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Serial topology of wide-band erbium-doped fiber amplifier for WDM applications

Serial topology of wide-band erbium-doped fiber amplifier for WDM applications
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  IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 9, SEPTEMBER 2001 939 Serial Topology of Wide-Band Erbium-Doped FiberAmplifier for WDM Applications M. Karásek and M. Menif   Abstract— We analyze the recently proposed serial topology of wide-banderbium-dopedfiberamplifier(EDFA)coveringboththeC- and L-bands and compare it with the parallel configuration of C-band and L-band amplifiers. The analysis is based on an ap-plication of a comprehensive large-signal numerical model, whichtakes into consideration propagation of wavelength-division mul-tiplexing (WDM) signals, bidirectional pump, and both the down-stream and upstream ASE power spectral components. We havefound that in multiwavelength regime the new topology can pro-vide output powerof 3 dBm/channel for 32 C-band and 40 L-band,0.8-nm-spaced signals with reasonable pump powers to the firstand the second stage. In comparison with the usual parallel config-uration of C-band and L-band EDFAs, this topology saves about20% of overall pump power, 10% of the necessary length of er-bium-dopedfiber(EDF)andachieveslowernoisefigureforL-bandsignals.  Index Terms— Erbium-doped fiber amplifiers, opticalfiber com-munication, wavelength-division multiplexing. I. I NTRODUCTION E RBIUM-DOPED fiber amplifiers (EDFAs) operating inthe long-wavelength band (L-band) of 1570–1610 nmhave doubled the capacity of dense wavelength-division-mul-tiplexed (WDM) communication systems [1]–[3]. The ideaof reconfiguring the EDFA for optimum performance at anaverage metastable level inversion of 35% and thus achievinganother 40-nm gain window dates back to 1990 [4]. Lownoise operation of the gain-shifted erbium-doped fiber (EDF)amplifier in the 1580-nm band has been described in 1992 [5].In the last three years, gain-flattened L-band EDFAs for WDMapplications have been reported [6]–[8].The dramatic increase in transmission capacity of WDM sys-tems has been achieved by employing a parallel configurationof gain-flattened conventional band (C-band, 1530–1560 nm)and L-band erbium-doped fiber amplifiers [1]–[3]. The inputand output ports of both the C-band and L-band EDFA unitswere connected to 1550/1580 nm-band WDM couplers formulti- and demultiplexing the channels in the 1530–1560-nmand 1570–1610-nm wavelength regions. In [9], attenuatorshave been used in front of the WDM multiplexer to equalize Manuscript received November 27, 2000; revised May 25, 2001. This workwas supported in part by the Grant Agency of the Czech Republic under Grant102/99/0393 and in part by a Grant from the Natural Science and EngineeringResearch Council of Canada and by Québec Téléphone.M. Karásek is with the Institute of Radio Engineering and Electronics,Academy of Sciences of the Czech Republic, 182 51 Prague ( Menif is with the Department of Electrical and Computer Engineering,Centre for Optics, Photonics and Lasers, Laval University, Québec, QC G1K7P4,Canada (e-mail: Item Identifier S 1041-1135(01)07511-5.Fig. 1. Schematic diagram of the serial wide-band EDFA topology. the flat gains of both EDFA units to the same level. Recently,a new broadband EDFA topology that provides 75 nm of gainbandwidthhasbeendemonstrated[10].Thenovelconfigurationschematically shown in Fig. 1 consists of two stages combined,in contrast to [1]–[3], [9], in series by a 1550/1580-nm WDMmultiplexer. The first stage [14 m of erbium-doped fiber (EDF),codirectionally pumped at 980 nm by110 mW] provides therequired C-band gain and a part of the L-band gain. The secondstage (85 m of EDF, pumped with 90 mW of copropagating and10 mW of counter-propagating 1480-nm power) amplifies onlythe L-band channels and provides the additional L-band gain.With total input power to the amplifier of 5 dBm, a minimumgain of 18 dB has been achieved across 75-nm bandwidth.Inthisletter,weoptimizetheparametersofthisnewtopologyfor WDM applications and compare it with the parallel config-uration of gain-flattened C-band and L-band EDFAs.II. R ESULTS AND  D ISCUSSION Fig. 1 shows the proposed two-stage serial configuration forC- and L-band. In the first stage, both the C- and L-band signalsareamplified. At the output of the first stage, the C-band signalsare separated by a 1550/1580-nm WDM coupler and combinedtogether by another WDM coupler at the output port of the am-plifier. The insertion loss of the WDM couplers was assumedto be 0.9 and 1.8 dB in the 1530–1560-nm and 1572–1600-nmtransmission signal regions, respectively, [9]. In difference tothe experimental layout of [10], we assume that both stages canbe bidirectionally pumped.The EDFA model used for the analysis has been describedin detail in [11] and is based on a three-level approximation of an erbium ion. Under the assumption of pumping the amplifierat 980- or 1480-nm bands the distribution of pump, multiplesignals and spectral components of forward and backwardpropagating ASE powers along the EDF are obtained throughnumerical solution of appropriate propagation equations.Atomic population densities and at the ground and 1041–1135/01$10.00 © 2001 IEEE  KARÁSEK AND MENIF: SERIAL TOPOLOGY OF WIDE-BAND EDFA 941 Fig. 4. Gain and noise figure of WDM channels at the output port of the serialwide-band topology (         m,      mW,      nm,     m,      mW,      nm) and parallel C-band (         m,      mW,      nm) and L-band topology: (parallel topology #   0      m,      mW,      nm) and (parallel topology #   0       m,     mW,      mW,      nm).Fig. 5. Optical spectrum at the output of a cascade of five wide-band EDFAswith serial topology (         m,      mW,      nm,     m,      mW,      nm). aforementioned number of L-band channels can be achievedwith m and mW (parallel topology # 1). Toachieve such a high pump power, several pump diodes wouldhave to be multiplexed. For nm, noise figure of the parallel topology # 1 is slightly higher than that of theserial topology. If we assume that output power of a single980-nm pump laser diode is limited to 180 mW, the same gainwith slightly higher noise figure can be achieved if we add, inaddition to the forward pump of mW an extra 90mW of counter-directional pump power (parallel topology #2). In comparison with parallel topology number 1, the noisefigure penalty is about 0.2 dB. The length averaged metastablelevel population of the parallel L-band EDFA was ,the gain ripple within the 40 L-band signals was %, forboth parallel topology # 1 and # 2.Finally, we investigate the interchannel power spread in acascade of five equal wide-band EDFAs with serial topology( m, mW, nm, m,mW, nm) concatenated with four trans-mission fibers of 20 dB span loss. The first EDFA is fed by 32C-band channels ( nm) and 40 L-band channels( nm) of dBm/channel. The opticalspectrum at the output of amplifier number 5 is shown in Fig. 5.The interchannel power spread accumulates along the cascadeand at the output port of EDFA number 5 the ratio of outputpower of the most favorite and the least favorite channels were5 and 4.9 dB over the 32 C-band channels and 40 L-band chan-nels, respectively.III. C ONCLUSION We have analyzed the recently proposed serial topology of wide-band EDFA for multiwavelength operation. The analysisis based on an application of space and frequency resolvedEDFA model. We have shown that the new topology canprovide 20 dB gain for 32 C-band and 40 L-band channels with0.8-nm spacing and input power of 17 dBm/channel withgain ripple less than 5%. The lowest noise figure is predictedfor pumping both the first and the second stage codirectionallyat 980 nm. In comparison with the usual parallel configurationof C-band and L-band EDFAs, the first EDFA of the newtopology requires 20% more pump power compared with theC-band EDFA of the parallel topology because it provides from7- to 15-dB amplification for L-band signals. The optimumEDF length and pump power of the second stage are smaller by10% and 33%, respectively, compared with the L-band EDFAof the parallel topology. The new topology saves about 20% of overall pump power, 10% of EDF length and achieves lowernoise figure in L-band than the parallel configuration.R EFERENCES[1] M. Jinno, T. Sakamoto, J. Kani, S. Aisawa, K. Oda, M. Fukui, H. Ono,and K. Oguchi, “First demonstration of 1580-nm wavelength bandWDM transmission for doubling usable bandwidth and suppressingFWM in DSF,”  Electron. Lett. , vol. 33, pp. 882–883, 1997.[2] T.Sakamoto,J.Kani,M.Jinno,S.Aisawa,M.Fukui,M.Yamada,andK.Oguchi, “Wide wavelength band (1535–1560 nm and 1574–1600 nm),   2    Gbit/sWDMtransmissionover320kmdispersionshiftedfiber,”  Electron. Lett. , vol. 34, pp. 392–394, 1998.[3] S. Aisawa, T. Sakamoto, M. Fukui, J. Kani, M. Jinno, and K. Oguchi,“Ultra-wideband, long distance WDM demonstration of 1 Tb/s (   2    Gbit/s), 600 km transmission using 1550 and 1580-nm wavelengthsbands,”  Electron. Lett. , vol. 34, pp. 1127–1129, 1998.[4] J. F. Massicott, J. R. Armitage, R. Wyatt, B. J. Ainslie, and S. P. Craig-Ryan, “High gain, broadband, 1.6    m   doped silica fiber ampli-fier,”  Electron. Lett. , vol. 26, pp. 1645–1646, 1990.[5] J. F. Massicott, R. Wyatt, and B. J. Ainslie, “Low noise operation of    doped silica fiber amplifier around 1.6    m,”  Electron. Lett. , vol.28, pp. 1924–1925, 1992.[6] H. Ono, M. Yamada, and Y. Ohishi, “Gain-flattened   -doped fiberamplifier for a WDM signal in the 1.57–1.60    m wavelength region,”  IEEE Photon. Technol. Lett. , vol. 9, pp. 596–598, May 1997.[7] H. Ono, M. Yamada, M. Shimizu, and Y. Ohishi,“Comparison ofampli-ficationcharacteristicsof1.58    mand1.55    mbandEDFAs,”  Electron. Lett. , vol. 34, pp. 1509–1510, 1998.[8] , “Signal output characteristics of 1.58    m band gain-flattened  -doped fiber amplifiers for WDM systems,”  Electron. Lett. , vol.34, pp. 1513–1514, 1998.[9] M. Yamada, H. Ono, T. Kanamori, S. Dudo, and T. Ohishi, “Broadbandand gain-flattened amplifier composed of a 1.55    m-band and a 1.58   m-band   -doped fiber amplifier in a parallel configuration,”  Elec-tron. Lett. , vol. 33, pp. 710–711, 1997.[10] D. Lowe, R. DiMuro, and S. Wilson, “75 nm of continuous gain usinga noval EDFA topology,” in  Proc. Euro. Conf. Optical Communications2000 , Munich, Germany, Paper 6.4.1.[11] M. Karásek and J. A. Vallés, “Analysis of channel addition/removal re-sponse in all-optical gain-controlled cascade of erbium-doped fiber am-plifiers,”  J. Lightwave Technol. , vol. 16, pp. 1795–1803, Oct. 1998.

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