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532 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 4, FEBRUARY 15, 2012 Generation and Transmission of 21.4-Gbaud PDM 64-QAM Using a Novel High-Power DAC Driving a Single I/Q Modulator Alan H. Gnauck, Fellow, IEEE, Peter J. Winzer, Fellow, IEEE, Agnieszka Konczykowska, Filipe Jorge, Jean-Yves Dupuy, Muriel Riet, Gabriel Charlet, B. Zhu, Member, IEEE, and David W. Peckham, Senior Member, IEEE Abstract—We generate a single-carrier 21.4-Gbaud polarization-division-multiplexed (PDM) 64-ary quadrature-amplitude-modulated (QAM) signal (256.8-Gb/s line rate) using a single in-phase/quadrature (I/Q) optical modulator driven by 8-level electrical waveforms from a novel high-power digital-to-analog converter (DAC). We measure a required optical signal-to-noise ratio of 29.5 dB (0.1-nm reference bandwidth; bit-error rate), 4.6-dB off the theoretical limit. Using ultra-large-area fiber, we achieve 400-km single-channel transmission. The DAC was also used to obtain excellent results with quadrature-phase-shift-keyed and 16-QAM signals at 21.4 Gbaud. Index Terms—Coherent detection, 100G ethernet, optical networking, quadrature amplitude modulation (QAM), transmission, wavelength-division multiplexing (WDM). I. INTRODUCTION C OMMERCIAL 100-Gb/s transmission systems available and in development today are largely based on single-carrier 28-Gbaud polarization-division-multiplexed (PDM) quadrature phase-shift keying (QPSK) [1]. Such signals can be operated on a 50-GHz frequency grid in wavelength-division-multiplexed (WDM) systems (i.e., at a spectral efficiency, SE, of 2 b/s/Hz), while still providing spectral margin for several reconfigurable optical add/drop multiplexers (ROADMs). In order to increase transport capacity, greater SE can be achieved using higher-level quadrature amplitude modulation (QAM). PDM 64-QAM has recently been demonstrated in WDM systems at line rates consistent with 100-Gb/s operation. Operating on a 12.5-GHz frequency grid, an SE of 8.4 b/s/Hz (assuming 7% overhead for forward error correction, FEC) was reached at a per-channel line rate of 9.4 Gbaud (112.8 Gb/s) [2], and an SE of 9.0 b/s/Hz was achieved at 10.03 Gbaud (120.4 Gb/s) [3]. These experiments used in-phase/quadrature (I/Q) modulators driven with 8-level electrical signals derived either from an opto-electronic scheme Manuscript received July 13, 2011; revised October 11, 2011; accepted October 22, 2011. Date of publication December 05, 2011; date of current version February 01, 2012. A. H. Gnauck and P. J. Winzer are with Bell Laboratories, Alcatel-Lucent, Holmdel, NJ 07733 USA (e-mail: [email protected]). A. Konczykowska, F. Jorge, J.-Y. Dupuy, and M. Riet are with III-IV Lab, Marcoussis, France. G. Charlet is with Bell Laboratories, Alcatel-Lucent, Nozay, France. B. Zhu and D. W. Peckham are with OFS Labs, Somerset, NJ 08873 USA. Digital Object Identifier 10.1109/JLT.2011.2175200 [2] or from a commercial arbitrary waveform generator [3]. Scaling per-channel transport bit rates beyond 100 Gb/s is important to accommodate the continuing increase in router interface rates, and to reduce the channel count in high-capacity transport systems. Unfortunately, increasing the channel rate at high SE such as provided by 64-QAM has proven challenging. A complex integrated modulator, comprised of three nested Mach–Zehnder modulators, was used to generate 20-Gbaud 64-QAM using six binary electrical drive signals [4]. Using this transmitter, back-to-back PDM operation with a coherent intradyne receiver resulted in a bit-error-rate (BER) . Another experiment used a commercial floor of digital-to-analog converter (DAC) to drive an I/Q modulator with 8-level electrical signals to produce 28-Gbaud 64-QAM [5]. BER measurements were not performed, but the obtained constellation suggests a similarly high BER floor. We have recently reported [6] 21.4-Gbaud 64-QAM generation using a novel 3-bit electronic high-power DAC circuit [7] driving an I/Q modulator with 8 electrical levels. In PDM operation this equates to a line rate of 256.8 Gb/s, or 240 Gb/s after accounting for 7% FEC overhead. Using a low-noise real-time oscilloscope, we obtained intradyne-detection performance . only 4.6-dB off the theoretical limit at a BER of Transmission over 400 km of ultra-large-area fiber (ULAF) was demonstrated, and a digital nonlinearity-compensation scheme, implemented in the digital signal processing algorithm at the receiver, was shown to improve the results. In this paper, we expand upon the work reported in [6], using the DAC to also obtain excellent results with QPSK and 16-QAM signals (requiring 2- and 4-level electrical drives, respectively). II. EXPERIMENT A. Transmitter Setup The experimental setup is shown in Fig. 1. A 1555.74-nm tunable external cavity laser (ECL) was modulated using an integrated LiNbO I/Q modulator with 30-GHz 3-dB bandwidth kHz, and V of 2.2 V. The ECL exhibited a linewidth of and was chosen due to the required narrow linewidth for 64-QAM [8]. To generate a 64-QAM optical signal, the I and Q branches of the modulator were driven by 21.4-Gbaud eight-level electrical signals from the DAC circuit described below. The DAC inputs were three copies of a 21.4-Gb/s true . The pseudo-random bit sequence (PRBS) of length second and third copies were decorrelated relative to the first 0733-8724/$26.00 © 2011 IEEE GNAUCK et al.: GENERATION AND TRANSMISSION OF 21.4-GBAUD PDM 64-QAM USING A NOVEL HIGH-POWER DAC DRIVING A SINGLE I/Q MODULATOR 533 Fig. 1. Experimental setup. TX: transmitter. PC: polarization controller. PBS: polarizing beamsplitter. EDFA: erbium-doped fiber amplifier. ULAF: ultra-largearea fiber. ATT: attenuator. LO: local oscillator. A/D: analog-to-digital converter. copy with delays of 19 and 43 bit periods, respectively. The two complementary 8-level DAC output signals, each with a swing of 1.6 V , were attenuated in these experiments in order to reduce the potential for distortion caused by small electrical impedance mismatches. They were then decorrelated by 12 symbol periods before being applied to the modulator. Polarization multiplexing was achieved by 3-dB splitting the optical signal, delaying one copy by 12 ns, and recombining it in a polarization beamsplitter (PBS) using manual polarization controllers (PCs). B. High-Power Digital-to-Analog Converter While the most popular high-speed DAC architecture is composed of a low-voltage digital-to-analog converter followed by a broadband linear amplifier, leading to a complex circuit with high power consumption as well as the need for wideband highfrequency amplifiers with high linearity and dynamic range requirements, our DAC concept [7] combines both functions in a single circuit: the DAC (i) creates multilevel signals from the 3 binary input data streams and (ii) amplifies them simultaneously (as necessary for driving a modulator), with all circuit elements operating in digital instead of mixed digital-analog mode. This architecture allows us to minimize unwanted signal distortions and, in particular, to avoid amplification of noise and other multilevel distortions. The three single-ended DAC input signals are first reshaped and retimed before being amplified by an active combiner. The output levels can be adjusted with three DC controls (corresponding to the weights of the 3 bits) from fully off to completely on. This functionality allows adjustments of the respective level positions, either for pre-distortion of the driving signal, or for generation of other modulation formats. The power consumption is 2 W when providing a 3.2-V differential output swing (1.6 V on each of the two DAC outputs). The circuit was realized using a 280-GHz Indium Phosphide HBT process. A microphotograph of the DAC is shown in Fig. 2, along with a picture of the packaged device. Single-ended 2-, 4-, and 8-level electrical output waveforms are also shown in Fig. 2. We note that input signals were not removed in 2- and 4-level operation, but rather the DC controls were adjusted to completely turn off the unused bit(s). C. Transmission System Transmission was performed in four backward-Raman-amplified 100-km ULAF spans, pumped to near transparency. The Fig. 2. Microphotograph of the DAC chip (top left), and photograph of the packaged DAC (top right). Single-ended 2-, 4-, and 8-level output waveforms (bottom). pump wavelengths for each span were 1429, 1447, and 1460 nm, and the pump powers were 270, 300, and 300 mW, respectively. At 1555 nm, the fiber loss, dispersion, and dispersion slope were 0.185 dB/km, 20 ps/nm/km, and 0.08 ps/nm /km, respectively. The effective area was 120 m , and the nonlinearity coefficient was 0.81 W km . 534 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 4, FEBRUARY 15, 2012 D. Intradyne Receiver At the receiver, the signal was amplified and passed through a 1-nm-bandwidth optical filter. The signal was then combined with a free-running ECL local oscillator (LO), within approximately 20 MHz of the signal carrier, in a polarization-diversity 90-degree hybrid, followed by 4 balanced detectors. The ECL exhibited a linewidth of kHz. The combined linewidth of kHz for the signal and LO lasers should have a negligible performance impact in 21.4-Gbaud 64-QAM operation, as the predicted penalty (at a BER of ) is only expected to reach 1 dB with a combined linewidth of 800 kHz [8]. The 4 signal components (I , Q , I , Q ) were sampled asynchronously and digitized using two Agilent 93204A 2-channel 80-GSamples/s real-time oscilloscopes with 33-GHz bandwidth and low-noise performance (achieving an effective number of bits ). All results are based on offline processing of samples from each balanced detector. Since our algorithms do not re-use any waveform samples for filter convergence and BER measurement, this corresponds to a total of symbols ( million bits for PDM 64-QAM) for error counting after the convergence of all receiver loops. Our intradyne-receiver algorithm [9] first corrected for front-end imperfections, e.g., sampling skew and hybrid phase errors, and performed chromatic-dispersion compensation in the frequency domain. The subsequent clock recovery oversampled a portion of the signal by a factor of 3 using zero-padding in the frequency domain and extracted the tone at the symbol rate (1/T) from the spectrum of the magnitude-squared signal. The original x and y polarizations of the signal were recovered using a 16-tap, T/2-spaced adaptive butterfly finite impulse response (FIR) filter. Filter pre-convergence was achieved with the constant modulus algorithm (CMA), followed by maximum-likelihood frequency- and phase-estimation. For 64-QAM, pre-convergence was much more sensitive than 16-QAM and QPSK to the initialization values for various loop and filter parameters. In particular, while the CMA was perfectly able to recover symbol timing for 16-QAM, an exhaustive search for the sampling phase needed to be performed for 64-QAM. Final filter adaptation used a decision-directed algorithm interleaved with a decision-directed phase-locked loop. No differential quadrature encoding [8] was used in our experiments, and no cycle slips were ever observed in any of the many captured waveforms. III. RESULTS AND DISCUSSION A. Back-to-Back Performance Fig. 3 shows the optical spectrum for single-polarization 64-QAM (as expected, the spectra for QPSK and 16-QAM were nearly identical to this and are not shown), as well as the directly detected eye patterns and recovered signal constellations for single-polarization QPSK, 16-QAM and 64-QAM. Driving the modulator with 2-, 4-, or 8-level electrical signals produced QPSK, 16-QAM, and 64-QAM optical signals, respectively. Fig. 4 shows the single-polarization back-to-back BER as a function of the optical signal-to-noise ratio (OSNR, 0.1-nm reference bandwidth, noise in both polarizations) for the three modulation formats. For these measurements, the DAC outputs Fig. 3. Optical spectrum of 21.4-Gbaud 64-QAM signal (top). Directly detected eye patterns and recovered signal constellations for single-polarization QPSK, 16-QAM, and 64-QAM (bottom). were attenuated by 3 dB (in order to reduce the effect of any electrical reflections) before being applied to the I/Q modulator. This produced a voltage swing of V at each output, and was found to give similar performance to the results reported in [6], where 9 dB of attenuation was used. No predistortion of the drive signal to compensate the modulator sinusoidal response was needed at this drive level. In fact, in later measurements it was found that modulator nonlinearity did not cause a measurable penalty even at the full output of 1.6 V . At a BER of , the experimental performances of QPSK, 16-QAM, and 64-QAM at 21.4 Gbaud are 0.9, 1.8, and 4.0 dB off their theoretical limits, respectively. As expected, implementation penalties due to hardware deficiencies at these high modulation speeds have a stronger impact at increasing constellation sizes. For example, note from Fig. 2 that the 8-level eyes do not have the exact same horizontal nor vertical shapes, which inherently results in inter-symbol interference (ISI). Since the high-speed waveform cannot be fully represented as a linear superposition of temporally shifted pulses (as is the case for ideal pulse amplitude modulation [10]), this ISI has a nonlinear component, which the linear receiver DSP cannot optimally compensate. Note further the slight cushion-shaped distortion of the received constellation (most pronounced in Fig. 5), which we isolated in the course of our experiments through performing measurements with various permutations of receiver components and consequently attribute it to imperfections of the receiver’s digitization circuitry. Fig. 5 reviews the back-to-back BER measurements for 64-QAM reported in [6]. In these measurements, a conservative voltage swing of mV was obtained by placing 9 dB of attenuation on the DAC outputs. The performance was measured for both single-polarization (solid circles and triangles for x- and y-polarizations) and PDM operation (open GNAUCK et al.: GENERATION AND TRANSMISSION OF 21.4-GBAUD PDM 64-QAM USING A NOVEL HIGH-POWER DAC DRIVING A SINGLE I/Q MODULATOR 535 Fig. 6. Bit-error rate as a function of launched power per span after 400 km. Open circles: without NLC. Solid circles: with NLC. Fig. 4. Back-to-back bit-error-rate results for single-polarization QPSK (triangles), 16-QAM (squares) and 64-QAM (circles). Theoretical limits are given by solid lines. below (the threshold for operation with advanced FEC with 7% overhead) even at a launched power of 0 dBm. IV. CONCLUSION We have reported the generation of 21.4-Gbaud QPSK, 16-QAM, and 64-QAM signals using an I/Q modulator drive by a novel 3-bit high-power DAC. In back-to-back single-polarization measurements, and at a BER of , the experimental performances of QPSK, 16-QAM, and 64-QAM at 21.4 Gbaud are 0.9, 1.8, and 4.0 dB off their theoretical limits. For PDM 64-QAM (a line rate of 256.8 Gb/s, equating to 240 Gb/s after accounting for 7% FEC overhead) the performance was 4.6 dB off the theoretical limit at a BER of . The PDM 64-QAM signal was successfully transmitted over 400 km of low-loss, low-nonlinearity ultra-large-area fiber. Fig. 5. Back-to-back bit-error-rate results for 64-QAM (top) and recovered signal constellations for PDM 64-QAM at full OSNR (bottom). circles). The single-polarization results diverge slightly due to small differences in the receiver hardware for the two polarization channels. The required OSNRs to achieve a BER of are 26.0 dB for the best single-polarization result and 29.5 dB for PDM. The theoretical limits for both single-polarization and PDM signals are also shown. For single-polarization and PDM, our results are 4.1 dB and 4.6 dB off the theoretical limit, respectively. We attribute the relatively large (0.5-dB) penalty for PDM to the greater parameter sensitivity and the slower convergence of the underlying intradyne algorithms. At full OSNR ( dB) a BER floor at is reached. The recovered signal constellations for PDM 64-QAM at full OSNR are also shown in Fig. 5. B. Transmission of PDM 64-QAM Signal We next performed 400-km PDM transmission. Fig. 6 shows the BER as a function of power launched into each span, both without (open circles) and with (solid circles) a simple digital nonlinearity compensation (NLC) scheme as part of the receiver DSP [11]. At a launched power of dBm per span, the OSNR after transmission was 31 dB, and the BER of without NLC shows a transmission penalty of dB (see Fig. 5). The recovered signal constellations under these operating conditions are shown in the inset of Fig. 6. With NLC the BER was well REFERENCES [1] C. S. R. Fludger, T. Duthel, D. van den Borne, C. Schulien, E.-D. Schmidt, T. Wuth, J. Geyer, E. De Man, G.-D. Khoe, and H. de Waardt, “Coherent equalization and POLMUX-RZ-DQPSK for robust 100-GE transmission,” J. Lightw. Technol., vol. 26, no. 1, pp. 64–72, Jan. 2008. [2] J. Yu, X. Zhou, Y.-K. Huang, S. Gupta, M.-F. Huang, T. Wang, and P. Magill, “112.8-Gb/s PM-RZ-64QAM optical signal generation and transmission on a 12.5 GHz WDM grid,” in Proc. OFC, San Diego, CA, 2010, paper OThM1. 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Konczykowska, F. Jorge, J.-Y. Dupuy, M. Riet, G. Charlet, B. Zhu, and D. W. Peckham, “Generation and transmission of 21.4-Gbaud PDM 64-QAM using a high-power DAC driving a single I/Q modulator,” in Proc. OFC, Los Angeles, CA, 2011, postdeadline paper PDPB2. [7] A. Konczykowska, J.-Y. Dupuy, F. Jorge, M. Riet, J. Moulu, V. Nodjiadjim, P. Berdaguer, and J. Godin, “42 GBd 3-bit power-DAC for optical communications with advanced modulation formats in InP DHBT,” Electron. Lett., vol. 47, no. 6, pp. 389–390, 2011. [8] T. Pfau, S. Hoffmann, and R. Noe, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightw. Technol., vol. 27, no. 8, pp. 989–999, Apr. 2009. 536 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 4, FEBRUARY 15, 2012 [9] P. J. Winzer, A. H. Gnauck, C. R. Doerr, M. Magarini, and L. L. Buhl, “Spectrally efficient long-haul optical networking using 112-Gb/s polarization-multiplexed 16-QAM,” J. Lightw. Technol., vol. 28, no. 4, pp. 547–556, Feb. 2010. [10] J. G. Proakis, Digital Communications, 4th ed. New York: McGrawHill, 2001. [11] E. Yamazaki, H. Masuda, A. Sano, T. Yoshimatsu, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, M. Matsui, and Y. Takatori, “Multi-staged nonlinear compensation in coherent receiver for 16 340-km transmission of 111-Gb/s no-guard-interval Co-OFDM,” in Proc. ECOC, Vienna, Austria, 2009, paper 9.4.6. Alan H. Gnauck (F’09) joined Bell Laboratories in 1982, where he is currently a Distinguished Member of Technical Staff in the Transmission Systems Research group. He has performed record-breaking optical transmission experiments at single-channel rates of from 2 to 400 Gb/s. He has investigated coherent detection, chromatic-dispersion compensation techniques, CATV hybrid fiber-coax architectures, wavelength-division-multiplexing (WDM) systems, and system impacts of fiber nonlinearities. His WDM transmission experiments include the first demonstration of terabit transmission in 1996. More recently, he demonstrated 25-Tb/s transmission in 2007. He is presently involved in the study of WDM systems with single-channel rates of 100 Gb/s and higher, using advanced modulation formats and coherent detection. He has authored or coauthored over 200 journal and conference papers, and holds 25 patents in optical communications. Mr. Gnauck is a Fellow of the Optical Society of America (OSA) and has served as an associate editor for IEEE PHOTONICS TECHNOLOGY LETTERS (2000–2009). He was a technical subcommittee member for the Optical Fiber Communications Conference (OFC) in 2000, 2001, and 2003, and served as subcommittee chair in 2004. He received the OSA Engineering Excellence Award in 2003. Peter J. Winzer (F’09) received the Ph.D. degree in electrical engineering from the Vienna University of Technology, Vienna, Austria, in 1998. Supported by the European Space Agency, he investigated space-borne Doppler lidar and laser communications using high-sensitivity digital modulation and detection. In 2000 he joined Bell Labs, focusing on many aspects of fiber-optic networks, including Raman amplification, optical modulation formats, advanced optical receiver concepts, digital signal processing and coding, as well as on robust network architectures for dynamic data services. He demonstrated several high-speed and high-capacity optical transmission records from 10 to 100 Gb/s and beyond, including the first 100G and the first 400G electronically multiplexed optical transmission systems and the first field trial of live 100G video traffic over an existing carrier network. He has widely published and patented and is actively involved in technical and organizational tasks with the IEEE Photonics Society and the Optical Society of America (OSA). He was promoted to Distinguished Member of Technical Staff at Bell Labs in 2007 and since 2010 heads the Optical Transmission Systems and Networks Research Department. Dr. Winzer is a member of the Optical Society of America. Agnieszka Konczykowska is in charge of microelectronic design activity at III-V Lab, Bell Labs, Thales Research and Technology and CEA/Leti joint laboratory, Marcoussis in France. She worked in different domains of CAD and design methodologies like: methods and applications of symbolic analysis; semiconductor device modeling; analysis and design of switched-capacitor circuits. Her present research interests are the design of very high speed circuits for optical communications in InP HBT technology. She is the (co)author of over 200 scientific publications and holds six patents. Dr. Konczykowska is a member of the editorial board of the International Journal of Circuit Theory and Applications. She has been a member of many technical committees of international conferences and serves as reviewer for numerous technical journals. From 1995 to 1999, she was the president of European Circuit Society. Filipe Jorge was born in Roubaix, France, in 1970. He received the Ph.D. degree in electronic engineering from the University of Lille 1, Villeneuve d’ascq, France, in 1999. The next year, he joined OPTO , an Alcatel Research and Innovation unit, as a Research and Development Engineer. He is currently involved in module design and characterization of high speed circuits for optical communication systems within the III-V Lab of Alcatel-Lucent Bell Labs. Jean-Yves Dupuy received the electrical engineering degree from the Ecole Nationale Supérieure de l’Electronique et de ses Applications (ENSEA), Cergy, France, in 2005. The same year, he joined Alcatel-Thales III-V Lab, now III-V Lab, as a Research Engineer to develop very high-speed monolithic microwave integrated circuits in InP DHBT technology dedicated to 40-Gb/s optical communication systems. His current research interests include the development of MMICs for next-generation optical and wireless communication systems. Muriel Riet was born in Choisy-le-Roi, France, in 1958. She received the Ph.D. degree in electronic engineering from the University of Paris XI, France, in 1985. The same year, she joined CNET, Research Center of France Telecom, where she has studied compound semiconductor heterojunction bipolar transistors. Since 1998, she has in charge of InP HBT technology for high-bit-rate optical communications up to 40 Gb/s in 3-5 Lab. Gabriel Charlet was born in Rueil Malmaison, France, in 1976. He received an engineering degree from the École Supérieure d’Optique, Orsay, France, in 1999, and the Ph.D. degree in physics from University Paris XI, Paris, France, in 2011. He joined Alcatel Research and Innovation (now Alcatel Lucent Bell Labs France) in 2000. Since then, he has been working on WDM transmission systems and realized several multi-terabit/s transmission records. He also addressed the topic of advanced modulation formats. He is the author of ten postdeadline papers in major conferences and more than 30 patents. Dr. Charlet received the “Fabry de Gramont” award for its work on fiber optics communication in 2007. In 2010, he was selected by the Technology Review from MIT as one the 35 innovators below 35 years for its work on 100 Gbit/s and coherent detection. In 2011, he was selected by Fast Company as one of the 100 most creative people in business. B. Zhu, biography not available at the time of publication. David W. Peckham (SM’04) received the B.S. and M.E. degrees in electrical engineering from the University of Florida, Gainesville. He started his career at the Bell Labs Transmission Media Laboratory in 1982, working on optical fiber measurement techniques. Since 1989, he has focused on the design, process development, and commercialization of optical fibers for high-capacity transmission systems at Bell Labs, Lucent, and currently OFS. He is currently a Consulting Member of Technical Staff/Research Fellow in the Optical Fiber Design Group at OFS in Norcross, GA. Mr. Peckham received the 2002 Optical Society of America Engineering Excellence Award recognizing his contributions in the design and commercialization of fibers enabling high-speed, wideband WDM networks.