LPO vs CPO: Which Will Dominate the Data Center Optical Interconnect?

In the ever-expanding realm of data centers, where speed and efficiency are paramount, the battle for dominance in optical interconnects has reached a crucial juncture. Two formidable technologies, Laser Phased-locked Oscillator (LPO) and Coherent Phased-locked Oscillator (CPO), have emerged as leading contenders in the race to revolutionize data center optical interconnectivity. This article delves deep into the intricacies of LPO and CPO, exploring their unique features, strengths, and challenges.

The Rise of CPO and LPO

The power consumption of a 10Gb optical module is only about 1W. With the upgrade of 400G and 800G optical modules, the power consumption of optical modules has soared, forcing it to 30 W. A switch can have more than one optical module. If it is fully loaded, there are often dozens of optical modules (if there are 48, it is 48 × 30 = 1440W). The power consumption of the optical module accounts for about 40% or more of the power consumption of the whole machine. The power consumption of the whole machine may exceed 3000 W. The surge in energy consumption of optical communication equipment also puts tremendous pressure on the energy consumption and cost of the entire data center, and it is urgent to reduce energy consumption and cost, so CPO/LPO solutions appear.

The emergence of CPO/LPO has quickly become two major buzzwords in the optical module industry, and major manufacturers have accelerated the layout of the corresponding program.

What Is CPO?

CPO (Co-Packaged Optics) is an innovative technology that refers to the electro-optical integration where the optical engine and the switch chip are encapsulated together, rather than using a pluggable optical module format. This approach allows for faster transmission of electrical signals between the engine and the chip by shortening the distance between the optical engine and the switch chip. It effectively reduces size, lowers power consumption, and enhances efficiency.

For more information about CPO, please read this article: A Comprehensive Overview of Co-Packaged Optics

Technology Development of CPO

Specifically, the sustainable technical route can be implemented in two development phases of switch silicon photonics technologies:

Phase 1: Near-packaged optics (NPO) technology. This technology provides low-cost low-power benefits in the shortest time before the co-packaged optics (CPO) ecosystem is ready.

Phase 2: CPO technology. This is the final form of switching silicon photonics technologies and can minimize network costs and power consumption.

An optical engine conducts a switch network's optical/electrical conversion function. It is most commonly in pluggable form. With the evolution of technologies, new product forms emerge. The CPO form assembles a switching chip and optical engine in a socketed slot, realizing the co-package of the chip and module. The NPO form decouples an optical engine and switching chip and assembles them on the same system board. Although a photoelectric module exists in both forms, it is packaged in different positions, the corresponding cabling distance varies, and the power consumption is also different.

The Advantages of CPO

Envision a scenario within a state-of-the-art telecommunications infrastructure, where space optimization and power efficiency are paramount to supporting increasing data traffic and speeds. The integration of CPO positions the system to deliver superior performance through faster, more reliable communication while also paving the way for streamlined, energy-conscious designs in the ever-evolving landscape of networking technology. Here are the advantages of CPO.

• Enhanced Signal Integrity: By dramatically cutting down the transmission distance for on-board electrical signals, CPO significantly diminishes the need for signal compensation. This advancement facilitates a more versatile PCB design, allowing for greater design freedom and improved performance.

• Compact Design and High Integration: CPO’s method of integrating both photonic and electronic components within the same packaging framework enables a size reduction compared to traditional, separate packaging of optical modules and electronic chips. This compactness is particularly beneficial for applications within high-density integrated circuits, where real estate is at a premium.

• Reduced Latency: Leveraging the CMOS process for the integration of both electrical and optical devices translates to lower power consumption, offering an improvement over conventional LPO technologies. With both optical modules and electronic chips enclosed in a single package, the path for signal transfers is shortened, allowing for markedly reduced latency compared to separate packaging methods.

LightCounting said in its December 2022 report that AI's demand for network rates is more than 10 times the current rate, and in this context, CPO is expected to reduce the power consumption of the existing pluggable optical module architecture by 50%, which will effectively solve the problem of high-speed, high-density interconnection transmission difficulties. Domestic vendors are also further developing CPO technology.

What Is LPO?

LPO, which stands for Linear Drive Pluggable Optics, refers to using direct-drive linear technology in fiber optic modules. This approach eliminates traditional components like DSP (Digital Signal Processor) and CDR (Clock Data Recovery) chips, which reduces power consumption and latency within the system. However, it should be noted that there is a trade-off, as these benefits come with a slight decrease in system error correction performance and a limitation on the transmission distance. This technology is particularly well-suited for short-range communication scenarios, such as those found within data centers, where high speed and low latency are paramount and the distances involved do not require the robust error correction typically provided by DSP/CDR chips.

 

Traditional pluggable optical module: The DSP chip processes high-speed signals to achieve low bit error rate, but the power consumption is large. Taking the 400G optical module as an example, the power consumption of 7nm DSP is about 4W, accounting for 50% of the power consumption of the entire module.

For more information about LPO, please read this article: What Is the LPO Transceiver?

The Advantages of LPO

Linear Drive Pluggable Optics (LPO) offers several advantages in the field of optical interconnects. These benefits position LPO as a promising solution for enhancing data center connectivity and meeting the ever-increasing demands of modern networking environments. Here are some key advantages of LPO:

• Hot-Swappable and Easy Maintenance: Unlike CPO, LPO retains the traditional modular form factor for optical components, making it a less invasive solution. If a device within a CPO system fails, it often necessitates disassembling the entire switch, which can be inconvenient for maintenance. In contrast, LPO’s design supports hot-swapping, which simplifies replacements and reduces downtime for users.

• Reduced Latency: LPO technology forgoes the use of digital signal processors (DSPs), thereby eliminating the need for signal restoration. This reduction in processing overhead significantly lowers system latency, making LPO well-suited for applications where timing is critical, such as inter-GPU communication in a high-performance computing center (HPC).

• Lower Costs: The absence of DSPs not only reduces power consumption but also lowers operational costs. DSP technology is a niche expertise held by a select few manufacturers like Broadcom and Inphi. By not relying on DSPs, to a certain extent, LPO also diminishes dependency on these limited sources, potentially diversifying supply chains and enhancing market competitiveness.

Stakeholders across the spectrum—from upstream chip manufacturers to switch vendors and down to end-users—are placing significant emphasis on the development and application of LPO (Linear Drive Pluggable Optics) technology, indicating strong prospects for commercial deployment. At OFC 2023, leading companies like Macom and Broadcom showcased their linear-drive solutions, signaling a robust industry trend. Notably, Arista's featured linear solution garnered endorsements from prominent switch manufacturers including Innovium and NVIDIA, reflecting a growing consensus on the technology's value proposition in cutting-edge networking scenarios.

CPO vs LPO: Which Will Dominate the Data Center Optical Interconnect?

LPO represents a strategic trade-off, tailored for particular use cases such as short-distance applications. By forgoing components like DSP/CDR, there may be a marginal performance trade-off in terms of bit error rate. Yet, it offers substantial benefits by lowering power usage, cost, and latency. This approach sets LPO apart from CPO with a distinct set of pros and cons. Despite its later arrival on the scene compared to CPO, LPO is expected to be adopted more rapidly. The differences between CPO and LPO are shown in the below table.

Future Development Prospects and Challenges of CPO/LPO

Development Prospect and Challenge of CPO

The Future Market Space of CPO Is Broad

CPO offers a direction for enhancing optical interconnect density and is expected to be progressively commercialized for 100T+ applications. LightCounting suggests that shipments of CPO are anticipated, to begin with 800G and 1.6T ports and are likely to enter commercial use between 2024 and 2025, with significant volume scaling happening from 2026 to 2027, primarily within short-reach, data communications scenarios for hyperscale cloud service providers. CIR projects that by 2027, the market revenue for co-packaged optics will reach $5.4 billion.

Global sales of CPO ports are projected to grow from 50,000 in 2023 to 4.5 million by 2027. In 2027, CPO ports are expected to account for nearly 30% of the total shipments in the 800G and 1.6T categories. According to a Yole report, the CPO market generated revenue of approximately $38 million in 2022, with projections reaching $2.6 billion by 2033. The compound annual growth rate (CAGR) for the period from 2022 to 2033 is estimated at 46%.

Challenges for CPO

While CPO holds promise for advanced optical interconnects, it faces challenges related to technology maturity, urgency of adoption, and business model considerations. Addressing these challenges will be essential for CPO to establish itself as a viable and widely adopted solution in the data center optical interconnect domain.

• Technological Pathways and Maturity: Coherent Phased-locked Oscillator (CPO) technology is still in its early stages of development, and the technological pathways need further maturation. While CPO shows promise, it has not yet reached a stage of urgent demand or widespread adoption. Traditional pluggable solutions continue to dominate the market, and it will take time for CPO to become a mainstream option.

• Urgency in Adoption: The demand for CPO form factors is relatively higher in the era of 3.2T optical modules. However, in the current era of 1.6T, pluggable optical modules already offer mature mainstream solutions with 8*200G configurations, effectively meeting the industry's requirements. This lack of immediate urgency for higher-speed CPO solutions may slow down its adoption.

• Business Model Challenges: Implementing CPO technology requires a significant reserve of silicon photonics technology. The close integration of fabrication processes in CPO makes it likely to be led by switch chip manufacturers. However, this dependency on specific manufacturers could potentially impact the progression and widespread application of CPO technology. Overcoming these business model challenges will be crucial for the successful integration of CPO into the market.

Development Prospect and Challenge of LPO

LPO Industry Progress: Domestic and International Parallel

LPO, as an emerging technology, is expected to reach mass production by the end of 2024. Major manufacturers have already unveiled related products, with main suppliers such as Macom, Semtech, and MaxLinear leading the way, and Broadcom advancing the research and development of respective products. Experts believe that North American cloud service providers are actively increasing their computing resources, and in the future, companies like Microsoft, Meta, AWS, and Google could gradually adopt LPO solutions.

Challenges for LPO

As Laser Phased-locked Oscillator (LPO) continues to evolve, LPO also faces the following challenges. By overcoming these hurdles, LPO can unlock its full potential and establish itself as a viable and competitive solution in the market.

• Collaboration Requirements: The implementation of Laser Phased-locked Oscillator (LPO) solutions necessitates close coordination between LPO providers and switch manufacturers. This places higher demands on optical module manufacturers to foster industry-wide collaboration, both upstream and downstream.

• Short Transmission Distance: One challenge with LPO is the removal of the Digital Signal Processing (DSP) component. This removal can result in increased bit error rates within the system, leading to reduced transmission distances. To overcome this limitation, efforts must be made to address the issue of limited range and optimize the system's error correction mechanisms.

• Standardization in Early Stages: Standardization for LPO is still at an early stage. There may be challenges with interoperability. Companies need to possess the technical expertise to define technical specifications and solutions, explore the boundaries between devices and modules, and conduct extensive integration and interoperability testing. In other words, LPO currently fits more closed systems with a single vendor. If a multi-vendor approach is chosen without strong internal capabilities, there may be issues with defining problems and pointing fingers, in which case the traditional DSP-based solutions might be preferable.

• Limitations in Electrical Channel Design: Experts point out that LPO presents certain challenges for the electrical channel design on the system side. The mainstream specification for SerDes is currently 112G, which is expected to soon upgrade to 224G. It is believed that LPO cannot meet the requirements for the upcoming 224G SerDes.

Summary

While Large Parallel Optics (LPO) and Co-Packaged Optics (CPO) are emerging as future trends for data center interconnection, pluggable optical modules remain the current standard. Catering to the intensifying requirements of AI compute power, FS (Fiberstore) offers 800G pluggable Ethernet and InfiniBand modules. These modules can support data transfer rates of up to 800 gigabits per second, a bandwidth capacity vital for the performance of AI-based applications. The table below is the product list.