Requirements for a 400g optical module

A 400G optical module supports 400 Gbps data rates using multi-lane PAM4 signaling, QSFP-DD or QSFP112 form factors, and single-mode or multimode fiber connectivity.

Form Factor and Interface

400G optical modules are typically available in QSFP-DD (Quad Small Form Factor Pluggable – Double Density) or QSFP112 form factors, which extend traditional QSFP interfaces from 4 lanes to 8 lanes, enabling 400G throughput while maintaining backward compatibility with QSFP/QSFP28 modules . Modules are mechanically compliant with standards like QSFP-DD Type 2A and support CMIS 4.1 or 5.3 for management . The electrical interface usually consists of 8 lanes at 50 Gbps each, often employing a gearbox DSP to convert lane speeds for compatibility with host devices .

Signaling and Modulation

400G modules use PAM4 (Pulse Amplitude Modulation 4-level) signaling, which doubles the data rate per lane compared to traditional NRZ signaling . Coherent 400G modules, such as Cisco's Ultra Long Haul (ULH) QSFP-DD, operate at up to 118 Gbaud and support C-band and L-band coherent transmission for long-distance DWDM networks .

Optical Connectivity

• Short-reach modules (DR4/VR4): Use parallel single-mode or multimode fiber with MPO/MTP-12 connectors, supporting distances from 50 meters to 500 meters .
• Medium-reach modules (FR4): Use duplex LC connectors over single-mode fiber, supporting distances up to 2 km .
• Ultra-long-haul coherent modules: Support duplex LC connectors with wavelength multiplexing, optimized for regional and long-haul DWDM networks .

Performance and Features

• Aggregate data rate: 400 Gbps, achieved via 8 lanes of 50 Gbps each or 4 lanes of 100 Gbps in PAM4 mode .
• Breakout capability: Modules can be logically divided into multiple lower-speed ports, e.g., 4x100G or 2x200G, for flexible deployment .
• Power consumption: Typically within 24W envelope for QSFP-DD modules, with low-power designs for data center efficiency .
• Management: Supports CMIS and Coherent CMIS for monitoring and configuration .
• Spectral efficiency: Coherent modules offer multiple application select codes for 75–112.5 GHz channel spacing .

Applications

400G optical modules are used in:

• High-performance data centers and AI/ML clusters for high-throughput connectivity .
• Regional and ultra-long-haul DWDM networks for coherent transmission .
• Flexible network architectures with breakout options for leaf/spine or multi-plane designs . In summary, 400G optical modules combine high-speed PAM4 signaling, multi-lane architecture, and flexible optical interfaces to support modern high-density, high-throughput networking environments, ranging from short-reach data center links to ultra-long-haul coherent optical networks .