LPO vs. CPO: Which Data Center Optical Interconnect Will Be the Next Generation Domination?

Data centers are growing, and speed and efficiency have become the core elements of their development. Against this background, the competition in optical interconnect technology has entered a white-hot stage. Laser Phase-Locked Oscillator (LPO) and Coherent Phase-Locked Oscillator (CPO) have become the leading technologies in this field. This article will analyze in detail the different features, advantages, and challenges of these two technologies to help you better understand their key role in data center optical interconnect.

 

The Rise of CPO and LPO

 

The power consumption of traditional 10Gb optical modules is around 1 W (copper module around 2 W). Still, with the introduction of 400G and 800G optical modules, the power consumption rapidly soars to 30 W. Worse, when the number of optical modules reaches dozens (e.g., 48 modules, the total power consumption may be as high as 48 × 30 = 1440 W), their power consumption often accounts for 40% or more of the total power consumption of the entire equipment. The surge in energy consumption of optical communication devices has brought tremendous pressure on data centers, especially in terms of energy costs. It has become a problem that cannot be ignored. Therefore, how to effectively reduce power consumption and save energy has become an urgent problem in the industry.

 

Because of this, CPO and LPO solutions have emerged and have quickly become two hot topics in the optical module industry, nowadays, many vendors have accelerated the development and layout of related technologies, hoping to take the lead in this wave of technological revolution.

 

What is CPO?

 

CPO, or Co-Packaged Optics, is a cutting-edge tech that combines optics and switching chips. It does away with the usual pluggable optical modules, bringing the optical engine and the switching chip closer together. This, in turn, boosts the speed of electrical signals between them, enhancing overall performance.CPO solutions are smaller and more energy-efficient than traditional optical modules, making them a great choice.

 

 

CPO Technology Development

 

The development of CPO technology can be divided into two stages specifically:

 

Phase I: Near-Package Optics (NPO) Technology

 

Before CPO technology is fully mature, NPO technology is a more convenient transition program. It offers the advantages of low cost and low power consumption in a relatively short period and is particularly suitable for initial system development and deployment.

 

Phase II: CPO Technology

 

This is the latest form of silicon photonic switching technology designed to minimize network costs and power consumption. In the CPO solution, the optical engine and switching chip are tightly packaged in the same module, which improves overall system performance and energy efficiency.

 

In practice, the optical engine is mainly responsible for optical/electrical conversion in the switching network. The most common form is the pluggable module, but new product forms are emerging as technology continues to evolve. The CPO form integrates the switching chip and the optical engine in a single slot, while the NPO places them separately on the same system board. While both utilize optoelectronic modules, they vary in power consumption and efficiency performance due to differences in package location and cabling distance.

 

 

Benefits of CPO

Picture a high-tech telecom infrastructure where space optimization and energy efficiency are key to handling the surge in traffic and speeds. By combining CPO technology, we can achieve faster and more reliable communications while also creating space for network technologies to grow smart and energy-efficiently. Here are the main benefits of CPO:

 

Improved Signal Integrity

CPO improves signal integrity by reducing the transmission distance of electrical signals, which means you won't need as much signal compensation. This advancement not only enhances the flexibility of PCB design but also optimizes the overall performance, resulting in more design freedom and efficiency.

 

Compact Design, High Integration

CPO technology integrates photonic and electronic components in the same package, this significantly reduces the size of the system compared to the traditional method of packaging optical modules and electronic chips separately. This compactness is especially important in high-density integrated circuits where every inch of space is critical.

 

Reduced Latency

CPO is able to significantly reduce power consumption and improve the efficiency of conventional LPO technology by integrating electronics and optics using a CMOS process. Since both the optical module and the electronics chip are packaged in the same frame, the signal transmission path is significantly shortened, reducing latency compared to separate packaging methods.

 

According to LightCounting's report in December 2022, the demand for network rates for HPC (high performance computing) is already more than 10 times higher than the current rate. Against the backdrop of such demand, CPO is expected to reduce the power consumption of the existing pluggable optical module architecture by 50%, effectively solving the problem of high-speed and high-density interconnect transmission. Meanwhile, domestic vendors are also actively researching and developing CPO technology to promote its continuous progress and maturity.

 

What is LPO?

 

LPO (Linear Drive Pluggable Optics) refers to a solution that employs direct drive linear technology in fiber optic modules. Unlike traditional optical module designs, LPO eliminates common legacy components such as DSPs (Digital Signal Processors) and CDRs (Clock Data Recovery), which effectively reduces system power consumption and latency. This design is great because it's energy-efficient and performs well, but it's got some tradeoffs. For one, it's not the best at fixing mistakes, and it can't go as far as other systems. So, LPO technology is better for short-distance communication, like inside data centers. It's especially good when you need to be super fast and don't need it to be 100% error-free.

 

 

Traditional pluggable optical modules usually rely on DSP chips to process high-speed signals to ensure a low BER, but this also brings higher power consumption. In the case of a 400G optical module, for example, the power consumption of a 7nm DSP is about 4W, which accounts for almost half of the power consumption of the entire module. In contrast, LPO reduces power consumption by omitting these extra components, but at the cost of reduced transmission distance and error correction capability.

 

 

Advantages of LPOs

 

Linear Drive Pluggable Optics (LPOs) offer a number of advantages in the optical interconnect space that make them ideal for improving data center connectivity and meeting the demands of modern networks. Below are a few of the key benefits of LPO technology:

 

Hot Swap and Simplified Maintenance

Unlike CPO, LPO maintains the traditional modular design of optical components, which makes it a low-interference, easy-to-maintain solution. If a component in a CPO system fails, the entire switch usually needs to be disassembled, creating a maintenance headache. However, the LPO supports hot-swapping, which is more convenient to replace failed modules without interrupting system operation, minimizing downtime.

 

Reduced Latency

The LPO forgoes the traditional digital signal processor (DSP), thus eliminating the need for a complex signal reduction process. This design results in a much lower system processing overhead, which effectively reduces latency. This is especially important for situations that require extremely low latency, like inter-GPU communication in HPC.

 

Reduced Cost

By not relying on DSPs, LPOs effectively reduce power consumption and overall operating costs.DSP technology is currently dominated by a handful of companies (e.g., Broadcom and Inphi), making it a high-cost, resource-limited expertise.LPOs' lack of reliance on DSPs helps to reduce the need for these scarce resources, which in turn opens up diversification opportunities in the supply chain and improves market competitiveness.

 

The high level of interest in developing and applying LPO (Linearly Driven Pluggable Optics) technology from all parties, from upstream chip makers to switching suppliers to end users, suggests that the business outlook is very promising. At OFC 2023, industry-leading companies such as Macom and Broadcom showcased their linear drive solutions, further emphasizing the trend of this technology. Meanwhile, the linear drive solution introduced by Arista has been recognized by prominent switch vendors, including Innovium and NVIDIA, reflecting the industry's consensus on the immense value of LPO technology in cutting-edge networking applications.

 

CPO vs. LPO: Which Technology Will Dominate Data Center Optical Interconnects?

 

LPO technology represents a strategic choice, tailored for specific scenarios such as short-haul applications. While LPO forgoes legacy components such as DSP/CDR, which may present a small performance tradeoff in terms of BER, the power, cost, and latency advantages it brings are significant. Unlike CPO, LPO emphasizes efficiency and cost reduction in specific applications, and thus, it has unique strengths and weaknesses.

 

Although LPO technology emerged a little later than CPO, it is expected to become more popular in the market at a faster rate. The differences between CPO and LPO are shown in the table below:

 

 

Future Prospects and Challenges of CPO

The Future Market of CPO is Promising

CPO technology provides a new direction for increasing the density of optical interconnects and is expected to be commercialized for applications above 100T. According to LightCounting, CPO shipments will begin with 800G and 1.6T ports and are expected to be commercialized between 2024 and 2025. By 2026 to 2027, CPO shipments are expected to increase significantly in short-range data communication scenarios for hyperscale cloud service providers, and market demand is expected to grow significantly. CIR expects the CPO market to reach $5.4 billion in total revenue by 2027.

The number of CPO ports is expected to grow from 50,000 in 2023 to 4.5 million in 2027 in terms of global sales. By 2027, CPO ports are expected to account for nearly 30 percent of shipments in the 800G and 1.6T categories. According to a report by Yole, the CPO market will generate revenues of approximately $38 million in 2022, and this is expected to grow to $2.6 billion by 2033, with a compound annual growth rate (CAGR) of 46%.

 

 

Challenges for CPO

 

While CPO technology offers new promise for advanced optical interconnects, it still faces challenges in terms of technology maturity, market adoption urgency, and business models. These issues must be addressed effectively for CPO to become a mainstream solution in the data center optical interconnect space.

 

Technology Path and Maturity: Coherent phase-locked oscillator (CPO) technology is still in the early stages of development, and its technology path needs further refinement. Although the future of CPO is quite bright, it has not yet reached the level of widespread application and still needs time to accumulate more practical cases and experiences. In contrast, traditional pluggable optical modules still dominate the market, and CPO may take a few years to replace them as the mainstream solution.

 

Urgency of Adoption: In the era of 3.2T optical modules, the form factor and size requirements for CPO technology were high. However, in the current 1.6T era, pluggable optical modules still provide a mature mainstream solution, and their 8x200G configurations already meet the needs of most industries. The current demand for higher-speed CPO solutions is not urgent, which may slow down the rapid rollout of CPOs in the market.

 

Business Model Challenges: The implementation of CPO technology requires a large reserve of silicon photonics technology, which means that its development will be highly dependent on the R&D and production capacity of the relevant technology. Dependence on specific manufacturers may become a major obstacle to the widespread application of CPO technology. First, the silicon photonics technology reserve and the complexity of the manufacturing process have concentrated CPO production and R&D in the hands of a few manufacturers. This dependence may lead to the emergence of a technology monopoly in the market, which in turn affects the competitiveness and technological innovation of different vendors. In addition, too much reliance on a single vendor may limit technological development, increase supply chain uncertainty, and affect the flexibility and freedom of end users in choosing CPO solutions.

 

While CPO technology shows great potential, it still faces challenges regarding technology maturity, market urgency, and the establishment of relevant business models.

 

LPO Development Prospects and Challenges

Progress of the LPO Industry: Parallelism at Home and Abroad

 

LPO technology, an emerging optical interconnect solution, is expected to achieve mass production by the end of 2024. As this technology continues to evolve, major global vendors have already launched related products. Leading vendors such as Macom, Semtech, and MaxLinear have made significant progress in technology development, and Broadcom is actively promoting its own LPO solution. Industry experts believe that as the demand for computing resources from North American cloud service providers increases, large companies such as Microsoft, Meta, AWS, and Google will gradually adopt LPO solutions to improve data transfer efficiency and reduce latency.

 

Challenges for LPO

 

While LPO shows great potential for optical interconnects, there are still a number of challenges to its development. Addressing these issues is imperative for LPO technology to be favored in the marketplace. The following are some of the key challenges that LPO is likely to face during its development:

 

Collaboration Requirements: LPO deployment requires close collaboration between LPO vendors and switch manufacturers. Optical module manufacturers must upgrade their technology and ensure close coordination between the upstream and downstream supply chains to drive collaboration and innovation across the industry.

 

Short Transmission Distances: LPOs are designed to eliminate the DSP component, which makes the BER higher and limits the distance the signal can be transmitted. LPOs may not be as stable as traditional optical modules when transmitting over long distances. Initially, LPOs can establish a connection within a few meters to a few tens of meters, and in the future, the transmission distance may be extended up to 500M. To address this issue, optimizing the transmission distance and strengthening the system's error correction mechanisms is important.

 

Standardization is at an early stage: The standardization process of LPO is still at an early stage, and the interoperability of the technology may encounter certain difficulties. At present, LPO is more applicable to single-vendor closed systems. If a multi-vendor program is adopted and the internal integration capability is insufficient, it may lead to system incompatibility and problems in the division of responsibilities. In this case, traditional DSP-based solutions may still be more advantageous.

 

Limitations of electrical channel design: LPOs place higher demands on the electrical channel design on the system side. Currently, SerDes mainly use 112G specifications and are expected to be upgraded to 224G in the future; however, LPO technology may not be able to meet the transmission requirements of the upcoming 224G SerDes, which may limit its performance in the future large-scale data transmission applications.

 

In conclusion, although LPO technology has many potentials and advantages, more technological innovations and industry cooperation are needed to break through the above challenges and realize widespread applications.

 

Conclusion

 

While Linear-drive Pluggable Optics (LPO) and Co-Packaged Optics (CPO) are emerging as future trends in data center interconnect technology, pluggable optical modules are still the industry's standard solution. As the demand for higher performance and lower latency in the data center continues to grow, vendors like QSFPTEK have introduced 800G pluggable Ethernet and InfiniBand modules. These modules are capable of delivering data rates of up to 800 Gigabits per second, providing critical bandwidth support for HPC applications. In the future, as technology evolves further and the industry adopts new technologies, the CPOs and LPOs may gradually replace existing pluggable optical modules, especially in hyperscale data centers and high-performance computing applications.