Inter-data Center Interconnect Research Articles

Dual-polarization carrier-assisted differential detection with asymmetric twin-SSB modulation and simplified receiver structure

Carrier-assisted differential detection (CADD) is a promising solution for high-capacity and cost-sensitive short-reach application scenarios, in which the optical field of a complex-valued double-sideband (CV-DSB) signal is reconstructed without using a local oscillator laser. In this work, we propose a polarization division multiplexed asymmetric twin single-sideband CADD (PDM-ATSSB CADD) scheme to realize the optical field recovery of the PDM CV-DSB signals. The polarization fading is solved by using a pair of optical bandpass filters (OBPFs) to suppress the unwanted other polarized offset carrier and signal, and the dual-polarization optical field is recovered by the CADD receiver. An asymmetric twin-SSB signal is used to relax the sharpness requirement of optical filter edges. We also propose a joint signal-signal beat interference (SSBI) iterative mitigation algorithm, which can effectively mitigate intra- and inter-polarization SSBI. The proposed PDM-ATSSB CADD scheme is validated for 30 Gbaud PDM asymmetric twin-SSB 16-ary quadrature amplitude modulation signals. We illustrate the parameter optimization process for PDM-ATSSB CADD, including optical delay and the number of iterations. The impacts of phase noise and relative intensity noise for laser and polarization impairment on PDM-ATSSB CADD are evaluated through numerical simulation. Compared with the PDM-symmetric-TSSB CADD (PDM-STSSB CADD), the required frequency gap to reach the 7% FEC threshold can be reduced by 1 GHz, and the OSNR sensitivity is improved by about 3 dB. Moreover, two simplified PDM-ATSSB CADD schemes are proposed and discussed, which could be considered hardware-efficient and integrative candidates for metro and inter-data center interconnects.

Paving the way toward 800 Gbps quantum-secured optical channel deployment in mission-critical environments

This article describes experimental research studies conducted toward understanding the implementation aspects of high-capacity quantum-secured optical channels in mission-critical metro-scale operational environments using quantum key distribution (QKD) technology. To the best of our knowledge, this is the first time that an 800 Gbps quantum-secured optical channel—along with several other dense wavelength division multiplexed channels on the C-band and multiplexed with the QKD channel on the O-band-was established at distances up to 100 km, with secret key-rates relevant for practical industry use cases. In addition, during the course of these trials, transporting a blockchain application over this established channel was utilized as a demonstration of securing a financial transaction in transit over a quantum-secured optical channel. The findings of this research pave the way toward the deployment of QKD-secured optical channels in high-capacity, metro-scale, mission-critical operational environments, such as Inter-Data Center Interconnects.

Real-Time FPGA Investigation of Potential FEC Schemes for 800G-ZR/ZR+ Forward Error Correction

Forward error correction (FEC) performance down to 1e-15 bit error rate (BER) of a open FEC code (OFEC), which was recently proposed for the 800G inter-data center interconnect (DCI) standard, is verified with a 100-piece-FPGA implementation at a record 400-Gbps throughput. Besides the OFEC, we also investigate the cons and pros of the concatenated staircase and Hamming code (CFEC) and multilevel codes (MLC) of a concatenated Low-Density Parity-Check (LDPC) and Turbo Product Code (TPC), which are the candidate schemes for the 800G-ZR FEC. The OFEC performance as a function of decoding iterations of Soft-in Soft-out (SISO) and Hard-in Hard-out (HIHO) is investigated, and FPGA emulations reveal the existence of an error flare of the OFEC with two SISO iterations and an severe error floor with one HIHO iteration even with four SISO iterations. Further investigations on different combinations of SISO and HIHO iterations, revealed that three SISO and two HIHO iterations are the optimal choice. Based on this iteration configuration, we studied the methods that decreasing the test patterns (TP) of SISO decoder and the soft bits to reduce the implementation complexity. Then we proposed the simplified OFEC scheme for 800G-ZR application, providing 1.81e-2 pre-FEC BER threshold with half power dissipation of standard OFEC, achieving a trade-off between the error correction performance and power dissipation. Finally, we investigated the performance of performance enhanced OFEC with 24.3% overhead and 2.5e-2 pre-FEC threshold, and finally, FEC candidates for the 800G-ZR+ applications are studied, indicating that the OFEC and channel polarized multilevel coding (CP MLC) are the most promising schemes.

Adaptive Bayesian neural networks nonlinear equalizer in a 300-Gbit/s PAM8 transmission for IM/DD OAM mode division multiplexing.

The strong stochastic nonlinear impairment induced by random mode coupling appears to be a long-standing performance-limiting problem in the orbital angular momentum (OAM) mode division multiplexing (MDM) of intensity modulation direct detection (IM/DD) transmission systems. In this Letter, we propose a Bayesian neural network (BNN) nonlinear equalizer for an OAM-MDM IM/DD transmission with three modes. Unlike conventional Volterra and convolutional neural network (CNN) equalizers with fixed weight coefficients, the weights and biases of the BNN nonlinear equalizer are regarded as probability distributions, which can accurately match the stochastic nonlinear model of the OAM-MDM. The BNN nonlinear equalizer is capable of adaptively updating its weights and biases sample-by-sample, according to the probability distribution. An experiment was conducted on a 300-Gbit/s PAM8 signal with three modes over a 2.6-km OAM-MDM RCF transmission. The experimental results demonstrate that the proposed BNN nonlinear equalizer exhibits promising solutions to effectively mitigate nonlinear distortions, which outperforms conventional Volterra and CNN equalizers with receiver sensitivity improvements of 1.0 dBm and 2.5 dBm, respectively, under hard-decision forward error correction (HD-FEC) thresholds. Moreover, compared with the Volterra and CNN equalizers, the complexity of the OAM-MDM is significantly improved through the BNN nonlinear equalizer. The proposed BNN nonlinear equalizer is a promising candidate for the high capacity inter-data center interconnects.

Scaling Optical Interconnects for Hyperscale Data Center Networks

Due to widespread applications and rapid development of cloud computing and Internet services, hyperscale data centers have experienced an unprecedented demand for networking. Data center networks are required not only to handle fast-growing traffic, which doubles almost every one to two years, but also to provide high flexibility and availability to support rapidly changing businesses as well. Therefore, hyperscale data center networks have become one of the main drivers for optical interconnect technologies in recent years. In this article, technologies for scaling optical interconnects for inter-data center interconnects (DCIs) are presented. We first describe hyperscale data center network architectures and requirements, as well as the differences between carrier’s optical transport networks and DCI optical networks, with the main focus on metro-DCI and campus-DCI networks. The scale and fast growth rates of DCI networks require innovations not only in data plane technologies but also in control and management planes as well. In data planes, high-capacity flexible coherent technology, pluggable wavelength-division-multiplexing (WDM) optical transponders, including 100G direct-detection four-level pulse-amplitude modulation (PAM4) and coherent 400ZR/800ZR, and flex-grid and disaggregated reconfigurable optical add-drop multiplexer (ROADM) optical networks are described. Control and management planes are an integral part of optical transport networks and crucial for effectively operating DCI networks. We show that standard protocols, data models, and modular design of a software platform are essential to building a scalable open and disaggregated optical network, which is fundamental to enabling high automation and intelligence in an optical network. An example control and management platform is discussed in detail. With current fiber capacity approaching the nonlinear Shannon limit, challenges to further scale DCI networks and some potential technologies to support ever-increasing traffic growth are discussed at the end.

Direct detection transmission of a PAM signal with power fading mitigation based on Alamouti coding and dual-drive MZM.

Power-fading impairment induced by fiber dispersion and photodiode detection imposes a fundamental limitation on the intensity-modulation direct-detection (IM-DD) transmission systems. In this work, we propose a cost-effective pulse-amplitude modulation (PAM) signal transmission scheme with power-fading mitigation enabled by Alamouti coding and dual-drive Mach-Zehnder modulator (DDMZM). By interleaving the symbol blocks in the time domain for upper- and lower-arm of the DDMZM, flat end-to-end frequency response can be obtained without spectral nulls after combining the photocurrents at odd and even time slots. For single channel demonstration, we experimentally transmit up to 160Gb/s PAM-4, 140Gb/s PAM-6, and 108Gb/s PAM-8 signals over 80 km standard single-mode fiber (SSMF) with bit-error rates (BERs) below the 20% soft-decision forward error correction (SD-FEC) threshold of 2.0×10-2. For wavelength division multiplexing (WDM) transmission, 8λ×150Gb/s PAM-4 signals spacing at 100 GHz can also achieve 80 km reach. Moreover, we compare the optical signal-to-noise ratio (OSNR) sensitivity with single sideband (SSB) scheme and evaluate the tolerance of bias deviation in numerical simulation. The proposed Alamouti coding-based scheme provides a hardware-efficient and dispersion-tolerant candidate for high-speed inter-data center interconnect (DCI) applications.

Polarization-transparent silicon photonic add-drop multiplexer with wideband hitless tuneability

Flexible optical networks require reconfigurable devices with operation on a wavelength range of several tens of nanometers, hitless tuneability (i.e. transparency to other channels during reconfiguration), and polarization independence. All these requirements have not been achieved yet in a single photonic integrated device and this is the reason why the potential of integrated photonics is still largely unexploited in the nodes of optical communication networks. Here we report on a fully-reconfigurable add-drop silicon photonic filter, which can be tuned well beyond the extended C-band (almost 100 nm) in a complete hitless (>35 dB channel isolation) and polarization transparent (1.2 dB polarization dependent loss) way. This achievement is the result of blended strategies applied to the design, calibration, tuning and control of the device. Transmission quality assessment on dual polarization 100 Gbit/s (QPSK) and 200 Gbit/s (16-QAM) signals demonstrates the suitability for dynamic bandwidth allocation in core networks, backhaul networks, intra- and inter-datacenter interconnects.

Reducing computation complexity by using elastic net regularization based pruned Volterra equalization in a 80 Gbps PAM-4 signal for inter-data center interconnects.

Volterra equalization (VE) presents substantial performance enhancement for high-speed optical signals but suffers from high computation complexity which limits its physical implementations. To address these limitations, we propose and experimentally demonstrate an elastic net regularization-based pruned Volterra equalization (ENPVE) to reduce the computation complexity while still maintain system performance. Our proposed scheme prunes redundant weight coefficients with a three-phase configuration. Firstly, we pre-train the VE with an adaptive EN-regularizer to identify significant weights. Next, we prune the insignificant weights away. Finally, we retrain the equalizer by fine-tuning the remaining weight coefficients. Our proposed ENPVE achieves superior performance with reduced computation complexity. Compared with conventional VE and L1 regularization-based Volterra equalizer (L1VE), our approach show a complexity reduction of 97.4% and 20.2%, respectively, for an O-band 80-Gbps PAM4 signal at a received optical power of -4 dBm after 40 km SMF transmission.

FPGA Investigation on Error-Flare Performance of a Concatenated Staircase and Hamming FEC Code for 400G Inter-Data Center Interconnect

Forward error correction (FEC) performance down to 10−15 bit error rate (BER) of a concatenated staircase and Hamming code (CSHC), which was recently proposed for the first 400G inter-data center interconnect standard, is verified with a 50-piece field-programmable gate array (FPGA) implementation at a record 200-Gb/s throughput. The effect of digital noise tail accuracy on the FEC emulation results is investigated. A converging criterion is proposed and demonstrated for achieving high confidence in artificial digital-noise-source-based FPGA emulations. The CSHC performance as a function of decoding iterations of the outer staircase code is investigated, and emulation results show that six staircase decoding iterations is the optimal choice. The FPGA emulations also reveal the existence of an error flare of the CSHC at 10−10 BER, which corrects the predicted BER threshold from 1.25 × 10−2 to 1.21 × 10−2. Further investigations on the effects of interleaving processes and staircase decoding buffer size show that the error flare behavior of the CSHC depends on the decoding buffer size of the outer staircase code. To remove the CSHC error flare, a proper choice of the staircase decoding buffer size is shown to be 6, which also moves the FEC threshold to 1.27 × 10−2.

Evaluation of Real-Time 8 × 56.25 Gb/s (400G) PAM-4 for Inter-Data Center Application Over 80 km of SSMF at 1550 nm

Leveraging client optics based on intensity modulation and direct detection for point-to-point inter-data center interconnect applications is a cost and power efficient solution, but challenging in terms of optical signal-to-noise ratio requirements and chromatic dispersion tolerance. In this paper, real-time 8 × 28.125 GBd dense wavelength division multiplexing four-level pulse amplitude modulation (PAM-4) transmission over up to 80 km standard single mode fiber in the C-Band is demonstrated. Using a combination of optical dispersion compensation and electronic equalization, results below a bit error rate of 1e−6 are achieved and indicate sufficient margin to transmit over even longer distances, if an forward error correction (FEC) threshold of 3.8e-3 is assumed. Moreover, single channel 28.125 GBd PAM-4 is evaluated against optical effects such as optical bandwidth limitations, chromatic dispersion tolerance, and optical amplified spontaneous emission noise.

Performance evaluation of multi-stratum resources integrated resilience for software defined inter-data center interconnect.

Inter-data center interconnect with IP over elastic optical network (EON) is a promising scenario to meet the high burstiness and high-bandwidth requirements of data center services. In our previous work, we implemented multi-stratum resources integration among IP networks, optical networks and application stratums resources that allows to accommodate data center services. In view of this, this study extends to consider the service resilience in case of edge optical node failure. We propose a novel multi-stratum resources integrated resilience (MSRIR) architecture for the services in software defined inter-data center interconnect based on IP over EON. A global resources integrated resilience (GRIR) algorithm is introduced based on the proposed architecture. The MSRIR can enable cross stratum optimization and provide resilience using the multiple stratums resources, and enhance the data center service resilience responsiveness to the dynamic end-to-end service demands. The overall feasibility and efficiency of the proposed architecture is experimentally verified on the control plane of our OpenFlow-based enhanced SDN (eSDN) testbed. The performance of GRIR algorithm under heavy traffic load scenario is also quantitatively evaluated based on MSRIR architecture in terms of path blocking probability, resilience latency and resource utilization, compared with other resilience algorithms.

Virtual Slice Assignment in Large-Scale Cloud Interconnects

Software-defined networking is an emerging method for providing flexible and scalable network connectivity in both intra- and inter-datacenter interconnects with regard to various requirements, including traffic-awareness, quality of service, energy efficiency, and renewable-power intermittency. This article investigates issues and solutions for the software-defined planning of a virtual slice that involves multiple virtual machines with interdependent constraints spanning a network of distributed data-centers. A flexible and optimized virtual-slice assignment that considers server consolidation and multipath forwarding can address large-scale cloud computing services. Simulations on large-scale testbeds, such as the GreenStar Network and the Green Telco Cloud, showed that the proposed model achieves good performance in terms of flexibility and energy efficiency.