Constructing a Free Space Optical Backplane: Challenges and Choices

We are delighted to introduce a Special Issue on Optical Interconnects for the IET Optoelectronics magazine. This issue presents the current status as well as some of the outstanding progress that has been witnessed in the rapidly evolving area of optical interconnects. Today, data centres (DC) and high-performance computers are struggling with limited interconnect capacity and increased power consumption while trying to meet overwhelming bandwidth demands. As photonics technologies have been rapidly maturing, optical interconnects have come into the spotlight as the means to cope with the emerging bandwidth, energy, and capacity needs. As such, a wealth of photonic interconnect solutions across the DC hierarchy has unfolded, covering from electro-optic boards to transceiver development, on-chip and on-board switches and interconnects as well as novel noninvasive monitoring methods for light controlling for compensating thermal crosstalk, fabrication tolerances and unstable environmental conditions. At the same time, the increased penetration of optical interconnects in short-reach communications, such as board and chip levels, has triggered new challenges in photonics technologies calling for new innovations with respect to integration techniques and packaging methods. This Special Issue of IET includes integrated circuits, assembly techniques, systems-level architectures, and controlling methods related to short reach ‘chip-to-chip’, ‘in-the-box’, and ‘box-to-box’ communications. Specific areas covered by this special issue include: Silicon photonic transceiver technologies and flat DC topologies; High-speed electro-optic hosting boards and fast, energy-efficient electronics; DC switches and co-packaged optic (CPO) technologies; and Noninvasive controlling techniques for complex photonic integrated circuits. The four papers in this special section promote high-performance optical interconnect architectures and key technologies to tackle the rising bandwidth demands of DCs. They cover (a) co-packaged DC optics, (b) silicon technology transceiver modules and high-speed hosting boards for multisocket server board (MSB) interconnects and (c) automated controlling architecture for complex photonic systems; Items (b) and (c) draw from impressive achievements accomplished through the recently completed European H2020 Project ICT-STREAMS. This report written by C. Mickenberg tries to shed light on motivating factors behind co-packaged optics (CPO), the trade-offs involved and their potential impact on data centre network architectures. The article reviews the evolution of DC switch application-specific integrated circuit (ASICs) and DC optics, provides a future road map and finally discusses the potential impact of CPO on the network architecture. This topic is covered by two articles. The first, written by M. Morallis-Pegios, proposes using silicon photonics to advance interconnect throughput, socket connectivity and energy efficiency in MSB layouts by enabling a flat-topology wavelength division multiplexing (WDM) point-to-point optical MSB interconnect scheme. WDM silicon photonic transceivers coassembled with their electronic circuits achieving a 50-Gb/s line rate and 400-Gb/s aggregate data transmission are demonstrated. The article also proposes silicon photonic Arrayed Waveguide Grating Router (AWGRs) that can offer collision-less time-of-flight connectivity for up to 16 nodes. The second article covering that area is written by T. Lamprecht and reports on the development of electro-optic hosting boards for on-board transceiver subassemblies which is required to eventually form the Tb/s multi-socket onboard communication. The article also discusses the electrical interface as well as photonic board requirements, including the manufacturing technique and the assembly concept that was followed to deliver terabyte per second multichip optical communication. This article, written by F. Zanetto, introduces a novel dithering technique for real-time control of multiple cascaded photonic integrated devices, proposing a calibration-free strategy to support the development and real-life diffusion of increasingly complex photonic systems. The article elaborates on the implementation of a calibration-free automated control loop exploiting integral controllers to close a real-time feedback and stabilise the optimum working point of the employed photonic devices. The effectiveness of the proposed architectural approach is experimentally validated with the error-free routing and transmission of a 30-Gbit/s optical signal traversing a cascade of micro-ring resonator filters. All of the articles selected for this Special Issue present a wide variety of key topics and significant results addressing optical interconnects in high-performance data communication systems, including new progress on architectures, systems and packaging techniques as well as novel optimised controlling methods. As guest editors, we would like to thank all of the authors for their high-quality submissions that significantly contribute to the optical interconnect field. We would also like to thank the reviewers for their valuable and constructive technical comments that greatly enhance the Special Issue, because their feedback and comments ensure the high quality of the IET Optoelectronics Journal. We hope that this IET Special Issue will provide a useful reference of the state-of-the-art in optical interconnects to readers, and we anticipate such technological perspectives will stimulate new advancements and technological outcomes that will in turn enrich the growing field of optical interconnects. This work was supported by the E.C. through the H2020-ICT-STREAMS research project (688172).

Large-core, tapered Al 2 O 3 :Nd3+ channel waveguides have been fabricated and optical amplification at a wavelength of 880 nm has been characterized. Amplification in optical backplanes has been demonstrated by inserting such Al 2 O 3 :Nd3+ channel waveguides in between two polymer waveguides. A maximum 0.21-dB internal net gain has been demonstrated in an Al 2 O 3 :Nd3+ waveguide coupled in between two polymer channel waveguides. The gain can be improved by increasing the pump power and adjusting the waveguide geometry and dopant concentration for the chosen pump power. Use of such rare-earth-ion-doped waveguide amplifiers can provide a solution for compensating the losses occurring in optical interconnects.

A novel optical backplane relying on standard MM 12-fiber ribbons and MPO connectors is proposed for high-capacity ICT apparatus. A modular, scalable and full-mesh optical interconnection between optoelectronic boards is guaranteed thanks to an optimized fiber ribbon layout routing that divides the overall backplane into different and independent optical sub-circuits. The automated assembly of fiber ribbons into sub-circuits with a robotic work-cell is described. System validation of the optical backplane performed with commercial MM 12-fiber transceivers @10Gb/s proved the feasibility of the proposed solution for future optical interconnections with terabit overall capacity.

The bandwidth of information and communication technology (ICT) systems is increasing and is predicted to reach 10 Tb/s and over. However, an electrical interconnect cannot achieve such bandwidth because of its density limits. To solve this problem, we propose a high-density optical backplane, which consists of a fiber cassette and eleven optical backplane connectors. The fiber cassette accommodates 768 optical fibers. The backplane connector housing can mount eight 48-fiber mechanically transferable (MT)-ferrules. Optical backplane is 89 mm wide and 13 mm thick; therefore, equipment can secure a sufficient area for cooling while achieving a 9.6-Tb/s aggregate bandwidth. It also eases assembly and testing. The insertion loss of the backplane including the two connectors results in 1 dB or less. We demonstrated an optical interconnection system using the optical backplane, which exhibited error-free transmission between boards up to 25.8 Gb/s. We also propose an optical connector using lensed ferrules instead of MT-ferrules. Due to the introduction of the lensed ferrules, the spring force of the connector can be reduced to 1/5 that with an MT-ferrule. The optical connector can be mounted in the front panel of ICT system equipment. These optical wiring components enable the development of ICT systems with bandwidth exceeding 10 Tb/s.

A 640 Gb/s ultra-high speed optical switch interconnection system based on wavelength division multiplexing (WDM) is proposed as a Tbit/s ATM switch interconnection system for future B-ISDN. The proposed optical interconnection system uses WDM in the switch interconnection to avoid interconnection limitation. The system consists of 8 wavelengths (each carrying a 10 Gb/s signal), eight optical transmitter modules (Tx), and an 8/spl times/8 arrayed waveguide (AWG) optical router which interconnects the 8/spl times/8 80 Gb/s WDM signal and 8 wavelengths receiver modules (Rx). Using a highly statistical WDM optical link, the proposed system can attain nearly non-blocking ATM switch interconnection. There are several advantages to the proposed optical interconnection system. First, by adopting a arrayed waveguide filter with a wide-channel-spacing (525 GHz), the system has a wide temperature margin of 10 degrees. Second, by setting a clock data recovery circuit in each Rx module, each 10 Gb/s highway interconnect unit switch can function independently. As a result, the proposed system is highly scalable. We fabricate an optical interconnection prototype system and succeed in demonstrating a 640 Gb/s optical interconnection. The proposed optical interconnection system can be applied to realize future broadband ATM networks.

Dramatically increasing telecommunication equipment bandwidth motivates exploration of new technologies for backplane implementation. Hence, it is necessary to consider optical backplane implementations for realizing petabit/s switches. Current optical backplane implementations usually guide the light from source to destination with embedded fibers. In this paper we present a new implementation variant for an optical backplane. wherein free space is used as the optical signal medium. This realization just requires fixed mirrors in the rear of the line cards and thus the solution is very cost-effective. The transmitter and receiver are realized using high-speed lasers and photodiodes, and each individual link can transport 25 Gb/s of traffic. Some experimental results are also presented. As an evolutionary step for increased bandwidth, the paper considers the use of an array of links. In this case, we also evaluate the possibility of implementing the parallel transmitters and receivers in an integrated device, which would further reduce space, power consumption and cost.

The popularity of using integrated planar-optic (IPO) packages as an optical backplane for board-to-board interconnection is growing because of their compactness, high bandwidth, flexibility, and efficiency. An efficient design strategy for such optical backplane interconnect packages is developed by maximizing the alignability of such packages. The alignability is a probability based measure of the ease with which an optical interconnect system can be aligned. An IPO package that is designed for the maximum alignability is most likely to achieve a high interconnect efficiency, robustness, and yield during manufacturing. The efficiency of power transfer and the alignability of IPO packages can be calculated as a function of the lateral and angular offsets in the input, the error in the spatial frequencies of the hologram, the distance between the boards, the thickness of the substrate, and the sizes of the holograms and the beam spot. From these calculations, guidelines on choosing these parameters are developed. The maximum alignability can be obtained by optimizing the reflection angle or the substrate thickness of the IPO package as a function of these parameters. A software package has been developed for performing such optimizations. The software can be fruitfully used for design and manufacturing of all types of IPO interconnect packages.

Rare-earth-ion-doped polymer waveguide amplifiers [1–2] have been studied due to their potential use in integrated optical applications. Neodymium can be of great interest for high-speed amplifiers in optical interconnects, because it emits around 850–930 nm on the 4F 3/2 → 4I 9/2 ground-state transition. Polymers are promising host candidates in this application due to their low cost, ease of fabrication, and potential integration with polymer waveguides in integrated optical systems, e.g. optical backplanes. In this work, Nd3+-complex-doped polymer channel waveguide amplifiers have been realized and gain at 850-930 nm has been demonstrated.

The high-performance server compute landscape is changing. The traditional model of building general-purpose enterprise compute boxes that end-users can configure with storage and networking to assemble their desired compute environments, has evolved to purpose-built systems optimized for specific applications. This tight integration of hardware and software components together with high-density midboard optical modules and an optical backplane allows for unprecedented levels of switching and compute efficiencies and has fueled the penetration of optical interconnects deep “inside the box,” particularly for switch scale-up. We briefly review earlier 40 G/port switching systems based on active optical cables, and present our newest system: An all-optically-interconnected 100 G/port 8.2 Tb/s InfiniBand packet switch ASIC with 41 ports running 100 Gb/s per port interconnected by 12-channel midboard optical transceivers with 25 Gb/s per channel per direction of optical I/O. Using a blind-mate optical backplane, these components enable systems with up to 50 Tb/s bandwidth in a 2U standard rack mount configuration with industry-leading density, efficiency, and latency. For even tighter co-integration of optical interconnects with switch and processor ASICs, we discuss photonic multichip module and interposer packaging technologies that will further improve system energy efficiencies and overcome impending system I/O bottlenecks.

In the next years, when the data throughput in the telecom equipment will increase, the electrical interconnections among the boards in a subrack will be a bottleneck for the signal transmission. For transmitting digital data at several tents of Gbps, it will be needed to pass to a transmission layer completely optical, where optical boards will be interconnected in the same subrack by means of optical backplanes. Actually the optical backplane design and manufacturing involves very sophisticated and expensive technologies. In this paper an example of low cost solution for optically interconnecting different boards in the same subrack is presented, highlighting the benefit from the data transmission point of view.

Efficient data movement is one of the key attributes for high performance computing. This paper advocates the use of stream descriptors to convey memory access patterns from the programmer to the compiler. This explicit separation of computation and data movement enables the compiler to manipulate the stream descriptors to match the system's interconnect capabilities. Data movement is optimized by manipulating stream descriptors to target specific optimizations such as bandwidth management and buffer allocation. In this paper, bandwidth improvements are shown for an example system performing video analysis using computer vision methods. The system includes key hardware mechanisms that use stream descriptors to prefetch and align data for stream processors.

Future bandwidth needs force the development of optical interconnects for data transmission at board level. The aim is to develop technology solutions for embedded optical architectures in data center and network systems to allow significant reduction in power consumption, increased energy efficiency, system density and bandwidth scalability, which is currently unfeasible in today's copper driven systems. A passive optical backplane comprising a printed circuit board (PCB) with integrated optical waveguides and standardized pluggable optical coupling interfaces to the line cards is proposed for future data communication systems. Our activities are focused on embedded glass-based electro-optical circuit boards (EOCB) with integrated gradient-index multimode waveguides, which exhibit very low propagation loss at key optical communication wavelengths. A pre-processed glass panel with planar integrated optical waveguides is embedded into a backplane by using proven industrial PCB processes. A pluggable optical connector and receptacle system is designed for connecting the optical fiber links of the line cards with the optical waveguide circuit of the backplane.

We simulated and compared optical and electrical interconnects for intrachip communications, focusing particularly on the dependence of the power consump tion versus the interconnect length and the transmis sion throughput. We conclude that optoelectronic interconnects today consume less energy when the interconnect length is (typically) longer than (23) mm. The competition is clearly open in the future. Optical interconnects might contribute to solve onchip power consump tion and crosstalk issues, provided that their technology becomes cheap, compatible with the CMOS processes, and that it will be possible to reduce the threshold current of the VCSEL emitters down to a few μAmps.

Nash-Q learning-based collaborative dispatch strategy for interconnected power systems

The rapid scaling of microprocessor technologies has led to unprecedented advancements in processing speed and transistor density. However, as device dimensions shrink below the nanometre scale, traditional electrical on-chip interconnects face critical limitations including increased signal delay, power consumption, and reduced bandwidth. These challenges have emerged as major bottlenecks in the performance and energy efficiency of modern microprocessors, especially in multi-core and many-core architectures. To overcome these limitations, optical interconnects have gained significant attention as a high-performance alternative for on-chip and chip-to-chip communication. Optical interconnects offer key advantages such as higher data rates, lower latency, reduced power dissipation, and immunity to electromagnetic interference. The integration of optical components like waveguides, modulators, photodetectors, and silicon photonics within the chip architecture has the potential to revolutionize interconnect design by addressing the shortcomings of conventional electrical wiring. This paper explores the evolution of on-chip interconnects, comparing the performance, scalability, and integration challenges of electrical and optical solutions in scaled microprocessors. It also discusses emerging hybrid interconnect architectures that combine the strengths of both technologies. A comparison is made between electrical and optical interconnects for different design criteria, based on projections about the future of optical devices. Around a tenth of the length of the chip's edge is the crucial dimension beyond which optical connectivity becomes preferable to electrical interconnect at the 22 nm technological node.