Optical Interconnects For Data Centers Research Articles

Silicon based integrated photonic chip technology for datacenter optical interconnection

With the rise of the modern Internet of Things, big data and other industries, the human society’s demand for information increases rapidly, and the demand for broadband network capacity shows a dramatic growth trend. As its core support, optical communication system and data center are facing major challenges. With the characteristics of small volume, light weight and low power consumption, optoelectronic integrated devices are the key devices to break through the bottleneck of optical communication system and data center optical interconnection. In this paper, silicon-based integrated optical switching technology and multiplexing technology applied for datacenter optical interconnection are studied. The implementation methods and technical bottlenecks of silicon-based integrated optical switching chips and mode multiplexing/demultipelxing chips are discussed. Moreover, the future of silicon-based integrated photonic technology is prospected.

Petabit-Scale Silicon Photonic Interconnects With Integrated Kerr Frequency Combs

Silicon photonics holds significant promise in revolutionizing optical interconnects in data centers and high performance computers to enable scaling into the Pb/s package escape bandwidth regime while consuming orders of magnitude less energy per bit than current solutions. In this work, we review recent progress in silicon photonic interconnects leveraging chip-scale Kerr frequency comb sources and provide a comprehensive overview of massively scalable silicon photonic systems capable of capitalizing on the large number of wavelengths provided by such combs. We first consider the high-level architectural constraints and then proceed to detail the corresponding fundamental device designs supported by both simulated and experimental results. Furthermore, the majority of experimentally measured devices were fabricated in a commercial 300 mm foundry, showing a clear path to volume manufacturing. Finally, we present various system-level experiments which illustrate successful proof-of-principle operation, including flip-chip integration with a co-designed CMOS application-specific integrated circuit (ASIC) to realize a complete Kerr comb-driven electronic-photonic engine. These results provide a viable and appealing path towards future co-packaged silicon photonic interconnects with aggregate per-fiber bandwidth above 1 Tb/s, energy consumption below 1 pJ/bit, and areal bandwidth density greater than 5 Tb/s/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Evolution of System Embedded Optical Interconnect in Sub-Top-of-Rack Data Center Systems

In this paper we review key technological milestones in system embedded optical interconnects in data centers that have been achieved between 2014 and 2020 on major European Union research and development projects. This includes the development of proprietary optically enabled data storage and switch systems and optically enabled data storage and compute subsystems. We report on four optically enabled data center system demonstrators: LightningValley, ThunderValley2, Pegasus and Aurora, which include advanced optical circuits based on polymer waveguides and fibers and proprietary electro-optical connectors. We also report on optically enabled subsystems including Ethernet-connected hard disk drives and microservers. Both are designed in the same pluggable carrier form factor and with embedded optical transceiver and connector interfaces, thus allowing, for the first time, both compute and storage nodes to be optically interchangeable and directly interconnectable over long distances. Finally, we present the Nexus platform, which allows different optically enabled data center test systems and subsystems to be interconnected and comparatively characterized within a data center test environment.

190Gbaud OOK signal generation with a sub-sampling rate of 0.63 using silicon photonic dual-drive Mach–Zehnder modulator

Intensity modulation with direct detection (IM/DD) has been served as a low-cost technology of data center optical interconnects. To achieve higher data rate, high-order partial response (PR) signaling is utilized to narrow the signal bandwidth below the Nyquist frequency of digital-to-analog converter (DAC) sampling rate. In this work, we experimentally demonstrate ultra-high speed optical interconnects based on a conventional silicon photonic traveling-wave Mach–Zehnder modulator (MZM) with a 3-dB bandwidth of 28.5 GHz. By applying PR narrowing and digital anti-aliasing brick-wall filter that serves to avoid spectral overlap, up to 190Gbaud OOK signal is generated using a 120GSa/s DAC with a sampling ratio as low as 0.63. Furthermore, a 180Gbaud OOK signal is generated and transmitted over 500 m standard single-mode fiber (SSMF) below 20% hard-decision forward error-correction (HD-FEC) threshold of 1.5 × 10−2.

On-chip Ge, InGaAs, and colloidal quantum dot photodetectors: comparisons for application in silicon photonics

The past twenty years have seen explosive growth in silicon photonics technology. It has revolutionized numerous fields such high-speed optical interconnects in data centers. A photodetector (PD) is one of the key building blocks in silicon photonics, enabling on-chip light detection. Here a comprehensive study has been demonstrated in which three materials, germanium (Ge), indium gallium arsenide (InGaAs), and colloidal quantum dots (CQD), are compared for a PD integrated with a waveguide in silicon photonics. Comparisons are conducted by assuming InGaAs and CQD PDs have the same interface quality as mature Ge PD technology. With this premise, we intend to predict future InGaAs and CQD PD performances. Figures of merit such as dark current, responsivity, and RF bandwidth are compared using simulations. With the premise that epitaxial InGaAs on silicon is as of high quality as epi-Ge, results found that the InGaAs PD is advantageous over the Ge PD with higher-efficiency bandwidth product and lower dark current. CQD PD, on the other hand, is slow but has the lowest dark current, which is suitable for medium-speed applications where ultralow noise is required.

Silicon Photonic Crystal Modulators for High-Speed Transmission and Wavelength Division Multiplexing

For next-era optical interconnects in data centers, development of compact, energy-efficient, low-cost, and high-speed optical transceivers are required, for which high-performance external modulators in silicon photonics will be key components. We present a silicon photonic crystal waveguide slow light Mach-Zehnder modulator suitable for this purpose. The enhancement in the modulation efficiency via the slow light effect reduces the halfwave voltage-length product VπL, maintaining a wide working spectrum over 15 nm. The frequency response of the slow light modulator is constricted by an electrooptic phase mismatch between slow light and RF signals. In this study, this was dramatically improved by matching the phase using meanderline electrodes that delay RF signals. The cutoff frequency was experimentally evaluated to be 32-38 GHz. Using this device, we demonstrated high-speed modulation, including 64-Gbps on-off keying, 100-Gbps pulse amplitude modulation, and 50-Gbps/ch wavelength division multiplexing in 170-200-μm- long devices.

O-Band Subwavelength Grating Filters in a Monolithic Photonics Technology

The data communications industry has begun transitioning from electrical to optical interconnects in datacenters in order to overcome performance bottlenecks and meet consumer needs. To mitigate the costs associated with this change and achieve performance for 5G and beyond, it is crucial to explore advanced photonic devices that can enable high-bandwidth interconnects via wavelength-division multiplexing (WDM) in photonic integrated circuits. Subwavelength grating (SWG) filters have shown great promise for WDM applications. However, the small feature sizes necessary to implement these structures have prohibited them from penetrating into industrial applications. To explore the manufacturability and performance of SWG filters in an industrial setting, we fabricate and characterize the first O-band subwavelength grating filters in a monolithic photonics technology at GLOBALFOUNDRIES (GF). We demonstrate a low drop channel loss of −1.2 dB with a flat-top response, a high extinction ratio of −30 dB, a 3 dB channel width of 5 nm and single-source thermal tunability without shape distortion. This filter structure was designed using elements from the product design kit provided by GF and functions in a compact footprint of 0.002 mm2 with a minimum feature size of 150 nm.

Widely tunable, heterogeneously integrated quantum-dot O-band lasers on silicon

Heterogeneously integrated lasers in the O-band are a key component in realizing low-power optical interconnects for data centers and high-performance computing. Quantum-dot-based materials have been particularly appealing for light generation due to their ultralow lasing thresholds, small linewidth enhancement factor, and low sensitivity to reflections. Here, we present widely tunable quantum-dot lasers heterogeneously integrated on silicon-on-insulator substrate. The tuning mechanism is based on Vernier dual-ring geometry, and a 47 nm tuning range with 52 dB side-mode suppression ratio is observed. These parameters show an increase to 52 nm and 58 dB, respectively, when an additional wavelength filter in the form of a Mach–Zehnder interferometer is added to the cavity. The Lorentzian linewidth of the lasers is measured as low as 5.3 kHz.

40 Gbps heterostructure germanium avalanche photo receiver on a silicon chip

Photodetectors are cornerstone components in integrated optical circuits and are essential for applications underlying modern science and engineering. Structures harnessing conventional crystalline materials are typically at the heart of such devices. In particular, group-IV semiconductors such as silicon and germanium open up more possibilities for high-performing on-chip photodetection thanks to their favorable electrical and optical properties at near-infrared wavelengths and processing compatibility with modern chip manufacturing. However, scaling the performance of silicon-germanium photodetectors to technologically relevant levels and benefiting from improved speed, reduced driving bias, enhanced sensitivity, and lowered power consumption arguably remains key for densely integrated photonic links in mainstream shortwave infrared optical communications. Here we report on a reliable 40 Gbps direct detection of chip-integrated silicon-germanium avalanche p-i-n photo receiver driven with low-bias supplies at 1.55 µm wavelength. The avalanche photodetection scheme calls upon fabrication steps commonly used in complementary metal-oxide-semiconductor foundries, alleviating the need for complex epitaxial wafer structures and/or multiple ion implantation schemes. The photo receiver exhibits an internal multiplication gain of 120, a high gain-bandwidth product up to 210 GHz, and a low effective ionization coefficient of ∼ 0.25 . Robust and stable photodetection at 40 Gbps of on–off keying modulation is achieved at low optical input powers, without any need for receiver electronic stages. Simultaneously, compact avalanche p-i-n photodetectors with submicrometric heterostructures promote error-free operation at transmission bit rates of 32 Gbps and 40 Gbps, with power sensitivities of − 12.8 d B m and − 11.2 d B m , respectively (for 10 − 9 error rate and without error correction coding during use). Such a performance in an on-chip avalanche photodetector is a significant step toward large-scale integrated optoelectronic systems. These achievements are promising for use in data center networks, optical interconnects, or quantum information technologies.

Research on Topoloty Reconstruction Mechanism Based on Traffic Identification

&lt;p align="justify"&gt;Due to the growing variety of data center services, the bursty and variability of data traffic is increasing. In order to make the network better meet the needs of upper-layer services, it is necessary to design a more flexible optical internet topology reconstruction mechanisms to adapt the changing traffic demands. In the past research on optical internet, all topology reconstruction mechanisms are designed based on global data traffic. Although these mechanisms can fully utilize the flexibility of the data center optical interconnection network topology and adjust topology in real time according to the traffic demands, but when the traffic is presented at the regional level, this mechanism does not give optimal results. This paper proposes a topology reconstruction mechanism for data center optical interconnection network based on traffic identification for the previously proposed data center optical switching architecture—OpenScale. The simulation results show that it utilizes the flexibility of the network to save bandwidth resources and increase the wavelength connection bandwidth utilization with a little sacrifice of throughput.&lt;/p&gt;

Flexible and Scalable Optical Interconnects for Data Centers: Trends and Challenges

Emerging applications such as machine learning place higher demands on the flexibility and scalability of data center networks. On one hand, it is difficult for the fixed network topology to match the traffic patterns of different neural networks. On the other hand, the rapid growth of training data, model size, and computing requirements has led to an ever-increasing scale of data center networks. Traditional electrical interconnects lack the ability to adjust the network topology and communication bandwidth, resulting in significant resource waste and high expansion costs. Optical interconnects are introduced to solve the above problems because of its high bandwidth, low power consumption, and reconfigurability. In this article, the optical interconnects in recent years are analyzed in detail, and the flexibility and scalability of different networks are compared qualitatively. The future trends and challenges of optical interconnects in data centers are discussed from the aspects of optical transmission technology, optical switching technology, and optical network control and management.

Low-Power (1.5 pJ/b) Silicon Integrated 106 Gb/s PAM-4 Optical Transmitter

As next-generation data center optical interconnects aim for 0.8 Tb/s or 1.6 Tb/s, serial rates up to 106 GBd are expected. However doubling the bandwidth of current 53 GBd 4-level pulse-amplitude modulation (PAM-4) transmitters is very challenging, and perhaps unfeasible with compact, non-travelling wave, lumped modulators or lasers. In our previous work, we presented an integrated 4:1 optical serializer with electro-absorption modulators (EAMs) in each path. Transmitter (TX) functionality was shown up to 104 GBd non-return-to-zero (NRZ) On-Off Keying (OOK) or PAM-4. However the performance for PAM-4 was limited by the distortion introduced by the EAM non-linearity. We also presented a real-time, DSP-free 128 Gb/s PAM-4 link with a silicon photonic transmitter using binary driven EAMs in a Mach-Zehnder interferometer (MZI) configuration. By combining two of such half-rate (53 GBd) transmitters in an integrated 2:1-serializer, improved 106 GBd PAM-4 performance is expected without needing to compensate the inherent modulator non-linearity and without requiring faster modulators or drivers. In this paper, we present a Silicon integrated 53 GBd PAM-4 TX as a candidate for integration into 106 GBd PAM-4 2:1 serialized TX. The presented TX consists of two EAMs in an MZI configuration, wirebonded to a low-power 55 nm 4-channel SiGe BiCMOS driver, operating at 1.5 pJ/b (excluding laser). With a reference receiver (RX), transmission at or below the KP4-FEC threshold is shown beyond 1km standard single-mode fiber (SSMF) and up to 2 km non-zero dispersion-shifted fiber (NZ-DSF) at 1565 nm. Furthermore, the integrated TX was combined with an Si-integrated RX consisting of the same EAM component, wirebonded to a 55 nm SiGe BiCMOS transimpedance amplifier (TIA). Both TX and RX were wirebonded on an RF-PCB, with electrical connectivity through transmission lines and 6-inch 50 GHz multi-coax cable connectors. With this electrically connectorized all-EAM TX and RX, PAM-4 link operation is shown up to 40 GBd at 3.9 pJ/b (excluding laser), without using DSP or equalization.

IM/DD Beyond Bandwidth Limitation for Data Center Optical Interconnects

Intensity modulation with direct detection (IM/DD) provides a low-cost solution for data center optical interconnects. To achieve higher data rate, Tomlinson–Harashima precoding (THP) is adopted to increase the operating symbol rate beyond the Nyquist rate defined by the system bandwidth. By applying THP, the signal is precoded by a feedback filter at the transmitter for the mitigation of the intersymbol interference caused by the faster-than-Nyquist signaling and the spectral power fading induced by the interaction of chromatic dispersion and direct detection. In this paper, high-performance THP-based IM/DD transmission is demonstrated, increasing the achievable data rate using a limited system bandwidth. In the experimental demonstration, we achieve a net bit rate of 185 Gb/s for up to 2-km fiber link, transmitting 74 GBd precoded eight-level pulse amplitude modulation (PAM-8) using an IM/DD system with 33 GHz brick-wall electrical bandwidth limitation.

Slope efficiency of integrated external cavity hybrid lasers: A general model and analysis

The integrated tunable external cavity diode laser (ECDL) plays an important role in many emerging applications, such as LiDAR, integrated free-space optical communication, integrated nonlinear optics, quantum information processing, and data center optical interconnects. Based on the III-V/silicon or III-V/silicon nitride hybrid integration, chipscale tunable ECDL could achieve a broad wavelength tuning range (&amp;gt;100nm) and very narrow linewidth (&amp;lt;10kHz) with the help of micro-ring resonators. However, most hybridly integrated ECDLs exhibit a low slope efficiency (&amp;lt;0.1W/A), which severely limits the laser performance. We have developed a general numerical model for the hybrid laser and investigated several methods to improve its slope efficiency. We show that the slope efficiency of the hybrid laser can be efficiently increased by controlling the external cavity phase, improving the coupling between the laser chip and passive chip, using optimized anti-reflection and high reflection coatings, and reducing the laser intrinsic loss. We have also experimentally built an arrayed waveguide grating (AWG) based hybrid laser and shown that the numerical simulation results match well with the experimental results.

4:1 Silicon Photonic Serializer for Data Center Interconnects Demonstrating 104 Gbaud OOK and PAM4 Transmission

With next-generation optical interconnects for data centers aiming for 0.8 Tb/s or 1.6 Tb/s, 100 Gbaud capable transmitters from a single-laser source will become indispensable. However, these lane rates would require bandwidths of 70 GHz or more, doubling the bandwidth requirements of the electrical and optical components with respect to the fastest current generation of optical interconnects running at 53 Gbaud pulse-amplitude modulation (PAM-4). In this paper, we propose an integrated 4:1 optical serializer topology to achieve 104 Gbaud On-Off Keying (OOK) and PAM-4 transmission using only quarter rate components at the transmitter. We show 104 (208) Gbit/s OOK (PAM4) transmission using four GeSi electro-absorption modulators (EAMs) over 1 km of single-mode fiber (SMF). For 104 Gbaud OOK, clearly open eyes are obtained, while for PAM-4 the performance is limited by the nonlinear E/O-transfer function of the EAM. However, adding pre-emphasis in the electrical driver or replacing the single EAM with our previously demonstrated optical DAC topology—consisting of two EAMs in parallel with a 90° phase difference between each—could substantially improve these results. Additionally, we discuss the possibility of a four channel transmitter (4 × 208 Gb/s) from a single mode locked laser, amounting to a 832 Gb/s rate based on the current demonstrator.

Recent advances in optical technologies for data centers: a review

Modern data centers increasingly rely on interconnects for delivering critical communications connectivity among numerous servers, memory, and computation resources. Data center interconnects turned to optical communications almost a decade ago, and the recent acceleration in data center requirements is expected to further drive photonic interconnect technologies deeper into the systems architecture. This review paper analyzes optical technologies that will enable next-generation data center optical interconnects. Recent progress addressing the challenges of terabit/s links and networks at the laser, modulator, photodiode, and switch levels is reported and summarized.

Real-Time and DSP-Free 128 Gb/s PAM-4 Link Using a Binary Driven Silicon Photonic Transmitter

Optical transmitters for four-level pulse amplitude modulation (PAM-4) have attracted a significant amount of research in recent years, in large part due to the standardization of the format for the 200 and 400 Gigabit Ethernet optical interconnects in data centers. However, combining low-power and linear operation of the electro-optical frontend with sufficiently large bandwidths has proven challenging, especially for the 100 Gb/s/λ links (i.e., employing 50 Gbaud PAM-4). The most straightforward solution has been to deal with the non-idealities of the modulator in the electrical domain: predistorting the signal levels and/or equalizing the frequency response with the help of digital signal processing (DSP). However, this typically requires fast digital-to-analog converters (DACs), either capable of delivering large swings (> 1 Vpp) or supplemented with an additional linear amplifier to drive the optical modulator. Both options substantially increase the power consumption and the complexity of the transceiver. Rather than allocating effort to linearize the electrical to optical conversion of a single modulator, we propose a topology that performs the DAC operation in the optical domain. Two compact electro-absorption modulators in an interferometer layout are driven with NRZ data to generate the four-level signal in the optical domain. Using this topology, we demonstrate the first real-time 128 Gb/s PAM-4 transmission with a silicon photonic transmitter in a chip-to-chip link. In a back-to-back setup, we obtained a bit-error ratio (BER) of 4 × 10−10 without requiring any DAC, DSP, or modulators with large traveling wave structures. Over 1 km of standard single mode fiber a BER of 8 × 10−6 is recorded, still well below the KP4 forward error-coding limit. These results correspond to the lowest BERs reported for any real-time PAM-4 link at 100 Gb/s or higher, illustrating the benefit of performing the DAC operation in the optical domain.

Software Defined Survivable Optical Interconnect for data centers

Software-defined networking (SDN) will be the key enabler of the coming 5G infrastructure. For service providers, extending the SDN concept from packet switching in Layers 2 and 3 to circuit switching in transport layers is a promising approach to meet the growing high burstiness and high bandwidth requirements. A multi-layer, multi-domain controller that can provide automated controller-based restoration and protection, even for unprotected links in a multi-administrative domain, would be a significant improvement. It would allow service providers to ensure provision of guaranteed Service Level Agreement (SLA) maintenance, with optimal bandwidth usage, high availability and reduced errors.In this paper, we propose a Software Defined Survivable Optical Interconnect (SDSOI) architecture for Data Centers (DCs). This unique architecture will address service providers' challenges related to bandwidth management, and optimize the time required while interconnecting numerous DCs to meet the high SLA demands. The architecture is built according to the overlay SDN concept, and categorizes the application layers into online, offline and third-party applications.Our work primarily focusses on the creation of business applications in the proposed SDSOI model, and on the development of the northbound interfaces used by the applications to interact with the controller. To verify the effectiveness of the proposed SDSOI architecture and its business applications, we simulated ‘Day Night Scheduling’, an application that combines characteristics of offline and online applications. Its primary function is to package and schedule varying DC bandwidths to service providers and optimize bandwidth usage at different times of day, along with the survivability of the interconnects.

High-Speed PAM4-Based Optical SDM Interconnects With Directly Modulated Long-Wavelength VCSEL

This paper reports the demonstration of high-speed PAM-4 transmission using a 1.5-{\mu}m single-mode vertical cavity surface emitting laser (SM-VCSEL) over multicore fiber with 7 cores over different distances. We have successfully generated up to 70 Gbaud 4-level pulse amplitude modulation (PAM-4) signals with a VCSEL in optical back-to-back, and transmitted 50 Gbaud PAM-4 signals over both 1-km dispersion-uncompensated and 10-km dispersion-compensated in each core, enabling a total data throughput of 700 Gbps over the 7-core fiber. Moreover, 56 Gbaud PAM-4 over 1-km has also been shown, whereby unfortunately not all cores provide the required 3.8 $\times$ 10 $^{-3}$ bit error rate (BER) for the 7% overhead-hard decision forward error correction (7% OH HDFEC). The limited bandwidth of the VCSEL and the adverse chromatic dispersion of the fiber are suppressed with pre-equalization based on accurate end-to-end channel characterizations. With a digital post-equalization, BER performance below the 7% OH-HDFEC limit is achieved over all cores. The demonstrated results show a great potential to realize high-capacity and compact short-reach optical interconnects for data centers.

A V-Shape Optical Pin Interface for Board Level Optical Interconnect

This paper introduces a new interface of an optical pin for Printed Circuit Boards (PCBs), the V-shape cut type which is an innovation from the 90-degree cut type optical pin. The effectiveness is determined by optical characteristics through OptiCAD and by experiment. The simulation used a model of ray tracing analysis which is a one to two (split) connection function model. For the experiment, a Polymer Optical Fibre (POF) V-shape optical pin has been fabricated. It was found that the V-shaped optical pin has a multi-branched function and is applicable to optical interconnection. Full Text: PDF ReferencesMikami, O., et al. Optical pin interface for 90-deg optical path conversion coupling to Printed Wiring Board. in Region 10 Conference (TENCON), 2016 IEEE. 2016. IEEE. CrossRef DeCusatis, C., Data center architectures, in Optical Interconnects for Data Centers. 2017, Elsevier. p. 3-41. CrossRef Duranton, M., D. Dutoit, and S. Menezo, Key requirements for optical interconnects within data centers, in Optical Interconnects for Data Centers. 2017, Elsevier. p. 75-94. CrossRef ITOH, Y., et al., Optical Coupling Characteristics of Optical Pin with 45° Micro Mirror for Optical Surface Mount Technology. Journal of The Japan Institute of Electronics Packaging, 2001. 4(6): p. 497-503. CrossRef Uchida, T. and O. Mikami, Optical surface mount technology. IEICE Transactions on Electronics, 1997. 80(1): p. 81-87. CrossRef Papakonstantinou, I., et al., Low-cost, precision, self-alignment technique for coupling laser and photodiode arrays to polymer waveguide arrays on multilayer PCBs. IEEE Transactions on Advanced Packaging, 2008. 31(3): p. 502-511. CrossRef Nakama, K., et al., Optical connection device. 2006, Google Patents. DirectLink Ramaswami, R., K. Sivarajan, and G. Sasaki, Optical networks: a practical perspective. 2009: Morgan Kaufmann. DirectLink Tong, X.C., Advanced materials for integrated optical waveguides. 2014: Springer. CrossRef