The Role of Multimode and Single-Mode Fiber in Short-Reach Data Centers

Multimode fiber (MMF) has historically been widely deployed for short-reach data center links because VCSEL-based transceivers and multimode cabling can provide economical solutions for relatively short distances.

OM3, OM4, and OM5 multimode fiber continue to support high-bandwidth data center applications. At the same time, single-mode fiber (SMF) has become increasingly important as data rates and reach requirements increase.

Single-mode architectures such as 400G-DR4, 400G-FR4, 800G-DR8, and 800G-FR4 provide a scalable optical platform for data center networks extending beyond the shortest MMF applications.

The design decision is therefore not simply "MMF versus SMF." Data center architects must select the appropriate fiber and transceiver architecture based on reach, bandwidth, fiber infrastructure, connector density, power, and lifecycle requirements.

Parallel Optics: The Foundation of High-Density Short-Reach Networks

Parallel optics have played a central role in the evolution of high-speed Ethernet. Instead of transmitting the entire aggregate data rate over a single optical channel, the total bandwidth is divided across multiple parallel optical lanes.

The architecture evolved from early 40GbE implementations using four optical lanes to today's 400G and 800G systems. The same architectural principle continues into higher-speed systems.

The important architectural trend is clear: future bandwidth increases cannot rely indefinitely on simply adding more optical lanes and fibers.

Why Lane Count Becomes a Problem at 1.6Tb/s

Moving from 800G to 1.6T immediately exposes the limitations of lane scaling.

One approach is to maintain the established 100G-per-lane electrical ecosystem and double the number of lanes:

This approach has an important advantage: it builds upon an established 100G-per-lane ecosystem. The disadvantage is the increase in electrical and optical lane count, connector complexity, fiber requirements, and thermal and packaging constraints.

The OSFP-XD form factor was developed to address this density problem. Its high-density architecture provides a migration path between 100G-per-lane and 200G-per-lane technologies.

OSFP-XD and the 1.6Tb/s Architecture

OSFP-XD is significant because it provides a high-density platform capable of supporting multiple generations of bandwidth. The architecture supports both 100G-per-lane and 200G-per-lane approaches to 1.6Tb/s.

Path 1: 16 × 100G

This architecture maximizes compatibility with the established 100G-per-lane electrical ecosystem, but requires 16 lanes.

Path 2: 8 × 200G

Here, the lane count is reduced by half, but the electrical and optical technology becomes substantially more demanding.

This illustrates the broader evolution of optical networking: capacity is increasingly being achieved by increasing the bandwidth carried by each lane rather than simply adding more lanes.

224G-PAM4: Increasing Bandwidth per Lane

The transition to 224G-PAM4 represents an important step in this evolution. PAM4 encodes two bits per symbol using four amplitude levels, allowing substantially more data to be transmitted at a given baud rate than NRZ signaling.

However, "224G" in 224G-PAM4 should not be interpreted as 224 GBaud. In the OSFP-XD architecture, 224G-PAM4 is associated with approximately 106.25 GBaud and a 200 Gb/s lane rate.

This distinction is increasingly important when discussing next-generation optical networks because baud rate, electrical lane rate, and aggregate module bandwidth are different parameters.

Thus:

and:

From 800G to 1.6T and 3.2T: What Changes?

Higher-Speed SerDes

Switch ASICs must support increasingly high-speed electrical interfaces while maintaining acceptable channel loss, crosstalk, jitter, and signal integrity.

Higher-Performance DSPs

Digital signal processors must compensate for increasingly severe electrical and optical impairments while operating within strict power budgets.

Optical Engine Scaling

Lasers, modulators, photodetectors, drivers, and optical packaging must support higher symbol rates while maintaining acceptable optical signal quality and manufacturing tolerances.

Thermal Management

As transceiver bandwidth increases, power density becomes an increasingly important design constraint. Higher-density modules therefore require increasingly sophisticated thermal and mechanical designs.

Fiber and Connector Density

Increasing the number of parallel lanes can increase fiber and connector complexity. This is one reason that higher per-lane bandwidth is attractive: it can increase aggregate capacity without requiring a proportional increase in fiber count.

Why Parallel Optics Will Not Disappear

It would be incorrect to conclude that higher-speed networking eliminates parallel optics.

Instead, parallel optics are evolving.

The industry is moving from architectures based primarily on:

toward:

where the technology and reach requirements permit.

The result is a hierarchy of architectures rather than a single universal solution. Current data center deployments can use 800G parallel single-mode architectures such as DR8, while multimode solutions remain relevant for shorter-reach applications. Other architectures use wavelength multiplexing to reduce the number of physical fibers.

The Road Ahead: 1.6T and 3.2T Data Center Optics

The transition beyond 800G is not simply a matter of developing a faster optical transceiver. It requires coordinated evolution across the complete electrical and optical link.

• Switch ASIC SerDes
• Electrical channel design
• PAM4 signaling
• DSP technology
• Lasers and modulators
• Photodetectors
• Optical packaging
• OSFP-XD and other high-density form factors
• Single-mode and multimode fiber infrastructure
• High-density connector architectures
• Thermal management
• Forward error correction

For network engineers and data center architects, the central design challenge is therefore to increase aggregate bandwidth while controlling power, fiber count, optical loss, signal integrity, thermal density, and operational complexity.

The evolution from 800G to 1.6T and eventually 3.2T represents a fundamental change in how short-reach optical systems are engineered. Higher lane rates reduce the need for ever-increasing parallelism, while high-density form factors provide the mechanical and electrical infrastructure needed to accommodate the next generation of optical interfaces.

Conclusion: The Evolution of Short-Reach Data Center Optical Networks

The evolution of short-reach data center optics is increasingly defined by higher bandwidth per lane, greater port density, and more efficient use of fiber infrastructure.

800G has established the importance of 100G-per-lane architectures, while 1.6T introduces the next major transition: moving from 100G lanes toward 200G lanes and higher-speed electrical interfaces.

OSFP-XD provides an important high-density platform for this transition, supporting both 16-lane 100G architectures and 8- or 16-lane 200G architectures.

As AI and accelerated computing continue to drive east–west traffic, understanding these optical architectures will become increasingly important for network architects, optical engineers, data center professionals, and anyone responsible for deploying or managing high-capacity fiber networks.