\(2 \times 100\) -gb/s nrz-ook integrated transmitter for intradata center connectivity
TRANSCRIPT
2078 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 20, OCTOBER 15, 2014
2 × 100-Gb/s NRZ-OOK Integrated Transmitter forIntradata Center Connectivity
Vasilis Katopodis, Panos Groumas, Ziyang Zhang, Jean-Yves Dupuy, Eric Miller, Antonio Beretta,Lefteris Gounaridis, Jung Han Choi, Detlef Pech, Filipe Jorge, Virginie Nodjiadjim, Raluca Dinu,
Giulio Cangini, Alberto Dede, Antonello Vannucci, Agnieszka Konczykowska, Norbert Keil,Heinz-Gunter Bach, Norbert Grote, Christos Kouloumentas, and Hercules Avramopoulos
Abstract— We demonstrate an integrated transmitter that cangenerate two 100-Gb/s optical channels with simple nonreturn-to-zero-ON-OFF keying format. The transmitter is based onthe combination of an ultrafast electro-optic polymer platformfor the photonic integration and the optical modulation withultrafast InP-double heterojunction bipolar transistor electronicsfor the multiplexing and the amplification of the 100-Gb/s drivingsignals. Through error-free transmission of 2 × 80-Gb/s signalsover 1 km of SMF and transmission of 2×100-Gb/s signals over500 m of single-mode fiber with error performance way belowthe forward error correction limit, we reveal the potential ofthe approach for parallel 100-GbE optical interfaces in smallfootprint transceivers for intradata center networks.
Index Terms— Data center networks, 100G transmitters,hybrid integration, electro-optic polymers, high-speed electronics.
I. INTRODUCTION
HYBRID architectures with an electronic packet switched(EPS) and an optical circuit switched (OCS) domain
form the basis of next generation intra data center networks(DCNs) [1], [2]. Within these architectures, the OCS domainwill rely on multiple 100 GbE interfaces at the top-of-rackswitches to provide fat pipes and shortcuts for rack-to-rackconnectivity over distances that may be longer than 1 km.For the implementation of the 100 GbE interfaces, solutionsbased on parallel 10×10 or 4×25 Gb/s links and non return-to-zero-on-off keying (NRZ-OOK) format [3], [4] are simple
Manuscript received May 14, 2014; revised July 14, 2014; acceptedAugust 6, 2014. Date of publication August 13, 2014; date of current versionSeptember 19, 2014. This work was supported by the European Union throughthe POLYSYS Project under Contract 258846.
V. Katopodis, P. Groumas, L. Gounaridis, C. Kouloumentas, andH. Avramopoulos are with the National Technical University of Athens,Athens 106 82, Greece (e-mail: [email protected]; [email protected];[email protected]; [email protected]; [email protected]).
Z. Zhang, J. H. Choi, D. Pech, N. Keil, H.-G. Bach, and N. Grote arewith Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute,Berlin 10587, Germany (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).
J.-Y. Dupuy, F. Jorge, V. Nodjiadjim, and A. Konczykowskaare with III-V Laboratory, Marcoussis 91460, France (e-mail:[email protected]; [email protected]; [email protected]; [email protected]).
E. Miller, R. Dinu, and G. Cangini are with GigOptix Inc., Both-ell, WA 98011 USA (e-mail: [email protected]; [email protected];[email protected]).
A. Beretta, A. Dede, and A. Vannucci are with Linkra Srl,Agrate Brianza 20864, Italy (e-mail: [email protected];[email protected]; [email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2014.2347244
Fig. 1. (a) Layout of 2×100 Gb/s transmitter. (b) Cross-section of EOpolymer platform. (c) Simulated interference pattern for the 1:2 MMI couplerand experimental results on the dependence of the insertion loss and powerimbalance on the length of the multi-mode region.
Fig. 2. Photograph of the packaged 2×100 Gb/s transmitter inside the bo×.
but not optimal, as they require large number of componentsand lead to large footprint, high power consumption andwasteful use of switch ports. Alternatives based on 25 Gbaudsystems with dual polarization and 4-pulse amplitude mod-ulation can reduce this number, but are more complex andwith shorter reach [5]. Finally, solutions based on phasemodulation and coherent detection have high performanceand long reach, but are not practical for intra DCNs dueto their high complexity, footprint and power consumption.Following a different approach with advantages in termsof number of components, simplicity and energy efficiency,we presented in [6] a 100 Gb/s NRZ-OOK transmitterbased on ultra-high speed electro-optic (EO) polymers andInP-double heterojunction bipolar transistor (InP-DHBT) elec-tronics, and we revealed its potential through back-to-back(b2b) measurements at 80-100 Gb/s. The interest for thisapproach was further confirmed through a recent work, whichinvolved, however, a lower speed modulator and for this reasonwas based on off-line signal processing [7].
In this letter, we make several steps forward compared toour previous work in [6], and we present a novel integratedtransmitter for 2×100 Gb/s NRZ-OOK operation. It is based
1041-1135 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
KATOPODIS et al.: 2 × 100-Gb/s NRZ-OOK INTEGRATED TRANSMITTER 2079
Fig. 3. Experimental setup. The indicated frequencies and data rates correspond to 80 Gb/s operation and should be scaled accordingly for operation at100 Gb/s. The picture on the right-hand side depicts the integrated pin-DEMUX receiver module that was utilized for the BER evaluation of the signals.
on monolithic integration of a multi-mode interference (MMI)coupler with two Mach-Zehnder modulators (MZMs) on a sin-gle EO polymer chip and hybrid integration of this chip witha laser diode and two InP-DHBT MUX-driver circuits insidea box. We evaluate its performance at 2×80 and 2×100 Gb/sand confirm error-free transmission of 2×80 Gb/s signalsover 1 km of dispersion uncompensated links. Apart fromduplicating the total throughput compared to [6], we revealwith these studies the potential of EO polymers to serve as anovel integration platform for complex photonic circuits andmultiple 100 GbE interfaces in small footprint packages, andthe viability of the overall approach for next generation DCNs.
II. CONCEPT AND DEVICE
Fig. 1(a) presents the layout of the transmitter. The device isdriven by four electrical data signals and two electrical clocksat half the final line rate, and provides at the output fiberstwo signals at the same wavelength with independent datastreams. Fig. 1(b) shows the cross-section of the single-modeEO polymer platform used for the monolithic integration ofthe MMI coupler and the two MZMs. The effective index ofthe transverse magnetic (TM) mode is 1.6603, the mode-fielddiameter is 2.8 µm in the vertical and 4.6 µm in the lateraldirection, and the propagation loss is 1.4 dB/cm at 1550 nm.Finally, Fig. 1(c) presents results from the optimization of the1:2 MMI coupler showing the dependence of the insertion lossand the power imbalance on the length of the MMI, as theseparameters were experimentally measured using test structureswith 12 µm width and variable lengths. As observed, for135 µm length the insertion loss becomes lower than 0.65 dBand the power imbalance at the output ports lower than 0.1dB.
Fig. 2 illustrates the assembly of the transmitter inside theFeNiCo package. Its optical subassembly is placed in thecenter of the package and consists of the EO polymer chip andthe hybridly integrated 1550 nm distributed feedback (DFB)laser. Since the EO effect is present only for TM modes [8], thetransverse electric (TE) emitting laser is rotated by 90o. Thecoupling loss between the laser and the polymer waveguideis lower than 3.5 dB. Two identical electronic circuits (MUX-DRV 1 and MUX-DRV 2) can be observed on either side ofthe polymer chip. Each one of them is fabricated in 0.7 µmInP-DHBT technology and integrates the 2:1 time division
multiplexing (MUX) and the driver amplification (DRV)functionalities on a single chip. Compared to the design thatwas described in [7], the new one employs an additionaldistributed amplifier that results in higher differential outputamplitude (2×3 V) at 100 Gb/s with 3.8 W total powerconsumption. The packaging also relies on alumina-basedstriplines, DC blocks for AC-coupling of the first outputof each MUX-DRV to the respective MZM (single-driveoperation), 50 Ohm terminations for the second output, activethermal management and lensed fibers. The optical loss inthe box is 13 dB including the laser coupling loss, the MMIinsertion loss, the MZM insertion loss and the fiber couplingloss, and results in -9 dBm output power of the continuouswave (cw) at the transmission peak of each MZM.
III. EXPERIMENTAL SETUP AND RESULTS
Fig. 3 presents the experimental set-up for the simulta-neous operation of the two transmitter channels at 80 and100 Gb/s. The detection of the signals is based on theintegrated pin-DEMUX receiver used in [6], which comprisesa pin-photodiode with bandwidth in excess of 100 GHz andresponsivity in excess of 0.5 A/W, and an InP-DHBT 1:2DEMUX circuit. The clock and data rates outlined in Fig. 3correspond to 2×80 Gb/s and should be scaled accordingly for2×100 Gb/s operation. Using a frequency doubler (f-doubler),a frequency divider (f-divider) and a number of RF powersplitters (S), the signal generator provides the clock signalsfor the pulse pattern generator (PPG), the external 4:1 MUX,the MUX-DRV circuits of the transmitter, the DEMUX ofthe receiver, the subsequent 1:4 DEMUX of the setup, theoscilloscope and the bit-error rate (BER) tester. The PPGgenerates a 231−1 long pseudo-random bit sequence (PRBS)at 10 Gb/s, which is split into four parts. These are driveninto the 4:1 MUX through parallel delay lines (DL) and phaseshifters (PS) that allow for pattern decorrelation and bit-levelsynchronization. The two electrical 40 Gb/s outputs are ampli-fied and split into two parts each, so as to serve after furtherde-correlation and bit-level synchronization as the data inputsof the two MUX-DRV circuits. It is noted that the DL in bothmultiplexing stages have been designed so as to preserve theorder of a 27−1 long PRBS at 100 Gb/s, but they are also ableto provide sufficient decorrelation of higher order PRBS during
2080 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 20, OCTOBER 15, 2014
Fig. 4. (a) Static transfer functions of MZM 1 and MZM 2. (b) Opticalspectrum of 80 Gb/s signal. (c) Optical spectrum of 100 Gb/s signal.
Fig. 5. Experimental eye-diagrams of: (a)-(d) Ch. 1 at 80 Gb/s, (e)-(h) Ch. 2at 80 Gb/s, (i)-(j) Ch. 1 at 100 Gb/s, and (k)-(l) Ch. 2 at 100 Gb/s.
the multiplexing. At the transmitter output, the 1553.4 nmoptical signal under evaluation is transmitted over single-mode fiber (SMF) spans with total length up to 1.25 km, andsubsequently is amplified by an erbium doped fiber amplifier(EDFA) and filtered by an optical band-pass filter (OBPF). Partof the signal is detected by a 70 GHz photodiode (PD) for eye-diagram-based studies and part of it by the integrated receiverwith its DEMUX receiving the electrical signal from the pin-photodiode and delivering the 40 Gb/s tributary that is timealigned with the input 40 GHz clock. This tributary is furtherdemultiplexed by the external 1:4 DEMUX, and the final10 Gb/s channels are evaluated by the BER tester. The EDFAhas 5 dB noise figure and provides 16.5 dBm output powerwith a gain of ∼ 31 dB to ensure sufficient power budget forthe system. Future work will focus on eliminating the need foran EDFA through optimization of the power budget inside thetransmitter and use of electrical amplification at the input ofnew DEMUX circuits with higher sensitivity. Fig. 4(a) presentsthe cw power at the outputs of the transmitter as a functionof the bias voltage of the respective MZMs. The maxima ofthe two curves have only 0.5 dB difference revealing the lowimbalance of the MMI coupler. Fig. 4(b)-(c) show the opticalspectra at the output of MZM 1 (channel 1) at 22oC for 80and 100 Gb/s modulation. Similar spectra were monitored atthe output of MZM 2 (channel 2). Fig. 5(a)-(h) present the
Fig. 6. Experimental BER curves of: (a) ch. 1 at 80 Gb/s, (b) ch. 2 at80 Gb/s, and (c) ch. 1 and ch. 2 at 100 Gb/s.
80 Gb/s eye-diagrams of channel 1 and 2 in b2b and after500, 1000 and 1250 m. The eye-diagrams remain wide openup to 1000 m, and get clearly affected by chromatic dispersiononly after 1250 m. The extinction ratio in all cases is higherthan 10 dB and the root mean square (rms) timing jitter lowerthan 1.2 ps, as measured with suitable timing jitter analysissoftware. Fig. 5(i)-(l) present the 100 Gb/s eye-diagrams ofthe two channels in b2b and after 500 m revealing a smallereye-opening at this rate.
Finally, Fig. 6 presents the corresponding BER measure-ments. For each channel, rate or distance, the BER curvecorresponds to the worst among the 8 demultiplexed tributaries(at 10 or 12.5 Gb/s). The reference curves refer to the b2bcurves of the single channel transmitter reported in [6]. Forboth channels at 80 Gb/s, BER lower than 10−9 is obtainedfor distances up to 1000 m, whereas an error floor at approxi-mately 3 · 10−8 is observed after 1250 m. At 100 Gb/s, an errorfloor at approximately 5 · 10−7 is present already in the b2bcase. This error floor after 500 m propagation is at 5 · 10−6,which is still a BER far below the forward error correction(FEC) limit. It is noted that the lower performance at 100 Gb/scompared to the reference measurement obtained in [6] isattributed to the lower quality of the 50 GHz clock that drivesthe two MUX-DRVs and the DEMUX due to the additionalRF components in the experimental setup, the additional lossesof the 100 Gb/s signals that drive the MZMs due to the DCblocks after the MUX-DRVs, and the additional optical noisedue to the higher optical insertion loss of the transmitter.
IV. CONCLUSIONS
We have demonstrated an integrated transmitter with twoNRZ-OOK channels and 200 Gb/s total capacity. The trans-mitter integrates an EO polymer chip with an MMI coupler andtwo ultra-high speed MZMs, a DFB laser and two MUX-DRVcircuits. Through simultaneous operation of both channelsat 2×80 and 2×100 Gb/s and error-free transmission of2×80 Gb/s signals over 1 km SMF, we have revealed thepotential of EO polymers and InP-DHBT electronics to serveas the basis of parallel 100 GbE optical interfaces inside smallfootprint and low complexity transceivers for intra DCNs.
REFERENCES
[1] N. Farrington et al., “Helios: A hybrid electrical/optical switcharchitecture for modular data centers,” in Proc. ACM SIGCOMM,Aug./Sep. 2010, pp. 339–350.
[2] G. Wang et al., “c-Through: Part-time optics in data centers,” in Proc.ACM SIGCOMM, Aug. 2010, pp. 327–338.
KATOPODIS et al.: 2 × 100-Gb/s NRZ-OOK INTEGRATED TRANSMITTER 2081
[3] M. A. Taubenblatt, “Optical interconnects for high-performance com-puting,” J. Lightw. Technol., vol. 30, no. 4, pp. 448–458, Feb. 15, 2012.
[4] N. Y. Li et al., “High-performance 850 nm VCSEL and photodetec-tor arrays for 25 Gb/s parallel optical interconnects,” in Proc. Opt.Fiber Commun. Collocated Nat. Fiber Opt. Eng. Conf. (OFC/NFOEC),San Diego, CA, USA, Mar. 2010, pp. 1–3, paper OTuP2.
[5] R. Rodes et al., “100 Gb/s single VCSEL data transmission link,”in Proc. OFC/NFOEC, Los Angeles, CA, USA, 2012, pp. 1–3,paper PDP5D.10.
[6] V. Katopodis et al., “Serial 100 Gb/s connectivity based on polymerphotonics and InP-DHBT electronics,” Opt. Exp., vol. 20, no. 27,pp. 28538–28543, Dec. 2012.
[7] J. Lee et al., “Serial 103.125-Gb/s transmission over 1 km SSMFfor low-cost, short-reach optical interconnects,” in Proc. OFC/NFOEC,San Francisco, CA, USA, 2014, paper Th5A.10.
[8] P. Groumas et al., “Complex monolithic and InP hybrid integrationon high bandwidth electro-optic polymer platform,” Opt. Lett., vol. 37,no. 16, pp. 3465–3467, Aug. 2012.