What are the functions of linear direct-drive optical modules

Linear direct-drive optical modules (LPOs) enable high-speed, low-latency optical interconnects by removing DSPs from the module and directly interfacing with host SerDes.

Overview

Linear direct-drive optical modules, also known as linear pluggable optics (LPOs), are designed to transmit and receive optical signals without the intermediate digital signal processing (DSP) typically used in conventional optical modules. Instead, the host ASIC's SerDes directly drives the optical transmitter and receives signals from the photodiode/TIA, creating a fully analog, linear signal path. This approach reduces module complexity, power consumption, and latency while maintaining a pluggable form factor compatible with existing network infrastructure.

Key Advantages

Lower Power Consumption: By eliminating the DSP, module power can drop by approximately 40%, reducing from ~14W to under 8W per module. Reduced Latency: Direct analog transmission avoids the re-timing and equalization delays introduced by DSPs, which is particularly beneficial for AI/ML and high-performance computing applications. Cost Efficiency: Fewer components and simpler module design lower manufacturing costs while preserving compatibility with standard optical form factors like OSFP, QSFP-DD, and QSFP112.

High Bandwidth Support: LPOs can handle high data rates (e.g., 112–224 Gbps per lane) and are suitable for 800G network systems.

Technical Considerations

Signal Integrity (SI): Removing the DSP shifts the responsibility for signal conditioning to the host SerDes. Designers must carefully manage crosstalk, reflection, inter-symbol interference (ISI), and channel loss across the electrical and optical path. Linear Performance: Each component in the link, including the modulator driver, optical transmitter, photodiode, and TIA, must operate as linearly as possible to maintain signal fidelity.

Hybrid Approaches: At very high speeds (e.g., 224 Gbps), nonlinearity in the optical domain may require additional components like Automatic Gain Control (AGC) and Clock Data Recovery (CDR) to ensure reliable transmission. System Integration: LPOs are often used in conjunction with co-packaged optics (CPO), where optical components are integrated closely with the host ASIC to minimize electrical trace losses and improve performance.

Applications

Linear direct-drive optical modules are particularly suited for: Data Center Networks: High-speed interconnects for AI/ML clusters and HPC systems. Enterprise and Telecom Networks: Short- and long-reach optical links with reduced power and latency.

Next-

Generation Ethernet Switches and NICs: Supporting 800G and beyond with efficient, pluggable modules.

Summary

Linear direct-drive optical modules represent a shift from DSP-based optical modules to a simpler, lower-latency, and more power-efficient architecture. By leveraging the host SerDes for direct analog driving of optical components, LPOs reduce complexity and cost while enabling high-speed, high-performance optical interconnects. Challenges remain in signal integrity, linearity, and high-speed nonlinearity management, but ongoing developments in SerDes design and co-packaged optics are addressing these issues.