01-1318924386-55864

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1 Institute of infomation science&engineeingHebei nivesity of science ad technology

Shjazhug, Chna [email protected], [email protected]

Ab stt -Fier-optic dispersion and its effect on optical transmission system are analyzed. The most commonly useddispersion compensation er (DCF) technology is studied in this article. Three schemes (pre-compensation, postcompensation, mix-compensation of dispersion compensation) ofdision compensation with DCF are proposed. Thesimulation model of the WDM ased on the Optisystem ispresented according to the aove principle. The simulationresults such as Q factor and BER are given and deeplyanalyzed. It is found that mixcompensation performance is the est. nd the input er power is taken as 9-10dB, thecorresponding BER performance is etter.

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I. NTRODUCTION

In cent yeas, with the apid gowth inteet business needs, people gently need moe capacity and netwok systems.[] So the demand fo asmissioncapacity ad badwidth e becoming moe d moechallenging to the cies ad sevice supplies. nde thesituation, with its huge bandwidth ad excellent tansmission pefomace, optical b is becoming themost favoable deliveing media ad laying moe and moeimpoant ole in infoation indus[2,3]. The optimaldesign ad application of optical be e vey impoant to the smission quality of optical be tasmissionsystem. Theefoe, it is ve necess to investigate the tansmission chacteistics of optical be. And the maingoal of co nication systems is to incease theansmission sac. Loss ispsion h majofacto that aect be-optical communication being the high-capaci develops. The EDF A is the gigantic change happened in the be-optical couication system; theloss is no longe the majo facto to esict the beoptical ansmission. Since EDFA woks in 1550 nm wave band, the aveage Single Mode Fibe (SMF) dispesionvalue in that wave band is vey big, about 15-20ps / ( nm.km)[4]. It is easy to se that the dispesion become the majo facto that esticts long distance be-optical tansfes[5].

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2 Tanspo DepaentsHebei Boadcasting and Television Bueau

Shjazhuag, Chna [email protected], [email protected]

In this study, we popose thee DCF compensationscheme, pe-compensation, de-compensation ad mixcompensation scheme. Simulation studies show that mixcompensation scheme is the best. ca eatly educe the

inuences of the be nonlineity and incease the tansmission distace eatly.The est of pape is ogaized as followed. In Section

II , the cause of be dispesion and its eects on optical tansmission is inoduced. Section III, pesent Fibedispesion compensation technology. In Section IV, pesent the eseach of DCF dispesion compensationscheme. Section V pesent tsmission system silation with sowae OPTISSTEM, aalyses ad compae each kind of compensation scheme, Section VI concludes the pap.

II. THE EFFECT OF FIBER-OPTIC DISPERSION ON

OPTIC TRANSMISSION

Loss ad dispesion e the majo facto that aectbe-optical communication being the high-capacitydevelops. The EDF A is the gigatic change happened in

the be-optical communication system; the loss is nolonge the majo facto to esict the be-optical tasmission. Since EDFA woks in 1550 nm wave bad, the aveage Single Mode Fibe (SMF) dispesion value in that wave band is ve big, about 15-20ps / ( nm. ). Itis easy to see that the dispesion become the majo factos that es ti ct long distance be-o ptical ta nsfes [6].Dispesion is deed as because of the dieent equencyo mode of light pulse in be tasmits at dieent ates,

so that these equency components o models eceive thebe temnas at ieent time t can cause intoeabeouts of distoions that ultimately lead to eos.

In single-mode be pefoance is pimaily limited by chomatic dispesion (also called goup velocitydispesion ), which occs because the index of the glassvaies slightly depending on the wavelength of the light,and light om eal optical tasmittes necessily has

nonzeo specal wid (due to modulation)[7,8].Polaization mode dispesion , aothe souce of limitation,occus because although the single-mode be ca sustainonly one asvese mode, it ca crthis mode with twodieent polizations, and slight impefections o

distoions in a be can alte the popagation velocities fo the two polizations. This phenomenon is called

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bieingence. Mode bieingence Bm is deed as thefollow Fomula:

l/x-/ yl B = = k o x y()

n, n e the eective eactive of the two ohogonal

polaizations. Fo a given Bm, its fast axis and slow axiscomponents will be fomed e phase dieence ae thelight waves asmission L

2q = k OBmL = -(N -Ny )L = (/x f )LA (2)

If the Bm is a constat, though the light waves in tansmission pocess the phase dieence between its fastaxis and slow axis will peiodicity epetition. The poweexchge also peiodically. The length that it leads to a

phase dieence of 2 o powe peiodic exchange iscalled polization beat length:

L

2 �

B-1/

-/-

Bm 3If the incident light has two polization components,due to eactive dieence between the fast axis and slowaxis, the asmit ate of two polization components will be dieent. Because the andomness of be bieingence chages, the goup velocity of dieent polization diection is also andom, this will esult in theouut pulse boadening. Deee of pulse boadening ca be expessed by dieent goup delay 8T.

LT � P )L= (n X-n y + Od ( n x-n y)JL ( 4) V d y c c d

Fom the equations pesent that the initial pulse is

boadened by the tasmission. The longe the opticalsignal tasmission distance, the eate the bedispesion coecient, the moe pulse boaden. The twoadjacent pulse esult oveheing, eo judgment willgeneated[9].

The inuence of dispesion on system pefomance isalso eected in the optical be nonlinea effects.Dispesion inceased the pulse shape distoion caused by

the self-phase modulation dispesion (SPM); the othe hand, dispesion in DM systems ca also incease ecss-phase modulation, fou-wave mixing ( FM) adothe nonlinea eects[,].

III. DCFDISPERSION COMPENSTION TECHNOLOGY

In ode to impove oveall system pefomance ad educed as much as possible the smission pefoanceinuenced by the dispesion, seveal dispesioncompensation technologies wee poposed[12]. Amongst

the vious techniques poposed in the liteatue, the ones that appea to hold immediate pomise fo dispesioncompensation and magement could be boadly classiedas: dispesion compensating bes (DC F), chiped be Bagg atings (FBG), ad high-ode mode (HOM)be[13].

The idea of using dispesion compensation be fodispesion compensation was poposed as ely as in 1980

but, until ae the invention of optical amplies, DCF began to be widespead atention ad study. As poducts of

41

DCF e moe matue, stable, not easily aected by tempeatue, wide badwid, DCF has become a most usel method of dispesion compensation ad has beenextensively studied.

Thee is positive second-ode and thid-odedispesion value in SMF (single mode be), while e

DCF dispesion value is negative. So by inseing a DCF, the aveage dispesion is close to zeo[14]. As the localdispesion of highe asmission link, FM and XPM

wee ignoed; only to conside the ole of SPM anddispesion, the signal asmission ca be simulate bysolve the nonline Schodinge equation.a(z,t) 02(z,t) a ( 5 ) ' i 2(Aj) ' i A z t A/ z,t)+-A/ z,t)=O

2 at 2

A ( z, t) is complex amplitude of j channel optical

pulse, /2A j) is the dispesion paete of j chael,r is the nonlinea coecient, is the loss coecient.

Ae -section dispesion compensation of DCF, thechael esidual dispesion c be expessed as

A = N L [(1- D A )+( . _ 8A d D A p J p D SMA d D A P» ]

, J p S M P } P d A D O F(A p) d A

6

In the foula, J is the dispesion compensation ate of p-channel.

ID A P)LCF

IJ= DS MFA p)LsMF ( 7 )

LSMF ad LCF e the conventional single-mode belength ad dispesion compensation be length within the

aplie spacing. A is the chanel wavelength spacing.D C(A p) a d DS MF(A p) e the dispe rsio n coefcie nt of conventional single-mode be ad dispesion

compensation be at the A p wavelength.

IV. DISPERSION COMPENSTION SCHEME

The dispesion compensation system in the DM isstudied in this pape. Based on optical ansmissionequation, consideing the vious types of nonline eectsad the impact of EDF A, system simulation models eestablished. Accoding to elative position of DCF and

single-mode be, post-compensation, pe-compensation,mix compensation is poposed. DCF Pecompensationscheme achieve dispesion compensation by place the DCF befoe a ceain conventional single-mode be, o ae the optical asmitte. Post -compensation scheme achievedispesion compensation by place the DCF ae a ceainconventional single-mode be, o befoe the optical tansmite. Mix compensation scheme is consist of postcompensation ad pe-compensation .Dieent location on the system will geneate dieent nonline effects. TheSimulation of tee dispesion compensation system isshown in Figue 1.

Figue 1 (a) shows the post-compensation schemes. TheDM system consists of eight chanels, each chanel wi

40Gb/s. e smlon mole nclues e asmission

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module, ansmission link and the eceive module.Simulation model use Mach-Zehnde exteal modulato to modulate the CW Lase. Eight dieent centeequency wavelength of the light cie wee poduced.The cente equency age of Lase is 192.6-194.01THZ.Tasmission code is the DPSK modulation code. 8-

chanel WDM badwidth is 80GHZ.Optical be tansmission link composed of a 160Km. The kind ofoptical be is G.655. Dispesion compensation isachieved with DCF. EDFA is used to compensate the powe loss geneating by SMF ad the DCF signal.Receive module includes demultiplexe and eceiveltes.

DE�D S 1FD 02 Tx �__�

8 Tx R

! posompensao

: o uu� :b) ompensaon� � S�� m -L 1 m opon

Figue 1 the thee dispesion compensation system

Figue 1 b) ad Figue c) espectively show the pe

compensation scheme ad the mixcompensation scheme.The simulation conguation is as simila as Figue 1 a).The diffeence is that at the asmitte DCF compensate80Km of single-mode be dispesion in Figue 1 b). InFigue c), thee is a mix- mix- compensation scheme.

V. U D ANY

In optical communication systems, only optical signal to noise atio (OSR) could not accately measue thesystem pefomace, especially in WDM systems.Typically, as a quality facto, Q is a one of the impotantindicatos to measue the optical pefomace by which tochaacteize the BER. Figue 2 display the inuence of

input optical powe on the pefomace of tasmissionsystem

397

053975 4 5

x c t-c _nr AC i o

6 7 9 3 4 5l i c fi b p (d�Figue 2 Compaison of tasmission influence of thee

compensation system

4

Figue 2 appe that the eect of lase aveage powe isjust con to the pevious situations. A modeate biggevalue of lase aveage powe is favoable to the pefomance of the ansmission system. And om thegue we can d that with the input optical poweinceased to about 9dB, the Q facto inceases. When the

input optical powe appoaches 9dB, the Q facto becomes the maximum. When the input optical powe is eate tha 9dB the quality facto deceased gadually and theeo pefomance is gadually degaded. This is becauseas the optical powe inceases, nonline eects incease, but the optical signal noise atio inceased. When the inputoptical powe is eate th 9dB, the nonline effectinceases apidly, maing the system BER pefomance isdeaded apidly.

As ca be seen om Figue 2 in the case of the beoptical powe equal, the quality facto of mixcompensation is eate th the two othe kinds ofdispesion compensation. Tough the whole system study

found that the pefomace of mix -compensation system is best in the long-distace high-speed WDM systems.

Figu3 give the simulation esults at dieent chanel. As we ca see om the gue3, the effect of dispesioncompensation is ve good. The signal quality is high,eye's shape is ve good, d the edge neat gaph issymmetical. And the eect of dispesion compensationquite good. The cuve of Q-facto changes with the opendeee of eye diaa as follows: Moe appoaches the point of lgest eye diaam opened, the Q-facto is bigge,and the Coesponding BER is smalle. Figue shows at thelagest eye diaga opened, the Q-factos of the fou

chanels achieve about 20. Accoding to the test, whenQ=6, the BER is about 10-9; when Q=7, the BER is about10-12. The gue shows this system have the big decisionscope unde the guaantee of the condition of system biteo ate. This indicated the DCF compensate dieentchanel's comatic dispesion geatly.

ly. Og AIr_

•-

D

j

Figu3 the eye diagam at dieent channel

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VI. CONCLUSION

On the basis of compaed and aalyzed the thee systemsimulation esults conclusions ae as the followings. singDCF fo dispesion compensation in 40Gb/s WDM systemis a eective solution. The atenuation of DCF be is not null. Thus, the attenuation of DCF be will poduceimpaiment to the signal quali as well as that of SMF. As

the pevious discusses, the inuence of atenuation ca becompensated with optical be aplie such as EDF A.Mix-compensation scheme can geatly educe the be nonline eects, this poam bette tha the pecompensation ad post compensation poga. Fo thiscompensation scheme, the eect of lase aveage powe isjust con to the pevious sitations. A modeate biggevalue of lase aveage powe is favoable to the pefomace of the asmission system the input be powe is taen as 9-10dB, the coesponding BER pefomance is bete.

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