LITERATURA PARA CONSULTA

As mentioned above, we evaluate the s heme with lasers having dierent phase

noise hara teristi s: (1) a 50 KHz linewidth ECL at 1547 nm, (2) a 10 MHz

linewidth DFB laserat 1540nm, and (3) anSG-DBR tunablelaser witha white

FMnoiselevel orrespondingtoa300 KHzlinewidth. Again, biasingthe passive

tuning se tions of the SG-DBR laser resulted in large ex ess

1/f

α

noise that

has been shown to signi antly degrade the performan e of oherent systems,

spe i ally at low baud rates as in previous se tions. Figure 4.24 shows the

example re overed symbol onstellations that dier only slightly for the three

dierent lasers with the baudrate-pilot-aided s heme. The symbols are slightly

ellipti al due to nonlinearity from the ele tri al driving ampliers and the IQ

opti al modulator whi h also exhibits non-perfe t orthogonality that ould not

be ompletely ompensated bythere eiverDSP.TheBERat-20dBmsensitivity

is well below the FEC limit of

3.8 × 10

−3

and is within a fa tor of two for all

three lasers. In ontrast, a oherent system employing 16-QAM would require

very narrow linewidth lasers su h as the ECL for 2.5 Gbaud.

Figure 4.25 ompares the ba k-to-ba k BER performan e of the pilot-aided

s heme for the three dierent lasers. The plots show similar performan e for

all three lasers despite their dierent phase noise hara teristi s. The operating

wavelength of the tunable SG-DBR laser varied from

λ = 1547.9nm

when the gain se tion was biased at

IGain

= 120mA

, to

λ = 1541.8nm

when both gain and ba k se tionswere biased, and

λ = 1551.5nm

whenbiasing allfourse tions, yieldingin reasinglevelsofex essFMnoise. Theperforman edieren ebetween

the lasers is attributed to the disparity in the demodulated noise enhan ement

that depends on the output power of the lasers, their RIN noise, and re eiver

noiseasdis ussedpreviously. Theexperimentalresults onrmthattheproposed

baudrate-pilot-aideds heme is highlyimmune tothe laser phase noise.

To re ap, a baudrate-pilot-aided transmission with dire t dete tion s heme

for high-order QAM formats has been proposed and demonstrated through ana-

lyti almodellingandexperimentalresults. The low omplexityre eiverishighly

immune to the laser phase noise. The proposed s heme was demonstrated for a

(b)

Figure4.22: (a)Experimentsetupforbaudrate-piloted-aideds hemefor16-QAM

at2.5Gbaud;(b)Opti alspe trumoftransmittedsignal(opti alresolutionband-

(b)

Figure 4.23: Trans eiver stru ture of baudrate-pilot-aideds heme for QAM: (a)

Transmitterstru ture; (b)Re eiverstru turein ludingsignalpro essing(indash

line), PD: photodiode, TIA:trans-impedan e amplier,LO:lo alos illator.

(a)

(b) ( )

Figure4.24: Re eived onstellationsofpilot-aided16-QAMat2.5Gbaudinstati

s enario with three dierent lasers: (a) ECL (

∆ν = 50KHz

), (b) DFB laser (

∆ν = 10MHz

), ( ) SG-DBR laser biasing 4se tions.

the three dierenttypes of lasers.

large ex ess phase noise.

4.4 Summary

Inthis hapter,wehaveextensivelyexaminedthesigni antimpa tsofthe om-

pli ated phase noise pro esses of the SG-DBR lasers in a oherent opti al om-

muni ationsystememployinghighordermodulationformats. We onrmedthat

while the whiteFM-noisehas been determinedtolargely dene the lowerbound

of BER performan e, the system operation ouldpotentially be impairedby the

ex ess laser phase noise. We have then examined the robustness of a se ond-

orderDD-PLLphasetra kingte hniqueina oherentsystememploying16-QAM

modulationformat at16Gbaudtoover omethis issue. The experimentalresults

presentthe abilitytoemploymonolithi tunablelasersforhighorderQAMmod-

ulation formats with oherent systems. The y le slipee ts of aSG-DBR laser

ina10.7GbaudQPSKtransmissionsystemwithwell-knownphasenoisetra king

algorithmsis alsoinvestigated. A new CS dete tion/ orre tion methodhas then

been developed tosolvethis problem.

Subsequently, we proposed and demonstrated a oherent self-heterodyne re-

eiverbasedonPMdete tionforDQPSKmodulationtoee tively over omethe

phasenoiseissue oftheSG-DBR lasers. Thenew oherentPMdete tionre eiver

not only oers the potential for a low- ost integrated solution but an also sig-

ni antly simplifythe front end of opti al oherent re eivers with the feasibility

of migrating toward higher order modulation formats. A baudrate-pilot-aided

noise. The proposed s heme was demonstrated for a 16-QAM system operating

at 2.5 Gbaud and employing an SG-DBR laser transmitter. Besides the above

advantages,thebaudrate-pilot-aidedtransmissions hemehasitsowndrawba ks.

Thesein ludewastingadditionalopti alpowerinthepilottoneandhavinghigher

requirement on the SNR of the re eived signal. These disadvantages would also

need to be taken into a ount when designing a ommuni ations link with the

proposed method.

As the opti al ommuni ations networks moving towards advan ed modula-

tionformatsandfastre- ongurablenetworks,thenext hapterwillbededi ated

to investigating the use of the SG-DBR lasers in a fast-swit hing opti al pa ket

network employing the above advan ed modulation formats. The performan e

degradationof su h asystem during and aftera wavelength-swit hing event of a

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swit hing and the real-time self-homodyne dete tion using polarization-

Opti al Pa ket Swit hing with

Advan ed Modulation Formats

As dis ussed in the previous hapters, to address the apa ity run h in opti al

networks,spe trally e ienttransmission s hemesemployinghighorder quadra-

tureamplitudemodulationformatswith oherentdete tionarebeingextensively

explored as the potential andidates for next generation opti al networks [1℄. In

addition, rapid re onguration of the opti al network with opti al pa ket/burst

swit hing te hnology allows the ampli ation bandwidth of the ber to be used

more e iently [2,3, 4℄. Combining oherent transmission te hniques with opti-

alpa ket/burst swit hingte hnology anenableopti alnetworksthatarehighly

e ient both temporally and spe trally. Opti al pa ket swit hing that employs

fully oherent dete tion for PM-QPSK [2℄, self-homodyne dete tion for DQPSK

[3, 4℄, 16-QAM polarization multiplexed pilot with self-homodyne oherent de-

te tion [5℄, or 16-QAM Baudrate-Pilot-Aided transmission s heme with dire t

dete tion [6℄ have been re ently reported.

As dis ussed in hapter 1,one of the potentialopti al networks onguration

employing opti al pa ket/burst swit hing with advan ed modulation formats is

as follows: At the opti al transportation layer, we make use of fast-tunable op-

ti al trans eivers in the opti al nodes. These tunable trans eivers an swit h to

or sele t an available wavelength on the ITU-grid that is used to transmit or

re eive the data opti al pa kets. Obviously, the key omponent in fast-tuning

trans eivers will be the fast-wavelength-swit hing tunable lasers, and the SG-

DBR laser is a potential hoi e as dis ussed previously. However the onsider-

ablephase/frequen ynoiseinSG-DBRlaserswouldu tuateevenmoreduringa

wavelengthswit hingtransientinthishighlydynami networks enario,espe ially

for short opti al pa kets. In this hapter, we will investigatethe performan e of

lationformatat10.7Gbaudinse tion5.1. Inse tion5.2, wethenmovetoopti al

pa ket swit hing systems employing higher order QAM modulationformat with

the proposed Baudrate-Pilot-Aidedtransmissions heme inthe previous hapter.

The lastse tion, se tion5.3 will on lude the hapter.

5.1 Opti al pa ket swit hing with Self-Homodyne

DQPSK

Wavelength swit hing times of less than 5ns have been demonstrated using the

SG-DBR laser [7℄ making it suitable for opti al pa ket swit hing, however its

signi antphase noiselevelmay redu eitssuitability ifphase-shift-keyingbased

advan edopti almodulationformatsareemployed intheopti alpa ketswit hed

network. EventhoughtheDQPSKmodulationformatwithdire tdete tionhasa

hightoleran e tolaser phasenoise,this transmissions hemestillsuersfromthe

high level of phase/frequen y instability during the wavelength swit hing event

of the SG-DBR laser. In this se tion, through detailed experimental studies,

this performan e degradation during the transient state of the emulated opti al

pa kets is investigated.

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