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40-gb/s transmission over 25 km of negative-dispersion fiber using asymmetric narrow-band filtering of a commercial directly Modulated DFB laser

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40-gb/s transmission over 25 km of negative-dispersion fiber using asymmetric narrow-band filtering of a commercial directly Modulated DFB laser
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  1322 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 6, JUNE 2005 40-Gb/s Transmission Over 25 km of Negative-Dispersion Fiber Using AsymmetricNarrow-Band Filtering of a CommercialDirectly Modulated DFB Laser L.-S. Yan  ,Student Member,IEEE  , C. Yu  ,Student Member,IEEE  , Y. Wang, T. Luo,L. Paraschis, Y.Shi  , Member,IEEE  ,and A. E. Willner  , Fellow, IEEE   Abstract— Using asymmetric narrow-band optical filteringto improve the response and reshape the optical spectrum of acommercial directly modulated distributed feedback laser, wedemonstrate 40-Gb/s error-free transmission over 25-km negativedispersion fiber without dispersion compensation.  IndexTerms— Directmodulation,dispersion,opticscommunica-tions, optical filter. I. I NTRODUCTION I TIS CLEAR thatcost-effectivetransmission is a key goalof the optical communications community. One area of height-ened interest is the reach extension of directly modulated lasers(DMLs), for which the removal of the external modulator andcontiguous electronics can significantly decrease the potentialcost. Moreover, implementing DMLs for short-reach 40-Gb/slinks may provide a low-cost high-capacity solution for metroand access systems.Previously, reported results on 40-Gb/s DML mainly work at 1310-nm transmission window for very-short-reach appli-cations [1] or use four-level signal transmission to extend thetransmission distance over 40 km of single-mode fiber (SMF)[2]. DML transmission at 1550 nm over 31-km SMF has beenachieved using four-level signaling and dispersion-supportedtransmission [3]. Very recently, transmission results at 1550 nmusing dispersion-managed links have been reported, including:1) 40 km of SMF and dispersion-compensating fiber (DCF)using a specially designed DML laser [4]; 2) over 42 km of SMF and DCF that used, interestingly, a commercially avail-able DML laser and achieved a bit-error-rate (BER) [5].In this letter, we demonstrate 40-Gb/s transmission over25 km of negative-dispersion fiber (NDF) at a BERusing narrow-band filtering of a commercially available DML. Manuscript received February 2, 2005; revised February 9, 2005. This work was supported in part by Cisco Systems Inc.L.-S. Yan, C. Yu, Y. Wang, T. Luo, and A. E. Willner are with the Departmentof Electrical Engineering-Systems, University of Southern California, LosAngeles, CA 90089 USA (e-mail: lianshay@usc.edu; changyuy@usc.edu;yawang@usc.edu; tluo@usc.edu; willner@csi.usc.edu).L. Paraschis is with Optical Networking, Advanced Technology andPlanning, Cisco Systems Inc., San Jose, CA 95134 USA (e-mail: loukas@cisco.com).Y. Shi is with the Boeing Satellite Systems, Los Angeles, CA 90009 USA.Digital Object Identifier 10.1109/LPT.2005.846618 Note that we do not use any dispersion compensation in ourtransmission link. Since a key challenge in a DML link isto overcome the chirped frequency spectrum that enhanceschromatic dispersion, we achieve our result by placing a0.65-nm-wide optical filter after the transmitter. This filtereffectively: 1) reduces the signal frequency bandwidth ornarrows the broadened spectrum due to the intrinsic chirpinside the DML [6]; 2) improves the frequency response of DML by converting induced frequency modulation (FM)into useful amplitude modulation (AM) [7], [8]; and 3) in- creases the extinction ratio (ER) by cutting off the low currentfrequency [9]. Without filtering, the commercial distributedfeedback (DFB) laser cannot reach error-free 40-Gb/s oper-ation even in the back-to-back condition, however, when thenarrow-band optical filter is applied off-center to the carrier,the DML performance is significantly improved and error-freetransmission distance up to 25 km over NDF with dispersion1.7 ps/nm/km is achievedwithout dispersion compensation.II. O PTICAL  F ILTERING IN  DML S YSTEMS Three major contributions enhance the performance in di-rectlymodulatedsystemsusingasymmetricnarrow-bandopticalfiltering: 1) the filter narrows the optical spectrum, reducing un-wanted frequency components; 2) the nonlinear phase responseof the slope edge of the optical filter will introduce chirp andinteract with the frequency chirp of the laser, thus convertingthe FM into useful AM and enhance the signal, especially afterpropagation along a certain distance of dispersive fiber; and3) the filter also improves the ER of the DML by cutting off low current frequencies. This concept has recently been appliedto 1400-km long-haul DML transmission at 10 Gb/s [10].Our experimental setup is shown in Fig. 1(a). A commer-cially available DFB laser with a wavelength of 1555.9 nm isdirectly modulated at 40 Gb/s with pseudorandom bi-nary sequence. The modulation voltage on the DML is 2.8 V(peak-to-peak), and the output power is 8 dBm. After theDML, an optical filter with 3-dB bandwidth of 0.65 nm is usedto improve the laser performance. The transmission spectrumand chromatic dispersion across the passband are shown inFig. 1(b). The transmission link is a single link using a spool of NDF without any optical amplification and dispersion compen-sation. The dispersion of NDF is 1.7 ps/nm/km. The signal 1041-1135/$20.00 © 2005 IEEE  YAN  et al. : 40-Gb/s TRANSMISSION OVER 25 km OF NDF USING NARROW-BAND FILTERING 1323 Fig. 1. (a) Experimental setup of 40-Gb/s DML transmission usingasymmetric narrow-band optical filtering. (b) Chromatic dispersion andtransmission spectrum of thin-film-based optical filter with 3-dB bandwidthof 0.65 nm. (c) Frequency response of the commercial DML (90-mA drivingcurrent is used in the experiment and the gray curve is measured after opticalfiltering). (d) Eye diagram of back-to-back case (without filtering): ER   2.2 dB. (e) Eye diagram of back-to-back case (after filtering): ER    3.0 dB. is detected using a PIN photodiode and demultiplexed into four10-Gb/s data streams and error analysis is performed at thisdata rate using an error detector.To be more specific, the bias current of the commercial DMLissettobe 90mA—atsuchhighcurrent,themodulationband-widthoftheDMLcanbeupto 30GHz.Thefrequencyresponsecurves at different driving currents are measured and shown in Fig. 2. BER performance of optical filtering effects. (a) Transmission of NDF with and without optical filtering. (Note that without optical filtering,error-free cannot be achieved even at back-to-back case.) (b) Optical filteringwith different bandwidths at back-to-back case. Fig. 1(c), where a gray curve shows a slight improvement inthe frequency response when using optical filtering. As pointedout in [7], the frequency response can be significantly improvedwhenthesignalpropagatesalongthedispersivefiberthroughFMto AM conversion. At the same time, since the optical filter cancutofflowcurrent-inducedfrequencies,theERcanbeimproved.As shown inFig. 1(d) and (e),the ERof thelaser increases from2.2 dB (without filtering) to 3 dB (after optical filtering).III. E XPERIMENTAL  R ESULTS In the back-to-back condition, without optical filtering,error-free reception cannot be achieved (BER floor at ).However, when we apply an off-center 0.65-nm optical filter,error-free is easily obtained with a receiver sensitivityof 5.3 dBm ( 8 dBm using external modulation in oursetup). The major contribution of this improvement is due tohigher ER using optical filtering. We use two spools of NDFfor the transmission, 11 and 25 km. The power penalties for 11-and 25-km transmission are 3.5 and 5 dB, respectively. Thecorresponding BER curves are shown in Fig. 2(a).Filters of varying bandwidth are also used to find the lim-itations of filtering. Here we compare the effects of threefilters with bandwidths of 0.30, 0.65, and 1.4 nm. As shownin Fig. 2(b), a 0.3-nm filter can improve transmission per-formance, but still cannot reach error-free (an error floor  1324 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 6, JUNE 2005 Fig. 3. Optical spectrum of filtering. (a) Comparison of spectrum with andwithout filtering; (b) slightly detuning or optimization is necessary when thelink length or dispersion varies (here the length changes from 11 to 25 km). at ). This bandwidth might be too narrow for DMLtransmission although strong prefiltering has been shown toimprove performance in high spectrally efficient transmissionsystems [11]. A 1.4-nm filter is quite similar to the case withoutfiltering with only slight improvement. These three filters areall thin-film based, and have similar Gaussian profiles, as wellas dispersion characteristics.Furthermore, wenotethatan optimizedfilterbandwidth(cor-responding to a narrowed spectrum) with optimized detuningfrequency(correspondingtoaspecificnonlinearphaseresponsefor FM to AM conversion) is essential to DML transmission.In our experiment, 0.65 nm turns out to be the best bandwidthwith a detuning frequency of 10 GHz. The spectrum compar-ison is shown in Fig. 3(a) for cases with and without filtering.We can see that the filter center is located at the left side of thecenter frequency of the carrier. This coincides with the realitythat our DML has positive chirp. Another effect that we shouldalso note is that the filter might need slight optimization underdifferent link conditions. For example, when the link length in-creasesfrom11to25km,thebest-casefilterdetuningfrequencyis slightly shifted to the left, as shown in Fig. 3(b), and morespecifically in the inserted graph in detail. We believe that fur-ther analysis of the contributions of the narrowing of the broad-enedspectrum,andtheinteractionbetweentheslopeofthefilteredge and link dispersion, are needed to allow full insight intothis observation, and have undertaken this investigation as ex-tension of this work.R EFERENCES[1] K. Sato, S. Kuwahara, Y. Miyamoto, and N. Shimizu, “40-Gbit/s di-rect modulation of distributed feedback laser for very-short-reach op-tical links,”  Electron. Lett. , vol. 38, no. 15, pp. 816–817, 2002.[2] A. Wonfor, J. K. White, R. V. Penty, and I. H. White, “Uncooled opter-ation of a 40 Gb/s directly modulatedmulti-level laser for datacoms ap-plications,” in  ECOC-IOOC  , Ramini, Italy, Sep. 2003, Paper Tu4.5.6.[3] B. Wedding, W. Idler, B. Franz, W. Pohlmann, and E. Lach, “40 Gbit/squaternary dispersion supported transmission over 31 km standard sin-glemode fiber without optical dispersion compensation,” in  ECOC’98 ,vol. 1, pp. 523–524.[4] K. Soto, S. Kuwahara, A. Hirano, M. Yoneyama, and Y. Miyamoto,“4 2   40 Gbit/s dense WDM transmission over 40-km SMF using di-rectlymodulatedDFBlasers,”in  ECOC2004 ,Stockholm,Sweden,Sep.2004, Paper We1.5.7.[5] B. Wedding and W. Poehlmann, “43 Gbit/s transmission over 40.5 kmSMF without optical amplifier using a directly modulated laser diode,”in  ECOC 2004 , Stockholm, Sweden, Sep. 2004, Paper We2.6.6.[6] C.-H. Lee, S.-S. Lee, H.-K. Kim, J.-H. Han, and C.-S. Shim, “Re-duction of chirping penalty in directly modulated multigigabit trans-mission systems by spectral filtering,” in  CLEO’95 , Paper CtuI10, pp.93–94.[7] M. McAdams, E. Peral, D. Provenzano, W. K. Marshall, and A. Yariv,“Improved laser modulation response by frequency modulation to am-plitude modulation conversion in transmission through a fiber grating,”  Appl. Phys. Lett. , vol. 71, no. 7, pp. 879–881, 1997.[8] H.-Y. Yu, D. Mahgerefteh, P. S. Cho, and J. Goldhar, “Improved trans-mission of chirped signals from semiconductor optical devices by pulsereshaping using a fiber Bragg grating filter,”  J. Lightw. Technol. , vol. 17,no. 5, pp. 898–903, May 1999.[9] P. A. Morton, G. E. Shtengel, L. D. Tzeng, R. D. Yadvish, T.Tanbun-Ek, and R. A. Morgan, “38.5 km error free transmissionat 10 Gbit/s in standard fiber using a low chirp, spectrally filtered,directly modulated 1.55    m DFB laser,”  Electron. Lett. , vol. 33, no.4, pp. 310–311, Feb. 1997.[10] L.-S. Yan, Y. Wang, B. Zhang, C. Yu, J. McGeehan, L. Paraschis, andA. E. Willner, “1400 km transmission using a directly modulated DFBlaser and optical sideband filtering in an 8 2   10 Gb/s WDM system,” in  ECOC 2004 , Stockholm, Sweden, Sep. 2004, Paper Mo.4.5.7.[11] N. Yoshikane and I. Morita, “160% spectrally-efficient 5.12-Tb/s(64 2   85.4 Gb/s RZ DQPSK) transmission without polarization demul-tiplexing,” in  ECOC 2004 , Stockhom, Sweden, Sep. 2004, PostdeadlinePaper Th4.4.3.
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