Requirements for 400G optical modules

400G optical modules are standardized under IEEE 802.3bs and OIF agreements, with key form factors including QSFP-DD, OSFP, CFP8, and coherent optics like 400ZR and OpenZR+.

IEEE 802.3bs Standard

The IEEE 802.3bs, ratified in December 2017, defines the 400 Gigabit Ethernet (400GbE) standard, specifying transmission over optical fiber using multiple physical coding sublayers (PCS) and physical medium dependent (PMD) interfaces . It supports four- and eight-lane electrical and optical implementations, including 400GBASE-SR16, 400GBASE-DR4, 400GBASE-FR8, and 400GBASE-LR8. The standard introduced PAM4 modulation, which doubles data rate efficiency per lane while maintaining lower power consumption compared to NRZ signaling, enabling cost- and energy-efficient 400G optics . IEEE 802.3bs ensures interoperability across vendors and backward compatibility with 100G and 200G technologies.

Main Form Factors

1. QSFP-DD (Quad Small Form Factor – Double Density): Extends the classic QSFP interface from 4 lanes to 8 lanes, enabling 400G while remaining backward-compatible with QSFP/QSFP28 modules .
2. OSFP (Octal Small Formfactor Pluggable): A new interface standard with 8 lanes, not backward-compatible with QSFP/XFP modules, designed for high-density data center deployments .
3. CFP8: Scales the older CFP4 standard to 8 channels, supporting 400G transmission .
4. CDFP and COBO: Early 400G form factors; COBO mounts optics directly onto the PCB but lacks hot-swap capability .

Coherent Optics Standards

400ZR and OpenZR+ are coherent optics standards defined by the Optical Internetworking Forum (OIF) for long-reach 400G transmission over DWDM networks . These modules, typically in QSFP-DD form factor, support point-to-point links up to 120 km (ZR) and regional to long-haul distances with amplification (OpenZR+). They are designed for multi-vendor interoperability, high channel density, and efficient optical signal-to-noise ratio (OSNR) performance .

Transmission Methods and Applications

• PAM4 Modulation: Used in most 400G modules, allowing each symbol to represent two bits, effectively doubling the data rate per lane .
• SR4/SR4.2 Modules: Short-range multimode fiber modules for intra-data center connections, supporting distances up to 150 meters on OM4 fiber .
• ZR/ZR+ Modules: Coherent modules for metro and long-haul networks, supporting 400G over DWDM with advanced FEC and modulation schemes .

Summary

The 400G optical module ecosystem is defined by IEEE 802.3bs for Ethernet, multiple pluggable form factors (QSFP-DD, OSFP, CFP8), and coherent optics standards (400ZR, OpenZR+) for extended reach. These standards ensure interoperability, high-density deployment, energy efficiency, and scalability for modern data centers, AI clusters, and carrier networks .