THE continuous growth of the internet traffic is boosting

JOURNAL OF LIGHTWAVE TECHNOLOGY 1 Real-Time 100 Gb/s Transmission using 3-Level Electrical Duobinary Modulation for Short-reach Optical Interconnects M. Verplaetse, R. Lin, J. Van Kerrebrouck, O. Ozolins, T. De Keulenaer, X. Pang, R. Pierco, R. Vaernewyck, A. Vyncke, R. Schatz, U. Westergren, G. Jacobsen, S. Popov, J. Chen, G. Torfs, J. Bauwelinck and X. Yin Abstract Electrical duobinary modulation is considered as a promising way to realize high capacity because of the low bandwidth requirement on the optical/electrical components and high tolerance towards chromatic dispersion. In this paper, we demonstrate a 100 Gb/s electrical duobinary transmission over 2 km standard single-mode fibre reaching a bit error rate under 7% HD-FEC threshold with the use of PRBS-7. This link is tested in real-time without any form of digital signal processing. Inhouse developed SiGe BiCMOS transmitter and receiver ICs are used to drive an electro-absorption modulated laser and decode the received signal from a PIN-photodiode. The performance of 50 Gb/s and 70 Gb/s non-return-to-zero and electrical duobinary transmission are investigated for comparison. Index Terms Electrical duobinary, Optical communication, Analog equalization I. INTRODUCTION THE continuous growth of the internet traffic is boosting the requirement of ultra-high-speed optical interconnects for data centres, which is largely driven by bandwidthconsuming applications, such as cloud computing, high definition video and Internet of Things. Currently the evolution from 100 Gb/s Ethernet to 400 Gb/s is under discussion within the IEEE P802.3bs 400 Gigabit Ethernet Task Force [1]. Among different approaches, the four lane 100 Gb/s scheme is particularly attractive for 500 m and 2 km single-mode fibre applications as it allows lower lane counts, thus offering higher spatial efficiency. For the intra-datacenter communication, avoiding complex transceivers is crucial in terms of cost and power consumption. Consequently, intensity modulation and direct detection (IMDD) links are preferred rather than coherent transmission technologies. On-off keying (OOK) [2] Manuscript received...; revised... (M. Verplaetse and R. Lin contributed equally to this work). M. Verplaetse, J. Van Kerrebrouck,T. De Keulenaer, R. Pierco, R. Vaernewyck, A. Vyncke, G. Torfs, J. Bauwelinck and X. Yin are with Ghent University - imec, IDLab, Department of Information Technology. M. Verplaetse has a PhD Fellowship of the Research Foundation - Flanders. O. Ozolins, X. Pang and G. Jacobsen are with NETLAB, Acreo Swedish ICT AB, SE- 164 25 Kista, Sweden. T. De Keulenaer,R. Pierco, R. Vaernewyck and A. Vyncke are with BiFAST, spin-off of Ghent University - imec, IDLab, Department of Information Technology, Ghent, Belgium. R. Lin, R. Schatz, U. Westergren, G. Jacobsen, S. Popov and J. Chen are with the School of ICT, KTH Royal Institute of Technology, Isafjordsgatan 22, 164 40 Kista, Sweden. Copyright (c) 2016 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org. [3] keeps the optical hardware simple but presents a big challenge to the bandwidth of transceivers in applications using 100 Gb/s and beyond. Advanced modulation formats, such as 4-level pulse-amplitude modulation (PAM-4) [4] [5] discrete multi-tone (DMT) [6] [7] and electrical duobinary (EDB) [8] [10] overcome this limitation by improving the spectral efficiency while maintaining the benefits of direct detection. However, most of the realized 100 Gb/s class DMT, PAM-4 and EDB demonstrations [3] [8] are based on off-line digital signal processing (DSP). The realization of 100 Gb/s class real-time DMT transmission [11] is hindered by the huge amount of calculations which are very energy demanding. In [12], a real-time 112 Gb/s PAM-4 optical link over 2 km standard single-mode fibre (SSMF) was demonstrated with a BiCMOS transceiver (including clock and data recovery (CDR)) and a high power consumption of 8.6 W. Recently, 3-level duobinary (i.e. EDB), is gaining a lot of attention [8] [10]. Compared to OOK, the three-level signaling in EDB or optical duobinary (ODB) narrows optical spectral bandwidth improving the CD tolerance and the equalization requirements of the high-speed serial link [10] [13]. Compared to ODB, several benefits can be found in the usage of EDB. The EDB scheme enables the usage of components and packaging technologies with lower bandwidth as the ODB receiver has no reduced bandwidth requirements compared to OOK in the electrical domain [13]. Moreover, the ODB transmitter requires an increased complexity (e.g. a dual-drive Mach-Zehnder Modulator) compared to more conventional and simple modulation in EDB. However, for the ODB receiver, a simple single threshold receiver can be used, yet requiring it with increased bandwidth, compared to a two threshold receiver for EDB. In terms of CD tolerance, ODB shows a better performance than EDB [13]. However, for the considered short ranges, the reduced complexity of EDB modulation exceeds the limited increase in CD tolerance of the ODB. In this paper, the EDB scheme is used to achieve realtime communication at 100 Gb/s over 2 km SSMF. In [8], 103.125 Gb/s is realized using the EDB scheme over 1 km SSMF with off-line processing. As in this paper communication up to 2 km SSMF is considered, a significant increase in the CD influence compared to [8] is introduced. This paper is an invited extension of our work presented in a post deadline paper of ECOC 2016 [14]. We present the developed EDB transmitter (TX) and receiver (RX) chipset, and

JOURNAL OF LIGHTWAVE TECHNOLOGY 2 100G EDB TX IC 100G EDB RX IC Fig. 1. Schematic representation of the used EDB architecture with TX and RX ICs photographs. experimentally demonstrate a real-time 100 Gb/s EDB optical link in the C-band up to 2 km SSMF. First, the used electrical and optical components are discussed in section II. Section III discusses the setup of the electro-optical link. Finally, section IV presents the results of real-time transmission at 50, 70 and 100 Gb/s. II. COMPONENTS FOR 100 GB/S SHORT-REACH OPTICAL INTERCONNECTS High speed optical communication up to 100 Gb/s demands careful design of both the electrical and optical devices. The design of the separate components will influence power and signal quality to great extents. In section II-A, the electrical transceiver chipset is presented, which provides the capability to equalize the transmitted signals and decode EDB signals in real-time. The used electro-absorption modulated laser (EML) is discussed in section II-B. A. Electrical transceiver To successfully obtain high speed electrical communication up to 100 Gb/s, a well designed transmitter architecture is required. An overview of the EDB architecture is found in Fig. 1. Both EDB TX and RX ICs were designed in-house and fabricated in a 0.13 µm SiGe BiCMOS technology. The TX IC consists of a 4-to-1 MUX capable of multiplexing signals up to 25 Gb/s into a serial 100 Gb/s non-return-to-zero (NRZ) stream and a 6-tap feed-forward equalizer (FFE). The FFE is designed as a tapped delay line with spacings around 9-10 ps where each gain cell can be independently tuned with a precision of 8 bit. The equalizer design is discussed in [15] (as 5-tap version). The FFE shapes the NRZ input waveform together with the roll-off of the subsequent components and the link. The cascade results in a EDB signal at the input of the RX. A simple approximation of the needed combined filter response is given in the z-domain by 1 + z 1, the delay-andadd filter. The TX IC occupies 1555 µm x 4567 µm including IOs. At a serial rate of 100 Gb/s, the chip consumes about 1 W of which 0.64 W is used to power the MUX and 0.36 W is used by the FFE. To decode the EDB signal, the receiver presented in [16] is used. The EDB decoding scheme is given at the bottom of Fig. 1. Controlling the levels of both thresholds V th1 and V th2 shifts the eyes up or down resulting in an extraction of the upper and lower eye of the EDB signal. Sending both signal through an XOR gate restores the transmitted NRZ stream under the condition of a precoded transmitted NRZ signal. The EDB RX IC consists of a transimpedance (TIA) input stage with a bandwidth of 41 GHz, two parallel level-shifting limiting amplifiers, an XOR stage and a 1-to-4 DEMUX. The decoding and DEMUX operation are combined to interleave the decoding step [16], significantly reducing power consumption. The EDB RX can be re-configured into an NRZ RX with four demuxed outputs. This NRZ decoding is also used in our experiments for comparison with NRZ signaling. The RX IC occupies 1926 µm x 2585 µm including IOs and consumes 1.2 W at 100 Gb/s. The combined electrical transmit and receive power will be 2.2 W resulting in 22 pj/bit in the electrical domain (without CDR). Compared to the 77 pj/bit in [12] (with CDR), a clear increase in power efficiency could be achieved. In our case, the equalizing is preferably done at the TX. This choice is driven by the ease of the input design of the FFE, which can be non-linear due to the NRZ input and the ease of the RX TIA design where some degree of non-linearity is tolerable. The exclusion of the noise shaping at the receiver by an FFE is an additional benefit. This choice challenges the automatic optimization in a practical implemented system. For this purpose, a (low-speed) back channel will be necessary. Updates itself can be guided by a LMS engine located at the receiver. B. Electro-absorption modulated laser The in-house fabricated monolithically integrated distributed feedback laser with traveling-wave electro-absorption modulator (DFB-TWEAM) was grown by metal organic vapor phase epitaxy (MOVPE) on n-doped InP substrate [17]. The gain section of the laser consists of 7 quantum wells (QWs), each 7 nm thick, whereas the modulator has 12 QWs with

JOURNAL OF LIGHTWAVE TECHNOLOGY 3 a thickness of 9 nm. The two components are integrated using a butt-joint technique. Fig. 2 shows the P(I) and P(V) characteristics of the transmitter operating at 22 C. The DFBlaser exhibits a threshold of 25 ma and an output slope of 0.4 W/A, which allows an output power of 3.2 mw with 120 ma driving current. The static P(V) characteristics of the transmitter module running at room temperature is shown in Fig. 2b. By adjusting the bias point below 3 V, a large extinction ratio can be obtained. The DFB in the EML emits at 1548.7 nm. T = 22 C λ = 1548 nm T = 22 C λ = 1548 nm (a) (b) Fig. 2. C(a) P(I) characteristics; (b) P(V) characteristics of the DFB-TWEAM module. III. EXPERIMENTAL SETUP The experimental setup is illustrated in Fig. 3. A Xilinx FPGA board is adopted to generate four electrical 25 Gb/s NRZ shifted 2 7 1 PRBS signal streams. The EDB TX IC multiplexes 4 25 Gb/s streams into a serial 100 Gb/s stream. To obtain a serial stream which is again the same periodic 2 7 1 PRBS signal, the four streams are shifted with a delay of respectively 0, 32, 64 and 96 bits. Precoding the serial stream, as introduced in section II, is generally not necessary as the precoding of a PRBS sequence yields the same sequence, but delayed. As a result, precoding is not implemented. Of course, a precoder implementation at 100 Gb/s is challenging. Methods using precoding on the parallel streams (e.g. similar to [18]) can be used to ease implementation. The TX IC pre-equalizes the serialized NRZ signal with the analog equalizer discussed in section II. The pre-emphasized signal was amplified by a 50 GHz amplifier to cope with the limited output swing of the TX IC (max. 400 mv). The amplified signal then drives the C-band 100 GHz EML which needs around 2 V swing for efficient modulation. Removing the amplifier would result in an increased link bandwidth but will cause decreased noise performance at the receiver. In the experiment, the laser was driven by 135.2 ma to ensure sufficient output power and the EAM was biased at -1.85 V, so it was operating in the linear regime. The optical output power of the EML was approximately 0 dbm. After transmission over an SSMF, the received optical signal was detected by a PIN-photodiode (PIN-PD) and the custom EDB RX IC. An erbium doped fibre amplifier (EDFA), a variable optical attenuator (VOA) and a power meter were used before the PIN-PD to adjust/record the received optical power for measurement purposes. The EDFA can be removed from the setup when a higher power EML becomes available. The PIN-PD is a high-speed InP-based O/E converter packaged prototype photodetector from U 2 T with a responsivity of 0.5 A/W and a bandwidth > 67 GHz (limited by measurement bandwidth of the available network analyzer). The singleended output of the diode is directly connected to the RX. The resulting received maximum peak-to-peak voltage (V pp ) was around 90 mv pp which is high enough to be detected by the RX IC. The RX has a maximum input range of ±250 mv pp such that a 9 db extra linear amplification can be useful. However, due to bandwidth limitations of the available amplifier, inferior performance is observed and this option is not considered. The RX IC demodulates 3-level signals with two separate threshold levels as discussed in section II. The levels are independently tuned to obtain the lowest bit-errorrate (BER). Due to the absence of CDR circuitry in the RX IC, a delayed version of the TX half-rate clock is used to demodulate the signal. The delay is tuned manually for each BER measurement to obtain the lowest BER. The demodulated signal is deserialized on-chip into 4 25 Gb/s NRZ outputs which can be used for real-time error detection. The error detection is implemented on the same FPGA board used for the PRBS generation by comparing the incoming data to a locked PRBS reference stream. Only one of the four outputs of the RX IC is used for error detection in this experiment. Due to the properties of (de)multiplexed PRBS streams, the measured BER will be representative for the full rate BER. The measured frequency response of the E/O/E components excluding and including the EDB ICs and PCBs are shown in Fig. 4. The response of the TX IC is measured as the insertion loss (IL) through the equalizer with only one tap active at its maximum value. This response is normalized at low frequency before cascading it with the back-to-back (B2B) E/O/E response. The frequency responses of fibres with a length of 500 m, 1 km and 2 km are measured and

EDB Receiver Error Counter (FPGA) JOURNAL OF LIGHTWAVE TECHNOLOGY 4 PRBS Generator (FPGA) 4x25G Data EDB Transmitter RF Amp DFB- EAM 100G EDB TX IC 0.5, 1, 2-km SSMF EDFA VOA PIN-PD Power Meter 4x25G Data Fig. 3. Experiment setup of real-time 100 Gb/s 3-level duobinary optical link. plotted in the same figure. As can be seen in Fig. 4, in the optical B2B case the bandwidth is mainly limited by the electrical amplifiers, interfaces and ICs. However, fibre operating in C-band severely degenerates the flatness of the frequency response, especially at 2 km showing the increased challenges compared to [8]. Together with the bandwidthlimited components and the dispersive fibre, the real-time FFE in the TX is engaged to create an equivalent channel response that transforms the NRZ from the serializer output into 3-level duobinary at the RX input. The limited bandwidth of 41 GHz in the RX TIA degenerates the flatness further and has an influence when higher bitrates are used. 2 km SSMF RF Amp + EML + PIN-PD 500 m SSMF RF Amp + EML + PIN-PD + TX IC 1 km SSMF Fig. 4. Measured frequency responses of the B2B optical link and various fibres. As shown in Fig. 3, both developed EDB TX and RX ICs are flip-chipped on test PCBs. Before performing the optical transmission experiment, the EDB TX/RX ICs have first been verified in a pure electrical B2B configuration. The electrical B2B measurement was performed continuously and the measured bit-error rate (BER) was less than 1E-14 for 100 Gb/s duobinary signals, which reveals a stable performance of the EDB TX/RX ICs. IV. RESULTS AND DISCUSSION The optical performance of the link is evaluated using the experimental setup shown in Fig. 3. To compare the performance, we evaluated both NRZ and the proposed 3- level duobinary at various rates in optical B2B configuration. Fig. 5a shows the electrical 100 Gb/s NRZ eye-diagram after the TX IC without FFE, which is clearly open. However, the received optical eye at the PIN-PD (Fig. 5b) was completely closed due to the bandwidth limitation as indicated in Fig. 4. As shown in Fig. 5c, by enabling the real-time 6-tap FFE in the EDB TX IC, the received optical eye has been shaped into 3-level duobinary and two separate eyes were clearly formed after the PIN-PD. As an example, in Fig. 6, the response of the uncompensated B2B case is shown in comparison with the compensated frequency response (both normalized to 0 db at low frequency). Clearly, more high frequency gain is introduced to perform the equalization. As comparison, the ideal 1 + z 1 filter is shown as well. The equalized response follows this filter response well up to 30 GHz, beyond that, there is a noticeable difference. Fortunately, at those high frequencies, the increased difference has less impact as the delay-and-add filter is a very broadband ideal EDB filter (not that much of bandwidth is actually needed). To fairly compare the different experiments, the TX FFE is always enabled and separately optimized for each rate. The optimization is performed via a sequential evaluation of a set of FFE coefficients. This set is determined via simulation as optimal filter parameters for different loss profiles. After the determination of the best filter, manual tuning is used to improve the BER. In the following discussions, the presented eye diagrams are captured at the input of the RX IC. As the bandwidth limitation of the RX TIA is not included, different filter parameters are used for BER measurements.

JOURNAL OF LIGHTWAVE TECHNOLOGY 5 (a) (b) (c) Fig. 5. Measured 100Gbit/s stream with (a) eye-diagram at the EDB TX IC output with TX FFE disabled (b) eye-diagram at the PIN-PD output with TX FFE disabled (c) 3-level duobinary eye-diagram at the PIN-PD output with TX real-time 6-tap FFE enabled. (a) (b) Fig. 6. Effect of the equalization on a B2B optical link for 100 Gb/s EDB communication. (c) (d) Fig. 7. Measured eye-diagrams of 70 Gb/s NRZ transmission at (a) B2B configuration, (b) 500 m, (c) 1 km, (d) 2 km. A. Measurements at 50 Gb/s and 70 Gb/s The measured BER curves for 50 Gb/s and 70 Gb/s NRZ signals are shown in Fig. 8. The optical power shown is measured at the input of the PIN-PD (after the VOA). The measurement times for the BER are limited to 5 s for the lowest BER values. The NRZ signaling works relatively well in B2B up to 70 Gb/s. However, after 2 km of SSMF, we noticed a decreased performance at 70 Gb/s (4.4 db penalty compared to the B2B at 7% HD-FEC limit). The different NRZ eye-diagrams for 70 Gb/s communication are shown in Fig. 7. The longer fibre lengths with the resulting CD introduce significant intersymbol interference (ISI) and close the NRZ eye gradually. In contrast, we can utilize this ISI effect and create the 3-level EDB signal by optimizing the TX FFE. In Fig. 7c and Fig. 7d EDB is already appearing. The measured BER for the 70 Gb/s EDB link over 2 km can reach a BER of 1E-10, which is well below the hard-decision forward error correction (HD-FEC) with 7% overhead (BER=3.8E-3 [19]) or RS(544,514,10) FEC (BER=3.09E-4), a FEC code defined for 802.3bs 400 Gigabit Ethernet Task Force [1] [20]. In addition, using EDB modulation, we obtained a sensitivity improvement of 3.3 db at 7% HD-FEC limit (or 4.5 db at the RS(544,514,10) limit) with respect to the 70 Gb/s NRZ transmission over 2 km distance. The optimized 3-level duobinary eye in Fig. 8 for the 2 km fibre shows the two open eyes. B. Measurements at 100 Gb/s When the rate is increased to 100 Gb/s, NRZ communication becomes nearly impossible due to the bandwidth limitations of the cascade as shown in Fig. 4. The resulting eye-diagrams for B2B, 500 m, 1 km, and 2 km SSMFs at 100 Gb/s EDB are depicted in Fig. 9a to Fig. 9d. The real-time BER measurements are shown in Fig. 10. For 500 m applications, negligible penalty was observed as could be expected from the measured frequency response. Up to 1 km SSMF, for powers higher than 5 dbm, a BER below the RS(544,514,10) FEC threshold is achieved. Over 2 km SSMF, the CD starts to introduce extra losses at frequencies below 50 GHz which influence the EDB performance significantly. Nonetheless, a 100 Gb/s EDB modulated signal can be received with BER of 3.7E-3, below the 7% HD-FEC threshold.

JOURNAL OF LIGHTWAVE TECHNOLOGY 6 3.3dB 7% HD-FEC 4.5 db RS(544,514,10) 7% HD-FEC RS(544,514,10) Fig. 8. Measured NRZ BER curve up to 70 Gb/s and 2km SSMF, compared to 70 Gb/s EDB modulation (measurements with PRBS7). Fig. 10. PRBS7). Measured BER curve for 100 Gb/s EDB (measurements with an improved BER below 1E-10. At a data-rate of 100 Gb/s, transmission of EDB over several fibre lengths is achieved. At 2 km, a BER of 3.7E-3 is demonstrated, which is below the 7% HD-FEC threshold. All signal processing is done on-chip in real-time and without any DSP, which proves the possibility for power efficient communication. (a) (b) ACKNOWLEDGMENT The authors would like to thank the support from the Industrial Research Fund (IOF) of Ghent University enabling the commercialization of the transceiver chipset through the BiFAST spin-off, Research Foundation - Flanders (FWO), National Natural Science Foundation of China, Natural Science Foundation of Guangdong Province, Swedish Research Council, the Swedish Foundation for Strategic Research, and Gran Gustafsson Foundation, and EU MC ICONE project (#60809). REFERENCES (c) Fig. 9. Measured eye-diagrams of 100 Gb/s EDB transmission at (a) B2B configuration, (b) 500 m, (c) 1 km, (d) 2 km. V. CONCLUSION In this paper, optical interconnects of 50 Gb/s, 70 Gb/s and 100 Gb/s over 2 km SSMF are discussed. A self developed chipset is used to provide the high speed electrical communication. The electrical signal is equalized via a 6-tap analog FFE before driving an electro-absorption modulated laser. Using a PIN-PD, the optical signal is received and demodulated by an RX chip which supports both NRZ and EDB signals. Over a 2 km fibre, 70 Gb/s NRZ communication below the threshold of the RS(544,514,10) FEC is achieved. The conversion to EDB modulation over the same channel and bitrate lead to (d) [1] IEEE P802.3bs 400 Gigabit Ethernet Task Force. [Online]. Available: http://www.ieee802.org/3/bs/ [2] W. Hartmann et al, 100 Gbit/s OOK using a silicon-organic hybrid (SOH) modulator, in European Conf. on Optical Communication, Valencia, 2015. [3] O. Ozolins et al, 100 GHz EML for High Speed Optical Interconnect Applications, in European Conf. on Optical Communication, Düsseldorf, 2016. [4] D. Sadot et al, Single channel 112 Gbit/sec PAM4 at 56 Gbaud with digital signal processing for data centers applications, in Optical Networking and Communication Conf., Los Angeles, 2015. [5] C. Yang et al, IM/DD-Based 112-Gb/s/lambda PAM-4 Transmission Using 18-Gbps DML, IEEE Photonics Journal, vol. 8, no. 3, 2016. [6] Y. Kai et al, 130-Gbps DMT Transmission using Silicon Mach-Zehnder Modulator with Chirp Control at 1.55-um, in Optical Networking and Communication Conf., Los Angeles, 2015. [7] K. Zhong et al, Experimental study of PAM-4, CAP-16, and DMT for 100 Gb/s Short Reach Optical Transmission Systems, Optics Express, vol. 23, no. 2, pp. 1176 1189, 2015. [8] J. Lee et al, Serial 103.125-Gb/s Transmission over 1 km SSMF for Low-Cost, Short-Reach Optical Interconnects, 2014, Optical Fiber Communications Conference and Exhibition (OFC), San Francisco, CA, MAR 09-13, 2014.

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