Introduction

400 G QSFP‑DD has become the cornerstone of high-bandwidth networking. Designed to deliver 400 Gbps per port, the Quad Small Form-factor Pluggable Double Density (QSFP-DD) interface doubles the lane density of standard QSFP modules, making it ideal for hyperscale data centers, cloud computing, HPC, and AI-driven network fabrics.

QSFP‑DD 400G Transceivers

Form Factor & Electrical Interface

Built upon the QSFP28 footprint, QSFP‑DD incorporates an 8-lane electrical interface (each 50 G using PAM4 or 25 G using NRZ), delivering up to 400 Gbps. Ascent Optics notes the dual-row 76-pin design enables backward compatibility with QSFP28/56 devices—a key trait for smooth network upgrades.

PAM4 Modulation & Optical Lane Structure

PAM4 Modulation

Modern 400 G QSFP‑DD optics rely on PAM4 (Pulse Amplitude Modulation with four levels). By encoding two bits per symbol, PAM4 doubles data throughput without increasing signal bandwidth—essential for high-speed links.

Optical lane configurations vary by module type:

  • SR4/SR8: 4 or 8 parallel multimode lanes (e.g., SR4 modules use 4 parallel lanes each operating at ~100 Gbps PAM4 (53.125 GBd), while SR8 modules utilize 8 lanes at ~50 Gbps PAM4 (26.5625 GBd), enabling high-density short-reach connections over multimode fiber.). Ideal for short-range (<100 m) interconnects in MMF backbone.

  • DR4: 4-lane single-mode MPO‑12 optics supporting ~500 m over SMF.

  • FR4: 4×100 G CWDM lanes using duplex LC, supporting 2 km on SMF.

  • LR4: Similar to FR4 but engineered for longer 10 km reaches at 1310 nm.

Thermal Design & Power Management

Operating at 400 G, efficient thermal management is vital. LINK‑PP’s LQD-CW400-LR4C, for example, supports up to ~12 W dissipation and employs a flat-top, SMT connector design, equipped with a flat-top heat sink and designed for improved airflow compatibility in high-density cages. These features maintain stable performance under heavy workloads.

Standards & Backward Compatibility

QSFP‑DD complies with the QSFP-DD MSA and IEEE 802.3bs and 802.3cd standards, supporting the 400GAUI-8 electrical interface — an 8-lane 50 Gbps PAM4 signaling protocol used for 400G Ethernet framing. While offering backward compatibility with 100/200/400 G devices via adapters or breakout cables. This flexibility simplifies deployment and stepwise upgrades.

LINK‑PP LQD‑CW400 Series: Practical Implementations

400G QSFP‑DD Optical Modules

LINK‑PP offers a full range of 400G QSFP‑DD Optical Modules tailored for diverse applications:

  • LQD-CW400-DR4C – QSFP-DD PAM4 500 m over SMF, Support 4×100G-DR;

  • LQD-CW400-FR4C – 2 km over SMF with duplex LC connector;

  • LQD-CW400-LR4C – Up to 10 km reach, 1310 nm CWDM optics

Each unit supports CMIS 4.0, digital optical monitoring, and operates from 0 °C to 70 °C with <10 W typical power. Free samples and OEM integration support are available via LINK‑PP’s official store at l‑p.com.

Applications & Benefits

  • Data Center & Spine–Leaf: High-density 400 G connections optimize space and performance.

  • HPC & AI clusters: Low latency and high throughput critical for training and simulation.

  • 5G Fronthaul & Transport: FR4 and DR4 modules support robust link lengths in urban networks.

  • Cloud‑scale & DCI: Scalable interfaces across campus and metro environments.

These modules offer twice the port density of previous generations, seamless 800 G migration readiness, and broad backward compatibility with existing 100G/200G infrastructures through breakout or adapter solutions.

Conclusion

400 G QSFP‑DD is now the standard bearer for next-generation network infrastructure. Its compact design, PAM4 modulation, diverse optical options, and thermal resilience deliver unmatched performance across data center, HPC, AI, and telecom applications.

For enterprises preparing to transition to 400 G networks, the LINK‑PP LQD‑CW400 series provides a portfolio of standards-compliant, interoperability-tested modules — spanning SR to LR4 — designed to integrate smoothly into existing ecosystems. Visit LINK‑PP’s official store to explore product spec sheets, request a quote, or request free evaluation samples.