Emerging Subsea Networks

Transcription

1 MODELLING OF NONLINEAR FIBER EFFECTS IN SYSTEMS USING CODIRECTIONAL RAMAN AMPLIFICATION Nlson Costa (Coriant Portugal), Lutz Rapp (Coriant R&D GmbH) Coriant Portugal, R. Irmãos Simns 1-1A, Amadora, Portugal Abstract: This papr proposs a nw tchniqu for dtrmining th impact of nonlinar fibr ffcts on signal propagation in unrpatrd transmission systms whn using codirctional Raman amplification. Th proposd concpt is basd on pr-calculatd nonlinar pnaltis for a st of powr profils by making us of accurat but computation-intnsiv tchniqus. Onc th prformanc of a spcific link nds to b dtrmind, an accurat stimat of th nonlinar pnalty is drivd from th ntris of th gnratd databas almost instantly. Th composition of th databas and th mthod to driv th nonlinar pnaltis from th ntris ar xplaind. 1. INTRODUCTION Accurat planning tools ar ssntial mans to provid a quick rspons to customr rqusts and to offr th bst suitd solution. In ordr to b comptitiv, th lowst cost configuration guaranting th rquird prformanc nds to b slctd. Charactrizing signal distortions arising from nonlinar fibr ffcts is on of th most tim consuming stps in prformanc calculation. In particular, accurat solutions can b obtaind by solving coupld diffrntial quations such as th nonlinar Schrödingr [1] or th Manakov quation [2]. Howvr, ths approachs ar not suitabl for providing fast rsponss to customr rqusts du to th larg simulation tim. Simplifid mthods providing fastr stimats of th nonlinar pnaltis at th xpns of lowr accuracy hav bn proposd in litratur. An xampl of such a mthod is th Gaussian nois (GN) approach applicabl to cohrnt modulation formats [3, 4]. Whil th most simplifid vrsions of th GN approach ar simpl and vry fast, th most accurat ons ar mor complx and may still lad to considrabl simulation tim [5]. Th fast and accurat prformanc stimation for transmission systms is vn mor challnging in unrpatrd long singl-span systms du to th high launch powrs. Morovr, with th trnd to highr data rats mor and mor systms hav bn quippd with codirctional Raman amplifirs lading to powr profils in th fibr that mak accurat prformanc modling vn mor difficult. Additionally, phas modulatd signals dtctd cohrntly xprinc nonlinar crosstalk from a largr numbr of nighboring channls, which furthr nhancs th difficulty to obtain a fast but still accurat prformanc stimat. In this papr, a nw tchniqu for dtrmining th prformanc of unrpatrd transmission systms using codirctional Raman amplification is proposd. Th impact of nonlinar fibr ffcts for a link is drivd from ntris of a databas gnratd bfor considring a spcific projct. In Copyright SubOptic216 Pag 1 of 6

2 particular, th databas contains information about nonlinar pnaltis for svral powr profils in th transmission fibr, whrin th pnaltis hav bn calculatd bforhand with high accuracy. Th powr profils ar chosn to rprsnt all kind of rlvant powr profils. Thus, th databas can b usd to stimat th prformanc of a larg variation of transmission systms in a fast but still accurat mannr. Th main advantag of this approach is that link prformanc can b dtrmind almost instantly with small computational powr, sinc rquird complx computations hav alrady bn don by a high-prformanc computr bforhand. Morovr, sinc similar transmission systms should lad to similar impact of nonlinar fibr transmission ffcts, th databas nds to compris a small numbr of powr profils, only. 2. MAPPING PROCEDURE Th ky problm to b solvd is mapping a givn powr profil to an appropriat powr profil for which th pnalty has alrady bn calculatd and driving th actual pnalty. Thrfor, a mapping rul has bn dvlopd. Th st of pr-calculatd scnarios will b rfrrd to as baslins in th rmaining of this papr. Various powr profils in th transmission fibr covring a wid rang of possibl applications nd to b considrd. Figur 1 shows svral diffrnt shaps of powr profils in th transmission fibr obtaind whn using codirctional Raman amplification. Th powr profils rprsnting diffrnt gain valus and pumping configurations hav bn normalizd to a launch powr of dbm. Howvr, th nonlinar pnalty nds to b stimatd for diffrnt launch powrs for ach of th curvs. A good charactrization in th rang of small nonlinar pnaltis is of major importanc sinc maximum prformanc is typically achivd in this rang. Morovr, th impact of disprsion prcompnsation should also b takn into account. Thrfor, in gnral, ach baslin consists of svral ntris containing th nonlinar pnalty for diffrnt pairs of launch powr and disprsion prcompnsation valus. Plas not that only th nonlinar pnalty is providd by th baslins, sinc dtrmining th availabl optical-tosignal nois ratio (OSNR) dos not rquir significant computational powr. Powr lvl [dbm] Fibr position [km] Figur 1. Exampl of powr profils along optical fibr whn using codirctional Raman amplification Saving th diffrnc in pnalty with rspct to an xponntial powr dcay for th sam launch powr is convnint, sinc it allows to align th simulation rsults with xprimntal data obtaind for a configuration with a high powr boostr. In fact, simulations provid a good stimat of th rlativ rach improvmnt achivd by codirctional Raman amplification ovr a configuration with a boostr amplifir. In contrast, th absolut transpondr prformanc dpnds on many paramtrs and largr dviations btwn simulation rsults and xprimntal data ar possibl. Thus, accurat rsults can b obtaind by masuring transpondr prformanc vrsus channl powr for a configuration with high powr boostr and using ths rsults for th furthr calculations, whras th Copyright SubOptic216 Pag 2 of 6

3 ffct of th codirctional Raman amplifir is takn into account by th simulation rsults. Hnc, only fw xprimntal data ar rquird for th calculations. Th concpt of th mapping procdur can b summarizd as follows: 1. First, th powr volution of th channl to b valuatd is calculatd, lading to a powr profil similar to th ons shown in Fig. 1. Th calculation of th OSNR availabl at th rcivr sid can also b don in this stp. 2. In th scond stp, an appropriat baslin is slctd for th actual powr profil. This can b achivd by assssing which baslin has th shap with th highst similarity to th actual normalizd powr volution of th considrd channl. Th similarity btwn th powr profils can b valuatd using a root man squar (RMS) dviation as critria. Aftr this stp, a baslin having a shap that is vry similar to th actual powr profil has bn slctd, but th rlvant launch powr is still not known. 3. Nxt, th quivalnt launch powr has to b dtrmind. For this purpos, th intgral of th powr profil calculatd according to q. 1 for th channl of intrst, whr z is th position along th fibr, L is th lngth of th fibr, and P(z) is th powr of th channl at position z is usd. In this quation, th powr is xprssd in linar units. Thn, two approachs can b usd to dtrmin th quivalnt launch powr : a) Data stimatd for svral launch powrs ar availabl for th slctd baslin and q. 1 can b usd to calculat I for ach powr lvl of th baslin. Thus, th rlation btwn th intgral I and th launch powr is known for th slctd baslin. Sinc th intgral of th actual powr profil has alrady bn dtrmind, th quivalnt launch powr can b dtrmind by applying intrpolation. b) Altrnativly, as th shap of th powr volution is dtrmind by th slctd baslin only and is indpndnt of th launch powr, w can writ: q. 2 whr th indx b rfrs to th slctd baslin. Using q. 1 to calculat I for th slctd baslin w gt: q. 3 Thn, by noticing that P launch is th quivalnt launch powr whn I=I b, w gt q In a fourth stp, th rquird OSNR (ROSNR) is calculatd for th quivalnt launch powr using th rlativ nonlinar pnalty providd by th corrsponding baslin (again dtrmind by mans of intrpolation) and by adding th masurd ROSNR for th cas without codirctional Raman amplification. Th margin of th transmission systm undr valuation is th diffrnc btwn th calculatd OSNR and th ROSNR. Th sris of stps forming th mapping rul is illustratd in Fig. 2. Copyright SubOptic216 Pag 3 of 6

4 Figur 2. Illustration of stps prformd to calculatd th rlativ nonlinar pnalty, th rquird OSNR, and th link margin Anothr paramtr that has not bn considrd so far and that may impair th accuracy of th prformanc stimat is th channl count. Whn considring intnsity modulatd channls, only nxt-nighbor channls influnc th channl undr considration and th baslin pnaltis can b calculatd taking into account th nonlinar distortions inducd by ths channls. In contrast, it has bn shown that th prformanc of phas modulatd channls using cohrnt dtction can b significantly affctd by channls having a quit larg frquncy spacing to th considrd channl [3, 6]. In ordr to mitigat th impact of this limitation, svral approachs can b followd: a) Th nonlinar pnalty and masurd ROSNR without codirctional Raman amplification can b obtaind for svral channl counts. This approach should lad to vry accurat rsults. Howvr, th simulation tim rquird to gnrat th baslins th baslins also hav to b calculatd for various channl counts and also th siz of th databas that nds to b gnratd will incras considrably. Morovr, it is difficult to tak into account th diffrnt launch powr lvls usd to compnsat for stimulatd Raman scattring (SRS) among th channls. b) A worst cas scnario can b considrd by calculating th nonlinar pnalty and by masuring th ROSNR without codirctional Raman amplification with th maximum channl count. Howvr, too consrvativ transmission systm prformanc stimats may b obtaind for rducd channl counts. c) Th nonlinar pnalty and th masurd ROSNR without codirctional Raman amplification can b obtaind only for a singl channl count that is not too high (.g. 16 channls). Evaluating th prformanc of th transmission systm for any numbr of channls considring only th information obtaind whn 16 channls ar transmittd is not vry accurat, but should lad to accptabl rsults for a larg rang of transmission systms. If a smallr numbr of channls is dployd, th prformanc stimat will b consrvativ. But th impact of possibly largr channl counts in th futur is alrady bing takn Copyright SubOptic216 Pag 4 of 6

5 Normalizd optical powr [dbm] 16 ] Actual powr volution B14 5 [d l 12 v 2 l 1 r w 6 3 o 8 p 7 d a liz 4 rm o 2 N Transmission distanc [km] Figur 3. Actual powr profil of transmittd channl (rd) and of baslins into account. If a channl count highr than 16 is dployd, th prformanc stimat will b optimistic. Howvr, th impact of multi-channl transmission is alrady capturd to a larg xtnt whn th transmission of 16 channls is considrd and, thrfor, only small inaccuracy should aris from such an approach. d) A rul that taks into account th impact of channl count can b usd. This is th approach that lads to th bst compromis btwn accuracy and simulation tim/databas siz. Th drivation of such a rul can b don by xploring th xponntial dpndnc of th OSNR pnalty on th numbr of channls [6, 7]. Morovr, th dpndnc of channl count on fibr typ can b takn into account by using th stratgy prsntd in [8]. 3. RESULTS AND DISCUSSION Th accuracy of th prformanc stimats obtaind using th tchniqu proposd in this papr is discussd in this sction. Th rd lin shown in figur 3 rprsnts th powr profil of on of th data channls undr analysis whn th transmission of 7 channls in a pur silica cor fibr (PSCF) charactrizd by an attnuation n1 tio 8 d v ia S 6 M R Baslin numbr Figur 4. Man RMS dviation btwn th powr profil of data channls and of ach baslin paramtr of.177 db/km at 155 nm is considrd. Only th powr profil of on of th data channl is shown bcaus th powr volutions of th 7 channls ar similar. Th powr profils of baslins that can b usd to stimat th nonlinar pnalty ar also shown in Fig. 3. Figur 4 shows th man RMS dviation btwn th powr profil of th transmittd channls and th powr profil of th diffrnt baslins. Th analysis of Fig. 4 shows that baslins 1, 2 and 3 should lad to vry accurat prformanc stimats whil stimats basd on th rmaining baslins may b mor inaccurat. Figur 5 shows th rror of th stimation of th transmission systm margin whn th diffrnt baslins ar mployd to ] 2 B1.5 [d r ro 1.5 n a tio -.5 stim -1 in -1.5 a rg M Baslin numbr Figur 5. Margin stimation rror whn using th diffrnt baslins Copyright SubOptic216 Pag 5 of 6

6 stimat th nonlinar dgradation arising from using codirctional Raman amplification. In ordr to strss th ffct of th baslin, it is assumd that th channls ar launchd at high optical powr lvl into th optical fibr which lads to quit larg pnaltis. Indd, a total nonlinar pnalty of about 3 db has bn stimatd for th transmission systm undr valuation. Th analysis of Fig. 5 shows that accurat prformanc stimats hav bn obtaind whn using 5 of th 7 diffrnt baslins. Only slcting baslins with powr profils vry diffrnt from th actual on lads to highr inaccuracy. Additional rsults hav shown that this rsult holds for a quit wid rang of transmission systm paramtrs. This obsrvation lads to th conclusion that th proposd mthod can b applid to a larg numbr of transmission systms using only a small databas siz. Th high accuracy of this approach can b attributd, in part, to only having to stimat th incras of nonlinar pnalty originating from th codirctional Raman amplification with rspct to th cas without codirctional Raman amplification instad of having to stimat th total nonlinar pnalty. 4. CONCLUSIONS A nw tchniqu to stimat th impact of nonlinar fibr transmission ffcts in unrpatrd transmission systms using codirctional Raman amplification has bn proposd. Th prsntd stratgy consists in prparing a databas containing information about th impact of codirctional Raman amplification on nonlinar pnalty for a small st of rfrnc scnarios. Aftrwards, th information containd in th databas will b usd to stimat th prformanc of any unrpatrd transmission systm mploying codirctional Raman amplification in a vry fast and, as has bn shown, accurat mannr. 5. Rfrncs [1] G. Agrawal, Nonlinar fibr optics, 3 rd d., Acadmic Prss, 21. [2] I. Kaminow, T. Li, and A. Willnr, Optical fibr tlcommunications A: Componnts and Subsystms, 5 th d., Elsvir, 28. [3] P. Poggiolini, Th GN modl of non-linar propagation in uncompnsatd cohrnt optical systm, IEEE J. of Lightwav Tchnology, vol. 3, no. 24, pp , Dc [4] P. Poggiolini, G. Bosco, A. Carna, V. Curri, Y. Jiang, and F. Forghiri, Th GN-modl of fibr non-linar propagation and its applications, IEEE J. of Lightwav Tchnology, vol. 32, no. 4, pp , Fb [5] A. Carna, G. Bosco, V. Curri, Y. Jiang, P. Poggiolini and F. Forghiri, EGN modl of nonlinar fibr propagation, OSA Optics Exprss, vol. 22, no. 13, pp , Jun [6] C. Xia, W. Schairr, A. Striglr, L. Rapp, M. Kuschnrov, J. Pina, and D. van dn Born, Impact of channl count and PMD on polarizationmultiplxd QPSK transmission, IEEE J. of Lightwav Tchnology, vol. 29, no. 21, pp , Aug [7] R. Pastorlli, G. Bosco, A. Carna, P. Poggiolini, V. Curri, S. Piciaccia, and F. Forghiri, Invstigation of th dpndnc on non-linar intrfrnc on th numbr of WDM channls in cohrnt optical ntworks, in Proc. ECOC 212, Amstrdam, Nthrlands, papr W.2.C.2. [8] A. Carna, V. Curri, G. Bosco, R. Cigliutti, E. Torrngo, P. Poggiolini, A. Nspola, D. Zolla, and F. Forghiri, Novl figur of mrit to compar fibrs in cohrnt dtction systms with uncompnsatd links, OSA Optics Exprss, vol. 2, no. 1, pp , Jan Copyright SubOptic216 Pag 6 of 6