Linear Pluggable Optics (LPO) and Linear Receive Optics (LRO) have emerged as frontrunners in the quest for optimizing data transmission. These technologies offer distinct benefits and operational efficiencies, not just within traditional network setups but also in active optical cables (AOCs) used across data centers and high-performance computing environments.
Here, we’re discussing the features and use cases of each of these formats.
Linear Pluggable Optics (LPO): Features and Benefits
LPOs are optical transceivers that do not incorporate DSP chips. They rely on simpler signal modulation techniques and straightforward electronic components. This leads to lower power consumption and lower production costs than DSP-based solutions. This simplicity also translates to reduced latency, making LPOs effective for high-speed environments.
Primary Use Cases
LPOs are well-suited for use in networking and computing equipment such as switches and Network Interface Cards (NICs), where high throughput and low latency are essential. They are also an attractive option for high-speed computing applications, including artificial intelligence (AI) and machine learning (ML).
Challenges and Limitations
- Calibration and Interoperability: Since LPOs do not include onboard DSPs to auto-correct signal imperfections, calibration depends on the host ASICs within the switches or NICs. This places a burden on end-users.
- Compatibility Issues: LPO faces challenges with standardization and compatibility. MSA (Multi-Source Agreement) compliance helps, but variations in implementation among different vendors can lead to interoperability issues.
Current Usage and Industry Standards
Despite these challenges, LPO modules are increasingly adopted in modern data centers and are becoming integral components of co-packaged optics (CPO) and near-package optics (NPO) solutions. These applications benefit from the direct, short connections between ICs and LPO optical modules, maximizing the efficiency enhancements offered by LPO technology.
Certain configurations of LPO modules support unidirectional 8 x 112 Gbps interfaces, which are ideal for specific high-speed computing applications where the directionality and bandwidth are crucial factors.
Linear Receive Optics (LRO) or Half-retimed Linear Optics (HALO)
Like LPOs, LRO modules lack a DSP on the receiving side. But unlike LPO, LRO modules retain a DSP in the transmitting path, ensuring that the signal meets standards of integrity in compliance with industry protocols like IEEE 802.3. This configuration allows LROs to strike a balance between performance and power efficiency.
The receive path in LROs typically incorporates high-linearity Transimpedance Amplifiers (TIAs), reducing the complexity and power consumption associated with DSPs while maintaining high performance.
Primary Use Cases
By keeping the DSP in the transmit path, LROs can achieve longer data transmission distances than standard LPOs. They are ideal for ultra-high-speed data transmission, both in data centers and AI/ML and supercomputing systems. These systems require rapid and reliable data processing capabilities, which LROs are well placed to meet.
Development and Industry Adoption
Credo’s Transmit-only DSP chips and the subsequent adoption by key industry players underline the critical role of LROs in modern optical networking. Demonstrations at major conferences like OFC also highlight the ongoing evolution of this technology.
LPO & LRO: Quick Comparison
Both LPO and LRO offer linear drive attributes for efficient, direct optical connections that enhance system performance without the complexities and costs of additional processing layers.
Here is a summary of key differences between them.
| Feature | Linear Pluggable Optics (LPO) | Linear Receive Optics (LRO) |
| DSP Inclusion | Does not incorporate DSP chips in either transmit or receive paths. | Includes a DSP chip in the transmit path but not in the receive path. |
| Power Consumption | More energy efficient due to the absence of DSPs. | Generally reduced power consumption than full DSP solutions, but slightly higher than LPO due to the DSP in the transmit path. |
| Cost | Lower cost due to simpler design and the elimination of DSPs. | Cost-effective but can be higher than LPO due to the inclusion of a DSP in the transmit path. |
| Latency | Reduced latency because there is no digital processing delay. | Lower latency compared to full DSP solutions, but slightly higher than LPO due to processing in the transmit path. |
| Calibration | Calibration depends heavily on the host ASICs since there is no DSP to auto-correct signals. | Modules can be factory-tuned for compliance and performance, reducing the burden on host system calibration. |
| Performance | Performance and calibration are variable, dependent on the host system’s capabilities. | Consistent performance and lower bit error rates due to transmit path DSP handling signal conditioning. |
| Interoperability | Can interoperate with DSP-based systems but requires powerful ASICs and well-designed transmission lines. | High interoperability and ease of deployment due to DSP presence in the transmit path ensuring standard compliance. |
| Ideal Applications | Suited for applications requiring high speed with lower power and cost, like AI and ML computing environments. | Ideal for high-reliability environments requiring consistent performance across various conditions, like data centers. |
| Industry Adoption | Growing interest, particularly in cost- sensitive and power-limited applications. | Increasing adoption due to its balance of performance and efficiency, suitable for a wider range of applications. |
Industry Trends and Future Outlook
The growing need for efficient, high speed optical solutions is driving a wave of innovation in optical technologies. As we saw at OFC 2024, the debate between using full DSP modules versus more linear solutions like LPO and LRO is shifting towards 200G/lane designs. This strongly suggests that future innovations in 2025 might bring even more efficient and powerful optical solutions.
Both formats are playing a crucial role in meeting the growing demands of AI and machine learning systems, where thousands of GPUs process vast amounts of data swiftly and efficiently.
Fast Photonics USA specializes in providing advanced optical interconnect solutions, including both Linear Pluggable Optics (LPO) and Linear Receive Optics (LRO).
Contact us to find out how our solutions can meet your data transmission needs.
