HFA Optimization for Nyquist WDM Transmission V. Curri, A. Carena OptCom DET, Politecnico di Torino, Torino, Italy. curri@polito.it Paper W4E.4

Motivations Need for larger capacity using the installed infrastructures Larger capacity means larger OSNR Some Raman pumping as a complement of EDFA – Hybrid Fiber Amplifier (HFA) - is a possible solution but some questions need answers How much pump power do I need? Co- or counter-propagating pump or both? What is the Raman merit on different fiber types? Supposing uniform and uncompensated links operated with NyWDM channel combs based on multilevel modulation formats and coherent Rx, the GN-model may help in giving proper answers Paper W4E.4

Outline The analyzed system scenarios System Figure of Merit including ASE noise and NLI Raman merit vs. EDFA GN-model and Raman amplification HFA optimization based on GN-model Results for span loss As = 20 dB Moderate pumping regime: a simplified scenario Summary results for different span losses Preliminary specific simulative validations Conclusions Paper W4E.4

…… NyWDM channel comb GTx(f) Bopt Rs Df GWDM f NyWDM channel comb Df = Rs PM-m-ary-PAM Flat PSD: GWDM W/Hz Pch= GWDM Rs Use on the entire C-band Bopt= Nch·Rs For calculations we consider Bopt = 4 THz Rs= 32 Gbaud Nch = 125 Paper W4E.4

Uniform and uncompesated link xNs Ls, adB, D, Aeff, n2 gR, RCF NyWDM Tx Polarization Diversity Rx DSP GEDFA FEDFA=4.5 dB Ppump,co Ac,in DOP=0 Ppump,counter Ac,out DOP=0 NZDSF adB = 0.222 dB/km D = 3.8 ps/nm/km Aeff = 70 mm2 SMF adB = 0.200 dB/km D = 16.7 ps/nm/km Aeff = 80 mm2 PSCF adB = 0.167 dB/km D = 21 ps/nm/km Aeff = 135 mm2 ap,dB = adB+0.05 dB/km - n2 = 2.5 x 10-20 m2/W - Flat gR = 0.33 x 10-13 m/W - RCF = -27 dB Paper W4E.4

Equivalent span: loss, ASE noise and NLI ASE noise PSD Energy per photon Total span loss GHFA + ASE noise Ac,in As=adB·Ls Ac,out HFA + NLI disturbance NLI PSD independent of other spans Transparency condition Note: so far no specific model for NLI evaluation Paper W4E.4

Co- and counter-pumping and percentage of RA Small-signal Raman gain Overall pump power Percentage of Raman gain in HFA Percentage of co-prop pump From full-EDFA to full Raman Co- and counter-prop RA Paper W4E.4

independent of format and BER System Figure of Merit Max OSNR (Min BER) Max reach Nph= GWDM /Eph is the average number of photons per symbol In general, Feq and GNLI depend on Nph, percRA and percco System FoM independent of format and BER Optimal power Optimal FoM Paper W4E.4

Raman merit Raman merit We define as Raman Merit DRA the increasing of optimal FoM when applying some Raman pumping, with respect to the full-EDFA setup, given the link and HFA configuration Raman merit Note that Raman merit besides depending on the link structure varies with (percRA, percco) Paper W4E.4

GN-Model and Raman amplification Can we use the GN-model to evaluate performance with RA? Several papers also at OFC15, e.g.: Invited paper M2I.6 by R. Pastorelli (Cisco Systems) Tutorial paper W1C.1 by W. S. Pelouch (Xtera Communications) Paper W4E.4

Using GN-model to evaluate DRA We need to evaluate Feq and GNLI vs. (Nph, percRA, percco) z 1 HFA Ls EDFA GRA Normalized NLI efficiency from GN-model Feq from accurate model of RA Paper W4E.4

HFA optimization For each system scenario, due to Raman pump depletion, G(z) depends on Nph, given (percRA, percco) Consequently, both Feq and GNLI depend on Nph MPI induced by Rayleigh backscattering degrades Feq for high pump power, so it must be considered In general, the HFA optimization needs an accurate characterization of Feq and GNLI vs. Nph Then, Fsys can be computed vs. Nph for every (percRA, percco) Fsys,opt is the maximum Fsys for every (percRA, percco) Paper W4E.4

The analysis Implemented analytical tools Considered system scenarios Accurate Raman amplification model giving G(z) and Feq for every pumping scheme and including MPI GN-model evaluating GNLI from the output of the RA model Considered system scenarios Links made of uniform fiber types: NZDSF, SMF and PSCF previously described Fiber span loss As = 20 ->30 dB, step 2 dB (Ls=As/adB) percRA = 0% (full EDFA) -> 100% (full RA), step 11% percco = 0% (counter-propagating pump), -> 100% (all co-propagating pump) step 25% Displayed results Results f or As = 20 dB and SMF Results comparing fibers for As = 20 dB Summary results vs. As Paper W4E.4

SMF – As=20 dB (Ls=100 km) - GRA vs. Pch ½ Raman ½ EDFA Full Raman Paper W4E.4

SMF – As=20 dB (Ls=100 km) - GNLI vs. Pch Full EDFA ½ Raman ½ EDFA Full Raman Paper W4E.4

SMF – As=20 dB (Ls=100 km) - Feq vs. Pch Full EDFA ½ Raman ½ EDFA Full Raman Paper W4E.4

SMF – As=20 dB (Ls=100 km) - Fsys vs. Pch Full EDFA Fsys reference value for Raman merit vs. EDFA ½ Raman ½ EDFA Full RAman Paper W4E.4

All fibers – As=20 dB - Pch,opt vs. percRA NZDSF SMF PSCF Paper W4E.4

All fibers – As=20 dB - DRA vs. percRA NZDSF SMF PSCF Paper W4E.4

All fibers – As=20 dB – Pch,opt vs. percRA Full Counter-prop ½ Counter-prop ½ Co-prop Full Co-prop Paper W4E.4

All fibers – As=20 dB – DRA vs. percRA Full Counter-prop ½ Counter-prop ½ Co-prop Full Co-prop Paper W4E.4

Comments (I) Full counter-prop HFAs (percco=0%) Experience low pump-depletion effect For moderate pumping-regime (roughly, percRA <66%) it is practically negligible Low pump-depletion means no major transients and enables the use of percco=0% HFAs in reconfigurable networks Are affected by negligible RA-induced NLI enhancement Are the best choice for moderate pumping-regime If co-propagating pump is used, the effect of pump depletion is more and more evident approaching percco=0%, as well as RA-induced NLI Paper W4E.4

Comments (II) The best choice in terms of DRA seems to be close to Full-RA (percRA =100%) for all fibers but the incremental benefit (∂DRA/∂Ppump) is larger for low percRA (<50%) Approaching percRA =100%, a minor percentage of co-propagating pump may improve DRA only of fraction of dB but reduces the optimal power per channel (e.g., 5 dB for SMF) Possible reduction of power consumption Raman merit vs. percRA is very similar for all fibers, in particular for high dispersive fibers like SMF and PSCF For full counter-prop HFAs, Raman merit vs. percRA is practically the same for all fibers Paper W4E.4

Moderate pumping regime If RA is a supplement to the EDFA gain, we can suppose the moderate pumping regime roughly corresponding to percRA <66% Supposing also full conter-prop pumping, pump depletion is negligible as well as MPI , consequently Feq and GNLI are ~ constant Raman merit and optimal power assume analytical close forms as for full-EDFA amplification DFeq=Feq,EDFA,dB-Feq,HFA,dB, always positive, is the RA noise reduction DGNLI=10log10(GNLI,HFA/GNLI,EDFA) always positive as well, is the RA NLI enhancement due to distributed gain If DGNLI can be neglected Paper W4E.4

All fibers - DRA vs. fiber-span loss NZDSF SMF PSCF Paper W4E.4

All fibers - Maximum DRA vs. fiber-span loss Maximum attainable (best) Raman merit Percentage of co-propagating pump giving the best Raman merit Percentage of RA giving the best Raman merit Paper W4E.4

Comments Increasing the fiber-span loss As, benefit of RA increases DRA gap between percco=25% and percco=0%, that is negligible for As = 20 dB, grows with As clearly showing that the best choice is to split the pump in 25% co and 75% counter DRA gap between the fiber types grows from 0.2 dB for As = 20 dB to 0.5 dB for As = 30 dB For all fibers and losses the best HFA is approaching the full Raman as percRA,opt is always  90 % Paper W4E.4

Preliminary simulative validation What about specific validations on the analyzed scenarios? Full max reach simulations are extremely time-consuming but we have preliminary results for NZDSF Paper W4E.4

Conclusions We presented a method for HFA optimization Results show that Raman pumping up to full-RA improves OSNR for all scenarios but incremental benefit (∂DRA/∂Ppump) is larger for low percRA (<50%) Counter-prop pumping always allows to reach system improvements close to the optimum Some co-pumping gives increasing extra DRA with span enalarging always reduces Pch,opt Prevalent co-pumping (percco >50%) never gives advantages Raman merit is similar for all fibers and practically equal in case of counter-prop pumping Preliminarily simulative validations confirm the proposed method Paper W4E.4

Acknowledgements Thanks to Mr. Carlos Gomez Universidad de Valladolid (Spain) Prof. Gabriella Bosco and Prof. Pierluigi Poggiolini OptCom, DET, Politecnico di Torino (Italy) Dr. Fabrizio Forghieri Cisco Photonics Italy Mr. Enrico Ghillino Synopsys (USA) Paper W4E.4