal and pump spectra are shown inFigure 12. The nonflatness of thesignal spectrum is due to the Raman gain shape. It can be overcome with gainflattening filters placed right after the EDFAs.


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Figure 12. Spectrum at Output of Transmission Fiber (Both

Directions)

It is quite interesting to look atFigure 13, which shows the signal and pumppropagation in both directions. The L-band signal launched in the backwarddirection at the far fiber end (z= 100 km) experiences a significant Ramanamplification of the backward-propagating C-band pump wave, which is alsolaunched at the far end. On the other hand, the C-band signal, which is launchedin the forward direction at the near fiber end (z= 0 km) experiences Raman

amplification of the forward-propagating L-band pump. Such signalamplification by foreign pump waves is possible because of the large bandwidthof Raman gain. Thus, the signal is amplified two times, one time by the foreignpump in the vicinity of the launch point and the second time by its own pump atthe fiber output.

This example also shows that careful modeling of pump-to-pump interactions isof importance. At the far fiber end, the C-band pump significantly amplifies theL-band pump. Pump depletion of the C-band pump occurs at the near fiber end.


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Figure 13. Propagation of C-Band and L-Band Signals and

Pumps

6. Raman Amplification and Fiber

Nonlinearities

This example presents results of a case study investigating the importance ofnonlinear propagation effects when deciding on optimum signal powerconditions. The considered DWDM system is shown inFigure 14.

Figure 14. DWDM System for Investigation of Optimized Span

Input Power Using Different Types of Hybrid EDF/RamanAmplification


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Advantages of hybrid amplification were investigated for a 40-channel DWDMsystem. Channels transmit at 10 Gbps and are placed equidistantly 50 GHz apart.The dispersion map consists of a span of 100 km dispersion shifted fiber (DSF) orSSMF. The accumulated fiber attenuation is 20 dB. Ideal precompensation ofchromatic dispersion is assumed for both cases.

Three different amplification scenarios are comparedfirst, backward Ramanamplification, second, bidirectional Raman amplification, and third, pure EDFamplification (with noise figure of 4 dB). The span power profile for the threescenarios was shown inFigure 6. To investigate impact of fiber nonlinearities onone hand and amplifier noise on the other hand, channel launch powers arevaried between -5 dBm and 20 dBm. The eye-closure of the central channel (1550nm) is measured after a receiver unit consisting of optical drop filter, photodiode,and post-detection filter before and after fiber propagation.

Figure 15shows the eye-closure penalty versus channel power for the threeinvestigated amplification schemes after propagation over the DSF.

Figure 15. Eye-Closure Penalty versus Channel Power for

Different Amplification Schemes after Propagation over DSF

Figure 15clearly indicates optimum values for the channel powers with respect toeye-closure penalty. At low channel powers, performance is limited by amplifiernoise, while for high channel powers, it is limited by fiber nonlinearities, namelycross-phase modulation (XPM) and four wave mixing (FWM). Regardless of theapplied amplification scheme, all three penalty curves rise with almost equalgradient.


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The systems using Raman amplification outperform the one using an EDFA bythe optimum achievable eye-closure penalty and the tolerance to powerfluctuations. For the given set of parameters, widest tolerance with respect to thelaunch power is found for the case of bidirectional Raman amplification.

For comparison,Figure 16shows the eye-closure penalty versus channel powerfor the three investigated amplification schemes after propagation over standardSMF.

Figure 16. Eye-Closure Penalty versus Channel Power for

Different Amplification Schemes after Propagation overStandard SMF

Again,Figure 16indicates optimum channel powers with respect to eye-closurepenalty. However, there is now a clear difference visible with respect to toleranceof fiber nonlinearities.

For both considered propagation fibers, the optimum launch powers differ by upto 7 dBm, depending on the applied amplification scenario. This has an impact onWDM systems using a high number of channels, as more channels can be

amplified with the same amount of pump power. Also, the minimum values ofeye-closure penalty differ, which indicates that different total transmissiondistances are possible.

The results of this example case study show the importance of includingnonlinear propagation effects in the system design process when deciding onoptimum signal and pump powers.


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7. Conclusions

First, this tutorial described how Raman amplification could be helpful whendesigning future fiber-optical communication systems requiring throughput oflarge capacity. Then, a general introduction of the Raman effect was presented,

and advantages of certain amplifier topologies were discussed. With the help ofthree application examples, general problems arising from the design of systemsconsidering Raman amplification were presented. It was shown that carefulmodeling of all relevant physical propagation effects is crucial for system design.

Self-Test

1. A Raman amplifier uses intrinsic properties of _____ fibers to obtain signalamplification.

a. silicab. erbium

c. pure

d. clear optical

2. The physical property behind DRAs is called _____.

a. EDFA

b. WDM

c. SRS

d. DSF

3. ___________ occurs when the pump photon gives up its energy to create anew photon at the signal wavelength, plus some residual energy, which isabsorbed as phonons.

a. convergence

b. amplification

c. vibration

d. positioning


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4. Raman amplification potentially can be achieved in every region of thetransmission window of the optical transmission fiber.

a. true

b. false

5. The Raman pump wave can be conveniently placed at the ________locations.

a. SRS

b. EDFA

c. DRA

d. WDM

6. Despite the many advantages of Raman amplification, there can be somedegradation effects.

a. true

b. false

7. Raman amplifiers are ____________ to design than doped-fiber amplifiers,as the existing transmission fiber can be used as a medium if properlypumped.

a. conceptually harder

b. more cost-effective

c. topologically simpler

d. more expensive

8. When building distributed Raman amplifiers, designers mustuse backwardpumping with respect to signal propagation.

a. true

b. false


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9. Further down the fiber, when the power of the low-wavelength pumps isreduced due to energy transfer to high wavelengths and fiber attenuation, theeffect of pump-to-pump amplification is reduced.

a. true

b. false

10. It is not necessary to include nonlinear propagation effects in the systemdesign process when deciding on optimum signal and pump powers.

a. true

b. false

Correct Answers

1. A Raman amplifier uses intrinsic properties of _____ fibers to obtain signalamplification.

a. silica

b. erbium

c. pure

d. clear optical

See Topic 2.

2. The physical property behind DRAs is called _____.

a. EDFA

b. WDM

c. SRS

d. DSF

See Topic 2.

3. ___________ occurs when the pump photon gives up its energy to create anew photon at the signal wavelength, plus some residual energy, which isabsorbed as phonons.

a. convergence


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b. amplification

c. vibration

d. positioning

See Topic 2.

4. Raman amplification potentially can be achieved in every region of thetransmission window of the optical transmission fiber.

a. true

b. false

See Topic 2.

5. The Raman pump wave can be conveniently placed at the ________locations.

a. SRS

b. EDFA

c. DRA

d. WDM

See Topic 2.

6. Despite the many advantages of Raman amplification, there can be somedegradation effects.

a. true

b. false

See Topic 3.

7. Raman amplifiers are ____________ to design than doped-fiber amplifiers,as the existing transmission fiber can be used as a medium if properly

pumped.

a. conceptually harder

b. more cost-effective

c. topologically simpler


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d. more expensive

See Topic 3.

8. When building distributed Raman amplifiers, designers mustuse backwardpumping with respect to signal propagation.

a. true

b. false

See Topic 3.

9. Further down the fiber, when the power of the low-wavelength pumps isreduced due to energy transfer to high wavelengths and fiber attenuation, theeffect of pump-to-pump amplification is reduced.

a. trueb. false

See Topic 4.

10. It is not necessary to include nonlinear propagation effects in the systemdesign process when deciding on optimum signal and pump powers.

a. true

b. false

See Topic 6.

Glossary

DCFdispersion compensating fiber

DRAdistributed Raman amplifier

DSFdispersion shifted fiber

DWDMdense wavelength division multiplexing


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EDFerbium-doped fiber

EDFAerbium-doped fiber amplifier

FBGfiber Bragg grating

FWMfour wave mixing

OSNRoptical signal-to-noise ratio

SRSstimulated Raman scattering

SSMFstandard single mode fiber

WDMwavelength division multiplexing

XPMcross-phase modulation

raman amplification design in wdm systems

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