LRO, LPO, and Silicon Photonics: Reducing Power Consumption in Optical Networks

Driven by the rise of AI clusters and cloud data centers, the demand for high-speed, efficient data transmission continues to grow. Computing engines—such as GPUs, CPUs, and storage systems—along with their supporting network infrastructure, consume significant amounts of power; in power-constrained environments, every watt consumed by the network is a watt diverted from computing. Consequently, minimizing network power consumption is critical to building efficient AI and data center infrastructure.
Linear Receive Optics (LRO) and Linear Pluggable Optics (LPO) are two key solutions currently being explored by engineers. Both offer reduced power consumption and fewer optical module components, making them increasingly attractive for high-speed AI clusters and data centers.
This article examines the advantages and challenges of LRO and LPO, as well as the pivotal role silicon photonics plays in enhancing the performance and cost-effectiveness of both solutions.

Architecture of Full Retiming, LPO, and LRO Modules

In Figure 1 below, you will notice how the optical module architecture evolves as we transition from the full retiming module to the LRO module and then to the LPO module.

What is LPO (Linear-drive Pluggable Optics)?

LPO (Linear Pluggable Optics) transceivers eliminate the full retiming (DSP) circuitry typically found in 400G, 800G, and 1.6T optical modules, shifting the responsibility for retiming and signal conditioning to the host. Removing the DSP reduces power consumption without compromising high-speed transmission performance, though careful design is required to ensure link robustness.

LPO Advantages:

LPO offers several advantages, including:

• Reduce power consumption: Retimers consume significant power; therefore, removing them from the module has a substantial impact on reducing power consumption. Figure 2 shows estimated power savings for 1.6T modules, comparing full retiming against LRO and LPO configurations.
• Cost-effectiveness: LPO reduces module costs by eliminating a key BOM element—the DSP—which accounts for more than 25% of a typical module's BOM cost.

Disadvantages of LPO:

Achieving a robust link in an LPO system presents significant challenges. The system must accommodate approximately 16 dB of electrical signal loss—and several decibels of optical loss—between the host switch and the module, while supporting arbitrary configuration; this means any LPO module must be able to interconnect and communicate reliably with any LPO switch, even across different vendors. Achieving this goal of "arbitrary interconnection" was already difficult at 100G per lane and becomes even more challenging at 200G per lane.

Two possible solutions: end-of-book links or project links.

If this ideal setup cannot be achieved, a "bookend" solution—using hardware from a single specific manufacturer at both ends of the link—may be required. Another option is an engineered link: a connection customized for a specific setup.
Bookend solutions may be easier to implement, but they limit flexibility and lead to vendor lock-in, representing a major obstacle to interoperability. Engineered links avoid this issue but introduce another: the complexity of customization increases costs, potentially making large-scale deployment unfeasible.

What is LRO (Linear Receiver Optics)?

The LRO module utilizes a single DSP at the transmit end, thereby eliminating the DSP typically found at the receive end of a full-retiming module. By doing so, the LRO reduces the module's overall power consumption while still providing some retiming functionality within the system link.
As with LPO, the responsibility for signal recovery shifts to the host system, which must manage signal integrity to ensure optimal performance.

LRO Advantages:

This shift has several benefits:

• Reduce power consumption: Compared to full-retiming modules, LRO modules consume less power because they do not require retiming at the receiver end; however, their power-saving performance is not as good as that of LPO modules.
• Cost-effectiveness: Compared to fully retimed modules, the absence of retiming circuitry at the receiver end reduces receiver module complexity; however, the impact is not as significant as it is with LPO modules. 

Disadvantages of LRO:

LRO offers distinct advantages and disadvantages compared to competing solutions. In short, LRO represents a compromise solution relative to LPO interfaces, offering roughly half the power and cost savings. Perhaps LRO's greatest advantage is that it significantly reduces risk regarding overall link performance; by spanning a single DSP between hosts, many interoperability challenges can be eliminated.

Optimizing LRO and LPO for Scalability: The Role of Silicon Photonics

Silicon photonics plays a key role in advancing LRO (Linear Receive Optics) and LPO (Linear Pluggable Optics) in the following areas:

1. Power Supply Efficiency

Silicon photonics reduces the power consumption of LRO and LPO modules by integrating optical components directly onto silicon chips. Traditional optical modules require separate components for signal generation, modulation, and detection—all of which consume power. Silicon photonics enables the miniaturization and integration of these components onto a single chip, thereby lowering energy consumption; this is particularly significant for LPO, where power efficiency is a key advantage.

2. Reduce Costs

Utilizing silicon photonics technology can reduce the production costs of LRO and LPO modules, as the technology relies on semiconductor manufacturing processes. Consequently, manufacturers can produce optical modules at a lower cost compared to traditional methods, thereby helping to lower the cost of LRO and LPO solutions.

3. High-density Integration

Silicon photonics enables a higher degree of integration of optical and electrical components on a single chip, resulting in more compact and scalable LRO and LPO modules. This is particularly advantageous for LPO, as simplifying transceiver design is a primary objective. Such integration allows for the development of smaller, more efficient transceivers capable of handling high data rates without the need for external retiming circuitry within the module.

4. Improve Linearity

Improvements in the linearity of silicon photonic modulators—such as advancements in Mach-Zehnder modulator (MZM) technology—ensure a more proportional output signal, thereby reducing distortion and enhancing signal integrity. These advancements position silicon photonics as a superior alternative for linear-drive pluggable optics (LPO), owing to their low power consumption, high level of integration with existing CMOS technology, and overall high performance. The result is a more efficient and reliable optical communication network.

5. 200G/Lane Availability

For cable applications such as Active Optical Cables (AOCs), the risk associated with LRO solutions is expected to be lower because the entire fiber infrastructure and the transceivers at both ends are shipped together as a unified package, effectively creating a turnkey solution. While these AOC products have historically relied on VCSEL technology, 200G-per-lane VCSELs are not yet available, nor is there a clear timeline for their release. Consequently, silicon photonics is ideally suited to replace VCSELs in these AOC applications.