4:15pm-4:30pm WAA3

Perfsrmarsse of Caseaded Wawelength Cemwerters Garoline C. Watrg

Opeti Telccoriiiiiiinicatii,lls Idtd., 20 1'10s~~ecI Sti'cct, llox Hill 3128, Australi;i E-niai I : Ciiroliiic. W o i i ~ ~ ~ ~ . c ~ ~ r i i . ~ i i ~

Mark A. Summerfield, and Rodney S . Tucker

Australiari Pliotoiiics Cl<(', l'lie IJniversity 0 1 Melboiiriic, Victol'ia :1010, Ausiro1i:i E-mail: ~n.sutiiincrlieltl @iece.org, r.tiickeraoce.tnu.ol..au

Introduction

Wavclcngtli~divisiori iiiiiltiplcxcd (WDM) rictworks may beiicfit l'roiii cmployiiniit of optical wavclcngtli coiivcrtcrs. Signal qualily iiiay degrade or iiii11rovc whcii uridcrgoiiig several stages of wavelength conversion in nu optical network. It is therefore necessary Lo optiiiiisc systciii aid signal ~paraincters for such systcnis to d i i c v c pcrforlnailcc. Cross-g;iiii iiiodulatioii (XGM) iii

semiconductor optical atnplilicrs (SOA's) is a simplc techriiquc Sor perfomiitig wavelength coiivcrsioii, capable of operating at bit-ratcs of tens of gigabits per second. Experimental a id theoretical investigations ol cascaded XGM wavclciigth coiivcrtcrs (XGM-WC's) have heen cmied out considering the effect of cxtiiictii)n ratio (HK) degradation in each stage /1,2/. The optical intensity noise, caiiscd hy aitiplificd spontaneous emission (ASB) i n the XGM-WC's, wluch limits tlic miniher of' cascaded stages, has not hecn considered in detail in previous analyses. In this paper, we present a Lhcorctical iiivestigatioii [if

pcrforinance of systems cniploying casciided wavclciigtli convcrtcrs. We consider ii pith consisting of' up to 21 stages of wavelength coriversion, in which arrayed- waveguide grating ~iiiiltiplcxeis (AWGM's) arc usct l Cor optical multiplexing, deinultiplcxing and filtering. Our systcin model includes the ASL! iioisc accuinulatioir withiii each stage, aiid tlic excess ASE noise and residual signal power which accumulate duc to imperfect filtering iti

realistic devices. Vol. tlic first time, we calculate the iiiipact ol'thcsc (actors oii tlic signal quality, iis measured hy the Q- factor aiid bit-cuor rate (BER) pxforiii;iiicc, aiid dcnioiistrate t h t 1 3 degradation nloric inay tiot be the iiltiinatc liiniting f;ctor in the cascadahility o1 XCiM-WC's.

System Miidel

The XGM-WC tnodcl used iii the iiivcstigatiiiti is Iiiised on a siriiplilicd wide-band static SOA model, wliicli wc liiivc showri to he iiccuratc to withiii 3 dR over 2111 iiqiut poww iange 01 SO dB, iiiid wavclcegtli raiigc of 40 iiin 131. Wc consider up to 21 stagcs of wavclciigth convcrsiori, aiid iissiiiiic that identical SUA'S arc used in all stages. AWGM's arc used as oiitpiit nioltiplcxcrs, aiid a lso Imivide band-piss filteriiig ii1 ASU iii)isc. We model 8x8 poit AWGM's with 100 OH7 cliiiiincl spciing, ZS t lU out-of- band rejection, a i d 0.3 iiin 3-dH ~~~ssbband width We

consider distortion due to tlic spcctml iiii~~owiiig in cascatlctl AWGMs 141. Our. systeiri niodel iricludea tlic accumulation of tlic SOA ASE rioisc gcnu;itcd at c;icli s l ~ g c 151 and we inodcl tlic traiislcr ot AStGiiidiiced iriteiisity noisr in ciicli XGM-WC tising a Chussiaii qipi'oxiimitioii lor tlic iioisc statistics. lliis ~ ~ ~ i ~ ~ r o x i i n ~ i t i ~ i i i is kiiown to be seasorialily accuratc, iiiid teiids to result iii

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0-7803-5947-W00/$10.00~2000 IEEE 63 1

tlic HE11 bciiig ovcrestiiii:itcd, tliiis n u resul ts ~ ~ c p ~ x " ail

uppc1 Iiounrl on the c r m p k " ; m c c 161. liiliiilly, o w iiiodcl includes the ASE noise, and ~csidual signal I J O W C ~ S

which leak t l i~~ougli the AWGM's due to: ( I ) the iii~itc out- of-band rejcctioii; aiid (2) add i t iod passb~intls occuiing at iiiultiples oC the AWGM f?cc-spcctral iai~gc, which contrihutc to the gaiii siitulation 01 sulncqucnt casc;idcd XGM-WC's. The receiver modcl includes Ciiaissiau post- detection mise iit il power lcvcl which results iii sciisitivity of -30 dUm lor a BER ol' 10.' iit 2.5 Ghls, with iiii ideal intcgrale-;iiid-duriil)" dccision circult /6/. 'Slic LnLal

wiivelcrigtli coiivcrsioii range cxaininctl is from ;ipproxi~tiately IS30 tiin to 1565 mil, ;uid we consider twn basic scenarios: (I) 21 stages of wavelength tlowii- conversion starting et 1564.92 nm; and (2)'LI stages (11 waveleiigth up-conversion starting at IS3 1.35 nni. In liotli cases. each sticcessivc conversion is by 200 GHz in tlic specified direction.

Results

I'.

I'igure 1 shows the output ER, Q' aild HEI< pcrl'olmalicc of tlic 21 cascaded duwri-convcrsioii stages. The powei levcla input to the tirst stage arc clioscri to hc 4 dllni and -40 dI3m Lor hit ' I ' and '0' respectively, giving iiii approximately ideal input ER of44 dU at the lirst stage. 'l'ht: pwhc power ii~put at eveiy stage is -8 dl%iii, wliicli ensures sufliciciit stimulated emission during data '0's to inaiiitairi high conversion speed /7/. 'l'lic black line lahellcd 'H' in I'igurc 1 (c) is Llic 1,ack-to-back lIUl< cuive, and tlic grey ciirvcs iiiid b l x k daslicd cii ivcs iirc tlic uiitpiit 111iI< ciiivcs at Llic outputs ol' Skigcs I to 20. TIic R E K ciiivc alter 21 stages is not shown 1ici.c bccmisc i t ~i i i s BUR > IO ', At the nutpiit o l tlic lint stage, tlic l iR is 10.33 dU, Q2 is 46.15 dR, mid tlie powc~' pcii'ilty (ciilcnlatcd lor IIER = 10~') is 0.94 dU. At the oiitIiu1 of the accoi~d stage, tlic ER is 7.21 d U . Q2 is 43 t lR, and tlic power pciialty is I .84 dll. 'lhc p w c r Imiiiltics due to 1!1< dcgIad;itioii iiloiic :it the tirst aiid secorid stage a1c calculated (using tlic cxprcssion givcri in /7/) to bc 0.7'Jdn :III~ 1.65dU respectively. The additiori;il penalties, of 0. IS (Ill ; inc l 0.19 dB rcslicctively, are due to iiitciisity iioisc, wliicl~ is caused hy the ASR iioisc gciicr;itcd iii c.ic11 XGM-WC. L'igiirc I sliows tliiit i i i the lirsi livc tu I O stages, tlic UI< degrades iapidly, resulting in sigiiilicarit IIOWCI ponallics. However, tlie HE11 ciirvcs in Piguic 1 (c) sliow IIO cvidcricc of nEl< tloors ;it this point. The Q-t ic to r rc~~iairis iclatively high, as iiidicetcd hy I'igurc 1 (II) , 1)ec;insc tlic lcvcl of iritciisity iioisc accuiriulatcd is low. Altliougli icccivctl optical powci' well in exccss (if tlic rcccivcr sensitivity is required. the crriwratc Iloor reinairis well IicIow IO-? l'hc viihc uf o2 c ~ i i r c s p ~ i t ~ d i ~ ~ g to a11 c r m late (loor O S 1 0 ~ ~ is ;ipproxiiiiately 15.5 dU, sl iow~i i n I'igurc I (11). ' lhc 0- tactor degradation is due to :I c~ii~il~iri~ition of tlir. 1IK

tlcg;alatiiiri. and acciirnulatioii 0 1 optical intensity tioisc. V i e I3ER llooi bccoiiics visible as curvature in tlic nHI< ciirvcs after 10 stages. ' l ' l ic lloor t iscs rapidly ;iftcr 1.5 stages. aiid i t rises above 10.' UBI( alter 16 stages.

-- 1 1

(b)

5 i o 15 20 Number of Stages

OO

increasing stage number

-3

4

-5 e K - 6 2 -7 -

-8 -9 -I 0

. -12 -14

30 -20 -10 a ROP (dBm)

(4 1,igore 1: i'wforniance niearures as :I fuiicticirr o l the nundier of stages of wavclengtli ciowii-couvel-sioii:

(a) ER; (h) Q2; (c) I1EK curvcs.

l h e BFX lloor is shown iti t'igure 2 as :I liinction ol the r i u t i i l m of stages of wwcleiigth conversion. We havc considered live different examples. The results (or thc system parametcl's considered ahovc at'c lahellcd " d o w ~ ~ conversion". 'rhc curve kibdk!d "tip coiivcrsind' wiis obtained using the swiic input signal Icvcls. init convcrtung upwards in wavclcrigth at each stage. l 'hc curve labelled "Pi = 2 dBni" is for wavelcngtli d[)\vti-coiiversiOti, with tlie itiput hit ' I ' power level retluced to 2 dBm (froin 4 dBiii), currcspnding to ail initid ER ol' 42 dU. For tlic c i i ivc lahelled = -10 di3nl", tlic input hit '0' power has Iheeti increaacd to -10 tlBiii (Iioiii 4 0 d13m), corrcspiitidilig to ari initial ER o i 14 dB. l h i l l y , to obtain the curvc lahcllcd

= -6 dDm" tlic XGM-WC probe pnwcr hiis I I C C I I increased to -6 dUni (fr.oiii -8 dBm), with the input ER rciiiainirig at 44 dR.. The results in Figure 2 caii b e sumimised and explained iis follows. l h e svsteni usilia wavelength upcoiivcrsiiin has ii

d;ihility t l i i in thc dowri-ci~iivcrsioii ciisc coiisidcicd ehovc. 'l'liis is duc t i i tlic Iiighei. dcgrce of F.I( dcgi.;icl;itiori that occurs wiicti conveiiing li-om shorter to longer w;ivclcngths using XGM iii SCIA's. Incrcasiii~; the bit '0' p w c r to i.cducc tlic i t iput El< to 14 (111 has only a sriiall iiiipiict on the cascatlability, Ircc;itise the dcgice of gain coinpressiori provided by the hit ' I ' power is uticllarigcd, rcsiiltiiig iii littlc cliaiigc to thc output P,l< o f tlic lirst stage. On the other hand, decreasing tlic bit '1' p w i x to 2 dHin di.astically reduces the cascadaibility of the XGM-WC's. This is because although tlic initial El< is high (42 dU), the dcgi-cr. of gain compression provitlctl by tlie Iiit ' I ' powcr is mluced, resulting in gi'catcr F,R degradation in each stage. Finally, increasing the probc powci reduces the c;iacadaliility because i t iiicrcascs the gain coinpression during input bit 'O's, wliicli results i u intire highly degriidcd output El<. llowcvcr, incrcased p d J c power inay he necessary to ciisiitc sufticiently last E l . . msicnt .' tatcs i n cxccss of 10 Ghls I l l .

rcsponsc tinics lor wavelength co~~vc~~si i i i i at bit

Down Conversion Up Conversion

P, = 2 dEm P. = -10 dBni

P,, = -6 dEm

.. 0 5 10 15 20

Number of Stages

Figure 2: Performance comparison for different system paranieters: Blii< tloor versus number of stages.

Canclusions

Wc havc tlicorctically investigated the perfortixiiicc of cascadcd stages of XGM waveleiigth convel-sim, using a ncw, highly accuratc SOA Inodcl, kuid including a detailed alalysis of the effects of accurnulatcd intensity noise, iuid the impact of noli-ideal filtcriiig aiid rnultiplexing components. Our results contim that E17 degiadatiort provides the dominant coittribution to power penalty cor short cascades of XGM-WC's. However, we have shown that if r.elativcly large puwcr pcndtics caii he tolerated, the cascadability of XGM-WC:'s Inay be g ~ a t c r than previously assuined. The ultimate limit uti the cascxdahility of XGM-WC's is imposcd hy the accuinulatiori iif intensity noise on tlic wavelcngtli- converted signals, which results in ari etror-late floor.

Kefereiiees

111 /2/ 131 I41 I51 I61 17/

A. E. Willricrcf. d, J1.1; vol. 13, pp. 771.781, 1995. 8. Mikkelscn et. r d , OF'(//#OC'Y9, 1999, paper HI. C. C:. Wang el. al, submitted to OAA'Z000. C. C;ispu et. 01, O l C 9 7 , 1997, papci' 'l'uE2. K. Inoue, PIT., vol. S, p ~ i , 888-890, 1996. D. Marcuse, ./L.?; vol. 9, pp, 505-513, 1991. J . M. Wicscriltld cr. nl, !'?'I., vol. 5 , I J ~ . 1300-1303, 1993.

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