520 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 39, NO. 2, JANUARY 15, 2021 Analog Coherent Detection for Energy Efficient Intra-Data Center Links at 200 Gbps Per Wavelength Takako Hirokawa , Student Member, IEEE, Student Member, OSA, Sergio Pinna , Member, IEEE, Navid Hosseinzadeh , Student Member, IEEE, Aaron Maharry , Hector Andrade , Junqian Liu, Thomas Meissner, Stephen Misak , Student Member, IEEE, Ghazal Movaghar, Luis A. Valenzuela , Student Member, IEEE, Yujie Xia, Shireesh Bhat, Fabrizio Gambini, Jonathan Klamkin, Senior Member, IEEE, Senior Member, OSA, Adel A. M. Saleh , Life Fellow, IEEE, Fellow, OSA, Larry Coldren , Life Fellow, IEEE, Fellow, OSA, James F. Buckwalter , Senior Member, IEEE, and Clint L. Schow, Fellow, IEEE, Fellow, OSA Abstract—As datacenters continue to scale in size, energy effi- ciency for short reach (<2 km) links is a major factor for networks that may connect hundreds of thousands of servers. We demon- strate that links based on analog coherent detection (ACD) offer a promising path to simultaneously achieving significantly larger link budgets and improved link energy efficiency. A complete anal- ysis is presented that considers the power consumption of all the photonic and electronic components necessary to realize an ACD link architecture based on 50 Gbaud (GBd) quadrature phase-shift keying (QPSK) signaling combined with polarization multiplexing to achieve 200 Gb/s/λ. These links utilize receivers that incorporate an optical phase-locked loop (OPLL) to frequency- and phase- lock the local oscillator (LO) laser to the incoming signal. QPSK modulation offers compelling advantages both in achievable link budget and in energy efficiency. Indeed, low-complexity electronics based on limiting amplifiers can be used as opposed to the linear front-ends, A/D converters, and digital signal processing (DSP) required for higher-order QAM or PAM formats. Our analysis indicates that links with 13 dB of unallocated budget operating at error rates of <10 -12 can be achieved and is compatible with higher error rates that require forward error correction (FEC). We present a comparison of silicon and InP platforms and evaluate both traveling-wave and segmented modulator designs, providing an illustration of the wide design space before converging on the Manuscript received July 14, 2020; revised September 21, 2020; accepted October 5, 2020. Date of publication October 12, 2020; date of current version January 15, 2021. This work was supported in part by the Advanced Research Projects Agency-Energy under Grant ARPA-E, and in part by the U.S. Depart- ment of Energy under Award DE-AR0000848. (Corresponding author: Takako Hirokawa.) Takako Hirokawa, Sergio Pinna, Navid Hosseinzadeh, Aaron Maharry, Hector Andrade, Junqian Liu, Thomas Meissner, Stephen Misak, Ghazal Movaghar, Luis A. Valenzuela, Yujie Xia, Fabrizio Gambini, Jonathan Klamkin, Adel A. M. Saleh, Larry Coldren, James F. Buckwalter, and Clint L. Schow are with the Electrical and Computer Engineering Department, University of California, Santa Barbara, CA 93106-9560 USA (e-mail: takako@ucsb.edu; pinna@ece.ucsb.edu; hosseinzadeh@ucsb.edu; amaharry@ucsb.edu; han- drade@ucsb.edu; junqian@ucsb.edu; thomas_meissner@ucsb.edu; smisak@ ucsb.edu; ghazalmovaghar@ucsb.edu; valenzuela@ucsb.edu; yujiexia@ ucsb.edu; fgambini@ucsb.edu; klamkin@ece.ucsb.edu; adelsaleh@ece. ucsb.edu; coldren@ece.ucsb.edu; buckwalter@ece.ucsb.edu; schow@ece. ucsb.edu). Shireesh Bhat was with the University of California, Santa Barbara, CA 93106-9560 USA. He is now with Juniper Networks, Sunnyvale, CA 94089 USA (e-mail: sbhat@ece.ucsb.edu). Color versions of one or more of the figures in this article are available online at https://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2020.3029788 most promising architectures that maximize energy efficiency and minimize laser power. We establish the theoretical potential to achieve picojoule-per-bit energy efficiency targets. Index Terms—Coherent detection, data center, energy efficiency. I. INTRODUCTION W ITH ever-increasing demand for cloud services, evaluat- ing interconnect technology benefits and tradeoffs antic- ipates future deployments of the data center through scaling baud rates, higher order modulation formats with more bits/symbol, polarization multiplexing, and adding additional wavelength division multiplexed (WDM) channels. Current data center links rely on intensity-modulated direct detection (IMDD) schemes due to their relative simplicity and correspondingly relatively low cost and power consumption. However, scaling IMDD links to 200 Gbps/lane will require a large jump in complexity and power consumption. A recent study showed the potential of a 100 GBd PAM-4 link to operate over a 400 m link distance [1]. However, heavy equalization was required, with 71 feedforward equalizer (FFE) taps and 15 decision feedback equalizer (DFE) taps, just to achieve a pre-FEC (Forward Error Correction) bit error ratio (BER) slightly below the soft decision (SD-FEC) limit of 2 × 10 -2 . With such power-hungry equalization, the required received optical power was > +7 dBm, likely de- manding an unfeasible output power from the transmitter (TX) source laser [1]. The limited prospects for scaling IMDD links to 200 Gbps/lane and beyond have driven substantial interest in developing a new generation of energy-efficient coherent links designed specifically for intra-datacenter applications [2]–[5]. A recent paper by authors from the Alibaba Group presents a detailed comparison of several variants of IMDD (PAM4, CAP16, DMT) against digital coherent (PDM-16QAM) for 400G links, backed up with experimental results, using metrics of minimizing laser and ASIC power consumption [4]. The authors conclude that coherent links have lower laser power requirements and comparable ASIC power dissipation and digi- tal signal processing (DSP) complexity compared to the IMDD approaches. Recent work from Google provides a comparison up to 1.6 Tb/s, analyzing in detail multiple digital coherent (16, 32, 0733-8724 © 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: Univ of Calif Santa Barbara. Downloaded on January 25,2021 at 22:50:15 UTC from IEEE Xplore. Restrictions apply.