Abstract
This article provides an extensive technical deep dive into optical I/O chiplets, their transformative benefits compared to electrical I/O, and how they can help eliminate interconnect bottlenecks in emerging high-throughput computing systems across numerous applications. Optical I/O chiplets aim to replace electrical I/O for chiplet-to-chiplet and chip-to-chip communications by monolithically integrating high performance silicon photonic devices alongside CMOS transistors using standard manufacturing processes.
Introduction to Optical I/O Chiplets
Electrical I/O using copper interconnects faces severe challenges in meeting the bandwidth density, power efficiency, latency, and reach requirements of next-generation computing architectures for cloud, high-performance computing (HPC), artificial intelligence (AI), networking, and more. Electrical SerDes cannot efficiently scale beyond ~100 Gbps, especially for chiplet-to-chiplet and chip-to-chip interconnects which require high density and energy efficiency within tight thermal budgets. Optical I/O based on silicon photonics integration can overcome these bottlenecks.
Companies like Ayar Labs have developed optical I/O "chiplets" with high performance photonic devices monolithically integrated alongside CMOS transistors using standard silicon manufacturing on 300mm wafers. Their TeraPHY optical I/O chiplets can achieve up to 2 Tbps bandwidth density at under 5 pJ/bit energy efficiency. TeraPHY also provides up to 2 km reach between chiplets/chips versus the few millimeters reach of electrical I/O. The improvements are orders of magnitude compared to electrical I/O.
These optical I/O chiplets can eliminate bottlenecks and enable innovative high-throughput computing architectures by removing traditional electrical I/O bandwidth density and efficiency constraints. This article provides examples for cloud data centers, HPC/AI acceleration, networking, and intelligent edge applications. Optical I/O chiplets unleash the full potential of advanced CMOS chiplets/chips and unlock innovation.
Limitations of Electrical I/O
To understand the critical need for optical I/O chiplets, it's important to examine the fundamental limitations of electrical I/O:
Power efficiency: Electrical I/O consumes very high power especially for longer reach links, severely limiting interconnect density/throughput due to thermal issues. Optical I/O can provide 10-100x better energy efficiency.
Latency: Electrical I/O requires power-hungry forward error correction (FEC) at high data rates, adding significant latency. Optical I/O can avoid FEC completely.
Bandwidth density: Electrical I/O bandwidth density peaks around 200-500 Gbps/mm. Optical I/O can reach Tbps/mm, a 10-100x density improvement.
Reach: Electrical I/O reach is limited to centimeters at best even with repeaters. Optical I/O can easily reach multiple kilometers.
These electrical I/O limitations create severe bottlenecks for high-throughput computing interconnects. Optical I/O chiplets can break through these barriers with transformative improvements in bandwidth density, efficiency, latency, and reach.
Technical Overview of Optical I/O Chiplets
This article provides an in-depth overview of Ayar Labs' breakthrough TeraPHY optical I/O chiplets which monolithically integrate high performance silicon photonic devices alongside CMOS transistors on a silicon chip:
Up to 8 optical ports operating at up to 256 Gbps per port providing 2 Tbps total bandwidth per chiplet.
Advanced Interface Bus (AIB) supporting a revolutionary 2 Tbps bandwidth density for in-package integration.
Operates at under 5 pJ/bit energy efficiency with 10 ns latency and up to 2 km reach between packages.
Manufactured using an economical 45nm SOI CMOS process on 300mm wafers with millions of transistors and hundreds of photonic components integrated together on chip.
TeraPHY chiplets connect to Ayar Labs' breakthrough SuperNova multi-wavelength tunable laser source that provides the modulated light beams for transmission over fiber links. The chiplets and light source together enable scalable, low-power, high-density optical I/O up to 8 Tbps with future generations.
Key Optical I/O Chiplet Differentiators
The PDF highlights unique differentiators of the TeraPHY optical I/O chiplets:
Monolithic integration of silicon photonics and CMOS transistors enables high bandwidth density along with small size and low cost.
Avoiding off-chip lasers via the SuperNova light source avoids thermal issues while enabling high port density.
Combining photonics and electronics in a single chip package reduces interface complexity vs. existing pluggable modules.
Supporting standard manufacturing processes ensures economy of scale for mass production vs. niche photonics fabs.
Providing active optical alignment and self-test capabilities on chip simplifies high-volume manufacturing.
These advantages make Ayar Labs' optical I/O chiplets stand out as an optimal solution versus hybrid approaches.
Photonic IC Design Automation
To make optical I/O chiplets viable, advanced photonic IC design automation tools will be crucial. Manual design of complex photonic ICs is not scalable or repeatable. Platforms like PIC Studio provide integrated EDA capabilities tailored for photonics including schematic capture, simulation, layout generation, verification, and more.
By automating PIC design with custom tools like PIC Studio's pSim circuit simulator and PhotoCAD layout editor, the development of optical I/O chiplets and systems can be accelerated. PIC Studio also enables co-design and co-simulation of electronics and photonics, facilitating analysis of complete links.
PIC Studio allows bi-directional design flows between schematic and layout views, ensuring synchronization. The availability of photonics-focused tools like PIC Studio that integrate simulation and synthesis will be key to unlocking the potential of optical I/O at scale across the industry.
Unleashing Architectural Innovation
A major benefit highlighted is how optical I/O chiplets can unleash innovation in computing architectures across numerous applications:
Cloud Computing
Enable powerful rack-scale architectures by interconnecting resources over high-density optical links. This removes throughput bottlenecks.
Support resource disaggregation and pooling by connecting chiplets/chips across data center racks like "Lego bricks".
Overcome density limits for high bandwidth switch fabrics and inter-rack networks.
Lower power consumption via 10-100x efficiency gains enabling greener operation.
Provide 1000x the bandwidth density at 1/10th the power vs. electrical I/O.
HPC and AI Acceleration
Low latency optical I/O allows HPC systems to run at full potential unconstrained by electrical I/O.
Higher bandwidth enables HPC/AI architectures with vast shared memory pools and lower latency access.
Facilitate flexible, distributed HPC/AI architectures linking resources over a low-latency optical fabric.
Networking and 5G Infrastructure
Multi-Tbps bandwidth density suits high-throughput switches, routers, and telco systems.
Low power connectivity between RF/antenna arrays and digital processing in remote units.
Replace copper cables with optical links in dense telecom infrastructure environments.
Intelligent Edge and Aerospace
Significant reductions in power consumption extend battery life via 10-100x efficiency gains.
Resistance to electromagnetic interference enables robust operation.
In-package integration suits SWaP-constrained applications like satellites, drones, and robots.
By breaking through electrical I/O density, efficiency, latency, and reach barriers, optical I/O chiplets open the door to new computing architectures not feasible previously across a diverse range of applications. This unlocks the full potential of advanced CMOS chiplets and systems.
Optical I/O Benefits for Cloud Computing
We provide an in-depth analysis of how optical I/O can transform hyperscale cloud computing systems and data centers. Key advantages include:
Enabling rack-scale architectures by seamlessly connecting resources such as CPUs, GPUs, memory, storage over high-density optical links rather than being constrained by electrical I/O port density. This removes system throughput bottlenecks.
Supporting resource disaggregation and flexible pooling by connecting chiplets/chips across data center racks in a "Lego brick" fashion rather than being limited by copper cable length.
Overcoming density limits for high bandwidth switch/network fabrics both within and across racks by replacing electrical cables with optical fiber.
Dramatically lowering power consumption via the 10-100x efficiency gains of optical I/O compared to electrical, enabling significantly greener data center operation.
Providing 1000x the bandwidth density at 1/10th the power consumption compared to electrical I/O based on copper interconnects. This is a transformational improvement.
Democratizing access to vastly more powerful computing resources by eliminating traditional performance bottlenecks imposed by electrical I/O density limits.
Effectively delivering the next phase of Moore's Law and overcoming the post-silicon scaling challenges by leapfrogging electrical I/O constraints through photonics integration.
These benefits make optical I/O chiplets an ideal solution for hyperscale cloud data centers to meet growing throughput demands within restricted power budgets. The transformative bandwidth density, efficiency, latency, and reach improvements enabled by optical I/O chiplets pave the way for new datacenter architectures not possible previously.
Optical I/O Benefits for HPC and AI Acceleration
We also dive into how optical I/O can revolutionize high performance computing and AI acceleration systems:
The ultra low latency of optical I/O allows HPC systems to run at their full potential rather than being artificially constrained by electrical I/O lag. This directly translates to real-world performance gains.
Providing significantly higher interconnect bandwidth enables HPC/AI architectures with vastly larger shared memory pools along with lower latency parallel data access.
Facilitates flexible, distributed HPC/AI architectures that efficiently link computational resources over a low-latency optical fabric rather than bottlenecked copper links.
Optical I/O supports advanced packaging techniques to tightly integrate processors, accelerators, memory modules.
The substantially lower power consumption enabled by optical I/O suits the rapidly growing demands of data-intensive AI computations and model training.
By delivering up to 1000x higher interconnect bandwidth density at 1/10th the power, optical I/O effectively continues compute performance scaling in the post-Moore's Law era. This is critical to advance HPC and AI to handle exponentially growing problem sizes and model complexities.
Optical I/O chiplets are poised to be a game changing technology for next-generation HPC and AI supercomputing by replacing electrical I/O bottlenecks with abundant low-latency interconnect bandwidth. This enables unlocking the full computational potential of advanced CMOS chiplets.
Technical Challenges and Solutions
There are some technical challenges involved in bringing optical I/O chiplets to widespread commercial adoption:
Packaging: Requires high-precision automated fiber alignment to optical ports during manufacturing. Ayar Labs demonstrates solutions with passive micro-optics and fiducial-based assembly processes.
Testing: Difficult to directly probe photonic signals within chips. Ayar Labs implements optical loopback test modes for characterization.
Thermal stability: Photonic devices require wavelength control. Ayar Labs integrates on-chip heaters for tunability and compensation.
Laser integration: Efficiently integrating lasers is difficult. Ayar Labs uses the external SuperNova source to simplify packaging.
While non-trivial, these engineering challenges have known solutions as evidenced by the rapid progress of companies like Ayar Labs. The benefits of optical I/O chiplets far outweigh the costs especially given the performance limits of electrical I/O.
Conclusion
Monolithically integrated optical I/O chiplets can provide transformative improvements in interconnect bandwidth density, power efficiency, latency and reach compared to electrical I/O. This enables eliminating major bottlenecks in high-throughput computing architectures across a diverse range of applications including cloud, HPC/AI, networking, aerospace, and more.
Optical I/O chiplets such as Ayar Labs' TeraPHY with up to 2 Tbps bandwidth per chiplet, 10-100x improved energy efficiency and 2 km reach effectively continue Moore's Law scaling through photonics integration. As electrical I/O constraints worsen, optical I/O chiplets on advanced CMOS nodes will become indispensable for future computing performance and efficiency gains. The rapid technical progress of this field proves the maturity and sizable benefits of silicon photonics-based optical I/O.
Reference
R. Danilak, “Why Energy Is A Big And Rapidly Growing Problem For Data Centers,” Forbes, 2017. [Online]. Available: https://www.forbes.com/sites/forbestechcouncil/2017/12/15/why-energy-is-a-big-and-rapidly-growing-problem-for-data-centers/?sh=64dd2d7f5a30
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