Optical Enabling Technologies
Revolutionizing data center connectivity through advanced fiber optic solutions and dci technology
Optical fiber, as the primary interconnect medium, has played a crucial role in data transmission within data centers. Various emerging optical technologies have become viable solutions for addressing the technical challenges faced by these networks during horizontal scaling, while improving the performance and efficiency of large-scale data centers. DCI technology continues to evolve as a key enabler in this domain.
The increasing demand for higher bandwidth and more efficient data transfer has made optical enabling technologies indispensable. With the exponential growth of data traffic, traditional copper-based solutions are reaching their physical limits, making dci technology and optical solutions more important than ever before.
Future Data Center Network Architecture
Figure 2.3 illustrates an example of a future data center network utilizing WDM transceivers as the primary entities in a modular data center. For links connecting to Pods and between Pods and core switches, traditional parallel optical transceivers will be replaced with integrated WDM transceivers (such as 40G, 100G, and 400G), enabling the aggregation of all electrical channels with the same destination over a single fiber.
To optimize power consumption, the interconnect bandwidth between Pods can be dynamically adjusted to match the required network bandwidth demands. This adaptive approach is a cornerstone of modern dci technology implementations, allowing for efficient resource utilization.
In structures built with EPS (Pod switches and core switches), interconnect technologies will require higher data rates (≥40G) while maintaining fixed costs, size, and bandwidth power. In this application domain, integrated multi-core fiber transceivers will be a highly effective method for scaling bandwidth, further enhancing dci technology capabilities.
Figure 2.3: Future Data Center Network
Integration of WDM transceivers in modular data center architecture with dci technology
Copper vs Optical Transition
As communication speeds within racks increase (>10G), traditional copper cables will be replaced by optical devices. At 10Gb/s and higher speeds, passive and active copper cables suffer from large volume, high power consumption, and high loss at high data rates, limiting their effective length to just a few meters.
Low-cost short-range optical devices using IC-type optical packaging (such as LightPeak) could revolutionize data centers. In the coming years, we will see commercial network interface cards (NICs) with low-cost X10G optical interfaces. Additionally, switch chips will feature integrated PHYs and support 10G serial interfaces to further reduce cost and power consumption, advancing dci technology capabilities.
This transition represents a significant shift in data center design philosophy, where dci technology and optical solutions become fundamental building blocks rather than specialized components, enabling unprecedented scalability and efficiency.
Copper Limitations
- High power consumption at speeds >10G
- Limited transmission distance (meters only)
- Large physical volume and weight
- High signal loss at higher frequencies
- Limited scalability for future bandwidth needs
Optical Advantages
- Lower power consumption for equivalent bandwidth
- Longer transmission distances (up to kilometers)
- Smaller physical footprint and lighter weight
- Minimal signal loss across a wide frequency range
- Superior scalability with dci technology advancements
Optical Interconnect Bandwidth and Scalability
Figure 2.4: Bandwidth Development Trend
Evolution of network and server I/O bandwidth showing exponential growth supporting dci technology
Fiber optic interconnects that can cover communication ranges from 10 meters to 2 kilometers are crucial for data centers. The need to continuously increase the total interconnect bandwidth within data centers persists, whether through horizontal or vertical scaling approaches.
To meet the growing bandwidth demands of servers and networks, as illustrated in Figure 2.4, all emerging optical technologies at the device level—from modulation to channel multiplexing and photonic packaging—need to achieve effective scaling in terms of data rate, power, cost, and space/density.
This scaling challenge has become a primary focus of dci technology research and development, as data center operators strive to keep pace with the exponential growth in data traffic. The doubling of bandwidth approximately every 18 months, as shown in the chart, creates immense pressure on optical interconnect technologies.
During this process, careful consideration must also be given to the choice of fiber (single-mode vs. multi-mode) and corresponding compatible technologies. Each decision point represents an opportunity to optimize for specific data center requirements while leveraging the latest advancements in dci technology.
High-Speed Optical Technologies
VCSEL, DFB, and Silicon Photonics
Low-power, low-cost vertical cavity surface emitting lasers (VCSELs) and multimode fiber (MMF) have played a crucial role in achieving 10 Gb/s communication speeds within data centers. While significant progress has been made in manufacturing higher-speed VCSELs using alternative materials, substantial challenges remain in increasing VCSEL speeds significantly beyond 10Gb/s while ensuring reliability and yield.
Furthermore, due to modal dispersion, traditional VCSELs coupled with MMF have a limited distance-bandwidth product. At 10Gb/s, their maximum communication distance is insufficient to cover entire data centers, and this maximum coverage decreases rapidly as data rates increase (see Figure 2.5). This limitation has driven innovation in dci technology to overcome these physical constraints.
Figure 2.5: Lens-Integrated Surface-Emitting DFB Laser
Advanced laser design enabling higher bandwidth in dci technology applications
To achieve coverage exceeding 300m at 10Gb/s, data centers now commonly use higher-power, more expensive distributed feedback (DFB) lasers with single-mode fiber (SMF). When scaling per-channel rates from 10Gb/s to 25Gb/s, DFB lasers using new quaternary materials (InGaAIAs/InP with larger wavelength shifts) can provide better high-temperature performance at higher speeds.
New DFB laser structures, such as short-cavity and lens-integrated surface-emitting DFB lasers, have also been validated. Compared to VCSELs, these approaches can provide higher device bandwidth and narrower spectral width, enabling increased interconnect bandwidth and coverage while maintaining low power consumption and cost—key factors in advancing dci technology.
These technological advancements are critical for dci technology, as they enable data centers to extend optical connectivity beyond traditional limits while maintaining the efficiency and cost-effectiveness required for large-scale deployments.
Silicon Photonics
Over the past decade, significant progress has been made in applying silicon photonics technology to address the energy efficiency and cost issues of traditional optical transceivers using III-V compound materials. Although silicon's indirect bandgap makes it less than ideal for semiconductor lasers, it offers excellent thermal conductivity, transparency to traditional telecom wavelengths, and low noise for avalanche multiplication (due to high electron/hole impact ionization rates).
Most importantly, silicon photonics processes can be made compatible with CMOS manufacturing processes developed by the electronics industry. Silicon photodetectors are among the oldest and best-understood silicon photonic devices. For wavelengths below 1000nm, silicon is a low-cost and highly efficient photodetector material.
Low-loss silicon-based optical waveguides for wavelengths above 1000nm have also been demonstrated, enabling waveguide devices with more functionality and chip-level interconnects for various components (photonic integrated circuits, PICs). These advancements have significantly benefited dci technology by providing a path toward higher integration and lower costs.
Other recent developments in silicon photonics include: high-efficiency germanium photodetectors, high-speed silicon modulators with minimal switching energy, and germanium-silicon lasers. The tight integration of electronics and photonics enables higher bandwidth at lower power levels.
Silicon photonics holds great potential for improving data center flexibility, enhancing energy efficiency, and reducing costs—all critical factors for advancing dci technology. However, realizing this potential depends on overcoming various packaging and integration challenges that currently hinder widespread adoption.
Multiplexing Technologies
Through the basic device improvements described above, optical link speeds can be increased to match electrical switching I/O rates. Additionally, multiplexing is an essential method for increasing interconnect bandwidth. Both approaches are integral to advancing dci technology capabilities.
Space Division Multiplexing (SDM) and Wavelength Division Multiplexing (WDM) can leverage the parallelism of data channels in computer architectures and switch chips, making them two widely used multiplexing technologies in data centers. Other multiplexing technologies such as Optical Orthogonal Frequency Division Multiplexing (O-OFDM), multi-level or advanced modulation can also expand the bandwidth and capacity of a single fiber.
Space Division Multiplexing
One of the simplest methods to increase bandwidth is to dedicate one fiber per channel, with arrays of lasers and photodetectors at both ends. Parallel optical transceivers using ribbon fibers and MPO connectors (see Figure 2.6(a)) are widely deployed in data centers and HPC environments.
However, MPO connectors and ribbon fibers account for a significant portion of the cost in the entire data center network, and increasing bandwidth in this parallel manner also results in excessive volume and size of the fiber infrastructure. Therefore, this method becomes impractical when longer-distance interconnects are required.
Beyond space division multiplexing using parallel ribbon cables, data centers have recently begun exploring multi-core fiber (MCF) technologies developed for long-haul telecommunications. This field and its associated components can also be used in data centers to expand the application scope and lifespan of space division multiplexing methods, further enhancing dci technology capabilities.
Figure 2.6: Multiplexing Technologies
Different approaches to increasing bandwidth through multiplexing, critical for dci technology
Wavelength Division Multiplexing
Over the past few decades, wavelength division multiplexing has been widely used in metropolitan and long-haul transmission, enabling the telecommunications industry to scale bandwidth relatively easily. It is clear that WDM will need to evolve from these traditional telecommunications applications to short-distance data center interconnects.
To reduce the aforementioned cabling overhead and continuously increase link bandwidth, next-generation data center transceivers will require more spectrally efficient optical devices. However, to meet the economic and scale requirements of data centers, while WDM is necessary, it is also desirable that associated power consumption and costs do not increase significantly.
WDM technology allows multiple optical signals to be transmitted simultaneously over a single fiber by using different wavelengths (colors) of laser light. This enables a dramatic increase in bandwidth without requiring additional fiber infrastructure, making it particularly valuable for dci technology implementations where space and cost are critical factors.
The adoption of WDM in data centers represents a significant advancement in dci technology, enabling unprecedented scaling of network capacity while maintaining efficient use of physical resources. As data demands continue to grow, WDM will play an increasingly central role in data center networking architectures.
The Future of Optical Enabling Technologies
As data centers continue to grow in size and complexity, optical enabling technologies and dci technology will play increasingly critical roles in meeting bandwidth demands while maintaining efficiency. The ongoing development of new materials, devices, and multiplexing techniques promises to deliver even greater performance, lower costs, and higher energy efficiency in the years ahead.